Isoproterenol directly stimulates the Na+-K+ pump in isolated cardiac myocytes

It has been suggested that catecholamines directly stimulate Na+-K+ pump activity in heart; however, these studies on multicellular preparations are confounded by possible alterations of extracellular K+ concentrations ([K+]o). We reinvestigated this problem by studying the effect of the beta-agonist isoproterenol (Iso) on intracellular Na+ activity (aiNa) in ventricular myocytes enzymatically isolated from rabbit heart. In 5 mM [K+]o, 0.1 microM Iso caused a 24.6 +/- 2.0% decrease of aiNa. Exposure to 1 microM Iso only caused a small additional decrease (27.8 +/- 2.4%), while a diminution of aiNa could already be noticed with only 10 nM Iso (12.8 +/- 1.9% diminution). Myocytes superfused with 15 mM [K+]o also exhibited a significant decrease of aiNa (22.9 +/- 3.6%) when exposed to 0.1 microM Iso. These data argue that accumulation of external K+ does not account for the effect of Iso on steady-state aiNa as postulated by Gadsby (Nature Lond. 306: 691-693, 1983). Furthermore, aiNa in myocytes superfused with 1.5 mM [K+]o decreased by only 8.7% on addition of 0.1 microM Iso. The latter observation suggests that the beta-agonist effect on aiNa regulation is directly or indirectly dependent on membrane potential and/or aiNa. Finally, kinetic analysis of the effect of 1 microM Iso on the decrease in aiNa on changing [K+]o from 1.5 to 5 mM demonstrated that the time course of aiNa recovery was accelerated by a factor of 1.9. This readily suggests that active Na+ transport is directly stimulated by Iso. The much greater relative effect of Iso on the time constant than on steady-state aiNa further indicates that Iso may also increase passive Na+ influx.