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Skeletal muscle Na+-K+-2Cl−cotransporter (NKCC) activity provides a potential mechanism for regulated K+uptake. β-Adrenergic receptor (β-AR) activation stimulates skeletal muscle NKCC activity in a MAPK pathway-dependent manner. We examined potential G protein-coupled pathways for β-AR-stimulated NKCC activity. Inhibition of Gs-coupled PKA blocked isoproterenol-stimulated NKCC activity in both the slow-twitch soleus muscle and the fast-twitch plantaris muscle. However, the PKA-activating agents cholera toxin, forskolin, and 8-bromo-cAMP (8-BrcAMP) were not sufficient to activate NKCC in the plantaris and partially stimulated NKCC activity in the soleus. Isoproterenol-stimulated NKCC activity in the soleus was abolished by pretreatment with pertussis toxin (PTX), indicating a Gi-coupled mechanism. PTX did not affect the 8-BrcAMP-stimulated NKCC activity. PTX treatment also precluded the isoproterenol-mediated ERK1/2 MAPK phosphorylation in the soleus, consistent with NKCC's MAPK dependency. Inhibition of isoproterenol-stimulated ERK activity by PTX treatment was associated with an increase in Akt activation and phosphorylation of Raf-1 on the inhibitory residue Ser259. These results demonstrate a novel, muscle phenotype-dependent mechanism for β-AR-mediated NKCC activation that involves both Gsand Giprotein-coupled mechanisms.

Although 3α-substituted metabolites of progesterone are well established to interact with GABAAreceptor/Cl−channels, the nature of the interaction(s) remains uncertain. We used patch-clamp recording to study the interaction with GABAAreceptor/Cl−channels expressed by embryonic hippocampal neurons differentiating in culture and nonneuronal cells transfected with GABAAreceptor subunits. Allopregnanolone primarily induced multiphasic current responses in neurons, which were eliminated by bicuculline, an antagonist of GABA at GABAAreceptor/Cl−channels. Similar multiphasic responses blocked by bicuculline were induced by allopregnanollone in nonneuronal cells transfected with α1and γ2subunits, indicating that the steroid activation of GABAAreceptor/Cl−channels occurred independently of GABA. Fluctuation analyses of current responses to allopregnanolone and GABA revealed underlying channel activities with similar estimated unitary properties. However, although both agonists activated Cl−channels with similar estimated short and long burst-length durations, most of those stimulated by the steroid were short, while most of those opened by GABA were long. Allopregnanolone potentiated GABA-evoked Cl−currents in nonneuronal cells transfected with α1and β2or β3subunits, which did not exhibit multiphasic responses to the steroid, indicating another, independent action of the steroid at activated receptors. Pertussis toxin treatment eliminated the low-amplitude current and attenuated the high-amplitude current induced by allopregnanolone in a reversible manner. Mastoparan, which activates G proteins directly, triggered a high-amplitude current after a delay, which was blocked by bicuculline. The results indicate that allopregnanolone interacts with GABAAreceptor/Cl−channels expressed by embryonic hippocampal neurons in multiple ways, some of which are mediated by G proteins.

This study examined the effects of D2-like dopamine receptor activation on Na+-K+-ATPase activity while apical-to-basal, ouabain-sensitive, amphotericin B-induced increases in short-circuit current and basolateral K+( IK) currents in opossum kidney cells were measured. The inhibitory effect of dopamine on Na+-K+-ATPase activity was completely abolished by either D1- or D2-like receptor antagonists and mimicked by D1- and D2-like receptor agonists SKF-38393 and quinerolane, respectively. Blockade of basolateral K+channels with BaCl2(1 mM) or glibenclamide (10 μM), but not apamin (1 μM), totally prevented the inhibitory effects of quinerolane. The K+channel opener pinacidil decreased Na+-K+-ATPase activity. The inhibitory effect of quinerolane on Na+-K+- ATPase activity was abolished by pretreatment of opossum kidney cells with pertussis toxin (PTX). Quinerolane increased IKacross the basolateral membrane in a concentration-dependent manner; this effect was abolished by pretreatment with PTX, S-sulpiride, and glibenclamide. SKF-38393 did not change IK. Both H-89 (protein kinase A inhibitor) and chelerythrine (protein kinase C inhibitor) failed to prevent the stimulatory effect of quinerolane on IK. The stimulation of the D2-like receptor was associated with a rapid hyperpolarizing effect, whereas D1-like receptor activation was accompanied by increases in cell membrane potential. It is concluded that stimulation of D2-like receptors leads to inhibition of Na+-K+-ATPase activity and hyperpolarization; both effects are associated with the opening of K+channels.

In this study, we analyzed in rat myometrial cells the signaling pathways involved in the endothelin (ET)-1-induced extracellular signal-regulated kinase (ERK) activation required for the induction of DNA synthesis. We found that inhibition of protein kinase C (PKC) by Ro-31–8220 abolished ERK activation. Inhibition of phospholipase C (PLC) by U-73122 or of phosphoinositide (PI) 3-kinase by wortmannin partially reduced ERK activation. A similar partial inhibition was observed after treatment with pertussis toxin or PKC downregulation by phorbol ester treatment. The effect of wortmannin was additive with that produced by PKC downregulation but not with that due to pertussis toxin. These results suggest that both diacylglycerol-sensitive PKC, activated by PLC products, and diacylglycerol-insensitive PKC, possibly activated by a Gi-PI 3-kinase-dependent process, are involved in ET-1-induced ERK activation. These two pathways were found to be activated mainly through the ETAreceptor subtype. ET-1 and phorbol ester stimulated Src activity in a PKC-dependent manner, both responses being abolished in the presence of Ro-31–8220. Inhibition of Src kinases by PP1 abrogated phorbol ester- and ET-1-induced ERK activation. Finally, ET-1 activated Ras in a PP1- and Ro-31–8220-sensitive manner. Altogether, our results indicate that ET-1 induces ERK activation in rat myometrial cells through the sequential stimulation of PKC, Src, and Ras.

Protease-activated receptor-2 (PAR-2) plays a role in inflammatory reactions in airway physiology. Proteases cleaving the extracellular NH2 terminus of receptors activate or inactivate PAR, thus possessing a therapeutic potential. Using RT-PCR and immunocytochemistry, we show PAR-2 in human airway epithelial cell lines human bronchial epithelial (HBE) and A549. Functional expression of PAR-2 was confirmed by Ca2+imaging studies using the receptor agonist protease trypsin. The effect was abolished by soybean trypsin inhibitor and mimicked by the specific PAR-2 peptide agonist SLIGKV. Amplitude and duration of PAR-2-elicited Ca2+ response in HBE and A549 cells depend on concentration and time of agonist superfusion. The response is partially pertussis toxin (PTX) insensitive, abolished by the phospholipase C inhibitor U-73122, and diminished by the inositol 1,4,5-trisphosphate receptor antagonist 2-aminoethoxydiphenyl borate. Cathepsin G altered neither the resting Ca2+ level nor PAR-2-elicited Ca2+response. Thermolysin, a prototypic bacterial metalloprotease, induced a dose-dependent Ca2+ response in HBE, but not A549, cells. In both cell lines, thermolysin abolished the response to a subsequent trypsin challenge but not to SLIGKV. Thus different epithelial cell types express different PAR-2 with identical responses to physiological stimuli (trypsin, SLIGKV) but different sensitivity to modifying proteases, such as thermolysin.

Extracellular ATP plays an important role in ischemic preconditioning (IPC) through the activation of P2ypurinoceptors. This study examined whether ATP-stimulated P2ypurinoceptors are coupled to pertussis toxin (PTX)-insensitive G protein and whether activation of this pathway enhances myocardial protection afforded by IPC. The rat was treated with PTX for 48 h, and the heart was then isolated and buffer perfused. The heart underwent IPC by three cycles of 5-min ischemia and 5-min reperfusion before 25 min of global ischemia. Isovolumic left ventricular function was measured, and functional recovery at 30 min after reperfusion was taken as an end point of myocardial protection. PTX pretreatment partially inhibited functional protection by IPC. Treatment with 100 μM 8-( p-sulfophenyl) theophylline (SPT) during IPC had no further effect on PTX-induced inhibition of functional protection by IPC, whereas suramin (300 μM) or reactive blue (RB) (10 μM) completely abolished myocardial protection in the preconditioned heart pretreated with PTX. Supplementation with adenosine (30 μM), ATP (30 μM), or UTP (50 μM) significantly enhanced IPC-induced functional protection, although preconditioning with these nucleotides without IPC had no protective effect. Adenosine-enhanced IPC was inhibited by pretreatment with PTX and SPT but not by suramin or RB, whereas ATP-enhanced IPC was inhibited by suramin or RB in combination with PTX pretreatment. On the other hand, UTP-enhanced IPC was not affected by PTX pretreatment but was inhibited by suramin or RB. Adenosine supplemented IPC without PTX pretreatment and ATP supplemented IPC with PTX pretreatment were not affected by nitric oxide synthase inhibitor Nω-nitro-l-arginine methyl ester (100 μM). Although the protein kinase C inhibitor Ro318425 (0.3 μM) or tyrosine kinase inhibitor genistein (50 μM) had no significant effect on the functional protection afforded by adenosine-supplemented IPC without PTX pretreatment and ATP-supplemented IPC with PTX pretreatment, the combination of Ro318425 and genistein attenuated functional protection afforded by both the purinoceptor agonist-supplemented IPC. These results suggest the crucial involvement of PTX-sensitive and -insensitive G protein coupled purinoceptors in enhanced IPC by supplementation with adenosine, ATP, and UTP.

