Sympathetic regulation of cardiac calcium current is due exclusively to cAMP-dependent phosphorylation

Abstract

THE positive inotropic effect of the sympathetic nervous system on the heart is partly mediated by an increase in the voltage-gated Ca²⁺ current (I_ca) This increase is generally attributed to β-adrenergic receptor-stimulated cyclic AMP-dependent phos-phorylation of the Ca²⁺ channel^1,2. It has been suggested that cAMP-dependent phosphorylation cannot explain all the effects of β-adrenergic agonists on I_Ca and that a parallel membrane-delimited pathway involving the 'direct' action of the G protein Gs also stimulates I_Ca (refs 3–7). A precedent exists for such a membrane-delimited pathway in the activation of a K⁺ channel by acetylcholine in heart^1,8. A membrane-delimited pathway for stimulation of I_Ca might be important in rapid beat-to-beat regulation of contraction by the sympathetic nervous system⁹, because isoproterenol may produce a biphasic increase in I_Ca with the rapid phase (τ = 150 ms) putatively mediated by the direct pathway and the slow phase (τ = 35s) by cAMP-dependent phosphorylation. Here we report that in frog, rat, and guinea pig ventricular myocytes I_Ca increases slowly and monophasically in response to isoproterenol. The increase is completely blocked by inhibitors of cAMP-dependent phosphorylation. Furthermore, the time course of the increase in I_Ca closely parallels the increase in contractile force produced by sympathetic nerve stimulation. These data refute earlier suggestions that regulation of Ca²⁺ channels by the sym-pathetic nervous system involves or requires a direct G-protein pathway.