Temperature effects on baroreflex control of heart rate in the toad, Rhinella schneideri.

For an adequate blood supply to support metabolic demands, vertebrates regulate blood pressure to maintain sufficient perfusion to avoid ischemia and other tissue damage like edema. Using a pharmacological approach (phenylephrine and sodium nitroprusside) we investigated baroreflex sensitivity at 15, 25, and 30°C in toads Rhinella schneideri. Baroreflex sensitivity presented a high thermal dependence (Q10=1.9-4.1), and the HR-baroreflex curve was shifted up and to the right as temperature increased from 15 to 30°C. Baroreflex variables, namely, HR range, gain50 (maximal gain) and normalized gain50 increased 206, 235, and 160% from 15 to 30°C, respectively. The cardiac limb of the baroreflex response to pharmacological treatments was significantly blunted after full autonomic blockade. In addition, there was a clear baroreflex-HR response mainly to hypotension at all three temperatures tested. These findings indicate that toads present temperature dependence for cardiac limb of the barostatic response and the cardiac baroreflex response in R. schneideri is primarily hypotensive rather than hypertensive as well as crocodilians and mammals. Thus, the cardiac baroreflex compensation to changes in arterial pressure might present different patterns among amphibian species, since the previously reported bradycardic compensation to hypertension in some anurans was not observed in the toad used in the present study.