Search

Systemic vascular resistance ([R.sub.sys]) of freshwater turtles increases substantially during anoxia, but the underlying mechanisms are not fully understood. We investigated whether hydrogen sulfide ([H.sub.2]S), an endogenously produced metabolite believed to be an [O.sub.2] sensor/ transducer of vasomotor tone, contributes to the increased [R.sub.sys] of anoxic red-eared slider turtles (Trachemys scripta). Vascular infusion of the [H.sub.2]S donor Naris in anesthetized turtles at 21[degrees]C and fully recovered normoxic turtles at 5[degrees]C and 21[degrees]C revealed [H.sub.2]S to be a potent vasoconstrictor of the systemic circulation. Likewise, wire myography of isolated turtle mesenteric and pulmonary arteries demonstrated [H.sub.2]S to mediate an anoxia-induced constriction. Intriguingly, however, Naris did not exert vasoconstrictory effects during anoxia (6 h at 21[degrees]C; 14 days at 5[degrees]C) when plasma [H.sub.2]S concentration, estimated from the colorimetric measurement of plasma acid-labile sulfide concentration, likely increased by ~3-and 4-fold during anoxia at 21[degrees]C, and 5[degrees]C, respectively. Yet, blockade of endogenous [H.sub.2]S production by DL-propargylglycine or hydroxylamine (0.44 mmol/kg) partially reversed the decreased systemic conductance ([G.sub.sys]) exhibited by 5[degrees]C anoxic turtles. These findings suggest that the signal transduction pathway of [H.sub.2]S-mediated vasoactivity is either maximally activated in the systemic circulation of anoxic turtles and/or that it is oxygen dependent. red-eared slider; anoxia; temperature; cardiovascular; systemic resistance; pulmonary resistance; blood pressure; heart doi: 10.1152/ajpregu.00521.2009.

The intrinsic heart rate of most vertebrates studied, including humans, is elevated during digestion, suggesting that a nonadrenergic-noncholinergic factor contributes to the postprandial tachycardia. The regulating factor, however, remains elusive and difficult to identify. Pythons can ingest very large meals, and digestion is associated with a marked rise in metabolism that is sustained for several days. The metabolic rise causes more than a doubling of heart rate and a fourfold rise in cardiac output. This makes the python an interesting model to investigate the postprandial tachycardia. We measured blood pressure and heart rate in fasting Python regius, and at 24 and 48 h after ingestion of a meal amounting to 25% of body wt. Digestion caused heart rate to increase from 25 to 56 min, whereas blood pressure was unchanged. The postprandial rise in heart rate was partially due to a doubling of intrinsic heart rate. The [H.sub.2]-antagonist did not affect heart rate of fasting snakes but decreased heart rate by 15-20 min at 24 h into digestion, whereas it had no effects at 48 h. Thus, the histaminergic tone on the heart rose from none to 30% at 24 h but vanished after 48 h. In anesthetized snakes, histamine caused a systemic vasodilatation and a marked increase in heart rate and cardiac output mediated through a direct effect on [H.sub.2]receptors. Our study strongly indicates that histamine regulates heart rate during the initial phase of digestion in pythons. This study describes a novel regulation of the vertebrate heart. reptile; digestion; heart rate; blood flow; blood pressure

[O.sub.2] chemoreceptors elicit cardiorespiratory reflexes in all vertebrates, but consensus on [O.sub.2]-sensing signal transduction mechanism(s) is lacking. We recently proposed that hydrogen sulfide ([H.sub.2]S) metabolism is involved in [O.sub.2] sensing in vascular smooth muscle. Here, we examined the possibility that [H.sub.2]S is an [O.sub.2] sensor in trout chemoreceptors where the first pair of gills is a primary site of aquatic [O.sub.2] sensing and the homolog of the mammalian carotid body. Intrabuccal injection of [H.sub.2]S in unanesthetized trout produced a dose-dependent bradycardia and increased ventilatory frequency and amplitude similar to the hypoxic response. Removal of the first, but not second, pair of gills significantly inhibited [H.sub.2]S-mediated bradycardia, consistent with the loss of aquatic chemoreceptors, mRNA for [H.sub.2]S-synthesizing enzymes, cystathionine [beta]-synthase and cystathionine [gamma]-lyase, was present in branchial tissue. Homogenized gills produced [H.sub.2]S enzymatically, and [H.sub.2]S production was inhibited by [O.sub.2], whereas mitochondrial [H.sub.2]S consumption was [O.sub.2] dependent. Ambient hypoxia did not affect plasma [H.sub.2]S in unanesthetized trout, but produced a [Po.sub.2]-dependent increase in a sulfide moiety suggestive of increased [H.sub.2]S production. In isolated zebrafish neuroepithelial cells, the putative chemoreceptive cells of fish, both hypoxia and [H.sub.2]S, produced a similar ~10-mV depolarization. These studies are consistent with [H.sub.2]S involvement in [O.sub.2] sensing/signal transduction pathway(s) in chemoreceptive cells, as previously demonstrated in vascular smooth muscle. This novel mechanism, whereby [H.sub.2]S concentration ([[H.sub.2]S]) is governed by the balance between constitutive production and oxidation, tightly couples tissue [[H.sub.2]S] to [Po.sub.2] and may provide an exquisitely sensitive, yet simple, [O.sub.2] sensor in a variety of tissues. hydrogen sulfide concentration