Regulators of G protein signaling (RGS proteins) interact with Gαq and Gαi and accelerate GTPase activity. These proteins have been characterized only within the past few years, so our understanding of their importance is still preliminary. We examined the effect of oxytocin on RGS2 mRNA expression to help determine the role of RGS proteins in oxytocin signaling in human myometrial cells in primary culture. Oxytocin increased RGS2 mRNA concentration maximally by 1 or 2 h in a dose-dependent and agonist-specific manner. RGS2 mRNA levels were also elevated by treatment with Ca2+ ionophore, phorbol ester, or forskolin. Oxytocin's effects were completely inhibited by an intracellular Ca2+ chelator and partially blocked by a protein kinase C inhibitor, indicating that intracellular Ca2+ concentration is the primary signal for oxytocin elevation of RGS2 mRNA levels. Use of pharmacological inhibitors indicated that part of oxytocin-stimulated RGS2 mRNA expression is mediated by Gi/tyrosine kinase activities. Although oxytocin does not stimulate increases in intracellular cAMP concentration, agents that elevate intracellular cAMP concentrations and cause myometrial relaxation may possibly cause heterologous desensitization to oxytocin via RGS2 expression. These results suggest that RGS2 may be important in regulating the myometrial response to oxytocin.

Parathyroid hormone (PTH) and dopamine (DA) inhibit Na-K ATPase activity and sodium-phosphate cotransport in proximal tubular cells. We previously showed that PTH and DA inhibit phosphate transport in opossum kidney (OK) cells through different signaling pathways. Therefore, we hypothesized that PTH and DA also inhibit Na-K ATPase through divergent pathways. We measured PTH and DA inhibition of Na-K ATPase activity in the presence of inhibitors of signaling pathways. PTH and DA inhibited Na-K ATPase in a biphasic manner, the early inhibition through protein kinase C (PKC)- and phospholipase A2(PLA2)-dependent pathways and the late inhibition through protein kinase A- and PLA2-dependent pathways. Inhibition of extracellular signal-regulated kinase (ERK) activation blocked early and late inhibition of Na-K ATPase by PTH but not by DA. Pertussis toxin blocked early and late inhibition by DA but not by PTH. Treatment with DA, but not PTH, resulted in an early downregulation of basolateral membrane expression of the α-subunit, whereas total cellular expression remained constant for both agonists. We conclude that PTH and DA regulate Na-K ATPase by different mechanisms through activation of divergent pathways.

Human airway smooth muscle cells treated with lysophosphatidic acid (LPA) and epidermal growth factor (EGF) exhibit synergistic stimulation of mitogenesis (Ediger TL and Toews ML. J Pharmacol Exp Ther 294: 1076–1082, 2000). The effects of LPA treatment of human airway smooth muscle cells on EGF receptor (EGFR) regulation have now been investigated. LPA treatment for 12–24 h resulted in a twofold increase in 125I-EGF binding and EGFR protein levels as assessed by Western blot analysis. Competition binding assays indicated single-site binding with an affinity of 3 nM, and the affinity was not changed by LPA treatment. EGFR upregulation was blocked by cycloheximide and actinomycin D, suggesting that LPA influences transcriptional regulation of EGFR expression. Inhibitor studies revealed a prominent role for activation of mitogen-activated protein kinase and p70 ribosomal S6 kinase. Both synergism and EGFR upregulation increased with increased cell density, whereas EGFR expression in control cells decreased. The similar requirements for exposure time, LPA concentrations, and cell confluence suggest that EGFR upregulation may be one contributing factor to the synergistic stimulation of mitogenesis seen with LPA plus EGF.

We examined the regulatory role of cytosolic phospholipase A2(cPLA2) and phosphatidylinositol (PI)-specific phospholipase C (PLC) in the degranulation of human eosinophils and leukotriene (LT) C4synthesis. Activation with formyl-Met-Leu-Phe + cytochalasin B (fMLP/B) caused a time-dependent release of eosinophil peroxidase (EPO) and LTC4, which was inhibited by pertussis toxin. By immunoblotting, eosinophil PLC-β2 and -γ2 isoforms were identified, and PLC activation was measured as a function of inositol 1,4,5-trisphosphate concentration. Stimulated release of EPO and intracellular Ca2+concentration was inhibited by ET-18-OCH3, a PI-PLC inhibitor, whereas trifluoromethylketone (TFMK), a cPLA2blocker, had no inhibitory effect. Both TFMK and ET-18-OCH3attenuated stimulated arachidonate release and LTC4secretion, suggesting that activation of both PLC and cPLA2is essential for LTC4synthesis caused by fMLP/B. The structurally unrelated protein kinase C inhibitors bisindolylmaleimide, Ro-31–8220, and Go-6976 all blocked fMLP/B-induced EPO release but not LTC4secretion. 1,2-bis(2-Aminophenoxy)ethane- N,N,N′,N′- tetraacetic acid acetoxymethyl ester, an intracellular Ca2+chelator, suppressed both EPO release and LTC4secretion. We found that fMLP/B-induced LTC4secretion from human eosinophils is regulated by PI-PLC through calcium-mediated activation of cPLA2. However, cPLA2does not regulate eosinophil degranulation.

Bordetella pertussisgenerates a bacterial toxin utilized in signal transduction investigation because of its ability to ADP ribosylate specific G proteins. We previously noted that pertussis toxin (PTX) directly activates endothelial cells, resulting in disruption of monolayer integrity and intercellular gap formation via a signaling pathway that involves protein kinase C (PKC). We studied the effect of PTX on the activity of the 42- and 44-kDa extracellular signal-regulated kinases (ERK), members of a kinase family known to be activated by PKC. PTX caused a rapid time-dependent increase in bovine pulmonary artery endothelial cell ERK activity that was significantly attenuated by 1) pharmacological inhibition of MEK, the upstream ERK activating kinase, 2) an MEK dominant-negative construct, and 3) PKC inhibition with bisindolylmaleimide. There was little evidence for the involvement of either Gβγ-subunits, Ras GTPases, Raf-1, p60src, or phosphatidylinositol 3′-kinases in PTX-mediated ERK activation. Both the purified β-oligomer binding subunit of the PTX holotoxin and a PTX holotoxin mutant genetically engineered to eliminate intrinsic ADP ribosyltransferase activity completely reproduced PTX effects on ERK activation, suggesting that PTX-induced ERK activation involves a novel PKC-dependent signaling mechanism that is independent of either Ras or Raf-1 activities and does not require G protein ADP ribosylation.

Intracellular signaling mechanisms by the angiogenesis inhibitors endostatin and angiostatin remain poorly understood. We have found that endostatin (2 μg/ml) and angiostatin (5 μg/ml) elicited transient, approximately threefold increases in intracellular Ca2+concentration ([Ca2+]i). Acute exposure to angiostatin or endostatin nearly abolished subsequent endothelial [Ca2+]iresponses to carbachol or to thapsigargin; conversely, thapsigargin attenuated the Ca2+signal elicited by endostatin. The phospholipase C inhibitor U-73122 and the inositol trisphosphate (IP3) receptor inhibitor xestospongin C both inhibited endostatin-induced elevation in [Ca2+]i, and endostatin rapidly elevated endothelial cell IP3levels. Pertussis toxin and SB-220025 modestly inhibited the endostatin-induced Ca2+signal. Removal of extracellular Ca2+inhibited the endostatin-induced rise in [Ca2+]i, as did a subset of Ca2+-entry inhibitors. Peak Ca2+responses to endostatin and angiostatin in endothelial cells exceeded those in epithelial cells and were minimal in NIH/3T3 cells. Overnight pretreatment of endothelial cells with endostatin reduced the subsequent acute elevation in [Ca2+]iin response to vascular endothelial growth factor or to fibroblast growth factor by ∼70%. Intracellular Ca2+signaling may initiate or mediate some of the cellular actions of endostatin and angiostatin.

Calcium transport across a monolayer of Madin-Darby canine kidney (MDCK) cells was measured in response to stimulation of the basal surface with calcium-sensing receptor (CaR) agonists. Stimulation of the CaR resulted in a time- and concentration-dependent inhibition of calcium transport but did not change transepithelial voltage or resistance. Inhibition of transport was not altered by pretreatment of cells with pertussis toxin but was blocked by the phospholipase C (PLC) inhibitor U-73122. To determine a potential mechanism by which the CaR could inhibit calcium transport, we measured activity of the plasma membrane calcium ATPase (PMCA). Stimulation of the CaR on the basal surface resulted in an inhibition of the PMCA in a concentration- and PLC-dependent manner. Thus stimulation of the CaR inhibits both calcium transport and PMCA activity through a PLC-dependent pathway. These studies provide the first direct evidence that calcium can inhibit its own transcellular absorption in a model of the distal tubule. In addition, they provide a potential mechanism for the CaR to inhibit calcium transport, inhibition of PMCA.

