Freshwater turtles (Chrysemys picta bellii Gray) were acclimated to temperatures of 5, 10, 20 and 30°C for at least 12 days, and pulmonary ventilation, oxygen uptake and arterial pH, PCOCO2 and POO2 were determined in completely unrestrained specimens. Oxygen uptake (V·OO2) increased overproportionately (6.7-fold) as compared to pulmonary ventilation (V·1, 4.4-fold) when the temperature increased from 10 to 30°C. The observed rise in arterial PCOCO2 from 13 (5°C) to 32mmHg (30°C) was the result of a decrease in V·1/V·OO2, whereas an increase of arterial POO2 from 12Torr at 5°C to about 60Torr at 20 and 30°C mainly resulted from the effects of intracardiac blood shunting combined with temperature-dependent shifts of the oxygen dissociation curve. Arterial pH fell with rising temperature significantly less (ΔpH/Δt =−0.010U/°C) than required for constant relative alkalinity and for constant dissociation of imidazole. The changes of cerebrospinal fluid pH with temperature, calculated from the mean arterial PCOCO2 values, were even smaller [ΔpH/ΔtCSF = −0.008). It is concluded that the observed temperature dependence of the acid-base status is not in agreement with the alphastat hypothesis.

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