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The biologic effect of cAMP and cGMP is terminated by phosphodiesterases and multidrug resistance proteins MRP4 and MRP5, which pump cyclic nucleotides out of the cell. Therefore, this study aimed to characterize the role of MRP inhibitor, MK 571 (3-[[[3-[(1 E )-2-(7-chloro-2-quinolinyl)ethenyl]phenyl][[3-(dimethylamino)-3-oxopropyl]thio]methyl]thio]propanoic acid), in the bladder, prostate, and urethra of male mice by means of functional assays, protein expression, and cyclic nucleotide quantification. The cumulative addition of MK 571 (1-30 µ M) produced only small relaxation responses (approximately 25%) in all studied tissues. In the bladder, isoprenaline/fenoterol and forskolin concentration-dependently relaxed and MK 571 (20 µ M) increased the maximal response values by 37% and 24%, respectively. When MK 571 was coincubated with fenoterol or forskolin, intracellular levels of cAMP and protein expression of phospho-vasodilator-stimulated phosphoprotein (p-VASP) Ser157 were significantly greater compared with bladders stimulated with fenoterol or forskolin alone. In the prostate and urethra, sodium nitroprusside concentration-dependently relaxed and MK 571 (20 µ M) significantly increased relaxation responses by 70% and 56%, respectively, accompanied by greater intracellular levels of cGMP and protein expression of p-VASP Ser239 in the prostate. Tadalafil and BAY 41-2272 (5-cyclopropyl-2-[1-[(2-fluorophenyl)methyl]-1 H -pyrazolo[3,4- b ]pyridin-3-yl]-4-pyrimidinamine) also relaxed the prostate and urethra, respectively, and MK 571 markedly enhanced this response. The stable analog of cGMP (8-Br-cGMP) induced concentration-dependent relaxation responses in the prostate and urethra, and MK 571 significantly increased the relaxation response. In conclusion, to our knowledge, this is the first study to show that efflux transporters are physiologically active in the bladder, prostate, and urethra to control intracellular levels of cAMP or cGMP., (Copyright © 2018 by The American Society for Pharmacology and Experimental Therapeutics.)

Milameline (E-1,2,5,6-tetrahydro-1-methyl-3-pyridinecarboxaldehyde, O-methyloxime monohydrochloride, CI-979, PD129409, RU35926) was characterized in vitro and evaluated for effects on central and peripheral cholinergic activity in rats and rhesus monkeys. In muscarinic binding studies, milameline displayed nanomolar affinity with an agonist ligand and micromolar affinity with antagonist ligands, with approximately equal affinities determined at the five subtypes of human muscarinic receptors (hM(1)-hM(5)) with whole cells or membranes from stably transfected Chinese hamster ovary (CHO) cells. On binding, milameline stimulated phosphatidylinositol hydrolysis in hM(1) and hM(3) CHO cells and inhibited forskolin-activated cAMP accumulation in hM(2) and hM(4) CHO cells. Additionally, it decreased K(+)-stimulated release of [(3)H]acetylcholine from rat cortical slices. Responses were not caused by the inhibition of acetylcholinesterase, and there was no significant binding to approximately 30 other neurotransmitter binding sites. In rats, milameline decreased spontaneous and scopolamine-induced swimming activity, improved water-maze performance of animals impaired by basal forebrain lesions, increased cortical blood flow, decreased core body temperature, and increased gastrointestinal motility. Electroencephalogram activity in both rats and monkeys was characterized by a predominance of low-voltage desynchronized activity consistent with an increase in arousal. Milameline also reversed a scopolamine-induced impairment of attention on a continuous-performance task in monkeys. Thus, milameline possesses a pharmacological profile consistent with that of a partial muscarinic agonist, with central cholinergic actions being produced in rats and monkeys at doses slightly lower than those stimulating peripheral cholinergic receptors.

