Heat produced by rabbit papillary muscle during anoxia and reoxygenation.

Resting heat rate was measured in superfused rabbit papillary muscles at 20 degrees C during 40 minutes of anoxia and subsequent reoxygenation. To reveal the nature of the reactions underlying energy output under such conditions, the data obtained were compared with values predicted from data on chemical change. Before and after the anoxic period, muscles were stimulated at 0.2 Hz, during which time the contraction-related heat rate was measured. During anoxia, muscles were kept at rest or stimulated at 1 Hz. Stimulation was switched off intermittently to determine resting heat rate. Before anoxia, resting heat rate was 8.7 +/- 1.1 (mean +/- SEM) mW.g dry wt-1. During anoxia, it decreased to 38% and 50% of the preanoxic level in resting and stimulated muscles, respectively (P < .05). In resting muscles, heat rate increased with reoxygenation in approximately 10 to 15 minutes to 1.3 times the preanoxic level, whereas this was 3.7 times in stimulated muscles. Resting heat rate returned within 65 (resting muscles) or 150 (stimulated muscles) minutes to the baseline. The ratio of force- and contraction-related heat rate, ie, the economy of contraction, was not different before and after anoxia. We estimated that the heat produced by muscles during anoxia was not different from the heat to be expected from the hydrolysis of creatine phosphate, the breakdown of nucleotides, and the formation of lactate. The overshoot in resting heat during reoxygenation of resting muscles could be accounted for by the resynthesis of the energy store. The much larger overshoot in resting heat of stimulated muscles was due to the contracture. The finding that the economy of contraction was not altered by anoxia and reoxygenation suggests that both sarcoplasmic reticulum Ca(2+)-ATPase and myofibrillar ATPase are depressed by anoxia and that the enhancement of cytosolic calcium transients with reoxygenation, reported in other studies on papillary muscle, results from reduced binding of calcium rather than from enhanced release.