1. Modulation of cardiac sodium currents (I(Na)) by the G protein stimulatory α subunit (Gsα) was studied using patch-clamp techniques on freshly dissociated rat ventricular myocytes. 2. Whole-cell recordings showed that stimulation of β-adrenergic receptors with 10 μM isoprenaline (isoproterenol, ISO) enhanced I(Na) by 68.4 ± 9.6% (mean ± S.E.M; n = 7, P < 0.05 vs. baseline). With the addition of 22 μg ml-1 protein kinase A inhibitor (PKI) to the pipette solution, 10 μM ISO enhanced I(Na) by 30.5 ± 7.0% (n = 7, P < 0.05 vs. baseline). With the pipette solution containing both PKI and 20 μg ml-1 anti-Gsα IgG or 20 μg ml-1 anti-Gsα IgG alone, 10 μM ISO produced no change in I(Na). 3. The effect of Gsα on I(Na) was not due to changes in the steady-state activation or inactivation curves, the time course of current decay, the development of inactivation, or the recovery from inactivation. 4. Whole-cell I(Na) was increased by 45.2 ± 5.3% (n = 13, P < 0.05 vs. control) with pipette solution containing 1 μM Gsα27-42 peptide (amino acids 27-42 of rat brain Gsα) without altering the properties of Na+ channel kinetics. Furthermore, application of 1 nM Gsα27-42 to Na+ channels in inside-out macropatches increased the ensemble-averaged I(Na) by 32.5 ± 6.8% (n = 8, P < 0.05 vs. baseline). The increase in I(Na) was reversible upon Gsα27-42 peptide washout. Single channel experiments showed that the Gsα27-42 peptide did not alter the Na+ single channel current amplitude, the mean open time or the mean closed time, but increased the number of functional channels (N) in the patch. 5. Application of selected short amino acid segments (Gsα27-36, Gsα33-42 and Gsα30-39) of the 16 amino acid Gsα peptide (Gsα27-42 peptide) showed that only the C-terminal segment of this peptide (Gsα33-42) significantly increased I(Na) in a dose-dependent fashion. These results show that cardiac I(Na) is regulated by Gsα via a mechanism independent of PKA that results in an increase in the number of functional Na+ channels. In addition, a 10 residue domain (amino acids 33-42) near the N-terminus of Gsα is important in modulating cardiac Na+ channels.
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