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 receptors in isoproterenol-induced relaxation. NO may be involved in acetylcholine-induced relaxation, but not 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.