Effects of Calcium and Sodium on Cardiac Contractility and Heat Production in Rabbit Papillary Muscle

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Abstract
The effects of altering the concentrations of sodium (Na+) and calcium (Ca2+) ions on the isometric energy output of rabbit papillary muscle at room temperature were examined by a myothermic technique. The well-known inotropic changes resulting from these alterations were not correlated with parallel changes in the magnitude of the tension-independent heat. Instead, the magnitude of the tension-independent heat was directly correlated with both increasing (Na+) concentration and with increasing (Ca2+) concentration, and these effects were additive. The energetics of ions pumping in cardiac muscle are discussed quantitatively and it is suggested that the combined enthalpy consumption of both the (Na+) and the (Ca2+) pumps constitutes the major determinant of cardiac tension-independent heat production.