Cytokines such as tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) are assumed to mediate anorexia during bacterial infections. To improve our understanding of the role that these two cytokines serve in mediating infection during anorexia, we investigated the ability of pentoxifylline (PTX), a potent inhibitor of TNF-alpha production, to block the anorectic effects of the bacterial products lipopolysaccharide (LPS) and muramyl dipeptide (MDP) in rats. Intraperitoneally injected PTX (100 mg/kg body wt) completely eliminated the anorectic effect of intraperitoneally injected LPS (100 microg/kg body wt) and attenuated the anorectic effect of a higher dose of intraperitoneally injected LPS (250 microg/kg body wt). Concurrently, PTX pretreatment suppressed low-dose LPS-induced TNF-alpha production by more than 95% and IL-1beta production 39%, as measured by ELISA. Similarly, high-dose LPS-induced TNF-alpha production was reduced by approximately 90%. PTX administration also attenuated the tolerance that is normally observed with a second injection of LPS. In addition, PTX pretreatment attenuated the hypophagic effect of intraperitoneally injected MDP (2 mg/kg body wt) but had no effect on the anorectic response to intraperitoneally injected recombinant human TNF-alpha (150 ug/kg body wt). The results suggest that suppression of TNF-alpha production is sufficient to attenuate LPS- and MDP-induced anorexia. This is consistent with the hypothesis that TNF-alpha plays a major role in the anorexia associated with bacterial infection.

Lipoxins (LX) are eicosanoids generated via transcellular biosynthetic routes during inflammation, hypersensitivity reaction, and after angioplasty. LXs are modulators of leukocyte trafficking and vascular tone. Their influence on the coagulation cascade has not been determined. In this study, we evaluated the influence of LXs on the expression of tissue factor (TF), a key regulator of coagulation. TF activity was measured in lysates of monocytes, human umbilical vein endothelial cells, and ECV304 cells using a one-stage clotting assay. LXA4stimulated TF activity in each cell type. The influence of LXA4on TF activity by ECV304 cells was studied further to explore the mechanism of induction of TF expression. LXA4-induced TF activity was dose dependent, cycloheximide sensitive, and associated with increased TF mRNA levels. Induction of TF activity was specific for LXA4and was not observed with LXB4, the other major lipoxin generated by mammalian cells. Furthermore, ECV304 cell TF expression was not influenced by 15( R/S)-methyl-LXA4or 16-phenoxy-LXA4, synthetic analogs of LXA4that activate the myeloid LXA4receptor, and was not modulated by SKF-104353, which blocks LXA4bioactivities transduced through the putative shared LXA4/LTD4receptor. LXA4-stimulated TF expression was blunted by pertussis toxin and by GF-109203X, an inhibitor of protein kinase C, and was not associated with degradation of IκBα. Our results establish that LXA4induces TF activity via cell signaling pathways with different structural and receptor requirements from those described for inhibition of leukocyte-endothelial cell interactions. They suggest a role for LXA4as a modulator of TF-related vascular events during inflammation and thrombosis.

We have previously demonstrated that pertussis toxin (PTX)-sensitive G protein (GPTX) plays a major role in coronary microvascular vasomotion during hypoperfusion. We aimed to elucidate the role of GPTX during increasing metabolic demand. In 18 mongrel dogs, coronary arteriolar diameters were measured by fluorescence microangiography using a floating objective. Myocardial oxygen consumption (MV˙o 2) was increased by rapid left atrial pacing. In six dogs, PTX (300 ng/ml) was superfused onto the heart surface for 2 h to locally block GPTX. In eight dogs, the vehicle (Krebs solution) was superfused in the same way. Before and after each treatment, the diameters were measured during control (130 beats/min) and rapid pacing (260 beats/min) in each group. Metabolic stimulation before and after the vehicle treatment caused 8.6 ± 1.8 and 16.1 ± 3.6% dilation of coronary arterioles 2 and coronary flow velocity were comparable between the vehicle and PTX groups. In four dogs, 8-phenyltheophylline (10 μM, superfusion for 30 min) did not affect the metabolic dilation of arterioles (15.3 ± 2.0% before and 16.4 ± 3.8% after treatment; 84.3 ± 11.0 μm at control, n = 8). Thus we conclude that GPTXplays a major role in regulating the coronary microvascular tone during active hyperemia, and adenosine does not contribute to metabolic vasodilation via GPTX activation.

Adenosine A2a-receptor activation enhances shortening of isolated cardiomyocytes. In the present study the effect of A2a-receptor activation on the contractile performance of isolated rat hearts was investigated by recording left ventricular pressure (LVP) and the maximal rate of LVP development (+dP/d tmax). With constant-pressure perfusion, adenosine caused concentration-dependent increases in LVP and +dP/d tmax, with detectable increases of 4.1 and 4.8% at 10−6M and maximal increases of 12.0 and 11.1% at 10−4M, respectively. The contractile responses were prevented by the A2a-receptor antagonists chlorostyryl-caffeine and aminofuryltriazolotriazinyl-aminoethylphenol (ZM-241385) but were not affected by the β1-adrenergic antagonist atenolol. The adenosine A1-receptor antagonist dipropylcyclopentylxanthine and pertussis toxin potentiated the positive inotropic effects of adenosine. The A2a-receptor agonists ethylcarboxamidoadenosine and dimethoxyphenyl-methylphenylethyl-adenosine also enhanced contractility. With constant-flow perfusion, 10−5M adenosine increased LVP and +dP/d tmaxby 5.5 and 6.0%, respectively. In the presence of the coronary vasodilator hydralazine, adenosine increased LVP and +dP/d tmaxby 7.5 and 7.4%, respectively. Dipropylcyclopentylxanthine potentiated the adenosine contractile responses with constant-flow perfusion in the absence and presence of hydralazine. These increases in contractile performance were also antagonized by chlorostyryl-caffeine and ZM-241385. The results indicate that adenosine increases contractile performance via activation of A2areceptors in the intact heart independent of β1-adrenergic receptor activation or changes in coronary flow.

Low oxygen (O2) is the key stimulus for expression of vascular endothelial growth factor (VEGF) in several adherent cells. Whether hypoxia also directs the release of VEGF protein from neutrophils (polymorphonuclear neutrophils; PMN) and platelets has not been investigated. We therefore compared VEGF release of platelets, PMN, and human vascular smooth muscle cells (HSMC) in response to hypoxia with that to activators of cellular degranulation. In contrast to HSMC, VEGF release from PMN and platelets or VEGF mRNA expression in PMN was not stimulated under hypoxic conditions (1% O2). Hypo- or hyperthermia and acidosis, other conditions potentially associated with ischemic and inflammatory tissue injury, also did not stimulate VEGF secretion from PMN. However, stimulation of platelets with thrombin and of PMN with phorbol 12-myristate 13-acetate induced a time-dependent release of VEGF, peaking after 30 and 60 min, respectively. This was blocked by the degranulation inhibitor pentoxifylline but not by the protein-synthesis inhibitor cycloheximide. We conclude that rapid release of VEGF from platelets and PMN may occur independently of oxygenation during inflammation and hemostasis.

Basal contractility and responses to β-adrenoceptor activation are compromised in hearts from rats with chronic portal vein stenosis. Here we report the effect of partial ligation of the portal vein on myocardial G protein expression, β-adrenoceptor-G protein coupling, and excitation-contraction coupling (ECC). Contractility (d T/d t) was reduced 30–50% in right and left ventricles, but the rate of relaxation (−d T/d t) was unaffected. Isoproterenol-induced positive inotropism was diminished, but there was no difference in ED50. The concentration-dependent increase in −d T/d t was unaffected. Gsα and Giα expression, cholera toxin- and pertussis toxin-induced ADP-ribosylation, and formation of the agonist-receptor-Gscomplex were unaffected by portal vein stenosis. Of the components of ECC examined, the caffeine-sensitive sarcoplasmic reticulum Ca2+pool was reduced 35%, although the Ca2+uptake and release processes were unchanged; the apparent density of L-type Ca2+channels decreased 60% with no change in affinity; the dihydropyridine Ca2+channel agonist BAY K 8644 produced relative changes in d T/d t that were similar in both groups, suggesting normal function in the remaining Ca2+channels; and Na+/Ca2+exchange was reduced 50% in the portal vein stenosis group. These data suggest that the effect of portal vein stenosis on the myocardium is the result of alterations to ECC.