Agonist stimulation of neonatal cardiac beta-adrenoceptors produces heterologous sensitization of adenylyl cyclase (AC) signaling, rather than desensitization, as seen in adults. We examined the ontogenetic patterns of AC expression and activity, and evaluated isoproterenol effects on this pattern. [(3)H]Forskolin binding showed an increase in AC concentration across the period (birth to 25 days of age) in which agonist-induced sensitization is replaced by desensitization; binding affinity also increased, suggesting a shift in conformation and/or isoform. Indeed, catalytic properties of AC changed substantially with development, as evaluated by AC responses to forskolin versus Mn(2+). In contrast, there were only minor changes in the levels of mRNAs encoding the two major isoforms. Neonates given repeated isoproterenol treatment showed an enhancement of [(3)H]forskolin binding B(max) and a precocious shift to the mature affinity state and corresponding catalytic properties. Although isoproterenol caused significant increases in AC mRNAs, the effects were small and showed no isoform preference. Thus, a primary mode for ontogenetic increases in cardiac cellular responsiveness to adrenergic stimulation is the increase in AC activity attendant upon an absolute increase in the membrane concentration of AC molecules, along with changes in the catalytic properties of AC. The lack of correlation between mRNA and AC protein suggests that the primary regulatory events are post-transcriptional. The induction of AC by beta-adrenoceptor stimulation in the fetus and neonate accounts for heterologous, agonist-induced sensitization, a phenomenon that preserves cellular responses during the period of the perinatal transition.

On the cellular level, opioid dependence is characterized by a significant elevation of adenylyl cyclase (AC) activity after drug withdrawal, a regulatory phenomenon termed "AC supersensitivity" or "cAMP overshoot." The present study examines the role of the stimulatory G protein (Gs) in the expression of naloxone precipitated opioid withdrawal in chronically morphine (10 microM; 3 days) treated neuroblastoma X glioma (NG108-15) hybrid cells. Determination of high-affinity [3H]forskolin binding to intact cells, which provides a direct parameter for the binding of the activated alpha-subunit of Gs (Gsalpha) to AC, revealed that the enhancement of AC activity after opioid withdrawal is not caused by an increased stimulation of effector activity by Gsalpha. Although not a direct function of Gs, the expression of AC supersensitivity required Gsalpha-mediated stimulation of AC, because 1) the enhancement of AC activity after opioid withdrawal was observed only in the presence of low, but not of high concentrations of forskolin, and 2) chemical inactivation of Gsalpha by low pH pretreatment abolished the induction of AC supersensitivity. Moreover, the regulatory mechanism underlying AC supersensitivity not only required the presence of activated Gsalpha per se, but functional intact stimulatory signal transduction pathways. Indeed, blockade of prostaglandin E1 receptor/Gs interaction in situ with a site-specific anti-Gsalpha antibody, as well as uncoupling of prostaglandin E1 receptor signaling by cholera toxin-catalyzed ADP-ribosylation of Gsalpha, prevented the expression of AC supersensitivity in membranes from opioid-withdrawn cells. These results suggest that the enhancement of AC activity in opioid-dependent cells, triggered by drug withdrawal, is not a direct Gsalpha effect, but involves a secondary regulatory event that requires costimulation of AC by acutely receptor-activated Gsalpha.

