Calcium currents in hair cells isolated from semicircular canals of the frog

L-type and R-type Ca2+ currents were detected in frog semicircular canal hair cells. The former was noninactivating and nifedipine-sensitive (5 mu M); the latter, partially inactivated, was resistant to omega-conotoxin GVIA (5 mu M), omega-conotoxin MVIIC (5 mu M), and omega-agatoxin IVA (0.4 mu M), but was sensitive to mibefradil (10 mu M). Both currents were sensitive to Ni2+ and Cd2+ (>10 mu M). In some cells the L-type current amplitude increased almost twofold upon repetitive stimulation, whereas the R-type current remained unaffected, Eventually, run-down occurred for both currents, but was prevented by the protease inhibitor calpastatin. The R-type current peak component ran down first, without changing its plateau, suggesting that two channel types generate the R-type current. This peak component appeared at -40 mV, reached a maximal value at -30 mV, and became undetectable for voltages greater than or equal to 0 mV, suggestive of a novel transient current: its inactivation was indeed reversibly removed when Ba2+ was the charge carrier. The L-type current and the R-type current plateau were appreciable at -60 mV and peaked at -20 mV: the former current did not reverse for voltages up to +60 mV, the latter reversed between +30 and +60 mV due to an outward Cs+ current flowing through the same Ca2+ channel. The physiological role of these currents on hair cell function is discussed.