Circadian variation of thermoregulatory responses during exercise in rats

Rats exercised on a treadmill at daytime lows and nighttime highs of circadian change in body temperature at two different work intensities [40 and 60% of maximal oxygen uptake (VO2max)] with the ambient temperature (Ta) at 24 degrees C. Immediately before exercise at 60% VO2max, rectal temperature (Tre) was 0.7 degrees C higher at night than during the day. During the exercise, Tre rose more during the day than at night, and Tre at the end of exercise was the same in the day as at night. Threshold Tre for tail vasodilation did not differ between day and night. Similar tendencies of Tre change and tail vasomotor response were observed at a work intensity of 40% VO2max, except that the rise in Tre was smaller than at the higher work intensity. On the other hand, threshold Tre for tail vasodilation spontaneously occurring in resting rats in a warm environment (Ta of 28 degrees C) was 0.7 degrees C higher at night than during the day. In conclusion, exercise in rats attenuates the differences in deep body temperature and threshold Tre for tail vasodilation seen between day and night.