Shifts of thermoeffector thresholds in heat-acclimated rats.

1. Heat-acclimated (HA) rats, kept under a 12:12 h light-dark regime, were subjected to an ambient temperature (Ta) of 33 degrees C for 5 h in the last half of the dark phase for 3 weeks. Control rats were kept under a 12:12 h light-dark regime at a constant Ta of 24 degrees C. 2. After the acclimation period, the rats were then placed in a metabolic chamber at a Ta of 26.5 degrees C, and body core temperature was gradually increased and decreased using an intravenous thermode. Thermoeffector thresholds were determined by the hypothalamic temperature (T(hy)) at the onset of warm-induced tail skin vasodilatation and cold-induced thermogenesis measured by oxygen consumption. 3. Each rat was subjected to the experiment twice, once in the first half and once in the last half of the dark phase, on different days in random order. 4. In HA rats, T(hy) at the onset of both skin vasodilatation and thermogenesis were significantly lower in the last half of the dark phase compared with the first half. In control rats, however, there were no such differences between the two halves. 5. The results suggest that in rats acclimated to daily heat exposure, thermoeffector thresholds shift to lower temperatures only during the period of day when the rats had previously been exposed to heat.