Inhibition of cardiac Na+ currents by isoproterenol

B. Schubert, A. M. Vandongen, G. E. Kirsch and A. M. Brown Department of Physiology and Molecular Biophysics, Baylor College of Medicine, Houston, Texas 77030. The mechanism by which the beta-adrenergic agonist isoproterenol (ISO) modulates voltage-dependent cardiac Na+ currents (INa) was studied in single ventricular myocytes of neonatal rat using the gigaseal patch-clamp technique. ISO inhibited INa reversibly, making the effect readily distinguishable from the monotonic decrease of INa caused by the shift in gating that customarily occurs during whole cell patch-clamp experiments (E. Fenwick, A. Marty, and E. Neher, J. Physiol. Lond. 331: 599-635, 1982; and J. M. Fernandez, A. P. Fox, and S. Krasne, J. Physiol. Lond. 356: 565-585, 1984). The inhibition was biphasic, having fast and slow components, and was voltage-dependent, being more pronounced at depolarized potentials. In whole cell experiments the membrane-permeable adenosine 3',5'-cyclic monophosphate (cAMP) congener 8-bromo-cAMP reduced INa. In cell-free inside-out patches with ISO present in the pipette, guanosine 5'-triphosphate (GTP) applied to the inner side of the membrane patch inhibited single Na+ channel activity. This inhibition could be partly reversed by hyperpolarizing prepulses. The nonhydrolyzable GTP analogue guanosine-5'-O-(3-thiotriphosphate) greatly reduced the probability of single Na+ channel currents in a Mg2(+)-dependent manner. We propose that ISO inhibits cardiac Na+ channels via the guanine nucleotide binding, signal-transducing G protein that acts through both direct (membrane delimited) and indirect (cytoplasmic) pathways.