Biophys J, March 2000, p. 1240-1254, Vol. 78, No. 3

Calcium Currents in Hair Cells Isolated from Semicircular Canals of the Frog

Istituto Nazionale per la Fisica della Materia, Dipartimento di Biologia dell'Università-Sezione di Fisiologia Generale, 44100 Ferrara, Italy

L-type and R-type Ca²⁺ currents were detected in frog semicircular canal hair cells. The former was noninactivating and nifedipine-sensitive (5 µM); the latter, partially inactivated, was resistant to -conotoxin GVIA (5 µM), -conotoxin MVIIC (5 µM), and -agatoxin IVA (0.4 µM), but was sensitive to mibefradil (10 µM). Both currents were sensitive to Ni²⁺ and Cd²⁺ (>10 µM). 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 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.

Biophys J, March 2000, p. 1240-1254, Vol. 78, No. 3
© 2000 by the Biophysical Society   0006-3495/00/03/1240/15  $2.00

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