Effects of estrus cycle on thermoregulatory responses during exercise in rats

In female rats, rectal temperature (T _re), tail vasomotor response, oxygen uptake \(\left( {\dot V_{{\text{O}}_{\text{2}} } } \right)\) , and carbon dioxide production \(\left( {\dot V_{{\text{CO}}_{\text{2}} } } \right)\) were measured in proestrus and estrus stages during treadmill running at two different speeds at an ambient temperature (T _a) of 24° C. Experiments were performed at 2.00–6.00 a.m., when the difference inT _re was greatest between the two stages;T _re at rest in the estrus stage was 0.54° C higher than in the proestrus stage. In a mild warm environment, thresholdT _re for a rise in tail skin temperature (T _tail) was also higher in the estrus stage than in the proestrus stage. In contrast, no difference was seen in the thresholdT _re and steady stateT _re at the end of exercise between proestrus and estrus stages. These values were higher at the higher work intensity. \(\dot V_{{\text{O}}_{\text{2}} } \) was also similar between the two stages, except in the second 5 min after the beginning of exercise, when \(\dot V_{{\text{O}}_{\text{2}} } \) was greater andT _re rose more steeply in the proestrus stage. These data indicate that deep body temperature during exercise is regulated at a certain level depending on the work intensity and is not influenced by the estrus cycle.