Thermoregulatory responses to acute heat loads in the FOK rat

FOK is an inbred rat strain with a genotypic adaptation to hot environments. The present study investigated the mechanism of the high heat tolerance of the FOK rat. Male FOK and WKAH rats were used. They were loosely restrained and placed individually in a direct calorimeter with an ambient temperature of 24 degrees C. Their hypothalamic temperature, evaporative and non-evaporative heat loss and heat production were measured. After thermal equilibrium had been attained, the rats were warmed for 30 min with a chronically implanted intraperitoneal electric heater (internal heating). At least 90 min after the heating, the jacket water temperature surrounding the calorimeter chamber was gradually raised from 24 degrees C to 36 degrees C in 80 min (external warming). During the internal heating, changes in the thermoregulatory parameters did not differ between the groups. During the external warming, the evaporative heat loss of the FOK rat was significantly greater than that of the WKAH rat, while changes in nonevaporative heat loss and heat production did not differ between the groups. The results suggest that in the FOK rat, the improved heat tolerance is attributable to an enhanced evaporative heat loss response, but not to a facilitation of nonevaporative heat loss or of metabolic depression.