The effect of rate of cooling,time of day and light regime on the shivering response in rabbits

In man hypothermia exists when core temperature (T_e) falls below 35 °C. The onset of hypothermia is often insidious in that it can occur without any particular symptoms of serious discomfort. There is evidence that this decrease in thermosensitivity is most likely to occur when the rate of body cooling is slow. In addition there is some evidence that the susceptibility to become hypothermic varies with the circadian rhythm of T_e, particularly in animals maintained under constant light conditions. A systematic investigation has been carried out to determine whether the thermoregulatory response to body core cooling is affected by the rate of change of body temperature, the time of day at which the cooling takes place and/or by the light regime under which the animals are maintained. The investigation was made in rabbits maintained either under a 12-h light/dark (LD) cycle or under conditions of continuous light (LL). Thermosensitivity (relationship between falling T_e and the induced increase in metabolic heat production) was determined at two different rates of body cooling (1 °C decrease in core temperature in either 30 or 160 min.) and at different times of the day. A chronically implanted intravascular heat exchanger was used to extract heat from the animals. The results indicate that neither the rate of body cooling nor the time of day at which the cooling took place had any clear effect on the shivering response. Likewise there was no clear difference in the shivering response of the animals maintained under (LD) conditions as compared to those maintained under (LL) conditions.