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

Simple Summary: Snakes are reptiles that have evolved over a period of approximately 170 million years, adapting to life in different habitats, including water (rivers and oceans), on the ground, in trees, and underground. The distance from the heart to the brain is known to be shorter in arboreal snakes compared to terrestrial ones, indicating that differences in habitat may also affect vascular response. In this study, we attempted to characterize vasoreactivity in the Tokara habu, a semi-arboreal snake. The Tokara habu snakes demonstrated a number of relaxation responses to various vasoactive substances, and they showed complex vasoreactivity, a contrast to the simple vasoreactivity seen in terrestrial snakes. These results suggest that the Tokara habu's distinctive, complex vasoreactivity may reflect adaptation to its semi-arboreal environment. Comparisons of vascular responses may be useful as a new approach to behavioral and ecological studies for species that are difficult to observe in the field. 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. [ABSTRACT FROM AUTHOR]