Modulation of cardiac sodium channels by cAMP receptors on the myocyte surface.

The phosphorylation of the cardiac sodium channel by adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase A leads to its inactivation. It was shown that extracellular cAMP can also modulate the sodium channel of rat, guinea pig, and frog ventricular myocytes in a rapid (less than 50 milliseconds), reversible, and dose-dependent manner. The decrease in the sodium current was accompanied by a 10- to 15-millivolt shift in the steady-state availability of the sodium channel toward more negative potentials and was inhibited by guanosine-5'-O-(2-thiodiphosphate) or pertussis toxin, suggesting that the extracellular modulation of the sodium channel by cAMP is mediated by a membrane-delimited mechanism that includes a pertussis toxin-sensitive G protein.