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

Abstract

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 (PA). This hypotension elicits systemic vasoconstriction and tachycardia, which generally reduce blood pooling and increase PA. Little is known about the mediation and importance of such cardiovascular adjustments that counteracts the haemodynamic effects of orthostasis in ectothermic vertebrates, and some discrepancies exist in the knowledge available on this subject. Thus, we sought to expand the 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 PA, heart rate (fH) and cardiac autonomic tones associated with 30° and 60° inclinations, before and after muscarinic blockade with atropine, double blockade with atropine and propranolol, and α1-adrenergic blockade with prazosin. Additionally, the animals’ fH variability was analysed. The results revealed that, in B. constrictor: (1) the orthostatic-tachycardia is initially mediated by a decrease in cholinergic tone followed later by an increase in adrenergic tone, a pattern that may be evolutionarily conserved in vertebrates; (2) the orthostatic-tachycardia is important for avoiding intense decrease in PA at the beginning of body inclinations; and (3) α1-adrenergic orthostatic vasomotor responses are important for the maintenance of PA in satisfactory values during long-term inclinations.