Hydrogen sulfide ([H.sub.2]S) is rapidly emerging as a biologically significant signaling molecule. Studies published before 2000 report low or undetectable [H.sub.2]S (usually as total sulfide) levels in blood or plasma, whereas recent work has reported sulfide concentrations between 10 and 300 [micro]M, suggesting it acts as a circulating signal. In the first series of experiments, we used a recently developed polarographic sensor to measure the baseline level of endogenous [H.sub.2]S gas and turnover of exogenous [H.sub.2]S gas in real time in blood from numerous animals, including lamprey, trout, mouse, rat, pig, and cow. We found that, contrary to recent reports, [H.sub.2]S gas was essentially undetectable (< 100 nM total sulfide) in all animals. Furthermore, exogenous sulfide was rapidly removed from blood, plasma, or 5% bovine serum albumin in vitro and from intact trout in vivo. To determine if blood [H.sub.2]S could transiently increase, we measured oxygen-dependent [H.sub.2]S production by trout hearts in vitro and in vivo. [H.sub.2]S has been shown to mediate ischemic preconditioning (IPC) in mammals. [PC is present in trout and, unlike mammals, the trout myocardium obtains its oxygen from relatively hypoxic systemic venous blood. In vitro, myocardial [H.sub.2]S production was inversely related to P[O.sub.2], whereas we failed to detect [H.sub.2]S in ventral aortic blood from either normoxic or hypoxic fish in vivo. These results provide an autocrine or paracrine mechanism for myocardial coupling of hypoxia to [H.sub.2]S in [PC, i.e., oxygen sensing, but they fail to provide any evidence that [H.sub.2]S signaling is mediated by the circulation. gasotransmitter; hydrogen sulfide metabolism; vascular signaling

Digestion of large meals in pythons produces substantial increases in heart rate and cardiac output, as well as a dilation of the mesenteric vascular bed leading to intestinal hyperemia, but the mediators of these effects are unknown. Bolus intra-arterial injections of python neurotensin ([[His.sup.3], [Val.sup.4], [Ala.sup.7]]NT) (1 - 1,000 pmol/kg) into the anesthetized ball python Python regius (n = 7) produced a dose-dependent vasodilation that was associated with a decrease in systemic pressure ([P.sub.sys]) and increase in systemic blood flow ([Q.sub.sys]). There was no effect on pulmonary pressure and conductance. A significant (P < 0.05) increase in heart rate ([f.sub.H]) and total cardiac output ([Q.sub.tot]) was seen only at high doses (>30 pmol/kg). The systemic vasodilation and increase in [Q.sub.tot] persisted after [beta]-adrenergic blockade with propranolol, but the rise in [f.sub.H] was abolished. Also, the systemic vasodilation persisted after histamine [H.sub.2]-receptor blockade. In unanesthetized pythons (n = 4), bolus injection of python NT in a dose as low as 1 pmol/kg produced a significant increase in blood flow to the mesenteric artery (177% [+ or -] 54%; mean [+ or -] SE) and mesenteric conductance (219% [+ or -] 74%) without any increase in [Q.sub.sys], systemic conductance, [P.sub.sys], and [f.sub.H]. The data provide evidence that NT is an important hormonal mediator of postprandial intestinal hyperemia in the python, but its involvement in mediating the cardiac responses to digestion may be relatively minor. blood flow; mesenteric artery; heart rate

Low [O.sub.2] levels in the lungs of birds and mammals cause constriction of the pulmonary vasculature that elevates resistance to pulmonary blood flow and increases pulmonary blood pressure. This hypoxic pulmonary vasoconstriction (HPV) diverts pulmonary blood flow from poorly ventilated and hypoxic areas of the lung to more well-ventilated parts and is considered important for the local matching of ventilation to blood perfusion. In the present study, the effects of acute hypoxia on pulmonary and systemic blood flows and pressures were measured in four species of anesthetized reptiles with diverse lung structures and heart morphologies: varanid lizards (Varanus exanthematicus), caimans (Caiman latirostris), rattlesnakes (Crotalus durissus), and tegu lizards (Tupinambis merianae). As previously shown in turtles, hypoxia causes a reversible constriction of the pulmonary vasculature in varanids and caimans, decreasing pulmonary vascular conductance by 37 and 31%, respectively. These three species possess complex multicameral lungs, and it is likely that HPV would aid to secure ventilation-perfusion homogeneity. There was no HPV in rattlesnakes, which have structurally simple lungs where local ventilation-perfusion inhomogeneities are less likely to occur. However, tegu lizards, which also have simple unicameral lungs, did exhibit HPV, decreasing pulmonary vascular conductance by 32%, albeit at a lower threshold than varanids and caimans (6.2 kPa oxygen in inspired air vs. 8.2 and 13.9 kPa, respectively). Although these observations suggest that HPV is more pronounced in species with complex lungs and functionally divided hearts, it is also clear that other components are involved. hypoxia; pulmonary circulation; systemic circulation

Incubation of heat-denatured plasma from the rattlesnake Crotalus atrox with trypsin generated a bradykinin (BK) that contained two amino acid substitutions ([Arg.sup.1] [right arrow] Val and [Ser.sup.6] [right arrow] Thr) compared with mammalian BK. Bolus intra-arterial injections of synthetic rattlesnake BK (0.01-10 nmol/kg) into the anesthetized rattlesnake, Crotalus durissus terrificus, produced a pronounced and concentration-dependent increase in systemic vascular conductance (Gsys). This caused a fall in systemic arterial blood pressure (Psys) and an increase in blood flow. Heart rate and stroke volume also increased. This primary response was followed by a significant rise in Psys and pronounced tachycardia (secondary response). Pretreatment with [N.sup.G]-nitro-L-arginine methyl ester reduced the NK-induced systemic vasodilatation, indicating that the effect is mediated through increased NO synthesis. The tachycardia associated with the late primary and secondary response to BK was abolished with propranolol and the systemic vasodilatation produced in the primary phase was also significantly attenuated by pretreatment, indicating that the responses are caused, at least in part, by release of cathecholamines and subsequent stimulation of [beta]-adrenergic receptors. In contrast, the pulmonary circulation was relatively unresponsive to BK. reptile; vasoactive kinin; catecholamines; nitric oxide; adrenergic receptor

The cardiovascular system of vertebrates is regulated by a vast number of regulatory factors, including histamine. In pythons, histamine induces a strong tachycardia and dilates the systemic vasculature, which resembles the cardiovascular response to the elevated metabolic rate during digestion. In fact, there is an important role of increased histaminergic tone on the heart during the initial 24 h of digestion in pythons. Whilst the cardiovascular effects of histamine are well studied in pythons, little is known about the effects in other groups of reptiles. The histaminergic effects on the heart vary among species and histamine may exert either pressor and depressor effects by causing either constrictive or dilatory vascular responses. Here, we investigated the cardiovascular effects of histamine in three species of reptiles with very different cardiovascular and pulmonary morphologies. Experiments were performed on both anesthetized and recovered animals. We show a species-dependent effect of histamine on the systemic vasculature with dilation in rattlesnakes and constriction in turtles and caimans but no effect on the pulmonary circulation. The histamine-induced dilation in rattlesnakes was mediated through an activation of H-receptors, whereas the histamine-induced constriction in caimans was mediated through both adrenergic signaling and H-receptors activation. In all three species, histamine-induced tachycardia by direct stimulation of histaminergic receptors as well as an indirect activation of adrenoreceptors. This finding highlights a more complex mechanism underlying the action of histamine than previously recognized in reptiles. [ABSTRACT FROM AUTHOR]