The role of cytoskeletal microtubules and microfilaments in modulating cAMP generation in S49 lymphoma cells was investigated using the agents colchicine and cytochalasin B, respectively, which are known to disrupt these structures. A 1-hr pretreatment of S49 cells with 10 microM colchicine typically enhanced maximal isoproterenol-(beta-adrenergic receptor) stimulated cAMP accumulation by 100%, whereas cytochalasin B increased isoproterenol-stimulated cAMP by 30%. The combination of colchicine and cytochalasin B synergistically enhanced agonist-stimulated cAMP to 225% over control values. A synergistic increase in cAMP accumulation was also observed in cells treated with the agonist prostaglandin E1 or cholera toxin (which activates the stimulatory guanine nucleotide regulatory (Gs) protein). Colchicine and cytochalasin B did not ablate the inhibitory effects of somatostatin or the stimulatory effect of pertussis toxin treatment on beta-receptor-stimulated cAMP accumulation, indicating that these cytoskeletal disrupting agents do not enhance responsiveness in S49 cells via alterations in the inhibitory guanine nucleotide regulatory protein pathway. Moreover, colchicine, but not cytochalasin B treatment, enhances expression of isoproterenol-promoted 3H-forskolin binding in intact cells (a measure of Gs/adenylyl cyclase coupling). Thus, colchicine and cytochalasin B appear to enhance signaling in the Gs/adenylyl cyclase pathway by alterations of components distal to hormone receptors, most likely at the Gs protein and/or via cAMP generation. These results imply that microtubules and microfilaments can interact in the regulation of this pathway and that increases in cellular cAMP may contribute to the action of drugs that alter function of these cytoskeletal elements.

Our study was designed to determine whether NKH477, a novel, potent and water-soluble forskolin derivative, may exert a positive inotropic effect in rats with chronic heart failure (CHF) after myocardial infarction. Cardiac output and stroke volume indices were decreased and systemic vascular resistance was increased 12 wk after left coronary artery ligation, suggesting that CHF has developed at this time. Dobutamine (4 micrograms/kg i.v.) increased the cardiac output and stroke volume indices in sham-operated rats (22.7 +/- 1.9 and 15.1 +/- 2.0% increase, respectively), whereas such increases were attenuated in rats 12 wk after the induction of myocardial infarction (cardiac output index: 4.0 +/- 1.4% increase and stroke volume index: 2.2 +/- 1.8% increase, respectively). In contrast to beta-adrenoceptor agonist, NKH477 (3, 10 and 30 micrograms/kg i.v.) increased cardiac output and stroke volume indices in the rats with CHF. The beta-adrenergic receptor density, measured by [3H] CGP-12177 binding assay, was reduced in homogenates of the failing heart. These results suggest that the decrease in cardiac beta-adrenergic receptor density may account, in part, for the reduction in the responsiveness to beta-adrenoceptor agonists. The primary defects in the signal transduction from beta-adrenergic receptor to adenylate cyclase, such as the receptor down-regulation and the failure in signaling from adenylate cyclase, may be present in the CHF heart. It may be possible to reverse the cardiac dysfunction associated with CHF with NKH477.

Aminoalkylcarbamate derivatives of forskolin have been synthesized at the 6- and 7-hydroxyl positions which have different selectivity for adenylyl cyclase and a glucose transporter, respectively. They were radioiodinated using the Bolton-Hunter reagent to yield [125I]-2-[3-(4-hydroxy-3-iodophenyl)propanamido]-N-ethyl-6- (aminocarbonyl)forskolin ([125I]6-IHPP-Fsk) and [125I]-2-[3-(4-hydroxy-3-iodophenyl)(propanamidol]-N-ethyl-7- (aminocarbonyl)-7-desacetylforskolin ([125I]7-IHPP-Fsk) and tested as autoradiographic probes for adenylyl cyclase and a glucose transporter. In slide-mounted rat brain sections [125I]6-IHPP-Fsk binding was potently inhibited by 1 microM 6-HPP-Fsk (95%) but unaffected by 500 mM D-glucose. In contrast, [125I]7-IHPP-Fsk was only partially inhibited by 1 microM 6-HPP-Fsk (37%), but residual [125I]7-IHPP-Fsk binding was further inhibited 56% by 500 mM D-glucose. These data suggest that while [125I]6-IHPP-Fsk binds exclusively to adenylyl cyclase, a significant fraction of [125I]7-IHPP-Fsk is binding to a glucose transporter in brain. Autoradiographic patterns of [125I]6-IHPP-Fsk and glucose-sensitive [125I]7-IHPP-Fsk binding were different. [125I]6-IHPP-Fsk binding was heterogeneously distributed and resembled [3H] forskolin binding. Highest densities of binding sites were noted in olfactory tubercle, caudate putamen, nucleus accumbens, pyramidal and granule cell layers of hippocampus, molecular layer of cerebellum and substantia nigra. In contrast, of glucose-sensitive [125I]7-IHPP-Fsk, binding appeared more homogeneous and similar to [3H]cytochalasin B, a compound which inhibits glucose transport. Highest densities of binding were noted in caudate putamen, nucleus accumbens, cerebral cortex and molecular layer of cerebellum.(ABSTRACT TRUNCATED AT 250 WORDS)

