Heat acclimation: cardiovascular response to hot/dry and hot/wet heat loads in rats

Body temperature, blood pressure (BP) and heart rate (HR) of rats before and after acclimation to heat were studied in chronically cannulated sedentary conscious rats in different hot environmental conditions [hot/dry: 40 degrees C, 20% relative humidity (RH) and hot/wet: 35 degrees C, 70% RH]. During exposure to heat stress, acclimated rats showed an attenuated increase in colonic temperature (Tc) compared to non-acclimated rats. Concomitantly, an abrupt decrease in HR, delayed and attenuated elevation in mean arterial BP and improved cardiac efficiency were recorded. Differences were observed upon exposure to the hot/wet and hot/dry climates: the hot/dry climate imposed a greater physiological burden than the hot/wet climate. The data suggest that for sedentary rats dry heat produces a greater load than humid heat. Thus, the conventional heat load indices do not apply universally but have a thermoregulatory pattern specificity.