Thermal homeostasis in pregnant rats during heat stress.

Rats exposed to inescapable heat stress maintained a controlled hyperthermia while increasing heat loss by cutaneous vasodilatation and by grooming behavior. In nonpregnant rats, the evaporation of saliva groomed onto the body surfaces increased exponentially as a function of ambient temperature above 36 degrees C. This was associated with a decrease in the body temperature threshold for salivary secretion from the submaxillary gland, which then began at approximately the same body temperature as cutaneous vasodilatation. In addition, the pregnant rats maintained a lower level of controlled hyperthermia during heat stress than did nonpregnant rats. This appeared to result from a decreased production of metabolic heat, reduced insulation on the ventral surface, and an increased motivation to keep cool during heat stress. These changes met the increased need for thermolysis during pregnancy and provided for thermal homeostasis both in the pregnant rat and in the unborn fetuses.