O reflexo barostático em teleósteos: controle cardiorrespiratório, influência do habitat e padronização do Método da Sequência para avaliação do ganho barorreflexo

Abstract

The regulation of arterial blood pressure by the baroreflex is a sine qua non condition for the maintenance of adequate tissue perfusion and for the prevention of organic lesions in most vertebrates. Fish, as well as other gnathostomates, regulate PA by adjusting heart rate and systemic vascular resistance, altering these variables in a manner inversely proportional to changes in PA. Concomitantly to these cardiovascular adjustments, some tetrapods also exhibit ventilatory adjustments inversely proportional to changes in PA, supposedly to enhance gas exchange in these animals’ respiratory surface. However, whether such ventilatory reflex responses based on PA occur in teleosts remains to be elucidated. In addition, despite the countless evidences on the presence of cardiovascular baroreflex responses in teleosts, no broad and robust description of these adjustments was performed in these animals. In this context, the present study aimed to: (1) verify the existence of baroreflex control of ventilation in teleosts; (2) perform the first broad and robust description of the baroreflex control of heart rate in teleosts, putting it in perspective with other species of different habitats, with emphasis on the influence of terrestriality on the evolution of the baroreceptor reflex; and (3) validate and standardize an alternative-complementary method for evaluating baroreflex sensitivity in teleosts, which is less invasive than the pharmacological method that constitutes the current paradigm for this task. To achieve the first objective, pharmacological manipulations of PA were performed in order to allow the evaluation of cardiac and possibly ventilatory baroreflex responses in the teleost with bimodal respiration Pangasianodon hypophthalmus. To achieve the second objective, identical pharmacological manipulations of PA were used to evaluate the cardiac baroreflex in exclusively aquatic teleosts (Hoplias malabaricus and Oreochromis niloticus – incapable of conducting terrestrial incursions) and amphibian teleosts (Clarias gariepinus and Hoplerythrinus unitaeniatus – capable of conducting terrestrial incursions). Finally, in order to reach the third objective, the pharmacological method for evaluation of cardiac baroreflex used to complete the previous objectives was employed in H. malabaricus and O. niloticus, as well as the alternative-complementary non-pharmacological method called "Sequence Method", for validation and standardization of this new approach for use in teleosts. The results demonstrated that: (1) unlike tetrapods, the teleost P. hypophthalmus does not present a respiratory component in the barostatic reflex, suggesting that the baroreflex control of ventilation evolved less than 450 million years ago, after the Actinopterygii-Sarcopterygii separation; (2) teleosts have a baroreflex control of heart rate as responsive as that of tetrapods, in addition, teleosts that perform terrestrial incursions (C. gariepinus and H. unitaeniatus) present greater baroreflex sensitivity and capacity to respond to hypotension compared to strictly aquatic teleosts (H. malabaricus and O. niloticus), suggesting that terrestriality represents an important driving force for the evolution of a more sensitive baroreflex in vertebrates; and (3) the Sequence Method provides reliable estimates of baroreflex sensitivity in teleosts, providing results similar to that of the pharmacological method, constituting an important alternative or complementary method for evaluation of baroreflex function in these animals.