Electrical activity of skeletal muscle of normal and acclimatized rats on exposure to cold.

T HE PRINCIPAL ALTERATION in heat balance which occurs in rats exposed to a moderately cold environment is characterized bY an increased produ ction of heat, rather than by a decrease in its loss. This has been demonstrated by the measurement of oxygen consumption, and by direct calorimetry (1-3). Increased oxygen consumption in the cold can be attributed to increased activity of skeletal muscle and also, in the acclimatized rat, to an increase in the metabolic activity of visceral tissues (4). On initial exposure, the non-acclimatized rat is able to increase its oxygen consumption to almost as high a level as that of the acclimatized animal, but under severe conditions lacks the same capability to continue producing heat at the high rate necessary to maintain the temperature of the body (5). In addition to the altered metabolism of visceral tissues already demonstrated, a difference between the acclimatized and nonacclimatized rat might also exist in the responses of the skeletal muscles to cold. Experiments were carried out in order to measure the electrical activity of the skeletal muscle of acclimatized and non-acclimatized rats and to relate these find ings to the rate of oxygen consump tion and to body tempera ture at normal and low ambient temperatures.