This study used whole cell patch clamp recordings in rat hypothalamic slice preparations to evaluate the effects of GABAB receptor activation on GABAA-mediated inhibitory postsynaptic currents (IPSCs) in paraventricular nucleus magnocellular neurons evoked by electrical stimulation in the suprachiasmatic nucleus (SCN). Baclofen induced a dose-dependent (1–10 μM) and reversible reduction in SCN-evoked IPSC amplitude (11/11 cells), blockable with 2-hydroxysaclofen (300 μM; 3/3 cells). IPSCs displayed paired-pulse depression (PPD), attenuated by both baclofen and 2-hydroxysaclofen, but neither altered resting membrane conductances or IPSC time constants of decay. Baclofen induced a significant dose-dependent (1–100 μM) reduction in frequency, but not amplitude, of spontaneous IPSCs and miniature IPSCs, reversible with 2-hydroxysaclofen pretreatment. Baclofen effects and PPD persisted in slices pretreated with pertussis toxin (PTX) and N-ethylmaleimide, implying that these GABABreceptors are coupled to PTX-insensitive G proteins. Responses were unaltered by barium (2 mM) or nimodipine, ruling out involvement of K+ channels and L-type Ca2+ channels. Thus pre- and postsynaptic GABAB and GABAA receptors participate in SCN entrainment of paraventricular neurosecretory neurons.

Opioid receptors can couple to K+and Ca2+channels, adenylyl cyclase, and phosphatidyl inositol turnover. Any of these actions may be important in the regulation of neurotransmitter and hormone release from excitable cells. GH3cells exhibit spontaneous oscillations of intracellular Ca2+concentration ([Ca2+]i) and prolactin release. Activation of cloned δ-opioid receptors stably expressed in GH3cells inhibits both spontaneous Ca2+signaling and basal prolactin release. The objective of this study was to examine a possible role for K+channels in these processes using the patch-clamp technique, fluorescence imaging, and a sensitive ELISA for prolactin. The selective δ receptor agonist [d-Pen2,d-Pen2]enkephalin (DPDPE) inhibited [Ca2+]ioscillations in GH3cells expressing both μ and δ receptors (GH3MORDOR cells) but had no effect on control GH3cells or cells expressing μ receptors alone (GH3MOR cells). The inhibition of [Ca2+]ioscillations by DPDPE was unaffected by thapsigargin pretreatment, suggesting that this effect is independent of inositol 1,4,5-triphosphate-sensitive Ca2+stores. DPDPE caused a concentration-dependent inhibition of prolactin release from GH3MORDOR cells with an IC50of 4 nM. DPDPE increased inward K+current recorded from GH3MORDOR cells but had no significant effect on K+currents recorded from control GH3cells or GH3MOR cells. The μ receptor agonist morphine also had no effect on currents recorded from control cells but activated inward K+currents recorded from GH3MOR and GH3MORDOR cells. Somatostatin activated inward currents recorded from all three cell lines. The DPDPE-sensitive K+current was inwardly rectifying and was inhibited by Ba2+but not TEA. DPDPE had no effect on delayed rectifier-, Ca2+-, and voltage-activated or A-type K+currents, recorded from GH3MORDOR cells. Ba2+attenuated the inhibition of [Ca2+]iand prolactin release by DPDPE, whereas TEA had no effect, consistent with an involvement of KIRchannels in these actions of the opioid.

Serotonin (5-HT) stimulates mitogenesis in rat renal mesangial cells through a G protein-coupled 5-HT2Areceptor. We tested the hypothesis that oxidants might be involved in the signal transduction pathway linking the receptor to extracellular signal-regulated protein kinase (ERK). 5-HT rapidly increased the activity and phosphorylation of ERK. These effects were blocked by the 5-HT2Areceptor antagonist ketanserin. The peak effect was noted at 5–10 min, and half-maximal stimulation was achieved at 10–30 nM 5-HT. Chemical inhibitor and activator studies supported the involvement of phospholipase C, protein kinase C (PKC), and reactive oxygen species (ROS, i.e., H2O2and superoxide) generated by an NAD(P)H oxidase-like enzyme in the ERK activation cascade. Mapping studies supported a location for the NAD(P)H oxidase enzyme and the ROS downstream from PKC. Our studies are most consistent with an ERK activation pathway as follows: 5-HT2Areceptor → Gqprotein → phospholipase C → diacylglycerol → classical PKC → NAD(P)H oxidase → superoxide → superoxide dismutase → H2O2→ mitogen-activated extracellular signal-regulated kinase → ERK. These studies demonstrate a role for the 5-HT2Areceptor in rapid, potent, and efficacious activation of ERK in rat renal mesangial cells. They support a role for oxidants in conveying the stimulatory signal from 5-HT, because 1) chemical antioxidants attenuate the 5-HT signal, 2) oxidants and 5-HT selectively activate ERK to a similar degree, 3) 5-HT produces superoxide and H2O2in these cells, and 4) a specific enzyme [NAD(P)H oxidase] has been implicated as the source of the ROS, which react selectively downstream of classical PKC.

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are potent lipid growth factors with similar abilities to stimulate cytoskeleton-based cellular functions. Their effects are mediated by a subfamily of G protein-coupled receptors (GPCRs) encoded by endothelial differentiation genes ( edgs). We hypothesize that large quantities of LPA and S1P generated by activated platelets may influence endothelial cell functions. Using an in vitro wound healing assay, we observed that LPA and S1P stimulated closure of wounded monolayers of human umbilical vein endothelial cells and adult bovine aortic endothelial cells, which express LPA receptor Edg2, and S1P receptors Edg1 and Edg3. The two major components of wound healing, cell migration and proliferation, were stimulated individually by both lipids. LPA and S1P also stimulated intracellular Ca2+mobilization and mitogen-activated protein kinase (MAPK) phosphorylation. Pertussis toxin partially blocked the effects of both lipids on endothelial cell migration, MAPK phosphorylation, and Ca2+ mobilization, implicating Gi/o-coupled Edg receptor signaling in endothelial cells. LPA and S1P did not cross-desensitize each other in Ca2+ responses, suggesting involvement of distinct receptors. Thus LPA and S1P affect endothelial cell functions through signaling pathways activated by distinct GPCRs and may contribute to the healing of wounded vasculatures.

The adrenal gland contains resident macrophages, some of which lie adjacent to the catecholamine producing chromaffin cells. Because macrophages release a variety of secretory products, it is possible that paracrine signaling between these two cell types exists. Of particular interest is the potential paracrine modulation of voltage-gated calcium channels ( ICa), which are the main calcium influx pathway triggering catecholamine release from chromaffin cells. We report that prostaglandin E2(PGE2), one of the main signals produced by macrophages, inhibited ICain cultured bovine adrenal chromaffin cells. The inhibition is rapid, robust, and voltage dependent; the activation kinetics are slowed and inhibition is largely reversed by a large depolarizing prepulse, suggesting that the inhibition is mediated by a direct G-protein βγ subunit interaction with the calcium channels. About half of the response to PGE2was sensitive to pertussis toxin (PTX) incubation, suggesting both PTX-sensitive and -insensitive G proteins were involved. We show that activation of macrophages by endotoxin rapidly (within minutes) releases a signal that inhibits ICain chromaffin cells. The inhibition is voltage dependent and partially PTX sensitive. PGE2is not responsible for this inhibition as blocking cyclooxygenase with ibuprofen did not prevent the production of the inhibitory signal by the macrophages. Nor did blocking the lipoxygenase pathway with nordihydroguaiaretic acid alter production of the inhibitory signal. Our results suggest that macrophages may modulate ICaand catecholamine secretion by releasing PGE2and other chemical signal(s).

The role of pertussis toxin (PTX)–sensitive G-proteins in corticostriatal synaptic transmission and long-term synaptic depression (LTD) was examined using extracellular field potential and whole cell voltage-clamp recordings in striatal slices. High-frequency stimulation (HFS) produced LTD, defined as long-lasting decreases both in synaptically driven population spikes (PSs) measured with field potential recording and in excitatory postsynaptic currents (EPSCs) measured with whole cell recording. Striatal LTD could not be induced in slices obtained from rats that had received a unilateral intrastriatal injection of PTX. However, LTD could be induced in slices obtained from paired control slices. Furthermore, striatal LTD was prevented by pretreatment with N-ethylmaleimide (NEM), another compound that disrupts the function of PTX-sensitive G-proteins. NEM, itself, also potentiated PS and EPSC amplitudes. In addition, NEM increased the frequency and amplitude of both spontaneous and miniature EPSCs and decreased the paired-pulse facilitation ratio, suggesting that it may act on both pre- and postsynaptic sites. The findings suggest that PTX-sensitive G-proteins have multiple roles at corticostriatal synapses, including regulation of synaptic transmission at both pre- and postsynaptic sites, and a key role in striatal LTD.

In newborn pig pial arterioles and cocultures of cerebral microvascular endothelial and smooth muscle cells, hypercapnia increases cAMP. In the intact cerebral circulation, both the increase in cAMP and the accompanying vasodilation require the presence of PGI2. Using piglet cerebral microvascular smooth muscle in primary culture, we addressed the hypothesis that, in the presence of PGI2, hypercapnia-induced changes in extracellular pH cause increases in cAMP. The stable PGI2-receptor agonist iloprost did increase production of cAMP in response to combined extracellular pH and pHi (11 ± 6 vs. 32 ± 10% in the absence and presence of 10−10 M iloprost, respectively). However, there was no positive dose-response relationship between iloprost concentration and stimulation of cAMP production by acidosis (e.g., 58 ± 9 vs. 41 ± 5% in the presence of 10−12 and 10−9 M iloprost, respectively). Rapid decreases in pHi stimulate the cAMP production. Decreases in extracellular pH do not appear to contribute further. The G protein inhibitor pertussis toxin did not augment cAMP production in response to decreasing pHi. We conclude that PGI2 receptor activation permits another mechanism to enhance cAMP generation in response to intracellular, but not extracellular, acidosis and that the mechanism of the permissive effect of PGI2 does not involve inhibition of a pertussis toxin-sensitive G protein.