To determine the costs of pulmonary ventilation without imposing severe oxygen limitations or acidosis that normally accompany exposures to hypoxia or hypercapnia, we opted to pharmacologically stimulate ventilation with doxapram (5 and 10 mg kg-1) in alligators. Doxapram is used clinically to alleviate ventilatory depression in response to anaesthesia and acts primarily on the peripheral oxygen-sensitive chemoreceptors. Using this approach, we investigated the hypothesis that pulmonary ventilation is relatively modest in comparison to resting metabolic rate in crocodilians and equipped seven juvenile alligators with masks for concurrent determination of ventilation and oxygen uptake. Doxapram elicited a dose-dependent and up to fourfold rise in ventilation, primarily by increasing ventilatory frequency. The accompanying rise in oxygen uptake was very small; ventilation in resting animals constitutes no more than 5% of resting metabolic rate. The conclusion that pulmonary ventilation is energetically cheap is consistent with earlier studies on alligators where ventilation was stimulated by hypoxia or hypercapnia. [ABSTRACT FROM AUTHOR]

The high blood pressure in giraffe leg arteries renders giraffes vulnerable to edema. We investigated in 11 giraffes whether large and small arteries in the legs and the tight fascia protect leg capillaries. Ultrasound imaging of foreleg arteries in anesthetized giraffes and ex vivo examination revealed abrupt thickening of the arterial wall and a reduction of its internal diameter just below the elbow. At and distal to this narrowing, the artery constricted spontaneously and in response to norepinephrine and intravascular pressure recordings revealed a dynamic, viscous pressure drop along the artery. Histology of the isolated median artery confirmed dense sympathetic innervation at the narrowing. Structure and contractility of small arteries from muscular beds in the leg and neck were compared. The arteries from the legs demonstrated an increased media thickness-to-lumen diameter ratio, increased media volume, and increased numbers of smooth muscle cells per segment length and furthermore, they contracted more strongly than arteries from the neck (500 ± 49 vs. 318 43 mmHg; n = 6 legs and neck, respectively). Finally, the transient increase in interstitial fluid pressure following injection of saline was 5.5 ± 1.7 times larger (n = 8) in the leg than in the neck. We conclude that 1) tissue compliance in the legs is low; 2) large arteries of the legs function as resistance arteries; and 3) structural adaptation of small muscle arteries allows them to develop an extraordinary tension. All three findings can contribute to protection of the capillaries in giraffe legs from a high arterial pressure. [ABSTRACT FROM AUTHOR]

Hypoxic pulmonary vasoconstriction (HPV) is an adaptive response that diverts pulmonary blood flow from poorly ventilated and hypoxic areas of the lung to more well-ventilated parts. This response is important for the local matching of blood perfusion to ventilation and improves pulmonary gas exchange efficiency. HPV is an ancient and highly conserved response, expressed in the respiratory organs of all vertebrates, including lungs of mammals, birds, and reptiles; amphibian skin; and fish gills. The mechanism underlying HPV and how cells sense low PO2 remains elusive. In perfused trout gills (Oncorhynchus mykiss), acute hypoxia, as well as H2S, caused an initial and transient constriction of the vasculature. Inhibition of the enzymes cystathionine- γ-synthase and cystathionine-γ-lyase, which blocks H2S production, abolished the hypoxic response. Individually blocking the four complexes in the electron transport chain abolished both the hypoxic and the H2S-mediated constriction. Glutathione, an antioxidant and scavenger of superoxide, attenuated the vasoconstriction in response to hypoxia and H2S. Furthermore, diethyldithiocarbamate, an inhibitor of superoxide dismutase, attenuated the hypoxic and H2S constriction. This strongly suggests that H2S mediates the hypoxic vasoconstriction in trout gills. H2S may stimulate the mitochondrial production of superoxide, which is then converted to hydrogen peroxide (H2O2). Thus, H2O2 may act as the "downstream" signaling molecule in hypoxic vasoconstriction. [ABSTRACT FROM AUTHOR]

Turtles of the genus Trachemys show a remarkable ability to survive prolonged anoxia. This is achieved by a strong metabolic depression, redistribution of blood flow and high levels of antioxidant defence. To understand whether nitric oxide (NO), a major regulator of vasodilatation and oxygen consumption, may be involved in the adaptive response of Trachemys to anoxia, we measured NO metabolites (nitrite, S-nitroso, Fe-nitrosyl and N-nitroso compounds) in the plasma and red blood cells of venous and arterial blood of Trachemys scripta turtles during normoxia and after anoxia (3 h) and reoxygenation (30 min) at 21 °C, while monitoring blood oxygen content and circulatory parameters. Anoxia caused complete blood oxygen depletion, decrease in heart rate and arterial pressure, and increase in venous pressure, which may enhance heart filling and improve cardiac contractility. Nitrite was present at high, micromolar levels in normoxic blood, as in some other anoxia-tolerant species, without significant arterial-venous differences. Normoxic levels of erythrocyte S-nitroso compounds were within the range found for other vertebrates, despite very high measured thiol content. Fe-nitrosyl and /V-nitroso compounds were present at high micromolar levels under normoxia and increased further after anoxia and reoxygenation, suggesting NO generation from nitrite catalysed by deoxygenated haemoglobin, which in turtle had a higher nitrite reductase activity than in hypoxia-intolerant species. Taken together, these data indicate constitutively high circulating levels of NO metabolites and significant increases in blood NO after anoxia and reoxygenation that may contribute to the complex physiological response in the extreme anoxia tolerance of Trachemys turtles. [ABSTRACT FROM AUTHOR]

