Properties of Na+ currents conducted by a skeletal muscle L-type Ca2+ channel pore mutant (SkEIIIK)

Four glutamate residues residing at corresponding positions within the four conserved membrane-spanning repeat of L-type Ca2+ channels are important structural determinants for the passage of Ca2+ across the selectivity filter. Mutation of the critical glutamate in Repeat III in the alpha(1S) subunit of the skeletal L-type channel (Ca(v)1.1) to lysine virtually eliminates passage of Ca2+ during step depolarizations. In this study, we examined the ability of this mutant Ca(v)1.1 channel (SkEIIIK) to conduct inward Na+ current. When 150 mM Na+ was present as the sole monovalent cation in the bath solution, dysgenic (Ca(v)1.1 null) myotubes expressing SkEIIIK displayed slowly-activating, non-inactivating, nifedipine-sensitive inward currents with a reversal potential (45.6 +/- 2.5 mV) near that expected for Na+. Ca2+ block of SkEIIIK-mediated Na+ current was revealed by the substantial enhancement of Na+ current amplitude after reduction of Ca2+ in the external recording solution from 10 mM to near physiological 1 mM. Inward SkEIIIK-mediated currents were potentiated by either +/- Bay K 8644 (10 mu M) or 200-ms depolarizing prepulses to +90 mV. In contrast, outward monovalent currents were reduced by +/- Bay K 8644 and were unaffected by strong depolarization, indicating a preferential potentiation of inward Na+ currents through the mutant Ca(v)1.1 channel. Taken together, our results show that SkEIIIK functions as a non-inactivating, junctionally-targeted Na+ channel when Na+ is the sole monvalent cation present and urge caution when interpreting the impact of mutations designed to ablate Ca2+ permeability mediated by Ca-v channels on physiological processes that extend beyond channel gating and permeability.