The adrenergic regulation of the cardiovascular system in the South American rattlesnake, Crotalus durissus

Adrenergic control of functional characteristics of the cardiovascular system in the South American rattlesnake, Crotalus durissus

In squamate reptiles, extensive innervation of the heart and vascular beds allows for continuous modulation of the cardiovascular system by the autonomic nervous system. The systemic vasculature is the main target of excitatory sympathetic adrenergic fibers, while the pulmonary circulation has been described as less responsive to both nervous and humoral modulators. However, histochemical evidence has demonstrated the presence of adrenergic fibers in pulmonary circulation. Besides, reduced responsiveness is intriguing since the balance of regulation between systemic and pulmonary vascular circuits has critical hemodynamic implications in animals with an undivided ventricle and consequent cardiovascular shunts. The present study investigated the role and functional relevance of α and β-adrenergic stimulation in regulating systemic and mainly the pulmonary circulations in a decerebrate, autonomically responsive rattlesnake preparation. The use of the decerebrate preparation allowed us to observe a new diverse functional modulation of vascular beds and the heart. In resting snakes, the pulmonary vasculature is less reactive to adrenergic agonists at 25 °C. However, the β-adrenergic tone is relevant for modulating resting peripheral pulmonary conductance, while both α- and β-adrenergic tones are relevant for the systemic circuit. Active dynamic modulation of both pulmonary compliance and conductance effectively counterbalances alterations in the systemic circulation to maintain the R-L shunt pattern. Furthermore, we suggest that despite the great attention given to cardiac adjustments, vascular modulation is sufficient to support the hemodynamic adjustments needed to control blood pressure.

Prenatal hypoxia affects scaling of blood pressure and arterial wall mechanics in the common snapping turtle, Chelydra serpentina

In reptiles, exposure to hypoxia during embryonic development affects several cardiovascular parameters. These modifications may impose different mechanical stress to the arterial system, and we speculated that the arterial wall of major outflow vessels would be modified accordingly. Since non-crocodilian reptiles possess a partially divided ventricle, ensuing similar systemic and pulmonary systolic pressures, we investigated how morphological and mechanical properties of segments from the left aortic arch (LAo) and the proximal and distal segments of the left pulmonary artery (LPAp and LPAd, respectively) change as body mass (M_b) increases. Eggs from common snapping turtles, Chelydra serpentina, were incubated under normoxia (21% O₂; N21) or hypoxia (10% O₂; H10), hatched and maintained in normoxia thereafter. Turtles (0.11-6.85 kg) were cannulated to measure arterial pressures, and an injection of adrenaline was used to increase pressures. Portions of the LAo, LPAp and LPAd were fixed under physiological hydrostatic pressures for histology and mechanical assessment. Arterial pressures increased with M_b for N21 but not for H10. Although mechanical and functional characteristics from the LPAp and LPAd were similar between N21 and H10, wall thickness from LAo did not change with M_b in the H10 group, thus wall stress increased in larger turtles. This indicates that larger H10 turtles probably experience an elevated probability of arterial wall rupture without concomitant changes in the cardiovascular system to prevent it. Finally, collagen content of the LPAp and LAo was smaller than in LPAd, suggesting a more distensible arterial wall could attenuate higher pressures from larger turtles.

A Decerebrate Preparation of the Rattlesnake, Crotalus durissus, Provides an Experimental Model for Study of Autonomic Modulation of the Cardiovascular System in Reptiles

AbstractThe South American rattlesnake, Crotalus durissus, has been successfully used as an experimental model to study control of the cardiovascular system in squamate reptiles. Recent technical advances, including equipment miniaturization, have lessened the impact of instrumentation on in vivo recordings, and an increased range of anesthetic drugs has improved recording conditions for in situ preparations. Nevertheless, any animal-based experimental approach has to manage limitations regarding the avoidance of pain and stress the stability of the preparation and duration of experiments and the potentially overriding effects of anesthesia. To address such aspects, we tested a new experimental preparation, the decerebrate rattlesnake, in a study of the autonomic control of cardiovascular responses following the removal of general anesthesia. The preparation exhibited complex cardiovascular adjustments to deal with acute increases in venous return (caused by tail lifting), to compensate for blood flow reduction in the cephalic region (caused by head lifting), for body temperature control (triggered by an external heating source), and in response to stimulation of chemoreceptors (triggered by intravenous injection of NaCN). The decerebrate preparation retained extensive functional integrity of autonomic centers, and it was suitable for monitoring diverse cardiac and vascular variables. Furthermore, reanesthetizing the preparation markedly blunted cardiovascular performance. Isoflurane limited the maintenance of recovered cardiovascular variables in the prepared animal and reduced or abolished the observed cardiovascular reflexes. This preparation enables the recording of multiple concomitant cardiovascular variables for the study of mechanistic questions regarding the central integration of autonomic reflex responses in the absence of anesthesia.