Pythons exhibit a doubling of heart rate when metabolism increases several times during digestion. Pythons, therefore, represent a promising model organism to study autonomic cardiovascular regulation during the postprandial state, and previous studies show that the postprandial tachycardia is governed by a release of vagal tone as well as a pronounced stimulation from nonadrenergic, noncholinergic (NANC) factors. Here we show that infusion of plasma from digesting donor pythons elicit a marked tachycardia in fasting snakes, demonstrating that the NANC factor resides in the blood. Injections of the gastrin and cholecystokinin receptor antagonist proglumide had no effect on double-blocked heart rate or blood pressure. Histamine has been recognized as a NANC factor in the early postprandial period in pythons, but the mechanism of its release has not been identified. Mast cells represent the largest repository of histamine in vertebrates, and it has been speculated that mast cells release histamine during digestion. Treatment with the mast cell stabilizer cromolyn significantly reduced postprandial heart rate in pythons compared with an untreated group but did not affect double-blocked heart rate. While this study indicates that histamine induces postprandial tachycardia in pythons, its release during digestion is not stimulated by gastrin or cholecystokinin nor is its release from mast cells a stimulant of postprandial tachycardia. [ABSTRACT FROM AUTHOR]

Abstract: Bullfrogs, Lithobathes catesbeianus, bearing a femoral artery cannula were held at 3 temperatures (10, 20 and 30°C) for 24h. Changes in heart rate were recorded before and after injection of cholinergic and adrenergic antagonists. Normal heart rate doubled for each temperature increment. Adrenergic tone on the heart varied around 20% at all 3 temperatures but cholinergic tone increased from −5% to 10% between 10 and 30°C. In contrast, cholinergic tone increased from 75% at 5°C to 329% at 25°C in Xenopus laevis. Injection of the neural tracer True Blue into the cervical vagus of the bullfrog revealed a single location for vagal preganglionic neurons (VPN) in the dorsal vagal motor nucleus (DVN), while Xenopus had 30% of its VPN in a ventro-lateral group outside the DVN. Broader comparative studies have suggested that differences in the extent of vagal tone may relate to the location of VPN in the brainstem and this may be the case in these amphibians. [Copyright &y& Elsevier]

The intrinsic heart rate of most vertebrates studied, including humans, is elevated during digestion, suggesting that a nonadrenergic-noncholinergic factor contributes to the postprandial tachycardia. The regulating factor, however, remains elusive and difficult to identify. Pythons can ingest very large meals, and digestion is associated with a marked rise in metabolism that is sustained for several days. The metabolic rise causes more than a doubling of heart rate and a fourfold rise in cardiac output. This makes the python an interesting model to investigate the postprandial tachycardia. We measured blood pressure and heart rate in fasting Python regius, and at 24 and 48 h after ingestion of a meal amounting to 25% of body wt. Digestion caused heart rate to increase from 25 to 56 mm, whereas blood pressure was unchanged. The postprandial rise in heart rate was partially due to a doubling of intrinsic heart rate. The H2-antagonist did not affect heart rate of fasting snakes but decreased heart rate by 15-20 mm at 24 h into digestion, whereas it had no effects at 48 h. Thus, the histaminergic tone on the heart rose from none to 30% at 24 h but vanished after 48 h. In anesthetized snakes, histamine caused a systemic vasodilatation and a marked increase in heart rate and cardiac output mediated through a direct effect on H2- receptors. Our study strongly indicates that histamine regulates heart rate during the initial phase of digestion in pythons. This study describes a novel regulation of the vertebrate heart. [ABSTRACT FROM AUTHOR]

Hydrogen sulfide (H2S) is rapidly emerging as a biologically significant signaling molecule. Studies published before 2000 report low or undetectable H2S (usually as total sulfide) levels in blood or plasma, whereas recent work has reported sulfide concentrations between 10 and 300 µM. suggesting it acts as a circulating signal. In the first series of experiments, we used a recently developed polarographic sensor to measure the baseline level of endogenous H2S gas and turnover of exogenous H2S gas in real time in blood from numerous animals, including lamprey, trout, mouse, rat, pig, and cow. We found that, contrary to recent reports, H2S gas was essentially undetectable (<100 nM total sulfide) in all animals. Furthermore, exogenous sulfide was rapidly removed from blood, plasma, or 5% bovine serum albumin in vitro and from intact trout in vivo. To determine if blood H2S could transiently increase, we measured oxygen-dependent H25 production by trout hearts in vitro and in vivo. H2S has been shown to mediate ischemic preconditioning (IPC) in mammals. IPC is present in trout and, unlike mammals, the trout myocardium obtains its oxygen from relatively hypoxic systemic venous blood. In vitro, myocardial H2S production was inversely related to Po2, whereas we failed to detect H2S in ventral aortic blood from either normoxic or hypoxic fish in vivo. These results provide an autocrine or paracrine mechanism for myocardial coupling of hypoxia to H2S in IPC, i.e., oxygen sensing, but they fail to provide any evidence that H2S signaling is mediated by the circulation. [ABSTRACT FROM AUTHOR]

Hypoxic pulmonary vasoconstriction (HPV) is an adaptive response that diverts pulmonary blood flow from poorly ventilated and hypoxic areas of the lung to better ventilated parts, matching blood perfusion to ventilation. HPV is an ancient and highly conserved response expressed in the respiratory organs of all vertebrates. However, the underlying mechanism and the role of the endothelium remain elusive. Isolated intrapulmonary arteries (internal diameter <346 µm) from the American alligator Alligator mississippiensis were mounted in microvascular myographs for isometric tension recording. Resting vessels and vessels contracted with either serotonin (5-HT) or endothelin-1 (ET-1) were exposed to sustained (45 mm) hypoxia (PO2<5 mmHg). In ET-1- contracted vessels, hypoxia induced a monophasic, sustained and fully reversible constriction, which was independent of the endothelium. In relaxed or in 5-HT-contracted vessels, hypoxia did not cause constriction. The effects of ET-1, ETA and ETB as well as the general ET-receptor antagonist were studied. ET-1 caused a contraction of the pulmonary arteries through stimulation of ETA-receptors. ETA and ETB immunoreactive staining revealed the location of both receptors in the smooth muscle layer and of ETB receptors in the endothelium. In conclusion, because precontraction with serotonin did not facilitate HPV, the required precontraction in alligators seems specific to ET-1, which implies that ET-1 plays an important permissive role for the HPV response In alligators. [ABSTRACT FROM AUTHOR]

