Background--Mutations in SCN5A, the gene coding for the human cardiac Na⁺ channel -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 KPQ). 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 KPQ channels was similarly inhibited by OAG exposure or stimulation of 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 -subunit. Sympathetic nerve activity may contribute directly to suppression of mutant channel bursting via -adrenergic receptor-mediated stimulation of PKC.