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The effects of extracellular potassium chloride concentrations (0, 2, 4, 4.6, 5, and 8 mM.) on simultaneously recorded transmembrane action potentials and contractions of human papillary muscle were investigated. In a potassium-free solution the early changes observed were a shortening of the action potential duration (APD) at the 25% level of repolariza-tion, and no change or a slight increase in the APD at the 90% level of repolarization. A temporal relation between the shortening of the APD at the 25% and 50% levels of repolarization and an increase in the force of contraction was observed. A direct relation between the increase in extracellular potassium concentration and an increase in the APD as-sociated with a corresponding decrease in the force of contraction was observed. Ouabain (10-9 and 10-8 M.) produced a shortening of the APD and an increase in the contraction similar to KCl-free solution. Ouabain was ineffective in producing its effects on the muscles prein-cubated in KCl-free solution. Exposure of ouabain treated muscles to a KCl-free solution containing ouabain resulted in a small increase in the contraction and a small shortening of the APD of the muscles. The combined effects of ouabain and KCl-free solution were not greater than the effects of a KCl-free solution alone. It is suggested that the changes in APD and force of contraction might be associated with potassium or oua-bain-induced changes in the adenosinetriphosphatase activity of the heart muscle.

Transmembrane Potential and Contraction of Human Heart in Relation to Potassium and Ouabain