Arterial wall thickening normalizes arterial wall tension with growth in American alligators, Alligator mississippiensis

Arterial wall tension increases with luminal radius and arterial pressure. Hence, as body mass (M_b) increases, associated increases in radius induces larger tension. Thus, it could be predicted that high tension would increase the potential for rupture of the arterial wall. Studies on mammals have focused on systemic arteries and have shown that arterial wall thickness increases with M_b and normalizes tension. Reptiles are good models to study scaling because some species exhibit large body size range associated with growth, thus, allowing for ontogenetic comparisons. We used post hatch American alligators, Alligator mississippiensis, ranging from 0.12 to 6.80 kg (~ 60-fold) to investigate how both the right aortic arch (RAo) and the left pulmonary artery (LPA) change with M_b. We tested two possibilities: (i) wall thickness increases with M_b and normalizes wall tension, such that stress (stress = tension/thickness) remains unchanged; (ii) collagen content scales with M_b and increases arterial strength. We measured heart rate and systolic and mean pressures from both systemic and pulmonary circulations in anesthetized animals. Once stabilized alligators were injected with adrenaline to induce a physiologically relevant increase in pressure. Heart rate decreased and systemic pressures increased with M_b; pulmonary pressures remained unchanged. Both the RAo and LPA were fixed under physiological hydrostatic pressures and displayed larger radius, wall tension and thickness as M_b increased, thus, stress was independent from M_b; relative collagen content was unchanged. We conclude that increased wall thickness normalizes tension and reduces the chances of arterial walls rupturing in large alligators.

Cholinergic regulation along the pulmonary arterial tree of the South American rattlesnake: vascular reactivity, muscarinic receptors, and vagal innervation

Vascular tone in the reptilian pulmonary vasculature is primarily under cholinergic, muscarinic control exerted via the vagus nerve. This control has been ascribed to a sphincter located at the arterial outflow, but we speculated whether the vascular control in the pulmonary artery is more widespread, such that responses to acetylcholine and electrical stimulation, as well as the expression of muscarinic receptors, are prevalent along its length. Working on the South American rattlesnake ( Crotalus durissus), we studied four different portions of the pulmonary artery (truncus, proximal, distal, and branches). Acetylcholine elicited robust vasoconstriction in the proximal, distal, and branch portions, but the truncus vasodilated. Electrical field stimulation (EFS) caused contractions in all segments, an effect partially blocked by atropine. We identified all five subtypes of muscarinic receptors (M1–M5). The expression of the M1 receptor was largest in the distal end and branches of the pulmonary artery, whereas expression of the muscarinic M3 receptor was markedly larger in the truncus of the pulmonary artery. Application of the neural tracer 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindo-carbocyanine perchlorate (DiI) revealed widespread innervation along the whole pulmonary artery, and retrograde transport of the same tracer indicated two separate locations in the brainstem providing vagal innervation of the pulmonary artery, the medial dorsal motor nucleus of the vagus and a ventro-lateral location, possibly constituting a nucleus ambiguus. These results revealed parasympathetic innervation of a large portion of the pulmonary artery, which is responsible for regulation of vascular conductance in C. durissus, and implied its integration with cardiorespiratory control.

