Abstract
Piaractus mesopotamicus were acclimated to 35°C (group I) and 15°C (group II). In group I, temperature was decreased from 35°C to 15°C, followed by a return to 35°C, while in group II, temperature rose to 35°C and returned to 15°C. For each temperature, oxygen uptake ( V ̇ O 2 ), ventilatory parameters ( V ̇ G , f R, V T, V ̇ G / V ̇ O 2 ), oxygen extraction (EO 2) and ECGs were obtained. Group II developed a higher V ̇ O 2 than group I, and the hysteresis presented by group I may characterize the attempt to reestablish the respiratory homeostasis. The V T remained almost constant and the increases in V ̇ G were achieved by larger increases in f R. Group II presented significantly higher f H values between 30°C and 20°C when compared to the warm-acclimated group. Neither thermal acclimation nor changes in temperature affected the QRS and the T wave amplitudes. The increases of QRS and T wave durations, A APD and V APD in group I suggest that low temperatures delayed the ionic diffusion within the sarcolemma and, thereby, the final depolarization and repolarization.
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