Autonomic control of cardiovascular adjustments associated with orthostasis in the scansorial snakeBoa 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 (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 α 1-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) α 1-adrenergic orthostatic vasomotor responses are important for the maintenance of P A at satisfactory values during long-term inclinations.

Analysis of heart rate variability in corn snakes (Pantherophis guttatus)

“…The LF and HF values and the LF/HF ratio were similar to those reported for Boa constrictor, in which the main LF peak occurred between 0.0 and 0.1 Hz and a smaller HF peak between 0.1 Hz and 0.2 Hz (Armelin et al, 2019). The peaks in each frequency range can vary in relation to the respiratory rate, as shown for the South American rattlesnake Crotalus durissus terrificus (Campbell et al, 2006).…”

“…Valentinuzzi et al, (1970) observed considerable variation in the T wave amplitude and duration over several days in snakes, although no immediate apparent cause of such variations was identified. The values for the ECG parameters in P. guttatus were lower than those for other species such as Boa constrictor (Conceição et al, 2014;Armelin et al 2019), Lampropeltis getuhts (Valentinuzzi et al, 1970) and Eunectes murinus (Strussmann, and Sazima,1991), but similar to those reported for Spilotes pullatus (Cruz and Junkes, 2008) and Crotalus durissus (Stuginski et al, 2011).…”

“…For the frequency domain, a non-parametric method based on the Fourier transformer algorithm, was used to decompose sequential series of RR intervals into different amplitudes and frequencies, thereby obtaining ultralow frequency (<0.003 Hz), very low frequency (VLF, 0.003-0.04 Hz), low frequency (LF, related primarily to the sympathetic system; 0.041-0.15 Hz) and high frequency (HF, related primarily to the parasympathetic system; 0.151-0.4 Hz) data. The LF/HF ratio provided another means of assessing the balance between the sympathetic and parasympathetic systems (Armelin et al 2019, Campbell et al 2006Vanderlei et al 2009…”

“…In addition, the animal´s position during ECG recording, e.g., whether in a horizontal or inclined position, can influence the level of stress and consequently affect the accuracy with which the vectors are determined (Armelin et al, 2019). These studies suggest that a detailed analysis of the mechanisms of cardiac regulation in a given species can improve our understanding of its adaptations to its natural habitat, whether terrestrial, aerial or aquatic (Shoemaker and Zandvliet, 2005).…”