Effects of gravitational profiles on the rat's thermoregulatory response to cold

Unrestrained rats were subjected to a 1-h period of cold exposure during centrifugation to characterize their ability to regulate core temperature T(c) and to determine if this regulation was dependent on the amplitude of the hypergravic field before the cold exposure. T(c) was measured in unrestrained rats by the use of a thermistor implanted adjacent to the carotid artery. One hour of cold exposure applied over the last hour of either a 1-, 4-, 7-, 13-, 19-, 25-, or 37-h period at 3 G evoked a decrease in T(c) of about 3 C. This fall in T(c) was significantly greater than changes in T(c) in cold-exposed rats at 1 G. No significant differences were found between the measured decreases in T(c) observed for the 1-h cold exposures during the first 37 h at 3 G. However, when rats were subjected concurrently to cold and acceleration after eight days at 3 G, they exhibited a smaller fall in T(c) suggesting a partial recovery of the acceleration-induced impairment of temperature regulation. In another series of experiments, the gravitational field profile was changed in amplitude in three different ways during the 3-h period preceding the 1-h cold exposure at 3 G. Despite the different gravitational field profiles before cold, the magnitude of the fall in T(c) over the 1-h period of cold exposure was the same in all cases. These results suggest that the thermoregulatory impairment has a rapid onset, is a manifestation of an ongoing effect of hypergravity, and is not dependent on the prior G profile. The inability of rats to maintain T(c) when cold exposed may be transient as indicated by the partial recovery of regulation by the eighth day.