Re-examination of Na+-dependent regulation of cAMP-dependent Cl- currents in the heart

Cardiac Cl- channels may play an important role in both the physiological as well as the pathophysiological regulation of the heart. Due to the present controversy regarding the role that Na+ may play in the regulation of the adenosine 3',5'-cyclic monophosphate (cAMP)-dependent Cl- channels in the heart, we have re-examined the effects of intracellular and extracellular Na+ on ICl,cAMP in isolated guinea-pig ventricular myocytes. In conventional, whole-cell patch-clamp experiments, exchanging of extracellular Na+ for N-methyl-d-glucamine (NMG) did not have a significant effect on the magnitude of the ICl,cAMP activated by external forskolin (FSK) in the presence of beta-adrenergic and muscarinic receptor blockade compared with normal rundown of the current with time. Utilization of the amphotericin B-perforated-patch technique prevented rundown of FSK-stimulated ICl, cAMP, and failed to reveal alterations in this conductance when NMG was substituted for extracellular Na+. Finally, intracellular dialysis of cells during whole-cell experiments with both 0 and 10 mM Na+-containing solutions failed to show any significant effect of intracellular Na+ on the magnitude of ICl,cAMP. We therefore conclude that intracellular or extracellular Na+ plays little or no direct regulatory role in modulation of cAMP-dependent Cl- channels in heart.