MODULATION OF CARDIAC NA-CELL LINE AND IN VENTRICULAR MYOCYTES BY PROTEIN-KINASE-C( CHANNELS EXPRESSED IN A MAMMALIAN)

Cardiac rH1 Na+ channel a subunits were expressed in cells of the Chin ese hamster lung 1610 cell line by transfection, and a stable cell lin e expressing cardiac Na+ channels (SNa-rH1) was isolated. Mean Na+ cur rents of 2.2 +/- 1.0 nA were recorded, which corresponds to a cell sur face density of approximately 1-2 channels active at the peak of the N a+ current per mum2. The expressed cardiac Na+ current was tetrodotoxi n resistant (K(d) = 1.8 muM) and had voltage-dependent properties simi lar to those of the Na+ current in neonatal ventricular myocytes. Acti vation of protein kinase C by 1-oleoyl-2-acetyl-sn-glycerol (OAG) (10 muM) decreased this current -33% at a holding potential of -114 mV and 56% at -94 mV. This reduction in peak current was caused in part by a n 8- to 14-mV shift of steady-state inactivation in the hyperpolarized direction. Na+ channel activation was unchanged. Effects of OAG in SN a-rH1 cells and in neonatal rat cardiac myocytes were similar, except that the time course of inactivation was slowed either transiently or persistently when protein kinase C was activated in myocytes bathed in low-Ca2+ (1 muM) or Ca2+-free solution but was unaffected in SNa-rH1 cells. The effects of OAG on cardiac Na+ current were blocked in cells that had been previously microinjected with a peptide inhibitor of pr otein kinase C but not with a peptide inhibitor of cAMP-dependent prot ein kinase, indicating that protein kinase C is responsible for the ef fects of OAG. Single-channel recordings from SNa-rH1 cells showed that the probability of channel opening was reduced by OAG, but the conduc tance was unaffected. OAG did not induce the late Na+ channel openings observed with PKC modulation of neuronal and skeletal muscle Na+ chan nels. Thus, the substantial reduction in Na+ current at normal diastol ic depolarizations with 10 muM OAG is due to failure of channel openin g in response to depolarization. Such Na+ current reductions may have profound effects on cardiac cell excitability.