EFFECTS OF ESTROGEN ON THERMOREGULATORY EVAPORATION IN RATS EXPOSED TO HEAT

The purpose of this study was to determine the effects of estrogen (E(2)) replacement on thermoregulation in ovariectomized rats exposed to heat. Female Sprague-Dawley rats were ovariectomized and splenectomized and implanted with a temperature-sensitive transmitter. Each rat was studied when E(2) treated (after an E(2) pellet implant) and untreated. Animals were divided into two groups with opposite order of treatment and were studied over a 9-wk period. Measurements of body core temperature (T-c) and evaporative water loss (EWL) were made on unrestrained animals resting at 38 degrees C air temperature. E(2)-treated animals increased EWL at all levels of T-c, reduced the threshold T-c for onset of saliva spreading, and regulated T-c at a lower level during heat exposure. E(2) treatment elevated plasma E(2) and reduced hematocrit but did not affect plasma osmolality. These effects of E(2) on evaporative cooling and T-c, in heat-stressed rats are similar to those that have been reported in human females. The mechanisms of the thermoregulatory effects of E(2) remain to be studied.