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Background— Mutations in SCN5A , the gene coding for the human cardiac Na + channel α-subunit, are associated with variant 3 of the long-QT syndrome (LQT-3). Several LQT-3 mutations promote a mode of Na + channel gating in which a fraction of channels fail to inactivate, contributing sustained Na + channel current ( I sus ), which can delay repolarization and prolong the QT interval. Here, we investigate the possibility that stimulation of protein kinase C (PKC) may modulate I sus , which is prominent in disease-related Na + channel mutations. Methods and Results— We measured the effects of PKC stimulation on Na + currents in human embryonic kidney (HEK 293) cells expressing 3 previously reported disease-associated Na + channel mutations (Y1795C, Y1795H, and ΔKPQ). We find that the PKC activator 1-oleoyl-2-acetyl- sn -glycerol (OAG) significantly reduced I sus in the mutant but not wild-type channels. The effect of OAG on I sus was reduced by the PKC inhibitor staurosporine (2.5 μmol/L), ablated by the mutation S1503A, and mimicked by the mutation S1503D. I sus recorded in myocytes isolated from mice expressing ΔKPQ channels was similarly inhibited by OAG exposure or stimulation of α 1 -adrenergic receptors by phenylephrine. The actions of phenylephrine on I sus were blocked by the PKC inhibitor chelerythrine. Conclusions— We conclude that stimulation of PKC inhibits channel bursting in disease-linked mutations via phosphorylation-induced alteration of the charge at residue 1503 of the Na + channel α-subunit. Sympathetic nerve activity may contribute directly to suppression of mutant channel bursting via α-adrenergic receptor–mediated stimulation of PKC.