Intracellularly applied sodium mimics the effects of ouabain in single cardiac myocytes

Sodium was applied intracellularly, in single myocytes from guinea-pig hearts via the potential-recording electrode. With electrodes containing 75 mM NaCl (but not with 15 mM) contractility increased and the action potential duration shortened. Aftercontractions, delayed afterpotentials and spontaneous activity as observed with toxic concentrations of ouabain (10 microM) appeared within 1 min when 150 mM NaCl was applied. The results demonstrate that sodium load without pump inhibition can mimic the effects of cardiac glycosides.