Potentiation of twitch tension and prolongation of action potential induced by reduction of temperature in rat and frog muscle.

URING D the past four years it has been observed that the rising time and the peak tension of the twitch of the in sitzc rat triceps surae were slightly greater in experiments carried out in the winter than in those done in the summer. The room temperature was about 26” C. in the winter and about 31’ C. in the summer. An effect of temperature changes on the modification of twitch response in the leg muscle suggested itself because it seemed likely that the temperature of anaesthetized animals may decrease more in the cooler room. No report on the effects of temperature changes on twitch tension of in situ mammalian muscle was found in the literature. However, it was found that an observation (I) on the effects of cooling on the isolated nerve-diaphragm preparation of the rat had been made. In the fatigued preparation cooling increased the rising time, tension and action potential duration in response to single indirect shocks. The increased duration of action potential produced by cooling skeletal muscle (2) is of interest in the present report because it has recently been shown (3) that the potentiation of tension and the increase of rising time of the twitch in rats given intraperitoneal injection of KC1 are accompanied by an increase in duration of the action potential. The present study was begun in order to determine the changes in mechanical and electrical response of in situ rat muscle induced by lowering the temperature approximately IO’ C. below normal. Discrepancies between our findings and the observations (4, 5> on isolated frog muscle suggested that experiments on frog muscle with intact circulation should be carried out. A preliminary report on this study was made before the American Physiological Society at the meeting in Minneapolis in September, 1948.