Thermoregulatory responses to acute heat loads in rats following spontaneous running

Earlier studies showed that spontaneous exercise training in rodents shifte d their core temperature and thermoeffector thresholds to high levels. The present study investigated heat loss and heat production responses to acute heat loads of exercise-trained rats. The exercise-trained rats were allowe d to run in a running wheel freely for 6 months, while the sedentary contro ls were denied access to the wheel during the same period. Then, they were loosely restrained and put in a direct calorimeter. After thermal equilibri um had been attained, they were warmed for 30 min with an intraperitoneal e lectric heater (internal heating). At least 2h I;ater, the rats were extern ally warmed for 90 min by raising the ambient temperature from 24 to 38 deg rees C (external warming). Hypothalamic temperature (T-hy), evaporative and nonevaporative heat loss (R+C +K) and heat production were measured. Inter nal and external heating significantly increased T-hy During internal heati ng, the magnitude of the increase in Thy was significantly smaller and the amount of increase in (R+C+K) was significantly greater in the exercise-tra ined rats than in the controls. The slope showing the relationship between T-hy and (R+C+K) in the trained rats was significantly steeper than that in the controls. During external warming, the magnitude of increase in T-hy O f the exercise-trained rats was significantly greater than that of the cont rols. The slope showing the relationship between T-hy and (R+C+K) in the tr ained rats was not different from that in the controls. Changes in evaporat ive heat loss and heat production during the two types of heat load did not differ between the two groups. The results suggest that, in rats, exercise training with voluntary running improves heat tolerance through enhancing nonevaporative heat loss response. However, this may be the case only when the rats are subjected to a direct internal heat load.