Pharmacological and transcriptomic characterization of the nitric oxide pathway in aortic rings isolated from the tortoise Chelonoidis carbonaria

In this study the nitric oxide (NO)-soluble guanylate cyclase (sGC) and phosphodiesterase-5 (PDE-5) pathway was characterized in tortoise Chelonoidis carbonaria aorta. Concentration response curves (CCR) to ATP, ADP, AMP, adenosine and histamine were performed in the presence and absence of L-NAME in aorta pre-contracted with ACh (3 μM). CCR to SNP, BAY 41-2272 (sGC stimulator), BAY 60-2770 (sGC activator) and tadalafil (PDE-5 inhibitor) were constructed in the presence and absence of ODQ (10 μM). ATP (pEC₅₀ 6.1 ± 0.1), ADP (pEC₅₀ 6.0 ± 0.2), AMP (pEC₅₀ 6.8 ± 0.1) and histamine (pEC₅₀ 6.8 ± 0.12) relaxed Chelonoidis aorta and the addition of L-NAME reduced their efficacy (p < .05). Adenosine effects (pEC₅₀ 6.6 ± 0.1) were not changed in the presence of L-NAME. SNP (pEC₅₀ 7.5 ± 0.7; Emax 102.2 ± 2.5%), BAY 41-2272 (pEC₅₀ 7.3 ± 0.2; Emax 130.3 ± 10.2%), BAY 60-2770 (pEC₅₀ 11.4 ± 0.1; Emax 130.3 ± 6.5%) and tadalafil (pEC₅₀ 6.7 ± 0.3; Emax 121.3 ± 15.3%) relaxed Chelonoidis aorta. The addition of ODQ reduced the SNP and tadalafil maximum response (p < .05) and promoted 63 fold right shift on BAY 41-2272 curve. In contrast, no alteration was observed on BAY 60-2770 response. Transcriptomic analysis for eNOS and sGC were found in aorta and brain libraries with high homology when compared with human transcripts. The NO-sGC-PDE-5 is functionally present in Chelonoidis aorta with a functional and genomic similarity to mammalian vessels. Unlike most of mammalian vessels, ACh did not cause endothelium-dependent relaxation in Chelonoidis carbonaria aortic rings.

Distribution and properties of cardiac and pulmonary β-adrenergic receptors in corn snakes (Pantherophis guttatus) and Boa constrictor (Boa constrictor)

The aim of the present study was to characterize β-adrenergic receptors in the snake heart and lung of corn and Boa constrictor snakes. The β-adrenergic receptor binding sites were studied in purified heart and lung membranes using the specific β-adrenergic receptor antagonist [¹²⁵J]-iodocyanopindolol (ICYP) and subtypes using selective β₁-adrenergic receptor antagonist CGP-20712A and selective β₂-adrenergic receptor antagonist ICI-118.551. A saturable and specific β-adrenergic receptor binding site was detected in cardiac membranes with maximal receptor density (B_max) of 43.99 ± 3.86 fmol/mg protein (corn snake) and 58.07 ± 2.88 fmol/mg protein (Boa constrictor) as well as K_D of 24.21 ± 7.38 pM (corn snake) and 21.48 ± 3.85 pM (Boa constrictor) and in lung membranes (B_max fmol/mg protein: 55.95 ± 16.28 (corn snake) and 107.00 ± 14.21 (Boa constrictor); K_D pM: 71.25 ± 21.92 (corn snake) and 55.04 ± 18.68 (Boa constrictor)). Competition-binding studies showed β-adrenergic receptors with low affinities to the β₂-selective adrenergic receptor antagonist and high affinity binding to β₁-selective adrenergic receptor antagonist in both heart and lung tissues of both snake species, suggesting the presence of high population of the post-synaptic β₁-adrenergic receptor subtype. It seems that the presence of the predominant β₁-subtype also in lung tissues may indicate the importance of the vascular system in the snake lung.

Autonomic control of cardiovascular adjustments associated with orthostasis in the scansorial snake Boa constrictor