A series of in vitro studies were conducted examining the adenylate cyclase stimulation, positive chronotropic and inotropic effects of forskolin and nine analogs which exhibited a range of [3H]forskolin binding site affinities (K1) from 0.020 to 3.174 microM. A significant (P less than .001) linear correlation (r = 0.94) was found between binding site affinity and adenylate cyclase stimulation (EC50) for forskolin and the nine structural analogs. Adenylate cyclase activity was also significantly correlated with the positive chronotropic and inotropic effects of these substances on isolated guinea pig atria. Compounds with K1 values between 0.020 and 1.136 microM produced concentration-dependent increases in heart rate and contractile force in isolated spontaneous and electrically paced guinea pig atria, respectively. In contrast, an analog with a K1 of 3.174 microM caused significant (P less than .05) negative chronotropic and inotropic effects at concentrations above 10 microM. The optimal separation between positive inotropic and chronotropic activity was found with compounds displaying potent [3H]forskolin binding site affinity but moderate adenylate cyclase stimulation, i.e., K1 and EC50 values of approximately 0.05 to 0.10 and 3 microM, respectively. The results of this study show that the forskolin analog, P87-7692 [7-desacetyl-7-(O-propionyl)-hydroxyl amino-carbonyl-forskolin], has marked activity with a wide separation between positive inotropic (248 +/- 41%) and chronotropic effects (43 +/- 13%) at 6.2 microM and may serve as a prototype for a forskolin-based cardiotonic.

Hormone-sensitive adenylate cyclase is believed to exist as a complex consisting of a catalytic subunit, guanine nucleotide binding regulatory unit and a hormone or neurotransmitter receptor. The diterpene compound, forskolin, is a potent stimulator of adenylate cyclase activity presumably interacting with a site directly on the catalytic subunit. Guanine nucleotides and sodium fluoride stimulate adenylate cyclase through a stimulatory guanine nucleotide binding regulatory subunit. In order to examine the role of the forskolin binding site in the rat brain, the distribution of [3H]forskolin binding sites has been compared with those of a radiolabeled guanine nucleotide analog. [3H]Forskolin densely labeled a few discrete brain regions including the caudate-putamen, nucleus accumbens, olfactory tubercle, globus pallidus and substantia nigra. Specific [3H]guanylyl-5'-imidodiphosphate ([3H]Gpp(NH)p) binding sites were found in high densities in not only these areas but also in the cerebral cortex, thalamus, hypothalamus and midbrain regions. In the hippocampal formation, guanine nucleotide binding sites were seen in the stratum oriens, stratum radiatum, stratum lacunosum molecular and the molecular layer of the dentate gyrus. On the other hand, forskolin labeled the hilus and the pyramidal cell layer of CA3 and CA4 with high density, a region where guanine nucleotide binding was relatively low. Sodium fluoride and Gpp(NH)p were found to enhance forskolin binding in regions in which [3H]Gpp(NH)p binding sites were present. These results indicate that most, but not all forskolin binding sites in the brain, are allosterically coupled with the stimulatory guanine nucleotide binding protein. Conversely, it has also been demonstrated that some forskolin binding sites in the hippocampus are probably not guanine nucleotide regulated.