Sodium channel isoform-specific effects of halothane: protein kinase C co-expression and slow inactivation gating

The modulatory effect of protein kinase C (PKC) on the response of Xenopus oocyte-expressed Na channel alpha-subunits to halothane (2-bromo-2-chloro-1,1,1-trifluroethane) was studied. Na currents through rat skeletal muscle, rat brain and human cardiac muscle Na channels were assessed using cell-attached patch clamp recordings. PKC activity was increased by co-expression of a constitutively active PKC alpha-isozyme. Decay of macroscopic Na currents could be separated into fast and slow exponential phases. PKC co-expression alone slowed Na current decay in neuronal channels, through enhancement of the amplitude of the slower phase of decay. Halothane (1.0 mM) was without effect on any of the three isoforms expressed alone but, after co-expression of PKC, there was enhancement of Na current decay with reduction in charge movement through skeletal muscle and neuronal channels. Cardiac channels were relatively insensitive to halothane. Enhanced Na current decay resulted from suppression of the slow phase, without effect on the faster phase or on either decay tau. Suppression of Na current through skeletal muscle channels was concentration-dependent over the therapeutic range and was described by third order reaction kinetics, with an IC(50) of 0.55 mM. We conclude that the halothane suppresses skeletal muscle and brain Na channel activity in this preparation through a reduction in the slow mode of inactivation gating, but only after PKC co-expression. Cardiac Na channels were relatively insensitive to halothane. The mechanism is likely to involve phosphorylation of the channel inactivation gate, although phosphorylation of other sites in the channel may account for the isoform specific differences.