Thermoregulatory responses in the rat to exercise in the heat following prolonged heat exposure.

To determine the effects of prolonged exposure to severe thermal stress on the subsequent ability to exercise in the heat, rats were exposed to a hot (35 degrees C) environment for 1, 2, 3, or 4 wk. At each of these weekly intervals the rats ran on a treadmill to hyperthermic exhaustion (41.5--43.0 degrees C), and tail-skin (Tt-sk) and rectal (Tre) temperatures were monitored. The results indicated that prolonged heat exposure did not enhance the rats' endurance capacity. Further, as the period of heat stress increased, there was a concomitant significant decrement in tail-skin vasodilation; indeed, after 3 and 4 wk at 35 degrees C Tt-sk reflects a complete shutdown of blood flow to the tail during exercise. Additionally, slight evaporative cooling from exogenous fluid (saliva or urine from the treadmill surface) might account for the low Tt-sk in relation to Tre and Ta. Hematocrit ratios ordinarily decreased from week to week during heat exposure, whereas body weights remained very consistent throughout the 4-wk interval. The mechanism of this decrement in vasodilation is undergoing further study.