Shift of threshold temperature for shivering and heat polypnea as a mode of thermal adaptation

Summary

Newborn guinea pigs were divided into three groups and kept for several weeks at one of the following environmental conditions: 1. 28°C, WA-animals; 2. 3°C, CA-animals; 3. 12 hrs daily at 3°C, 12 hrs at 28°C, CWA-animals. At the age of 4–7 weeks threshold temperatures were determined for shivering (electrical muscle activity) and heat polypnea, and the maximum amount of nonshivering thermogenesis was measured using the noradrenaline test. In the CA-animals both shivering and heat polypnea threshold were found to be decreased in comparison with WA-animals by about 1°C; for these studies the animals were placed and immobilized in a climatized respiratory chamber. In another series of studies, in which the animals were unrestrained, the mean colon temperature of CA-animals was about 0.5°–1°C lower than in WA-animals when exposed to ambient temperatures of 22.5 and 30°C for 24 and 2 hrs, respectively. At 15°C ambient temperature there was a similar but smaller temperature difference. Hence, both types of studies would indicate that the “set point” of the temperature control system was set to a lower level in CA-animals. In CWA-animals the shivering threshold was decreased as in CA-animals; the heat polypnea threshold, however, remained as high as in the WA-animals. This “widening of the interthreshold zone” in CWA-animals is shown to provide a more economical temperature regulation when the animals are subjected to fluctuating environmental conditions, as they are enabled to tolerate body temperature changes to some extent before they actuate their cold or heat defense mechanisms. With regard to the ability of NST (i.e. metabolic cold-adaptation) CWA-animals were not different from CA-animals.