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Center for Neuroscience, Department of Otolaryngology, University of California at Davis, Davis, California 95616
Correspondence: Address reprint requests to Ebenezer N. Yamoah, University of California at Davis, Ctr. for Neuroscience, Dept. of Otolaryngology, 1544 Newton Court, Davis, CA 95616. Tel.: 530-754-6630; Fax: 530-54-7183/754-5046; E-mail: enyamoah{at}ucdavis.edu.
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 (V1/2) 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 V1/2 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 V1/2 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.
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