PH-Dependent Regulation of rNaV1.2 Channel Inactivation

Abstract

Brain sodium channels play a crucial role in neuronal excitability. Ischemia leads to decreased sodium influx that, as suggested by previous studies, may be caused by channel protonation. Using whole-cell recordings we studied effects of low extracellular pH on the biophysical properties of rNaV1.2 stably expressed in CHO cells. We confirmed that acidification causes a decrease of peak Na currents with no measurable effects on the voltage dependence of activation. By contrast, low pH has a significant effect on rNaV1.2 inactivation properties. At pH 6.0 the effective charge of the steady state fast inactivation curve is significantly reduced, whereas the midpoint voltage is unchanged. The kinetics of fast inactivation are accelerated and shifted to less negative potentials, indicating a destabilization of the fast inactivated state. This was confirmed by first-order two-state Eyring model fit to τ(V) dependence. Slow inactivation is enhanced at low pH, as demonstrated by experiments using cumulative inactivation of rNaV1.2 with 45Hz stimulation. Thus, our data suggest Na+ channel fast and slow inactivation, but not activation, might be the target for protonation at low pH and might play role in rNaV1.2 down-regulation in low extracellular pH during ischemic events.