Abstract: The present study investigates adrenergic regulation of the systemic and pulmonary circulations of the anaesthetised South American rattlesnake, Crotalus durissus. Haemodynamic measurements were made following bolus injections of adrenaline and adrenergic antagonists administered through a systemic arterial catheter. Adrenaline caused a marked systemic vasoconstriction that was abolished by phentolamine, indicating this response was mediated through α-adrenergic receptors. Injection of phentolamine gave rise to a pronounced vasodilatation (systemic conductance (G sys) more than doubled), while injection of propranolol caused a systemic vasoconstriction, pointing to a potent α-adrenergic, and a weaker β-adrenergic tone in the systemic vasculature of Crotalus. Overall, the pulmonary vasculature was far less responsive to adrenergic stimulation than the systemic circulation. Adrenaline caused a small but non-significant pulmonary vasodilatation and there was tendency of reducing this dilatation after either phentolamine or propranolol. Injection of phentolamine increased pulmonary conductance (G pul), while injection of propranolol produced a small pulmonary constriction, indicating that α-adrenergic and β-adrenergic receptors contribute to a basal regulation of the pulmonary vasculature. Our results suggest adrenergic regulation of the systemic vasculature, rather than the pulmonary, may be an important factor in the development of intracardiac shunts. [Copyright &y& Elsevier]

Synbranchus marmoratus is a facultative air-breathing fish, which uses its buccal cavity as well as its gills for air-breathing. S. marmoratus shows a very pronounced tachycardia when it surfaces to air-breathe. An elevation of heart rate decreases cardiac filling time and therefore may cause a decline in stroke volume (Vs), but this can be compensated for by an increase in venous tone to maintain stroke volume. Thus, the study on S. marmoratus was undertaken to investigate how stroke volume and venous function are affected during air-breathing. To this end we measured cardiac output (a), heart rate (fH), central venous blood pressure (PCV), mean circulatory filling pressure (MCFP), and dorsal aortic blood pressures (PDA) in S. marmoratus. Measurements were performed in aerated water (PO2>130 mmHg), when the fish alternated between gill ventilation and prolonged periods of apnoeas, as well as during hypoxia (PO2⩽50 mmHg), when the fish changed from gill ventilation to air-breathing. Q increased significantly (luring gill ventilation compared to apnoea in aerated water through a significant increase in both fH and Vs. PCV and MCFP also increased significantly. During hypoxia, when the animals surface to ventilate air, we found a marked rise in fH, PCV, MCFP, Q and VS, whereas FDA decreased significantly. Simultaneous increases in PCV and MCFP in aerated, as well as in hypoxic water, suggests that the venous system plays an important regulatory role for cardiac filling and Vs in this species. In addition, we investigated adrenergic regulation of the venous system through bolus infusions of adrenergic agonists (adrenaline, phenylephrine and isoproterenol; 2 µg kg-1). Adrenaline and phenylephrine caused a marked rise in FCV and MCFP, whereas isoproterenol led to a marked decrease in FCV, and tended to decrease MCFP. Thus, it is evident that stimulation of both α- and β-adrenoreceptors affects venous tone in S. marmoratus. [ABSTRACT FROM AUTHOR]

Abstract: The roles that nitric oxide (NO) plays in the cardiovascular system of reptiles are reviewed, with particular emphasis on its effects on central vascular blood flows in the systemic and pulmonary circulations. New data is presented that describes the effects on hemodynamic variables in varanid lizards of exogenously administered NO via the nitric oxide donor sodium nitroprusside (SNP) and inhibition of nitric oxide synthase (NOS) by l-nitroarginine methyl ester (l-NAME). Furthermore, preliminary data on the effects of SNP on hemodynamic variables in the tegu lizard are presented. The findings are compared with previously published data from our laboratory on three other species of reptiles: pythons (Skovgaard, N., Galli, G., Taylor, E.W., Conlon, J.M., Wang, T., 2005. Hemodynamic effects of python neuropeptide γ in the anesthetized python, Python regius. Regul. Pept. 18, 15–26), rattlesnakes (Galli, G., Skovgaard, N., Abe, A.S., Taylor, E.W., Wang, T., 2005. The role of nitric oxide in the regulation of the systemic and the pulmonary vasculature of the rattlesnake, Crotalus durissus terrificus. J. Comp. Physiol. 175B, 201–208) and turtles (Crossley, D.A., Wang, T., Altimiras, J., 2000. Role of nitric oxide in the systemic and pulmonary circulation of anesthetized turtles (Trachemys scripta). J. Exp. Zool. 286, 683–689). These five species of reptiles possess different combinations of division of the heart and structural complexity of the lungs. Comparison of their responses to NO donors and NOS inhibitors may reveal whether the potential contribution of NO to vascular tone correlates with pulmonary complexity and/or with blood pressure. All existing studies on reptiles have clearly established a potential role for NO in regulating vascular tone in the systemic circulation and NO may be important for maintaining basal systemic vascular tone in varanid lizards, pythons and turtles, through a continuous release of NO. In contrast, the pulmonary circulation is less responsive to NO donors or NOS inhibitors, and it was only in pythons and varanid lizards that the lungs responded to SNP. Both species have a functionally separated heart, so it is possible that NO may exert a larger role in species with low pulmonary blood pressures, irrespective of lung complexity. [Copyright &y& Elsevier]

