Influence of energy supply and calcium on the low sodium induced changes in the transmembrane potential and contraction of guinea pig papillary muscle

The effects of changes in extracellular concentrations (25, 50, 100, 140 mM) of NaCl on the simultaneously recorded action potential (AP) and contraction of guinea pig papillary muscle in the presence of various concentrations of glucose (0, 5, 20, 30, 50 mM) and CaCl2 (0, 0.5, 1.0, 2.5 mM) were investigated. Low sodium (25 mM) produced a marked shortening of the action potential duration (APD) associated with a marked increase in the force of contraction but a small change in AP amplitude. A direct relation between extracellular NaCl and cardiac contraction was observed. There was no relation between AP amplitude and cardiac contraction. Low sodium induced shortening of the APD was greater in the absence of glucose than in its presence while the increase in the force contraction induced by low Na was markedly reduced in the absence of glucose. Low sodium was practically ineffective in restoring the contraction of papillary muscle in the absence of CaCl2. A calcium concentration dependent increase in the force of contraction was associated with a corresponding shortening of the APD in low sodium. Anoxia markedly reduced the positive inotropic effects of low sodium and enhanced the low sodium induced shortening of the APD in the papillary muscle. Glucose produced a marked lengthening of the APD which was shortened during anoxia but it was effective only slightly in lengthening the APD shortened by low sodium. The effects of ouabain in the muscles in the presence of low sodium were reduced. It is suggested that the changes in the APD accompanied by changes in the contraction in the guinea pig papillary muscle might be associated with NaCl-induced changes in the membrane ATPase. The low sodium induced increase in the force of contraction seems to be dependent upon both calcium and energy supply.