Seasonal changes in the cardiorespiratory responses to hypercarbia and temperature in the bullfrog, Rana catesbeiana

We assessed the seasonal variations in the effects of hypercarbia (3 or 5%, inspired CO2) on cardiorespiratory responses in the bullfrog Rana catesbei ana at different temperatures (10, 20 and 30 degrees C). We measured breath ing frequency, blood gases, acid-base status, hematocrit, heart rate, blood pressure and oxygen consumption. At 20 and 30 degrees C, the rate of oxyge n consumption had a tendency to be lowest during winter and highest during summer. Hypercarbia-induced changes in breathing frequency were proportiona l to body temperature during summer and spring, but not during winter (20 a nd 30 degrees C). Moreover, during winter, the effects of CO2 on breathing frequency at 30 degrees C were smaller than during summer and spring. These facts indicate a decreased ventilatory sensitivity during winter. PaO2 and pH(a) showed no significant change during the year, but PaCO2 was almost t wice as high during winter than in summer and spring, indicating increased plasma bicarbonate levels. The hematocrit values showed no significant chan ges induced by temperature, hypercarbia or season, indicating that the oxyg en carrying capacity of blood is kept constant throughout the year. Decreas ed body temperature was accompanied by a reduction in heart rate during all four seasons, and a reduction in blood pressure during summer and spring. Blood pressure was higher during winter than during any other seasons where as no seasonal change was observed in heart rate. This may indicate that pe ripheral resistance and/or stroke volume may be elevated during this season . Taken together, these results suggest that the decreased ventilatory sens itivity to hypercarbia during winter occurs while cardiovascular parameters are kept constant. (C) 1999 Elsevier Science Inc. All rights reserved.