We examined actions of arginine vasopressin (AVP) and amastatin (an inhibitor of the aminopeptidase that cleaves AVP) on synaptic currents in slices of rat parabrachial nucleus using the nystatin-perforated patch recording technique. AVP reversibly decreased the amplitude of the evoked, glutamate-mediated, excitatory postsynaptic current (EPSC) with an increase in paired-pulse ratio. No apparent changes in postsynaptic membrane properties were revealed by ramp protocols, and the inward current induced by a brief application of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid was unchanged after AVP. The reduction induced by 1 μM AVP could be blocked by a V1AVP receptor antagonist, [d(CH2)51-O-Me-Tyr2-Arg8]-vasopressin (Manning compound, 10 μM). Bath application of an aminopeptidase inhibitor, amastatin (10 μM), reduced the evoked EPSC, and AVP induced further synaptic depression in the presence of amastatin. Amastatin’s effects also could be antagonized by the Manning compound. Corticotropin-releasing hormone slightly increased the EPSC at 1 μM, and coapplication with AVP attenuated the AVP response. Pretreatment of slices with 1 μg/ml cholera toxin or 0.5 μg/ml pertussis toxin for 20 h did not significantly affect AVP’s synaptic action. The results suggest that AVP has suppressant effects on glutamatergic transmission by acting at V1AVP receptors, possibly through a presynaptic mechanism involving a pertussis-toxin- and cholera-toxin-resistant pathway.

Extracellular stimuli induce cytoskeleton reorganization (stress-fiber formation) in cells and Ca2+sensitization in intact smooth muscle preparations by activating signaling pathways that involve Rho proteins, a subfamily of the Ras superfamily of monomeric G proteins. In airway smooth muscle, the agonists responsible for cytoskeletal reorganization via actin polymerization are poorly understood. Carbachol-, lysophosphatidic acid (LPA)-, and endothelin-1-induced increases in filamentous actin staining are indicative of actin reorganization (filamentous-to-globular actin ratios of 2.4 ± 0.3 in control cells, 6.7 ± 0.8 with carbachol, 7.2 ± 0.8 with LPA, and 7.4 ± 0.9 with endothelin-1; P < 0.001; n = 14 experiments). Although the effect of all agonists was blocked by C3 exoenzyme (inactivator of Rho), only carbachol was blocked by pertussis toxin. Although carbachol-induced actin reorganization was blocked in cells pretreated with antisense oligonucleotides directed against Gαi-2alone, LPA- and endothelin-1-induced actin reorganization were only blocked when both Gαi-2and Gqα were depleted. These data indicate that in human airway smooth muscle cells, carbachol induces actin reorganization via a Gαi-2pathway, whereas LPA or endothelin-1 induce actin reorganization via either a Gαi-2or a Gqα pathway.

Involvement of Akt/Protein kinase B (PKB), a serine/threonine kinase with a pleckstrin-homology domain, in angiotensin II (ANG II)-induced signal transduction was investigated in cultured vascular smooth muscle cells (VSMC). Stimulation of the cells with ANG II led to a marked increase in the kinase activity of Akt/PKB, which coincided with Ser-473 phosphorylation. ANG II-stimulated Akt/PKB activation was rapid, concentration dependent, and inhibited by the AT1-receptor antagonist CV-11974, but not by pertussis toxin. Akt/PKB activity was stimulated by the Ca2+ionophore ionomycin, suggesting the possible involvement of Ca2+in ANG II-stimulated Akt/PKB activation. However, blockade of Ca2+mobilization by BAPTA-AM only partially inhibited ANG II-stimulated Akt/PKB activation. ANG II-stimulated Akt/PKB activation was inhibited by the tyrosine kinase inhibitors genistein and herbimycin A and by the phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin and LY-294002. These results indicate that ANG II stimulates Akt/PKB activity via AT1receptors in VSMC and that the activities of tyrosine kinase and PI3K are required for this activation.

Thyroid hormone [l-thyroxine (T4)] rapidly induced phosphorylation and nuclear translocation (activation) of mitogen-activated protein kinase (MAPK) in HeLa and CV-1 cells in the absence of cytokine or growth factor. A pertussis toxin-sensitive and guanosine 5′- O-(3-thiotriphosphate)-sensitive cell surface mechanism responsive to T4and agarose-T4, suggesting a G protein-coupled receptor, was implicated. Cells depleted of MAPK or treated with MAPK pathway inhibitors showed reduced activation of MAPK and of the signal transducer and activator of transcription STAT1α by T4; they also showed reduced T4potentiation of the antiviral action of interferon-γ (IFN-γ). T4treatment caused tyrosine-phosphorylated MAPK-STAT1α nuclear complex formation and enhanced Ser-727 phosphorylation of STAT1α, in the presence or absence of IFN-γ. STAT1α-deficient cells transfected with STAT1α containing an alanine-for-serine substitution at residue 727 (STAT1αA727) showed minimal T4-stimulated STAT1α activation. IFN-γ induced the antiviral state in cells containing wild-type STAT1α (STAT1αwt) or STAT1αA727; T4potentiated IFN-γ action in STAT1αwtcells but not in STAT1αA727cells. T4-directed STAT1α Ser-727 phosphorylation is MAPK mediated and results in potentiated STAT1α activation and enhanced IFN-γ activity.

We recently isolated and characterized 86-amino acid CCK-releasing peptide from porcine intestinal mucosa. The sequence of this peptide is identical to that of porcine diazepam-binding inhibitor (DBI). Intraduodenal administration of DBI stimulates the CCK release and elicits pancreatic secretion in rats. In this study we utilized a murine tumor cell line (STC-1 cells) that contains CCK to examine if DBI directly acts on these cells to stimulate CCK release. We investigated the cellular mechanisms responsible for this action. We showed that DBI33–50, a biologically active fragment of DBI1–86, significantly stimulated CCK secretion in STC-1 cells. This action was abolished by Ca2+-free medium. The mean basal intracellular Ca2+concentration ([Ca2+]i) was 52 nM in fura 2-loaded STC-1 cells. DBI33–50(1–1,000 nM) elicited Ca2+oscillations; DBI33–50(10 nM) increased the oscillation frequency to 5 cycles/10 min and elicited a net [Ca2+]iincrease (peak − basal) to 157 nM. In contrast, bombesin and forskolin caused an initial transient [Ca2+]ifollowed by a small sustained [Ca2+]iplateau. Withdrawal of extracellular Ca2+abolished Ca2+oscillations stimulated by DBI33–50. L-type Ca2+channel blockers nifedipine and diltiazem (3–10 μM) markedly attenuated DBI-stimulated Ca2+oscillations. In other cell types L-type Ca2+channels are activated by cAMP-protein kinase A. DBI33–50failed to stimulate cAMP formation in STC-1 cells. Similarly, DBI33–50had no effect on myo-inositol 1,4,5-trisphosphate concentration ([IP3]), whereas bombesin caused an eightfold increase in [IP3] over basal. In addition, inhibitors of phospholipase C (U-73122), phospholipase A2(ONO-RS-082), and protein tyrosine kinase (genistein) did not alter the Ca2+oscillations elicited by DBI33–50. It appears that DBI33–50acts directly on STC-1 cells to elicit Ca2+oscillations via the voltage-dependent L-type Ca2+channels, resulting in the secretion of CCK. Mediation of this action is by intracellular mechanisms independent of the traditional signal transduction pathways, including phospholipase C, phospholipase A2, protein tyrosine kinase, and cAMP systems.

The GTP-binding proteins from cultured A6 epithelia were examined in isolated membrane preparations. Binding of [35S]GTPγS revealed a class of binding sites with an apparent Kdvalue of 100 nM and a Bmaxof 220 pmol/mg protein. Short-term aldosterone treatment of the cells did not modify the binding kinetics, whereas pertussis toxin (PTX) decreased Bmaxby 50%. The mRNA levels for Gαi-3, Gα0, Gαs, and Gαqwere not increased after aldosterone. The patterns of small MrG proteins and of PTX-ribosylated proteins were identical in membranes of both control and aldosterone-treated cells. Cross-linking of [α-32P]GTP, in control membranes, showed either no labeling or a faint band of Mr59.5 kDa. This protein became prominent after aldosterone, and its labeling decreased with spironolactone. Thus short-term aldosterone does not promote increased expression of known heterotrimeric G proteins in epithelial membranes but activates resident PTX-sensitive Giproteins and stimulates the expression of a specific GTP-binding protein of Mr59.5 kDa.