The effects of adrenergic stimulation on mean circulatory filling pressure (MCFP), central venous pressure (PCV) and stroke volume (Vs), as well as the effects of altered MCFP through changes of blood volume were investigated in rattlesnakes (Crotalus durissus). MCFP is an estimate of the upstream pressure driving blood towards the heart and is determined by blood volume and the activity of the smooth muscle cells in the veins (venous tone). MCFP can be determined as the plateau in PCV during a total occlusion of blood flow from the heart. VS decreased significantly when MCFP was lowered by reducing blood volume in anaesthetised snakes, whereas increased MCFP through infusion of blood (up to 3 ml kg-1) only led to a small rise in Vs. Thus, it seems that end-diastolic volume is not affected by an elevated MCFP in rattlesnakes. To investigate adrenergic regulation on venous tone, adrenaline as well as phenylephrine and isoproterenol (α- and β-adrenergic agonists, respectively) were infused as bolus injections (2 and 10 μg kg-1). Adrenaline and phenylephrine caused large increases in MCFP and PCV, whereas isoproterenol decreased both parameters. This was also the case in fully recovered snakes. Therefore, adrenaline affects venous tone through both α- and β-adrenergic receptors, but the α-adrenergic receptor dominates at the dosages used in the present study. Injection of the nitric oxide donor SNP caused a significant decrease in PCV and MCFP. Thus, nitric oxide seems to affect venous tone. [ABSTRACT FROM AUTHOR]

The effects of endothelin-1 (ET-1) on systemic and pulmonary circulation were investigated in anaesthetised freshwater turtles (Trachemys scripta) instrumented with arterial catheters and blood flow probes. Bolus intra-arterial injections of ET-1 (0.4–400 pmol kg-1) caused a dose-dependent systemic vasodilatation that was associated with a decrease in systemic pressure (Psys) and a rise in systemic blood flow (Qsys), causing systemic conductance (Gsys) to increase. ET-1 had no significant effects on the pulmonary vasculature, heart rate (fH) or total stroke volume (VStot). This response differs markedly from mammals, where ET-1 causes an initial vasodilatation that is followed by a pronounced pressor response. In mammals, the initial dilatation is caused by stimulation of ETB-receptors, while the subsequent constriction is mediated by ETA-receptors. In the turtles, infusion of the ETB-receptor agonist BQ-3020 (150 pmol kg-1) elicited haemodynamic changes that were similar to those of ET-1, and the effects of ET-1 were not affected by the ETA-antagonist BQ-610 (0.15 μmol kg-1). Conversely, all effects of ET-1 were virtually abolished after specific ETB-receptor blockade with the ETB-antagonist BQ-788 (0.15 μmol kg-1). The subsequent treatment with the general ET-receptor antagonist tezosentan (15.4 μmol kg-1) did not produce effects that differed from the treatment with ETB-antagonist, and the blockade of ET-1 responses persisted. This present study indicates, therefore, that ETB-receptors are responsible for the majority of the cardiovascular responses to ET-1 in Trachemys. [ABSTRACT FROM AUTHOR]

Abstract: The effects of python neuropeptide gamma (NPγ) on hemodynamic parameters have been investigated in the anesthetized ball python (Python regius). Bolus intra-arterial injections of synthetic python NPγ (1–300 pmol kg−1) produced a dose-dependent decrease in systemic arterial blood pressure (Psys) concomitant with increases in systemic vascular conductance (Gsys), total cardiac output and stroke volume, but only minor effects on heart rate. The peptide had no significant effect on pulmonary arterial blood pressure (Ppul) and caused only a small increase in pulmonary conductance (Gpul) at the highest dose. In the systemic circulation, the potency of the NK1 receptor-selective agonist [Sar9,Met(02)11)] substance P was >100-fold greater than the NK2 receptor-selective agonist [βAla8] neurokinin A-(4–10)-peptide suggesting that the python cardiovascular system is associated with a receptor that resembles the mammalian NK1 receptor more closely than the NK2 receptor. Administration of the inhibitor of nitric oxide synthesis, L-nitro-arginine-methylester (L-NAME; 150 mg kg−1), resulted in a significant (P<0.05) increase in Psys as well as a decrease in Gsys, but no effect on Ppul and Gpul. Conversely, the nitric oxide donor, sodium nitroprusside (SNP; 60 μg kg−1) produced a significant (P<0.05) decrease in Psys along with an increase in Gsys and pulmonary blood flow. However, neither L-NAME nor indomethacin (10 mg kg−1) reduced the cardiovascular responses to NPγ. Thus, nitric oxide is involved in regulation of basal vascular tone in the python, but neither nitric oxide nor prostaglandins mediate the vasodilatory action of NPγ. [Copyright &y& Elsevier]

The functional role of nitric oxide (NO) was investigated in the systemic and pulmonary circulations of the South American rattlesnake,Crotalus durissus terrificus. Bolus, intra-arterial injections of the NO donor, sodium nitroprusside (SNP) caused a significant systemic vasodilatation resulting in a reduction in systemic resistance (Rsys). This response was accompanied by a significant decrease in systemic pressure and a rise in systemic blood flow. Pulmonary resistance (Rpul) remained constant while pulmonary pressure (Ppul) and pulmonary blood flow (Qpul) decreased. Injection ofL-Arginine (L-Arg) produced a similar response to SNP in the systemic circulation, inducing an immediate systemic vasodilatation, while Rpul was unaffected. Blockade of NO synthesis via the nitric oxide synthase inhibitor, L-NAME, did not affect haemodynamic variables in the systemic circulation, indicating a small contribution of NO to the basal regulation of systemic vascular resistance. Similarly, Rpul and Qpul remained unchanged, although there was a significant rise in Ppul. Via injection of SNP, this study clearly demonstrates that NO causes a systemic vasodilatation in the rattlesnake, indicating that NO may contribute in the regulation of systemic vascular resistance. In contrast, the pulmonary vasculature seems far less responsive to NO. [ABSTRACT FROM AUTHOR]

Incubation of heat-denatured plasma from the rattlesnake Crotalus atrox with trypsin generated a bradykinin (BK) that contained two amino acid substitutions (Arg¹ → Val and Ser6 → Thr) compared with mammalian BK. Bolus intra-arterial injections of synthetic rattlesnake BK (0.01-10 nmol/kg) into the anesthetized rattlesnake, Crotalus durissus terrificus, produced a pronounced and concentration-dependent increase in systemic vascular conductance (Gsys). This caused a fall in systemic arterial blood pressure (Psys) and an increase in blood flow. Heart rate and stroke volume also increased. This primary response was followed by a significant rise in Psys and pronounced tachycardia (secondary response). Pretreatment with NG-nitro-L-arginine methyl ester reduced the NK-induced systemic vasodilatation, indicating that the effect is mediated through increased NO synthesis. The tachycardia associated with the late primary and secondary response to BK was abolished with propranolol and the systemic vasodilatation produced in the primary phase was also significantly attenuated by pretreatment, indicating that the responses are caused, at least in part, by release of cathecholamines and subsequent stimulation of β-adrenergic receptors. In contrast, the pulmonary circulation was relatively unresponsive to BK. [ABSTRACT FROM AUTHOR]

The article presents a tone regulation in giraffe retinal arterioles. Topics discussed include extremes of auto regulation of blood flow and capillary pressure in giraffe. It reports that giraffe has the highest known mean arterial blood pressure among mammals, which in the erect position overcomes hydrostatic pressure gradient in the neck and ensures a perfusion pressure of the head including the eye similar to other mammals.

