Vestibular signals are relayed to the CNS by Type I and Type II hair cells. While Type II hair cells are contacted by several “bouton-like” afferent nerve terminals, Type I hair cells are characterized by their basolateral membrane being enveloped in a single large afferent nerve terminal, named calyx, whose functional meaning is still unknown. Furthermore, Type I hair cells express an outward rectifying K+ current, IK,L, which is active at unusually negative membrane voltages. The molecular nature of IK,L still escapes. By using the patch-clamp whole-cell technique, we examined the voltage- and time-dependent properties of IK,L in Type I hair cells of the mouse semicircular canal. We found that the biophysical properties of IK,L were affected by an unstable K+ equilibrium potential (VeqK+). Both the outward and inward K+ currents shifted VeqK+ consistent with K+ accumulation or depletion, respectively, in the extracellular space. We attributed this phenomenon to a residual calyx attached to the basolateral membrane of the hair cell. We therefore optimized the hair cell dissociation protocol in order to isolate mature Type I hair cells without their calyx. In these cells, the uncontaminated IK,L showed a half-activation at –73.5 mV and a steep voltage dependence (3.1 mV). IK,L also showed complex activation and deactivation kinetics, which we faithfully reproduced by an allosteric channel gating scheme where the channel is able to open from all (five) closed states. The “side” open states substantially contribute to IK,L activation at negative voltages. This study provides the first complete description of the “native” biophysical properties of IK,L in adult mouse vestibular Type I hair cells.
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