Voltage-Gated Calcium Channel Currents in Type I and Type II Hair Cells Isolated From the Rat Crista

1 The Bobby R. Alford Department of Otorhinolaryngology and Communicative Sciences, Baylor College of Medicine, Houston, Texas 77030; and 2 Department of Neurobiology, Pharmacology, and Physiology, University of Chicago, Chicago, Illinois 60637 Submitted 3 April 2002; accepted in final form 13 March 2003 When studied in vitro, type I hair cells in amniote vestibular organs have a large, negatively activating K + conductance. In type II hair cells, as in nonvestibular hair cells, outwardly rectifying K + conductances are smaller and more positively activating. As a result, type I cells have more negative resting potentials and smaller input resistances than do type II cells; large inward currents fail to depolarize type I cells above –60 mV. In nonvestibular hair cells, afferent transmission is mediated by voltage-gated Ca 2 + channels that activate positive to –60 mV. We investigated whether Ca 2 + channels in type I cells activate more negatively so that quantal transmission can occur near the reported resting potentials. We used the perforated patch method to record Ca 2 + channel currents from type I and type II hair cells isolated from the rat anterior crista (postnatal days 4–20). The activation range of the Ca 2 + currents of type I hair cells differed only slightly from that of type II cells or nonvestibular hair cells. In 5 mM external Ca 2 + , currents in type I and type II cells were half-maximal at –41.1 ± 0.5 (SE) mV ( n = 10) and –37.2 ± 0.2 mV ( n = 10), respectively. In physiological external Ca 2 + (1.3 mM), currents in type I cells were half-maximal at –46 ± 1 mV ( n = 8) and just 1% of maximal at –72 mV. These results lend credence to suggestions that type I cells have more positive resting potentials in vivo, possibly through K + accumulation in the synaptic cleft or inhibition of the large K + conductance. Ca 2 + channel kinetics were also unremarkable; in both type I and type II cells, the currents activated and deactivated rapidly and inactivated only slowly and modestly even at large depolarizations. The Ca 2 + current included an L-type component with relatively low sensitivity to dihydropyridine antagonists, consistent with the subunit being Ca V 1.3 ( 1D ). Rat vestibular epithelia and ganglia were probed for L-type -subunit expression with the reverse transcription-polymerase chain reaction. The epithelia expressed Ca V 1.3 and the ganglia expressed Ca V 1.2 ( 1C ). Address for reprint requests: R. A. Eatock, Dept. of Otolaryngol