Effects of heat and cold on the albino rat: crossed resistance or crossed sensitization?

¹ Department of Physiology and Physical Environment Unit, University of Illinois, Urbana, Illinois

Two experiments were carried out to test whether or not cold-acclimatized rats adjust better to heat and heat-acclimatized rats adjust better to cold than rats not previously exposed to either environmental extreme. In both, groups of rats were exposed to 94.5°–96.5°F (hot), 33°–40°F (cold) and 76°–78°F (control). In the first experiment 45 female Holtzman littermates were used. After 50 days, ‘hot’ rats were abruptly transferred to cold, ‘cold’ rats to heat. Control animals were also placed in each environment. In the second experiment 55 identical rats were similarly treated; the cross, however, was made after 29 days. Rats exposed to 95°F exhibited sustained hypothermia. Rats exposed to 36°F developed a transient hypothermia which lasted longer in the first experiment than in the second and was accompanied by more severe cold injury. Heat was not a stress in the sense of Selye, but cold was. There was crossed sensitization rather than crossed resistance.