Orthostatic hypotension is a phenomenon triggered by a change in the position or posture of an animal, from a horizontal to a vertical head-up orientation, characterised by a blood pooling in the lower body and a reduction in central and cranial arterial blood pressure (P _A). This hypotension elicits systemic vasoconstriction and tachycardia, which generally reduce blood pooling and increase P _A Little is known about the mediation and importance of such cardiovascular adjustments that counteract the haemodynamic effects of orthostasis in ectothermic vertebrates, and some discrepancies exist in the information available on this subject. Thus, we sought to expand our knowledge on this issue by investigating it in a more elaborate way, through an in vivo pharmacological approach considering temporal circulatory changes during head-up body inclinations in unanaesthetised Boa constrictor To do so, we analysed temporal changes in P _A, heart rate (f _H) and cardiac autonomic tone associated with 30 and 60 deg inclinations, before and after muscarinic blockade with atropine, double blockade with atropine and propranolol, and α₁-adrenergic receptor blockade with prazosin. Additionally, the animals' f _H variability was analysed. The results revealed that, in B. constrictor: (1) the orthostatic tachycardia is initially mediated by a decrease in cholinergic tone followed by an increase in adrenergic tone, a pattern that may be evolutionarily conserved in vertebrates; (2) the orthostatic tachycardia is important for avoiding an intense decrease in P _A at the beginning of body inclinations; and (3) α₁-adrenergic orthostatic vasomotor responses are important for the maintenance of P _A at satisfactory values during long-term inclinations.

Contribution of active atrial contraction to cardiac output in anesthetized American alligators (Alligator mississippiensis)

Ventricular filling may occur directly from the venous circulation during early diastole or via atrial contraction in late diastole. The contribution of atrial contraction to ventricular filling is typically small in mammals (10-40%), but has been suggested to predominate in reptiles. We investigated the importance of atrial contraction in filling of the ventricle in American alligators (Alligator mississippiensis) by bypassing both atria (with the use of ligatures to prevent atrial filling) and measuring the resultant effects on cardiac output in anesthetized animals. Atrial ligation had no significant effects on total systemic blood flow before or after adrenaline injection. Unexpectedly, pulmonary flow was increased following atrial ligation prior to adrenaline treatment, but was unaffected after it. These findings suggest that the atria are non-essential (i.e. redundant) for ventricular filling in alligators, at least under anesthesia, but may serve as important volume reservoirs.

In situ cardiac perfusion reveals interspecific variation of intraventricular flow separation in reptiles

The ventricles of non-crocodilian reptiles are incompletely divided and provide an opportunity for mixing of oxygen-poor blood and oxygen-rich blood (intracardiac shunting). However, both cardiac morphology and in vivo shunting patterns exhibit considerable interspecific variation within reptiles. In the present study, we develop an in situ double-perfused heart approach to characterise the propensity and capacity for shunting in five reptile species (turtle: Trachemys scripta, rock python: Python sebae, yellow anaconda: Eunectes notaeus, varanid lizard: Varanus exanthematicus, and bearded dragon: Pogona vitticeps). To simulate changes in vascular bed resistance, pulmonary and systemic afterloads were independently manipulated and changes in blood flow distribution amongst the central outflow tracts were monitored. As previously demonstrated in Burmese pythons, rock pythons and varanid lizards exhibited pronounced intraventricular flow separation. As pulmonary or systemic afterload was raised, flow in the respective circulation decreased. However, flow in the other circulation, where afterload was constant, remained stable. This correlates with the convergent evolution of intraventricular pressure separation and the large intraventricular muscular ridge, which compartmentalises the ventricle, in these species. Conversely, in the three other species, the pulmonary and systemic flows were strongly mutually dependent, such that the decrease in pulmonary flow in response to elevated pulmonary afterload resulted in redistribution of perfusate to the systemic circuit (and vice versa). Thus, in these species, the muscular ridge appeared labile and blood could readily transverse the intraventricular cava. We conclude that relatively minor structural differences between non-crocodilian reptiles result in the fundamental changes in cardiac function. Further, our study emphasises that functionally similar intracardiac flow separation evolved independently in lizards (varanids) and snakes (pythons) from an ancestor endowed with the capacity for large intracardiac shunts.

Links between thermoregulation and aging in endotherms and ectotherms

While the link between thermoregulation and aging is generally accepted, much further research, reflection, and debate is required to elucidate the physiological and molecular pathways that generate the observed thermal-induced changes in lifespan. Our aim in this review is to present, discuss, and scrutinize the thermoregulatory mechanisms that are implicated in the aging process in endotherms and ectotherms. Our analysis demonstrates that low body temperature benefits lifespan in both endothermic and ectothermic organisms. Research in endotherms has delved deeper into the physiological and molecular mechanisms linking body temperature and longevity. While research in ectotherms has been steadily increasing during the past decades, further mechanistic work is required in order to fully elucidate the underlying phenomena. What is abundantly clear is that both endotherms and ectotherms have a specific temperature zone at which they function optimally. This zone is defended through both physiological and behavioral means and plays a major role on organismal senescence. That low body temperature may be beneficial for lifespan is contrary to conventional medical theory where reduced body temperature is usually considered as a sign of underlying pathology. Regardless, this phenomenon has been targeted by scientists with the expectation that advancements may compress morbidity, as well as lower disease and mortality risk. The available evidence suggests that lowered body temperature may prolong life span, yet finding the key to temperature regulation remains the problem. While we are still far from a complete understanding of the mechanisms linking body temperature and longevity, we are getting closer.

