Arterial baroreflex function in toads Rhinella schneideri: temperature effect, hydration state and role of renin-angiotensin system

Abstract

Various environmental or anthropogenic stressors can influence amphibians' life cycle. Fluctuations in temperature can cause substantial changes in cardiorespiratory variables in these animals, reflecting effects on peripheral vascular resistance, cardiac contractile activity, intrinsic sinoatrial node firing rate, as well as ventilation. Few studies report baroreflex responses in different species of vertebrates and little is known about thermal influences on the feedback loop regulation of blood pressure (BP) in amphibians. Interactions between cardiovascular and respiratory systems operate coordinately to match oxygen delivery to demand in an ectotherm and thereby increasing oxygen consumption can be achieved when animal experience high environmental temperatures. It has been suggested that ventilation might be part of the BP feedback loop regulation in mammals, as well as in amphibians; however, no data exist related to relationship between BP and ventilation against different metabolic demands imposed by temperature. We have recently reported an increase in cardiac baroreflex sensitivity along with a clear cardiac reflex response mainly to hypotension at all three temperatures tested (15, 25 and 30°C) in the South American savanna-related toad Rhinella schneideri. These findings indicate that these toads present temperature dependence for cardiac limb of the barostatic response and the cardiac baroreflex response is primarily hypotensive rather than hypertensive, similar to crocodilians and mammals. Thus, the cardiac baroreflex response and ventilation will be determined by increases induced by phenylephrine (alpha 1-adrenergic agonist) and decreases caused by sodium nitroprusside (a nitric oxide donor) in systemic arterial BP of toads. The cardiac limb of barorreflex evaluation will be comprised by the sigmoid curves analyses in addition to ventilation measurements (breathing frequency and tidal volume) at 25 and 35oC. Animals will also be submitted to hyperoxia to assess the possible involvement of peripheral chemoreflex. (AU)