Effects of calcium and sodium on cardiac contractility and heat production in rabbit papillary muscle.

The effects of altering the concentrations of sodium (Na) and calcium (Ca + ) 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 Ca 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 Ca + pumps constitutes the major determinant of cardiac tension-independent heat production. ADDITIONAL