Time course of ouabain uptake in isolated myocardial cells: dependence on extracellular potassium and calcium concentration.

1 Spontaneously beating myocardial cells isolated from newborn rats have been used to evaluate the time course of cellular ouabain uptake. 2 The rate of cellular uptake and the amount of ouabain bound at equilibrium were computed by fitting the experimental data to the conventional exponential equation for receptor binding of drugs. 3 At normal extracellular potassium and calcium concentrations a biexponential equation was the best fit to the experimental data, indicating two receptor sites of ouabain with different rates of uptake. 4 Increasing extracellular potassium or calcium concentrations decreased the amounts of ouabain bound at equilibrium. 5 High and low extracellular concentrations of potassium or calcium decreased the rate of ouabain uptake. 6 It is well known that ouabain changes ionic fluxes. Changes in the extracellular potassium and calcium concentrations also influence the amount of ouabain taken up by myocardial cells, as demonstrated in the present study.