Calcium dependent shifts of Na+ channel activation correlated with the state dependence of calcium-binding to the pore
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Calcium ions block the open configuration and antagonise the tonic binding of TTX to the closed state of sodium channels in very different ranges of extracellular concentration, [Ca]O. We measured the open-state block in channels expressed in Xenopus oocytes by α-subunits from rat brain (rBIIa) or adult rat skeletal muscle (rSkM1). Recordings of instantaneous tail-currents from cell-attached macro patches show that the binding of Ca2+ to the blocking site has a dissociation constant of about 20 mM at 0 mV and senses about 30% of the membrane potential drop, whereas the concentration of half-inhibition of TTX-binding is less than 1 mM and voltage-insensitive. Assuming that both effects involve a single binding site, a simple model predicts that the state-dependency of the dissociation constant entails positive shifts of activation and faster kinetics of deactivation at increasing [Ca]O. The shifts of activation measured for rBIIA and rSkM1 channels are comparable in size to those predicted by the model, which accounts also for the observed larger shifts of the rBIIA-mutant K226Q as a consequence of its reduced voltage-sensitivity. Shifts attributable to surface-charge screening effects seem smaller in the oocyte than in native cell-membranes. The experimental [Ca]O-dependence of deactivation kinetics is also consistent with the model and with the idea that Ca2+-binding changes to the same extent, but in opposite directions, the activation free-energies of both opening and closing transitions.
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