Presence of functional angiotensin II receptor and angiotensin converting enzyme in the aorta of the snake Bothrops jararaca

Habu snakes (Protobothrops flavoviridis) show variation in thoracic aortic vasoreactivity between adjacent Japanese islands

Habu snakes (Protobothrops flavoviridis) are pit vipers found in the geographically adjacent but ecologically divergent islands of Tokunoshima and Amami-Oshima in southwestern Japan. Abiotic factors can cause variation in animal populations between the two islands, and Habu snakes may show such intraspecific physiological variation. We therefore evaluated the vasoreactivity in aortas isolated from the Habu of both islands. Tokunoshima Habu showed significantly greater contractile responses to angiotensin (Ang) II, acetylcholine (ACh) and noradrenaline, and significantly higher affinities (pEC50) for Ang II and ACh, than Amami-Oshima Habu. ACh caused contractions in aortas from both populations, a finding previously unreported in snakes. Our findings indicate that vasoreactivity may differ between Tokunoshima and Amami-Oshima Habu.

Characterization of Vasoreactivity in a Semi-Arboreal Snake, the Tokara Habu (Protobothrops tokarensis)

Vasoreactivity is relatively well documented in terrestrial snakes but has previously been investigated in only one semi-arboreal snake species. Consequently, the extent to which vasoreactivity is common across snake taxa or varies by habitat is unclear. The Tokara habu (Protobothrops tokarensis) is a semi-arboreal snake endemic to only two small adjacent Japanese islands, and hence a useful species for further investigation of vasoreactivity. We evaluated responses to known vasoactive substances in thoracic aortas isolated from Tokara habu. Under resting tension, noradrenaline and angiotensin II induced concentration-dependent contraction, but acetylcholine, serotonin (5-hydroxytriptamine; 5-HT), and isoproterenol induced relaxation followed by contraction. Histamine and rattlesnake bradykinin had no effect. Experiments with receptor-specific antagonists suggest that M1 and M3 receptors are involved in the acetylcholine-induced response; 5-HT1, 5-HT2, and 5-HT7 receptors in the serotonin-induced response; and β1 and β2 adrenoceptors in isoproterenol-induced relaxation. This is the first report on such response patterns in snakes (including serotonin- and isoproterenol-induced relaxation). Nitric oxide may be involved in acetylcholine-induced relaxation but not in the responses to serotonin or isoproterenol. In contrast to the uniform vasoreactivity observed in terrestrial snakes, the vasoreactivity of semi-arboreal snakes may be governed by diverse regulatory mechanisms.

The vasopressor action of angiotensin II (ANG II) in ball pythons (Python regius)

Angiotensin II (ANG II) is part of the renin-angiotensin system (RAS) in vertebrates and exert vasoconstriction in all species studied. The present study examines the vasopressor effect of ANG II in the ball python (Python regius), and examines whether ANG II exert its effect through direct angiotensin receptors or through an activation of α-adrenergic receptors. The studies were conducted in snakes with chronic arterial catheters that had recovered from anesthesia. In addition to demonstrating a clear and pronounced dose-dependent rise in arterial blood pressure upon repeated injections of boluses with ANG II (0.001-1 μg/kg), we demonstrate that the pressor response persisted following α-adrenergic blockade using the α-adrenergic antagonist phentolamine (2.5 mg/kg). Unfortunately, it proved impossible to block the ANG receptors using losartan (1, 3 or even 10 mg/kg). The pressor response to ANG II was associated with a significant rise in heart rate at the higher dosages, pointing to a resetting of the barostatic mechanism for heart rate regulation. The responses were similar in fasting and digesting pythons despite the expected rise in baseline values for blood pressure and heart rate of the digesting snakes.

Chronic captopril treatment reveals the role of ANG II in cardiovascular function of embryonic American alligators (Alligator mississippiensis)

Angiotensin II (ANG II) is a powerful vasoconstrictor of the renin-angiotensin system (RAS) that plays an important role in cardiovascular regulation in adult and developing vertebrates. Knowledge of ANG II's contribution to developmental cardiovascular function comes from studies in fetal mammals and embryonic chickens. This is the first study to examine the role of ANG II in cardiovascular control in an embryonic reptile, the American alligator (Alligator mississippiensis). Using chronic low (~ 5-mg kg embryo^-1), or high doses (~ 450-mg kg embryo^-1) of captopril, an angiotensin-converting enzyme (ACE) inhibitor, we disrupted the RAS and examined the influence of ANG II in cardiovascular function at 90% of embryonic development. Compared to embryos injected with saline, mean arterial pressure (MAP) was significantly reduced by 41 and 72% under low- and high-dose captopril treatments, respectively, a greater decrease in MAP than observed in other developing vertebrates following ACE inhibition. Acute exogenous ANG II injection produced a stronger hypertensive response in low-dose captopril-treated embryos compared to saline injection embryos. However, ACE inhibition with the low dose of captopril did not change adrenergic tone, and the ANG II response did not include an α-adrenergic component. Despite decreased MAP that caused a left shifted baroreflex curve for low-dose captopril embryos, ANG II did not influence baroreflex sensitivity. This study demonstrates that ANG II contributes to cardiovascular function in a developing reptile, and that the RAS contributes to arterial blood pressure maintenance during development across multiple vertebrate groups.