THE CONTRIBUTION OF THE SODIUM-PUMP TO THE BASAL-METABOLISM OF ISOLATED CARDIAC-MUSCLE - A CALORIMETRIC STUDY

The rate of heat production of thin ventricular trabeculae isolated from guinea-pig heart was measured using a calorimetric technique. The preparations were mounted in a Perspex tube of 0.8 mm i.d. and 0.1 mm wall thickness. The tube was perfused with physiological salt solution at a constant rate of 1-mu-l s-1. The temperature difference between two points on the Perspex tube, one upstream and one downstream of the preparation, was measured with constantan/chromel thermocouples, and was shown to be proportional to the rate of heat production of the preparation. Inhibition of the sodium pump in quiescent trabeculae with the cardiac glycoside dihydro-ouabain produced a decrease in the resting heat production rate of 4.1 +/- 1.0%. This probably represents the heat production related to the activity of the Na,K ATPase in the steady state, including the heat production due to the resynthesis of ATP via oxidative phosphorylation. From this figure, the turnover rate of the Na,K ATPase in resting cardiac muscle was calculated to be about 8 nmol cm-3 s-1. Superfusion of beating preparations with dihydro-ouabain resulted in an approximately three-fold increase in the measured heat production rate. This effect was related to the positive inotropic action of the drug and may be a consequence of the increased activity of the actomyosin ATPase, the Ca ATPase, and the Na,K ATPase.