Sphingosine 1-phosphate (S-1- P) is a bioactive sphingolipid that is released from activated platelets. Extracellular S-1- P augments an inwardly rectifying potassium conductance in cultured atrial preparations, but the electrophysiological effects of this compound in the ventricle are unknown. The electrophysiological effects of S-1- P were examined in single myocytes from rat ventricular muscle. Action potential waveforms and underlying ionic currents in the presence and absence of 3 μM S-1- P (1–6 min) were recorded. S-1- P increased the minimum stimulus current needed to elicit an action potential by ∼100 pA. Pertussis toxin or preexposure to S-1- P did not alter this effect. The action potential waveform was unchanged by S-1- P. The inward sodium current ( I Na) was examined in a range of membrane potentials just negative to the potential for firing an action potential. S-1- P reversibly inhibited peak I Na by ∼50 pA, whereas the inward rectifier potassium current was not significantly changed. The results of this study suggest that S-1- P inhibits rat ventricular excitability by reducing I Na.

It is widely held that only one prostacyclin (IP) receptor exists that can couple to guanine stimulatory nucleotide binding proteins (Gs) leading to activation of adenyl cyclase. Although IP receptor mRNA is expressed in vascular arterial smooth muscle cells and platelets, with lower level expression in mature thymocytes, splenic lymphocytes, and megakaryocytes, there is no molecular evidence for IP receptor expression in renal epithelial cells. The purpose of the present study was to obtain molecular evidence for the expression and localization of the IP receptor and to study the signaling pathways of IP receptor in rat medullary thick ascending limb (MTAL). Biochemical studies showed that IP prostanoids do not increase cAMP in rat MTAL. However, in the presence of vasopressin, inhibition of cAMP formation by prostacyclin (PGI2) analogs is pertussis toxin sensitive and does not activate protein kinase C. In situ hybridization studies localized IP receptor mRNA expression to MTAL in the rat kidney outer medulla. The results of RT-PCR of freshly isolated RNA from MTAL, with primers specific for the mouse IP receptor cDNA, produced an amplification product of the correct predicted size that contained an expected Nco I endonuclease restriction site. We conclude that rat renal epithelial cells express the IP receptor, coupled to inhibition of cAMP production.

Tian, Ning, Thomas N. Hwang, and David R. Copenhagen. Analysis of excitatory and inhibitory spontaneous synaptic activity in mouse retinal ganglion cells. J. Neurophysiol. 80: 1327–1340, 1998. Spontaneous inhibitory and excitatory postsynaptic currents (sIPSCs and sEPSCs) were identified and characterized with whole cell and perforated patch voltage-clamp recordings in adult mouse retinal ganglion cells. Pharmacological dissection revealed that all cells were driven by spontaneous synaptic inputs mediated by glutamate and γ-aminobutyric acid-A (GABAA) receptors. One-half (7/14) of the cells also received glycinergic spontaneous synaptic inputs. Both GABAA and glycine receptor–mediated sIPSCs had rise times (10–90%) of A receptor–mediated sIPSCs were comparable with those of the glycine receptor–mediated sIPSCs. The average decay time constant for monoexponentially fitted sIPSCs was 63.2 ± 74.1 ms (mean ± SD, n = 3278). Glutamate receptor–mediated sEPSCs had an average rise time of 0.50 ± 0.20 ms ( n = 109) and an average monoexponential decay time constant of 5.9 ± 8.6 ms ( n = 2705). Slightly more than two-thirds of the spontaneous synaptic events were monoexponential (68% for sIPSCs and 76% for sEPSCs). The remainder of the events was biexponential. The amplitudes of the spontaneous synaptic events were not correlated with rise times, suggesting that the electrotonic filtering properties of the neurons and/or differences in the spatial location of synaptic inputs could not account for the difference between the decay time constants of the glutamate and GABAA/glycine receptor–mediated spontaneous synaptic events. The amplitudes of sEPSCs were similar to those recorded in tetrodotoxin (TTX), consistent with the events measured in control saline being the response to the release of a single quantum of transmitter. The range of the sIPSC amplitudes in control saline was wider than that recorded in TTX, consistent with some sIPSCs being evoked by presynaptic spikes having an average quantal size greater than one. The rates of sIPSCs and sEPSCs were determined under equivalent conditions by recording with perforated patch electrodes at potentials at which both types of event could be identified. Two groups of ganglion cell were observed; one group had an average sEPSCs/sIPSCs frequency ratio of 0.96 ± 0.77 ( n = 28) and another group had an average ratio of 6.63 ± 0.82 ( n = 7). These findings suggest that a subset of cells is driven much more strongly by excitatory synaptic inputs. We propose that this subset of cells could be off ganglion cells, consistent with the higher frequency of spontaneous action potentials found in off ganglion cells in other studies.

mIMCD-k2 cells are derived from the inner medullary collecting duct of a mouse and exhibit electrogenic sodium absorption and cAMP- and vasopressin (AVP)-stimulated electrogenic chloride secretion [N. L. Kizer, B. Lewis, and B. A. Stanton. Am. J. Physiol. 268 ( Renal Fluid Electrolyte Physiol. 37): F347–F355, 1995; and N. L. Kizer, D. Vandorpe, B. Lewis, B. Bunting, J. Russell, and B. A. Stanton. Am. J. Physiol. 268 ( Renal Fluid Electrolyte Physiol. 37): F854–F861, 1995]. The purpose of the present study was to determine how peptide YY (PYY) affects electrogenic Na+and Cl−current in mIMCD-k2 cells. Short-circuit currents ( Isc) were measured across monolayers of mIMCD-k2 cells mounted in Ussing-type chambers. PYY did not alter baseline Isc, nor did it alter Iscin chloride-free conditions, indicating no effect on electrogenic sodium transport. Baseline chloride current in these cells is low; therefore, chloride short-circuit current[Formula: see text] was stimulated with AVP (10 nM) added to the basolateral surface and 10 μM amiloride added to the apical surface. Although apical applications of PYY had no effect, basolateral application of PYY caused attenuation of[Formula: see text], with the maximal inhibitory dose (100 nM) causing 52 ± 1.3% inhibition (IC50= 0.11 nM). Inhibition by PYY of [Formula: see text] is mediated through the Y2receptor subtype, as PYY-(3–36) was the only PYY analog tested that caused inhibition and was equipotent to PYY. Inhibition by PYY of[Formula: see text] was abolished following incubation with pertussis toxin. We also show that PYY inhibits AVP-stimulated cAMP accumulation, with a maximal inhibitory dose (100 nM) causing a 38% ± 6% inhibition (IC50= 0.16 nM), comparable to inhibition by PYY of [Formula: see text]. We conclude that PYY acts through either Gior Goto inhibit adenylate cyclase activity, leading to a decrease in AVP-stimulated chloride current.

The signal transduction that mediates CCK-induced contraction of gallbladder muscle was investigated in the cat. Contraction was measured by scanning micrometry in single muscle cells isolated enzymatically with collagenase. Production ofd- myo-inositol 1,4,5-trisphosphate (IP3) and sn-1,2-diacylglycerol (DAG) was quantitated using HPLC and TLC, respectively. Protein kinase C (PKC) activity was determined by measuring the phosphorylation of a specific substrate peptide from myelin basic protein, Ac-MBP-(4—14). CCK-induced contraction was blocked by incubation in strontium medium, pertussis toxin (PTx), and antibodies against Giα3or βγ-subunits but was not blocked by Ca2+-free medium or by antibodies against Gq/11α, Giα1–2, or Goα. The contraction induced by CCK was inhibited by the phospholipase C (PLC) inhibitor U-73122, anti-PLC-β3 antibody, and the IP3receptor antagonist heparin but was not inhibited by the the phospholipase D inhibitor propranolol or antibodies against PLC-β1 or PLC-β2. Western blot analysis of gallbladder muscle revealed the presence of PLC-β2 and PLC-β3 but not PLC-β1. CCK caused a 94% increase in IP3generation and an 86% increase in DAG generation. A low dose of CCK caused PKC translocation, and CCK-induced contraction was blocked by the PKC inhibitor H-7. A high dose of CCK, however, caused no PKC translocation, and its contraction was blocked by the calmodulin antagonist CGS9343B. In conclusion, CCK contracts cat gallbladder muscle by stimulating PTx-sensitive Gi 3protein coupled with PLC-β3, producing IP3and DAG. Low doses activate PKC, whereas high doses activate calmodulin.