Abstract: Hydrogen sulfide (H2S), nitric oxide (NO) and nitrite (NO2 −) are formed in vivo and are of crucial importance in the tissue response to hypoxia, particularly in the cardiovascular system, where these signaling molecules are involved in a multitude of processes including the regulation of vascular tone, cellular metabolic function and cytoprotection. This report summarizes current advances on the mechanisms by which these signaling pathways act and may have evolved in animals with different tolerance to hypoxia, as presented and discussed during the scientific sessions of the annual meeting of the Society for Experimental Biology in 2011 in Glasgow. It also highlights the need and potential for a comparative approach of study and collaborative effort to identify potential link(s) between the signaling pathways involving NO, nitrite and H2S in the whole-body responses to hypoxia. [Copyright &y& Elsevier]

Endothelin-1 (ET-1) is a very potent vasoactive peptide released from endothelial cells, and ET-1 plays an important role in the maintenance and regulation of blood pressure in mammals. ET-1 signaling is mediated by two receptors: ET A and ET B. In mammals, ET A receptors are located on vascular smooth muscle where they mediate vasoconstriction. ET B receptors located on the endothelium mediate vasodilatation through the release of nitric oxide, whereas stimulation of ET B receptors placed on vascular smooth muscle leads to vasoconstriction. Less is known about ET-1 signaling in reptiles. In anaesthetized alligators, ET-1 elicits a biphasic blood pressure with a long-lasting initial decrease followed by a smaller increase in systemic blood pressure. In anaesthetized freshwater turtles, ET-1 causes a dose-dependent systemic vasodilatation mediated through ET B receptors. In the present study, we investigated the cardiovascular effects of ET-1 on the systemic and pulmonary vasculature of pythons. The presence of ET A and ET B receptors in the vasculature of pythons was verified by means of immunoblotting. Myography on isolated vessels revealed a dose-dependent vasoconstrictory response to ET-1 in both mesenteric and pulmonary arteries. Pressure measurements in recovered specimens revealed an ET-1-induced rise in systemic blood pressure supporting our in vitro findings. In conclusion, our study shows that ET-1 induces a strong pressor effect in the systemic circulation. [ABSTRACT FROM AUTHOR]

Background and purpose Hydrogen sulfide (H 2 S) is an endogenously produced gas with many physiological functions including involvement in regulation of the blood pressure. The present study was designed to investigate the effect of H 2 S in small arteries and the mechanisms underlying H 2 S-induced vasodilation. Experimental approach Mesenteric and pulmonary small arteries from rats were isolated and mounted in microvascular myographs for isometric tension recordings. The endogenous production of H 2 S was blocked with enzyme inhibitors dl-propargylglycine (PPG) and amino-oxyacetate (AOA) with and without the presence of blockers of nitric oxide (NO) synthase (nitro- l -arginine) and cyclooxygenase (indomethacin). NaHS (a H 2 S donor) was applied in arteries with and without endothelium as well as in the presence of different blockers. Simultaneous measurements of intracellular calcium concentration and force were conducted by fura-2 AM fluorescence. Key results In mesenteric arteries acetylcholine (ACh)-induced vasodilation was reduced in the presence of PPG and AOA both in the absence and the presence of indomethacin and nitro- l -arginine. In pulmonary arteries, the combination of indomethacin and nitro- l -arginine inhibited ACh-induced vasodilation, and there was no further effect by adding PPG and AOA. Application of NaHS relaxed mesenteric arteries with an EC 50 of 385 μM. Removing endothelium, inhibition of K ATP -channels or NO synthase did not affect H 2 S-induced vasodilation in mesenteric or pulmonary arteries. Simultaneous measurements of calcium and relaxation revealed that NaHS induces relaxation without changes in smooth muscle calcium. Conclusions and implications H 2 S induces vasodilation in both rat mesenteric and pulmonary arteries, but is only involved in ACh-induced vasodilatation in rat mesenteric arteries. H 2 S-induced vasodilation was endothelium-independent and may involve desensitization of the contractile apparatus in the vascular smooth muscle. [ABSTRACT FROM AUTHOR]

Endogenous hydrogen sulfide (H2S) is involved in the regulation of vascular tone. We hypothesized that the lowering of calcium and opening of potassium (K) channels as well as calcium-independent mechanisms are involved in H2S-induced relaxation in rat mesenteric small arteries. Amperometric recordings revealed that free [H2S] after addition to closed tubes of sodium hydrosulfide (NaHS), Na2S, and GYY4137 [P-(4-methoxyphenyl)-P-4-morpholinyl-phosphinodithioic acid] were, respectively, 14%, 17%, and 1% of added amount. The compounds caused equipotent relaxations in isometric myographs, but based on the measured free [H2S], GYY4137 caused more relaxation in relation to released free H2S than NaHS and Na2S in rat mesenteric small arteries. Simultaneous measurements of [H2S] and tension showed that 15 µM of free H2S caused 61% relaxation in superior mesenteric arteries. Simultaneous measurements of smooth muscle calcium and tension revealed that NaHS lowered calcium and caused relaxation of NE-contracted arteries, while high extracellular potassium reduced NaHS relaxation without corresponding calcium changes. In NE-contracted arteries, NaHS (1 mM) lowered the phosphorylation of myosin light chain, while phosphorylation of myosin phosphatase target subunit 1 remained unchanged. Protein kinase A and G, inhibitors of guanylate cyclase, failed to reduce NaHS relaxation, whereas blockers of voltage-gated KV7 channels inhibited NaHS relaxation, and blockers of mitochondrial complex I and III abolished NaHS relaxation. Our findings suggest that low micromolar concentrations of free H2S open K channels followed by lowering of smooth muscle calcium, and by another mechanism involving mitochondrial complex I and III leads to uncoupling of force, and hence vasodilation., (Copyright © 2015 by The American Society for Pharmacology and Experimental Therapeutics.)

