Cardiovascular, respiratory and metabolic responses to temperature and hypoxia of the winter frog Rana catesbeiana.

The objective of the present study was to determine the effects of hypoxia and temperature on the cardiovascular and respiratory systems and plasma glucose levels of the winter bullfrog Rana catesbeiana. Body temperature was maintained at 10, 15, 25 and 35 degrees C for measurements of breathing frequency, heart rate, arterial blood pressure, metabolic rate, plasma glucose levels, blood gases and acid-base status. Reducing body temperature from 35 to 10 degrees C decreased (P < 0.001) heart rate (bpm) from 64.0 +/- 3.1 (N = 5) to 12.5 +/- 2.5 (N = 6) and blood pressure (mmHg) (P < 0.05) from 41.9 +/- 2.1 (N = 5) to 33.1 +/- 2.1 (N = 6), whereas no significant changes were observed under hypoxia. Hypoxia-induced changes in breathing frequency and acid-base status were proportional to body temperature, being pronounced at 25 degrees C, less so at 15 degrees C, and absent at 10 degrees C. Hypoxia at 35 degrees C was lethal. Under normoxia, plasma glucose concentration (mg/dl) decreased (P < 0.01) from 53.0 +/- 3.4 (N = 6) to 35.9 +/- 1.7 (N = 6) at body temperatures of 35 and 10 degrees C, respectively. Hypoxia had no significant effect on plasma glucose concentration at 10 and 15 degrees C, but at 25 degrees C there was a significant increase under conditions of 3% inspired O2. The arterial PO2 and pH values were similar to those reported in previous studies on non-estivating Rana catesbeiana, but PaCO2 (37.5 +/- 1.9 mmHg, N = 5) was 3-fold higher, indicating increased plasma bicarbonate levels. The estivating bullfrog may be exposed not only to low temperatures but also to hypoxia. These animals show temperature-dependent responses that may be beneficial since during low body temperatures the sensitivity of most physiological systems to hypoxia is reduced.