Proliferation and immunoglobulin secretion of B lymphocytes are regulated by specific antigens and numerous accessory immunomodulatory factors. Lysophosphatidic acid (LPA) is a glycerophospholipid mediator that is released from activated blood platelets, attains high levels in serum, and exerts potent stimulatory effects on, e.g., neutrophils, monocytes, and T lymphocytes. LPA is also generated by a secretory, cytokine-inducible phospholipase A2present in high concentrations in inflammatory exudates and septic states. We investigated effects of LPA on human Epstein-Barr virus-immortalized B lymphoblasts, a model for immunoglobulin-secreting B cells. Intracellular Ca2+was determined with fura 2 and the formation of inositol 1,4,5-trisphosphate by anion-exchange chromatography. LPA stimulated an increase in inositol 1,4,5-trisphosphate levels and induced a transient rise in intracellular free Ca2+concentration from 105 ± 17 to 226 ± 21 nM. This Ca2+signal resulted from Ca2+mobilization and Ca2+influx and was subject to homologous desensitization. Pertussis toxin inhibited these responses by ∼70%. Furthermore, LPA stimulated a 27.5% increase in guanosine 5′- O-(3-thiotriphosphate) binding to permeabilized B lymphoblasts, which suggests the direct activation of pertussis toxin-sensitive G proteins by LPA. LPA stimulated a strong increase in the specific phosphorylation of the mitogen-activated protein kinase (immunoblot analysis) that was prevented by the MEK inhibitor PD-98059. Finally, LPA triggered a 2-fold increase in DNA synthesis ([3H]thymidine incorporation) and a 2-fold increase in B lymphoblast number and evoked a 20- to 50-fold increase in immunoglobulin formation. By RT-PCR we detected specific mRNA transcripts for the recently cloned human LPA receptor. Thus our data suggest that LPA behaves as a B cell growth factor.

From video imaging of fura 2-loaded baby hamster kidney (BHK) cells stably expressing the cloned human glucagon receptor, we found the Ca2+response to glucagon to be specific, dose dependent, synchronous, sensitive to pertussis toxin, and independent of Ca2+influx. Forskolin did not elicit a Ca2+response, but treatment with a protein kinase A inhibitor, the Rp diastereomer of 8-bromoadenosine-3′,5′-cyclic monophosphothioate, resulted in a reduced glucagon-mediated Ca2+response as well as Ca2+oscillations. The specific phospholipase C inhibitor U-73122 abolished the Ca2+response to glucagon, and a modest twofold increase in inositol trisphosphate (IP3) production could be observed after stimulation with glucagon. In BHK cells coexpressing glucagon and muscarinic (M1) acetylcholine receptors, carbachol blocked the rise in intracellular free Ca2+concentrations in response to glucagon, whereas glucagon did not affect the carbachol-induced increase in Ca2+. Furthermore, carbachol, but not glucagon, could block thapsigargin-activated increases in intracellular free Ca2+concentration. These results indicate that, in BHK cells, glucagon receptors can activate not only adenylate cyclase but also a second independent G protein-coupled pathway that leads to the stimulation of phospholipase C and the release of Ca2+from IP3-sensitive intracellular Ca2+stores. Finally, we provide evidence to suggest that cAMP potentiates the IP3-mediated effects on intracellular Ca2+handling.

We previously demonstrated, using rat PC-12 pheochromocytoma cells differentiated to a sympathetic neuronal phenotype with nerve growth factor (NGF), that neuropeptide Y (NPY) inhibits catecholamine synthesis as well as release. Inquiry into the mechanisms of these inhibitions implicated distinct pathways involving reduction of Ca2+influx through voltage-activated Ca2+channels. In the present investigation the effects of NPY on whole cell Ba2+currents were examined to obtain direct evidence supporting the mechanisms suggested by those studies. NPY was found to inhibit the voltage-activated Ba2+current in NGF-differentiated PC-12 cells in a reversible fashion with an EC50of 13 nM. This inhibition was pertussis toxin sensitive and resulted from NPY modulation of L- and N-type Ca2+channels. The inhibition of L-type channels was not seen with 2+or when protein kinase C (PKC) was inhibited by chelerythrine or PKC-(19—31). Furthermore, the effect of NPY on L-type channels was mimicked by the PKC activator phorbol 12-myristate 13-acetate. These studies demonstrate that, in addition to inhibition of N-type Ca2+channels, in NGF-differentiated PC-12 cells NPY inhibits L-type Ca2+channels via an intracellular Ca2+- and PKC-dependent pathway.

To determine whether M2muscarinic receptors are linked to the monomeric G protein Rho, we studied the effect of carbachol on actin reorganization (stress fiber formation) in cultured human airway smooth muscle cells that expressed mainly M2muscarinic receptors by dual- fluorescence labeling of filamentous (F) and monomeric (G) actin. F-actin was labeled with FITC-labeled phalloidin, and G-actin was labeled with Texas Red-labeled DNase I. Carbachol stimulation induced stress fiber formation (increased F-actin staining) in the cells and increased the F- to G-actin ratio 3.6 ± 0.4-fold (mean ± SE; n = 5 experiments). Preincubation with pertussis toxin, Clostridium C3 exoenzyme, or tyrosine kinase inhibitors reduced the carbachol-induced increase in stress fiber formation and significantly decreased the F- to G-actin ratio, whereas a mitogen-activated protein kinase inhibitor, a phosphatidylinositol 3-kinase inhibitor, and a protein kinase C inhibitor were without effect. This study demonstrates that in cultured human airway smooth muscle cells, muscarinic-receptor activation induces stress fiber formation via a pathway involving a pertussis-sensitive G protein, Rho proteins, and tyrosine phosphorylation.

Neuropeptides are known to modulate the excitability of mammalian sympathetic neurons by their actions on various types of K+ and Ca2+ channels. We used whole cell patch-clamp recording methods to study the actions of substance P (SP) on dissociated adult guinea pig stellate ganglion (SG) neurons. Under current-clamp conditions, SG neurons exhibited overshooting action potentials followed by afterhyperpolarizations (AHP). The K+ channel blocker tetraethylammonium (1 mM), the Ca2+ channel blocker Cd2+ (0.1–0.2 mM), and SP (500 nM) depolarized SG neurons, decreased the AHP amplitude, and increased the action potential duration. In the presence of Cd2+, the effect of SP on membrane potential and AHP was reduced. Under voltage-clamp conditions, several different K+ currents were observed, including a transient outward K+ conductance and a delayed rectifier outward K+ current ( I K) consisting of Ca2+-sensitive [ I K(Ca)] and Ca2+-insensitive components. SP (500 nM) inhibited I K. Pretreatment with Cd2+ (20–200 μM) or the high-voltage-activated Ca2+ channel blocker ω-conotoxin (10 μM) blocked SP’s inhibitory effects on I K. This suggests that SP reduces I K primarily through the inhibition of I K(Ca) and that this may occur, in part, via a reduction of Ca2+ influx through voltage-dependent Ca2+ channels. SP’s actions on I K were mediated by a pertussis toxin-insensitive G protein(s) coupled to NK1 tachykinin receptors. Furthermore, we have confirmed that 500 nM SP reduced an inward Cd2+- and ω-conotoxin-sensitive Ba2+ current in SG neurons. Thus the actions of SP on I K(Ca) may be due in part to a reduction in Ca2+influx occurring via N-type Ca2+channels. This study presents the first description of ionic currents in mammalian SG neurons and demonstrates that SP may modulate excitability in SG neurons via inhibitory actions on K+ and Ca2+ currents.

Whole cell L-type Ca2+current was recorded in ventricular myocytes dissociated from guinea pigs that were bred at ambient temperatures ranging between daily averages of 4 and 29°C. The dynamic voltage range of inactivation, as measured using 400-ms conditioning pulses and a holding potential of −40 mV, extended from −50 to −20 mV in myocytes prepared in summer. In winter, the inactivation curve was shifted to more negative potentials than in summer. Double-pulse experiments revealed that the negative shift was due to slow-inactivation kinetics. The negative shift of inactivation could be induced in myocytes prepared from animals that had been kept at 5°C for >3 wk in the summer. The negative shift in Ca2+current inactivation could be abolished by adding guanosine 5′- O-(2-thiodiphosphate) (5 mM) to the pipette solution, but not by adding staurosporine (2 μM) or 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (100 μM) to the bath. The cold acclimation may introduce the slow inactivation of the cardiac L-type Ca2+channel through an unknown pertussis toxin-insensitive G protein.

To determine whether the phosphoinositol/Ca2+pathway interacts with the adenylate cyclase/adenosine 3′,5′-cyclic monophosphate (cAMP) pathway in the cardiac κ-receptor, the effects of U-50488, a specific κ-receptor agonist, on the intracellular Ca2+concentration ([Ca2+]i) and forskolin-induced accumulation of cAMP in rat ventricular myocytes were determined after interference of the phosphoinositol/Ca2+pathway. U-50488 suppressed the forskolin-induced accumulation of cAMP and elevated [Ca2+]i, which were blocked by norbinaltorphimine, a specific κ-receptor antagonist, and pertussis toxin. The effects of U-50488 were qualitatively similar to those of A-23187, a Ca2+ionophore, but opposite to those of 1,2-bis(2-aminophenoxy)ethane- N, N, N′, N′-tetraacetic acid (BAPTA)-acetoxymethyl ester (AM), a [Ca2+]ichelator. Abolition of U-50488-induced elevation of [Ca2+]iby BAPTA-AM also abolished the effect of U-50488 on forskolin-induced accumulation of cAMP. Inhibition of the phospholipase C by specific inhibitors, U-73122 and neomycin, abolished the effects of U-50488 on both [Ca2+]iand forskolin-induced accumulation of cAMP. The results showed for the first time that κ-receptor stimulation may suppress cAMP accumulation via activation of the phosphoinositol/Ca2+pathway in the rat heart.

