Strain difference in thermoregulation of rats surviving extreme heat.

The survival times of unanesthetized rats in 42.5 degree C. 48% rh were studied in 12 different strains. In males, Sprague-Dawley rats (P less than 0.01) and Fisher 344/MK (P less than 0.05) showed significantly higher heat tolerance than the other 9 strains. Among Sprague-Dawley rats, females tolerated heat longer than males (P less than 0.05). There was no difference in lethal body temperature according to strains and exposure temperatures (38.5-48.5 degree C). Maximum survivable body temperature was 43.1 degree C in males and 43.3 degree C in females. The body weight loss in heat was greater in Sprague-Dawley, Fisher 344/MK, and JCL:Wistar strains. The degree of saliva spreading during the equilibrium period just below the maximum survivable body temperature correlated significantly with heat tolerance and was found to be the index of strain difference in heat tolerance. These findings demonstrated that the thermoregulatory system of rats is controlled genetically, though survival times of individuals in different strains sometimes overlap.