Effects of permeant ion concentrations on the gating of L-type Ca2+ channels in hair cells

We determined the gating and permeation properties of single L-type Ca2+ channels, using hair cells and varying concentrations (5-70 mM) of the charge carriers Ba2+ and Ca2+. The channels showed distinct gating modes with high- and low-open probability. The half-activation voltage (V-1/2) shifted in the hyperpolarizing direction from high to low permeant ion concentrations consistent with charge screening effects. However, the differences in the slope of the voltage shifts (in VM-1) between Ca2+ (0.23) and Ba2+ (0.13), suggest that channel-ion interaction may also contribute to the gating of the channel. We examined the effect of mixtures of Ba2+ and Ca2+ on the activation curve. In 5 mM Ca2+, the V-1/2 was, -26.4 +/- 2.0 mV compared to Ba2+, -34.7 +/- 2.9 mV, as the charge carrier. However, addition of 1 mM Ba2+ in 4 mM Ca2+, a molar ratio, which yielded an anomalous-mole fraction effect, was sufficient to shift the V-1/2 to -34.7 +/- 1.5 mV. Although Ca2+-dependent inactivation of the L-type channels in hair cells can yield the present findings, we provide evidence that the anomalous gating of the channel may stem from the closed interaction between ion permeation and gating.