Brain and abdominal temperatures at fatigue in rats exercising in the heat

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Abstract
We measured brain and abdominal temperatures in eight male Sprague-Dawley rats (350–450 g) exercising voluntarily to a point of fatigue in two hot environments. Rats exercised, at the same time of the day, in three different trials, in random order: rest 23°C, exercise 33°C; rest 23°C, exercise 38°C; and rest 38°C, exercise 38°C. Running time to fatigue was 29.4 ± 5.9 (SD), 22.1 ± 3.7, and 14.3 ± 2.9 min for the three trials, respectively. Abdominal temperatures, measured with intraperitoneal radiotelemeters, at fatigue in the three trials (39.9 ± 0.3, 39.9 ± 0.3, and 39.8 ± 0.3°C, respectively) were not significantly different from each other. Corresponding brain temperatures, measured with thermocouples in the hypothalamic region (40.2 ± 0.4, 40.2 ± 0.4, and 40.1 ± 0.4°C), also did not differ. Our results are consistent with the concept that there is a critical level of body temperature beyond which animals will not continue to exercise voluntarily in the heat. Also, in our study, brain temperature was higher than abdominal temperature throughout exercise; that is, selective brain cooling did not occur when body temperature was below the level limiting exercise.