The rate of resting heat production of rat papillary muscle.

The rate of resting heat production of quiescent rat left ventricular papillary muscles was measured myothermically. The effects of contractile activity, stretch, oxygen partial pressure, temperature, amino acids and time were examined. The rate of basal heat production was the same throughout the day whether or not muscles contracted isotonically under a small pre-load. Passive stretch increased the rate of resting heat production; the stretch-induced increment was highly variable from muscle to muscle. The resting heat rate per se was only moderately sensitive to oxygen partial pressure and temperature, and was insensitive to the presence of amino acids in the bathing medium. The stretch-induced increase in resting heat rate was independent of these three factors. The rate of resting heat production declined exponentially with time to reach a plateau about 4 h following cardiectomy.