Yang, Lie and Larry S. Benardo. Epileptogenesis following neocortical trauma from two sources of disinhibition. J. Neurophysiol. 78: 2804–2810, 1997. Intracellular and field potential recordings were obtained from superficial and deep neurons from both intact coronal rat somatosensory slices, and from slices which had been acutely divided into a superficial strip of cortex (∼450 μm from the pia) and a deep segment. Membrane properties for cells in the traumatized slices were similar to those of their counterparts in intact slices. However, synaptic hyperexcitability developed in the deep segments in which a majority of cells likely underwent dendrotomy. This hyperexcitability was manifested by epileptiform activity in 54% of traumatized slices. Measurements of fastGABAergic inhibitory strength showed these slices were disinhibited. Superficial delivery of tetrodotoxin to the upper 450 μm of intact slices led to disinhibition of fast GABAergic transmission as well as an attendant increase in excitatory postsynaptic potential strength but not epileptogenesis. Pharmacological maneuvers aimed at preventing glutamate-triggered increases in intracellular calcium [glutamate ionotropic antagonists, dantrolene, and bis-( o-aminophenoxy)- N,N,N′,N′-tetraacetic acid (BAPTA)-AM] showed that a 1 h treatment in these agents conferred protection against epileptogenesis. These results demonstrate that the seizure-like activity developing in deep dendrotomized cortical segments resulted from two sources of GABAergic disinhibition: the physical removal of important superficial inhibitory circuits and glutamate-triggered increases in intracellular calcium.

The mineralocorticoid aldosterone is the most important hormone for the regulation of Na+ and K+ homeostasis in mammals and is thereby involved in the regulation of extracellular volume and blood pressure. Because aldosterone is a steroid hormone, the classical way of action involves transcription, translation, and protein synthesis. We previously reported a rapid, nongenomic, and Zn2+-sensitive action of aldosterone on Na+/H+exchange in renal epithelial [Madin-Darby canine kidney (MDCK)] cells (M. Gekle, N. Golenhofen, H. Oberleithner, and S. Silbernagl. Proc. Natl. Acad. Sci. 93: 10500–10504, 1996). Here we show that, in the absence of Na+ (i.e., with inactive Na+/H+exchange), aldosterone induces a membrane potential-dependent and Zn2+-sensitive cytoplasmic acidification in MDCK cells within 2–4 min. This aldosterone-induced activation of a proton conductance is insensitive to the inhibitor of the classical genomic pathway, spironolactone. Furthermore, the inhibitor of serine/threonine kinases and staurosporine, as well as the specific inhibitor of protein kinase C (PKC), calphostin C, prevented proton conductance activation. Activation of PKC by phorbol esters mimicked the effect of aldosterone. Furthermore, preincubation of the cells with pertussis toxin reduced the effect of aldosterone significantly. We propose a new nongenomic mechanism of action for aldosterone, independently of the intracellular type 1 mineralocorticoid receptor: G protein-dependent stimulation of PKC by aldosterone leads to the activation of a plasma membrane proton conductance that enhances the activity of Na+/H+exchange. This rapid nongenomic effect could explain the observation that aldosterone may alter renal Na+ and K+ excretion within 5–10 min.

We have previously observed that gastrin has a cholecystokinin B (CCK-B) receptor-mediated growth-promoting effect on the AR42J rat pancreatic acinar cell line and that this effect is paralleled by induction of expression of the early response gene c- fos. We undertook these experiments to elucidate the mechanism for induction of c- fos and the linkage of this action to the trophic effects of gastrin. Gastrin (0.1–10 nM) dose dependently induced luciferase activity in AR42J cells transfected with a construct consisting of a luciferase reporter gene coupled to the serum response element (SRE) of the c- fos promoter. This effect was blocked by the specific CCK-B receptor antagonist D2 but not by the specific CCK-A receptor antagonist L-364,718 or by pertussis toxin, indicating that gastrin targets the SRE via specific CCK-B receptors through a mechanism independent of Gi. Inhibition of protein kinase C (PKC) either by prolonged (24 h) exposure of the cells to the phorbol ester 12- O-tetradecanoylphorbol 13-acetate (100 nM) or by incubation with the selective inhibitor GF-109203X (3.5 μM) resulted in an 80% reduction in luciferase activity. Similar results were observed in the presence of the specific extracellular signal-regulated kinase (ERK) kinase (MEK) inhibitor PD-98059 (50 μM). We measured ERK2 activity in AR42J cells via in-gel kinase assays and observed that gastrin (1 pM-100 nM) induced ERK2 enzyme activity in a dose-dependent manner. Addition of GF-109203X and PD-98059, either alone or in combination, produced, respectively, partial and total inhibition of gastrin-induced ERK2 activity. Gastrin induction of ERK2 activity also resulted in a threefold increase in the transcriptional activity of Elk-1, a factor known to bind to the c- fos SRE and to be phosphorylated and activated by ERK2. PD-98059 blocked the growth-promoting effect of gastrin on the AR42J cells, demonstrating that this effect depends on activation of MEK. Our data lead us to conclude that the trophic actions of gastrin are mediated by ERK2-induced c- fos gene expression via PKC-dependent and -independent pathways.

The role of β1- and β2-adrenergic receptor stimulation in modulating adenosine 3′,5′-cyclic monophosphate (cAMP)-regulated Cl−and Ca2+currents was investigated with use of guinea pig ventricular myocytes. Activation of the Cl−current by the nonselective β-receptor agonist isoproterenol (Iso) was not affected by the β2-receptor antagonist ICI-118,551 (ICI), but it was blocked by the β1-receptor antagonist atenolol. The inability of β2-receptor stimulation to activate the Cl−current was confirmed by the lack of response to the selective β2-receptor agonists salbutamol and zinterol. Responses to β2-adrenergic receptor stimulation were also looked for in pertussis toxin (PTX)-treated myocytes because PTX increases the sensitivity of responses to Iso, and PTX has been reported to increase the responsiveness to β2- but not β1-receptor stimulation. PTX treatment increased the sensitivity of the Cl−current to activation by Iso in the presence of ICI, indicating that PTX increases β1-receptor responsiveness. PTX treatment also resulted in the ability of salbutamol to activate the Cl−current. However, the response to salbutamol was blocked by atenolol but not by appropriate concentrations of ICI, suggesting that salbutamol was activating β1-receptors. These results indicate that PTX treatment increases the sensitivity to β1-receptor stimulation, without affecting β2-responsiveness. To verify that the lack of response to β2-receptor stimulation was not unique to the Cl−current, the effects of β2-receptor agonists on the L-type Ca2+current were also examined. The Ca2+current was only affected by high concentrations of zinterol or salbutamol, and such responses were blocked by atenolol, but not by ICI, suggesting that activation of β1-receptors was involved. These results indicate that β1- but not β2-adrenergic receptor stimulation plays an important role in modulating the cAMP-regulated Cl−and Ca2+currents in guinea pig ventricular myocytes.

PC-12 cells depolarize during hypoxia and release dopamine. The hypoxia-induced depolarization is due to inhibition of an O2-sensitive K+current. The role of dopamine released during hypoxia is uncertain, but it could act as an autocrine to modulate membrane conductance during hypoxia. The current study was undertaken to investigate this possibility. Reverse transcription-polymerase chain reaction and sequence analysis revealed that the D2isoform of the dopamine receptor is expressed in rat PC-12 cells. Exogenously applied dopamine and the D2agonist quinpirole elicited inhibition of a voltage-dependent K+current ( IK) that was prevented by sulpiride, a D2receptor antagonist. Dopamine and quinpirole applied during hypoxia potentiated the inhibitory effect of hypoxia on IK. We also found that quinpirole caused reversible inhibition of a voltage-dependent Ca2+current ( ICa) and attenuation of the increase in intracellular free Ca2+during hypoxia. Our results indicate that dopamine released from PC-12 cells during hypoxia acts via a D2receptor to “autoregulate” IKand ICa.

Low- and high-density lipoproteins (LDL and HDL, respectively) stimulate alveolar type II cells to secrete surfactant. Increases in phosphoinositide hydrolysis, cytosolic Ca2+, and membrane-associated protein kinase C activity precede LDL- and HDL-stimulated secretion. We report three lines of evidence supporting the hypothesis that Gi mediates LDL- and HDL-stimulated surfactant secretion and signal transduction in type II cells. First, pertussis toxin (PTX) inhibited secretion stimulated by the apolipoprotein ligands for either the LDL receptor or the HDL binding protein. Second, PTX inhibited protein kinase C activity in cell membranes stimulated by LDL or HDL. Third, treatment of cell membranes with LDL or HDL inhibited PTX-catalyzed labeling of substrates corresponding in molecular mass to Gi alpha. These observations suggest that receptor-mediated activation of Gi is required for LDL- and HDL-stimulated secretion and that LDL and HDL activate Gi. These studies in type II cells are the first to support the hypothesis that Gi mediates the effects of LDL or HDL on important phenotype-specific functions of differentiated cells.