Nature and expression of dihydropyridine-sensitive and -insensitive calcium currents in hair cells of frog semicircular canals

Abstract

Ca²⁺ currents in hair cells of the frog crista ampullaris were studied using the whole-cell patch-clamp technique. Currents were recorded in situ from hair cells in peripheral, intermediate and central regions of the sensory epithelium. Two types of Ca²⁺ currents were found: a partially inactivating current that was expressed by nearly all central cells and by about 65% of intermediate and peripheral cells, and a sustained current expressed by the remaining cell population. The mean Ca²⁺ current amplitude was larger in intermediate cells than in central or peripheral cells. The two types of Ca²⁺ currents were composed of two components: a large, nifedipine-sensitive (NS) current and a small, nifedipine-insensitive (NI) current. The latter was resistant to SNX-482, ω-conotoxin MVIIC and ω-agatoxin IVA and to ω-conotoxin GVIA, antagonists of R, P/Q and N-type Ca²⁺ channels. The amplitude of NS and NI currents varied among peripheral cells, where the current density gradually increased from the beginning of the region toward its end. No significant variation of Ca²⁺ current density was detected in hair cells of either intermediate or central regions. These results demonstrate the presence of regional and intraregional variations in the expression of L and non-L Ca²⁺ channels in the frog crista ampullaris. Finally, immunocytochemical investigations revealed the presence of Ca²⁺ channel subunits of the α_1D type and the unexpected expression of α_1B-subunits.