The effects of temperature on the energetics of rat papillary muscle.

The mechanical and myothermic responses of left ventricular papillary muscles from adult rats have been examined at 20 degrees and at 27 degrees C. Contraction trains of six isometric or isotonic twitches at 1/6 Hz were used to establish the heat-stress and load-enthalpy relations respectively. Peak isometric stress was slightly higher at 20 degrees than at 27 degrees C (45 vs. 41 mN/mm2) and was inversely related to muscle cross-sectional area. The stress-independent heat component, identified with the activation heat, was 75% greater at the lower temperature. The stress-dependent heat component, identified with the heat of actin-myosin interaction, was unaffected by temperature. In isotonic experiments the external work performance was similar at both temperatures but the heat liberation was significantly enhanced at the lower temperature so that mechanical efficiency (external work/enthalpy) was reduced. Evidence is presented suggesting that the preparations were not O2-diffusion limited at either temperature. The results are discussed in terms of known functional anomalies of rat cardiac tissue.