1. Hair cells were isolated from apical and basal regions of the embryonic chick's cochlea. Outward potassium currents were recorded using whole cell tight-seal voltage clamp. 2. Outward currents in basal hair cells activated and inactivated rapidly. The average time to half-maximum at 0 mV was 2.9 ms. The time constant of inactivation at 0 mV was 71 ms. Boltzmann fits to conductance-voltage curves gave an average half-activation voltage of -36 mV, and steady-state inactivation was half-maximal at -62 mV. 3. Potassium currents in apical hair cells had slower kinetics, with a time to half-maximum of 6.7 ms and an inactivation time constant of 242 ms at +10 mV. The half-activation voltage derived from Boltzmann fits was -16 mV and that for inactivation was -43 mV. 4. With respect to kinetic and voltage- dependent properties, the rapidly and slowly activating potassium currents of embryonic cells were similar to the rapidly inactivating 'A' current of mature short hair cells and to the delayed rectifier of mature tall hair cells. However, unlike the adult currents, the embryonic currents did not show differential sensitivities to tetraethylammonium chloride and 4- aminopyridine. As early as the tenth day of embryogenesis, hair cells at the apical and basal extremes of the cochlea produced functionally distinct voltage-gated potassium currents.
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