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The effects of extracellular rubidium chloride (1, 2, 4, and 8 mM) on simultaneously recorded transmembrane action potentials and contractions of human papillary muscle in potassium-free solutions were investigated. In KCl-free solution, reduction in the duration of the action potential at the 25% and 50% levels of repolarization was associated with an increase in the force of contraction. Exposure of the muscle to rubidium chloride led to an increase in the duration of the action potential at 25% and 50% levels of repolarization associated with a corresponding decrease in the force of contraction. A decrease in the force of contraction was more usually associated with an increase in the duration of the action potential at the 25% level of repolarization than at the 50% level. No appreciable change in the duration of the action potential at the 90% level of repolarization was observed. Increases in the duration associated with corresponding decreases in the force of contraction were concentrationdependent. Rubidium chloride (8 mM) produced a conduction delay and inexcitability in the muscle. It has been suggested that in KCl-free solution the reduction in the duration of the action potential and an increase in the force of contraction were associated with inhibition of membrane ATPase. Also, the increases in the duration of the action potential accompanied by decreases in the force of contraction were associated with rubidium chloride-induced stimulation of membrane ATPase in human myocardium.

Effect of Replacement of Potassium by Rubidium on the Transmembrane Action Potential and Contractility of Human Papillary Muscle