Ablation of the ability to control the right-to-left cardiac shunt does not affect oxygen consumption, specific dynamic action or growth in rattlesnakes, Crotalus durissus

The morphologically undivided ventricle of the heart in non-crocodilian reptiles permits the mixing of oxygen-rich blood returning from the lungs and oxygen-poor blood from the systemic circulation. A possible functional significance for this intra-cardiac shunt has been debated for almost a century. Unilateral left vagotomy rendered the single effective pulmonary artery of the South American rattlesnake, Crotalus durissus, unable to adjust the magnitude of blood flow to the lung. The higher constant perfusion of the lung circulation and the incapability of adjusting R-L shunt in left-denervated snakes persisted over time, providing a unique model for investigation of the long-term consequences of cardiac shunting in a squamate. Oxygen uptake recorded at rest, during spontaneous and forced activity, was not affected by removing control of the cardiac shunt. Furthermore, metabolic rate and energetic balance during the post-prandial metabolic increment, plus the food conversion efficiency and growth rate were all similarly unaffected. These results show that control of cardiac shunting is not associated with a clear functional advantage in adjusting metabolic rate, effectiveness of digestion or growth rates.

Responses to temperature variation: integration of thermoregulation and metabolism in vertebrates

Many vertebrates regulate their body temperature in response to thermal variability of the environment. Endotherms maintain relatively stable body temperatures by adjusting metabolic heat production in response to varying environmental heat loads. Although most ectotherms do not display adaptive thermogenesis, they do acclimate cellular metabolism to compensate for environmental temperature variation. The components of the thermoregulatory systems in endotherms and ectotherms are evolutionarily conserved, and I suggest that metabolic acclimation in ectotherms relies on the same regulatory pathways as adaptive thermogenesis in endotherms. Both groups rely on transient receptor potential ion channels to sense environmental temperatures. Thermosensory (afferent) information is relayed to the hypothalamus, which initiates a sympathetic efferent response. Cardiovascular responses to heat are similar in ectothermic crocodiles and in mammals, and are mediated by the autonomic nervous system in both cases. The sympathetic nervous system also modulates cellular metabolism by inducing expression of the transcriptional regulator peroxisome proliferator activated receptor γ coactivator 1α (PGC-1α), which interacts with a range of transcription factors that control glycolysis, fatty acid oxidation, gluconeogenesis, mitochondrial biogenesis and bioenergetics, and metabolic rate. PGC-1α is best known from mammalian model species but there is increasing evidence that it is also instrumental in non-mammalian vertebrates. Hence, endothermic adaptive thermogenesis may result from the same regulatory pathways as ectothermic metabolic acclimation, and both could be considered as adaptive metabolic responses to temperature variation.

The unequal influences of the left and right vagi on the control of the heart and pulmonary artery in the rattlesnake,Crotalus durissus

