Dose-dependent alteration of rat cardiac sodium current by isoproterenol: Results from direct measurements on multicellular preparations

Conflicting results have been reported in literature about the influence of-adrenergic stimulation on the fast cardiac sodium current (I_Na⁺). To elucidate these mechanisms in multicellular preparations we used the loose-patch-clamp technique to evaluate the effect of the-adrenergic agonist isoproterenol 1–1000 nmol/l. Isoproterenol enhancedI_Na+ at all membrane potentials by elevation of the maximal availableI_Na⁺. Only at the high concentration of 1 mol/l wasI_Na⁺ slightly depressed after depolarizing conditioning clamps. The most marked increase of the maximal availableI_Na⁺ was 30 ± 9 % after application of 100 nmol/l isoproterenol. To learn about the mechanisms in view of sodium channel modulation we combined isoproterenol with the sodium channel blocker lidocaine (47 mol/l). Under these circumstances the effects of both drugs were completely independent. This investigation shows clearly that low concentrations of isoproterenol increaseI_Na+ in multicellular preparations by a gating-independent mechanism.