Recent research has shown that the endogenous gas hydrogen sulphide (H2S) is a signalling molecule of considerable biological potential and has been suggested to be involved in a vast number of physiological processes. In the vascular system, H2S is synthesized from cysteine by cystathionine-γ-lyase (CSE) in smooth muscle cells (SMC) and 3- mercaptopyruvate sulfuresterase (3MST) and CSE in the endothelial cells. In pulmonary and systemic arteries, H2S induces relaxation and/or contraction dependent on the concentration of H2S, type of vessel and species. H2S relaxes SMC through a direct effect on KATP-channels or Kv-channels causing hyperpolarization and closure of voltage-dependent Ca2+-channels followed by a reduction in intracellular calcium. H2S also relaxes SMC through the release of endothelium- derived hyperpolarizing factor (EDHF) and nitric oxide (NO) from the endothelium. H2S contracts SMC through a reduction in nitric oxide (NO) availability by reacting with NO forming a nitrosothiol compound and through an inhibitory effect on endothelial nitric oxide synthase (eNOS) as well as a reduction in SMC cyclic AMP concentration. Evidence supports a role for H2S in oxygen sensing. Furthermore, reduced endogenous H2S production may also play a role in ischemic heart diseases and hypertension, and treatment with H2S donors and cysteine analogues may be beneficial in treatment of cardiovascular disease.

Hydrogen sulfide (H(2)S) is rapidly emerging as a biologically significant signaling molecule. Studies published before 2000 report low or undetectable H(2)S (usually as total sulfide) levels in blood or plasma, whereas recent work has reported sulfide concentrations between 10 and 300 microM, suggesting it acts as a circulating signal. In the first series of experiments, we used a recently developed polarographic sensor to measure the baseline level of endogenous H(2)S gas and turnover of exogenous H(2)S gas in real time in blood from numerous animals, including lamprey, trout, mouse, rat, pig, and cow. We found that, contrary to recent reports, H(2)S gas was essentially undetectable (<100 nM total sulfide) in all animals. Furthermore, exogenous sulfide was rapidly removed from blood, plasma, or 5% bovine serum albumin in vitro and from intact trout in vivo. To determine if blood H(2)S could transiently increase, we measured oxygen-dependent H(2)S production by trout hearts in vitro and in vivo. H(2)S has been shown to mediate ischemic preconditioning (IPC) in mammals. IPC is present in trout and, unlike mammals, the trout myocardium obtains its oxygen from relatively hypoxic systemic venous blood. In vitro, myocardial H(2)S production was inversely related to Po(2), whereas we failed to detect H(2)S in ventral aortic blood from either normoxic or hypoxic fish in vivo. These results provide an autocrine or paracrine mechanism for myocardial coupling of hypoxia to H(2)S in IPC, i.e., oxygen sensing, but they fail to provide any evidence that H(2)S signaling is mediated by the circulation.

The effects of endothelin-1 (ET-1) on systemic and pulmonary circulation were investigated in anaesthetised freshwater turtles (Trachemys scripta) instrumented with arterial catheters and blood flow probes. Bolus intra-arterial injections of ET-1 (0.4-400 pmol kg(-1)) caused a dose-dependent systemic vasodilatation that was associated with a decrease in systemic pressure (P(sys)) and a rise in systemic blood flow (Q(sys)), causing systemic conductance (G(sys)) to increase. ET-1 had no significant effects on the pulmonary vasculature, heart rate (fh) or total stroke volume (Vs(tot)). This response differs markedly from mammals, where ET-1 causes an initial vasodilatation that is followed by a pronounced pressor response. In mammals, the initial dilatation is caused by stimulation of ET(B)-receptors, while the subsequent constriction is mediated by ET(A)-receptors. In the turtles, infusion of the ET(B)-receptor agonist BQ-3020 (150 pmol kg(-1)) elicited haemodynamic changes that were similar to those of ET-1, and the effects of ET-1 were not affected by the ET(A)-antagonist BQ-610 (0.15 micromol kg(-1)). Conversely, all effects of ET-1 were virtually abolished after specific ET(B)-receptor blockade with the ET(B)-antagonist BQ-788 (0.15 micromol kg(-1)). The subsequent treatment with the general ET-receptor antagonist tezosentan (15.4 micromol kg(-1)) did not produce effects that differed from the treatment with ET(B)-antagonist, and the blockade of ET-1 responses persisted. This present study indicates, therefore, that ET(B)-receptors are responsible for the majority of the cardiovascular responses to ET-1 in Trachemys.

Incubation of heat-denatured plasma from the rattlesnake Crotalus atrox with trypsin generated a bradykinin (BK) that contained two amino acid substitutions (Arg1 --> Val and Ser6 --> Thr) compared with mammalian BK. Bolus intra-arterial injections of synthetic rattlesnake BK (0.01-10 nmol/kg) into the anesthetized rattlesnake, Crotalus durissus terrificus, produced a pronounced and concentration-dependent increase in systemic vascular conductance (Gsys). This caused a fall in systemic arterial blood pressure (Psys) and an increase in blood flow. Heart rate and stroke volume also increased. This primary response was followed by a significant rise in Psys and pronounced tachycardia (secondary response). Pretreatment with N(G)-nitro-L-arginine methyl ester reduced the NK-induced systemic vasodilatation, indicating that the effect is mediated through increased NO synthesis. The tachycardia associated with the late primary and secondary response to BK was abolished with propranolol and the systemic vasodilatation produced in the primary phase was also significantly attenuated by pretreatment, indicating that the responses are caused, at least in part, by release of cathecholamines and subsequent stimulation of beta-adrenergic receptors. In contrast, the pulmonary circulation was relatively unresponsive to BK.