Autonomic control of the cardiovascular system in reptiles includes sympathetic components but heart rate (fH), pulmonary blood flow (Q̇pul) and cardiac shunt patterns are primarily controlled by the parasympathetic nervous system. The vagus innervates both the heart and a sphincter on the pulmonary artery. The present study reveals that whereas both the left and right vagi influence fH, it is only the left vagus that influences pulmonary vascular resistance. This is associated with the fact that rattlesnakes, in common with some other species of snakes, have a single functional lung, as the other lung regresses during development. Stimulation of the left cervical vagus in anaesthetised snakes slowed the heart and markedly reduced blood flow in the pulmonary artery whereas stimulation of the right cervical vagus slowed the heart and caused a small increase in stroke volume (VS) in both the systemic and pulmonary circulations. Central stimulation of either vagus caused small (5–10%)reductions in systemic blood pressure but did not affect blood flows or fH. A bilateral differentiation between the vagi was confirmed by progressive vagotomy in recovered snakes. Transection of the left vagus caused a slight increase in fH (10%) but a 70%increase in Q̇pul, largely due to an increase in pulmonary stroke volume (VS,pul). Subsequent complete vagotomy caused a 60% increase in fHaccompanied by a slight rise in Q̇pul, with no further change in VS,pul. By contrast, transection of the right vagus elicited a slight tachycardia but no change in VS,pul. Subsequent complete vagotomy was accompanied by marked increases in fH, Q̇puland VS,pul. These data show that although the heart receives bilateral vagal innervation, the sphincter on the pulmonary artery is innervated solely by the left vagus. This paves the way for an investigation of the role of the cardiac shunt in regulating metabolic rate, as chronic left vagotomy will cause a pronounced left–right shunt in recovered animals,whilst leaving intact control of the heart, via the right vagus.

Rhythmic contractility in the hepatic portal "corkscrew" vein of the rat snake

Terrestrial, but not aquatic, species of snakes have hepatic portal veins with a corkscrew morphology immediately posterior of the liver. Relatively large volumes of venous blood are associated with this region, and the corkscrew vein has been proposed to function as a bidirectional valve that impedes gravitational shifts of intravascular volume. To better understand the functional significance of the corkscrew anatomy, we investigated the histology and contractile mechanisms in isolated corkscrew segments of the hepatic portal vein of a yellow rat snake (Pantherophis obsoletus). Morphologically, the corkscrew portal vein is here shown to have two distinct layers of smooth muscle--an inner circular layer, and an outer longitudinal layer, separated by a layer of collagen--whereas only a single circular layer of smooth muscle is present in the adjacent posterior caval vein. Low frequency (approximately 0.3 cycles*min(-1)) spontaneous and catecholamine-induced rhythms were observed in 11% and 89% of portal vein segments, respectively, but neither spontaneous nor agonist-induced cycling was observed in adjacent posterior (non-corkscrew) caval veins. Catecholamines, angiotensin II, or stretch increased the amplitude and/or frequency of contractile cycles. Ouabain, verapamil or indomethacin, but not tetrodotoxin, alpha-, or ss-adrenergic receptor antagonists, inhibited cyclical contractions indicating a dependence of these cycles on Na+/K+ ATPase, extracellular Ca2+ and prostanoid(s). These data suggest that the rhythmic contractility of the corkscrew segment of the ophidian portal vein may act in conjunction with its morphological features to improve venous return and to prevent retrograde shifts of blood that might otherwise pool in posterior veins.

Physiological importance of the coronary arterial blood supply to the rattlesnake heart

The reptilian heart consists of a thick inner spongy myocardium that derives its oxygen and nutrient supply directly from the blood within the ventricular cavity, which is surrounded by a thin outer compact layer supplied by coronary arteries. The functional importance of these coronary arteries remains unknown. In the present study we investigate the effects of permanent coronary artery occlusion in the South American rattlesnake (Crotalus durissus) on the ability to maintain heart rate and blood pressure at rest and during short term activity. We used colored silicone rubber(Microfil) to identify the coronary artery distribution and interarterial anastomoses. The coronary circulation was occluded and the snakes were then kept for 4 days at 30°C. Microfil injections verified that virtually all coronary arteries had successfully been occluded, but also made visible an extensive coronary supply to the outer compact layer in untreated snakes. Electrocardiogram (ECG), blood pressure (Psys) and heart rate (fH) were measured at rest and during enforced activity at day 1 and 4. Four days after occlusion of the coronary circulation, the snakes could still maintain a Psys and fH of 5.2±0.2 kPa and 58.2±2.2 beats min–1, respectively, during activity and the ECG was not affected. This was not different from sham-operated snakes. Thus, while the outer compact layer of the rattlesnake heart clearly has an extensive coronary supply, rattlesnakes sustain a high blood pressure and heart rate during activity without coronary artery blood supply.

Hypoxia-induced vasoconstriction in alligator (Alligator mississippiensis) intrapulmonary arteries: a role for endothelin-1?

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 &lt;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 min)hypoxia (PO2&lt;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.