Differential expression of potassium currents by hair cells in thin slices of frog crista ampullaris

1. Electrical responses in hair cells located in the peripheral regions and in the central region of the frog crista ampullaris were investigated in thin slice preparations by using the whole-cell configuration of the patch-clamp technique. 2. Hair cells from the peripheral regions exhibited mostly a club-like shape and had an average resting potential of -46 mV, whereas cells from the central region had mostly a cylindrical shape and a more negative resting potential (-57 mV). 3. Voltage-clamp recordings revealed that ionic conductances differed in the two epithelial regions. Cells from the peripheral regions exhibited a transient K+ current of A-type (IA) in conjunction with a slow rectifier outward K+ current (IK). Cells from the central region showed little or no IA and generated an IK together with an inward rectifier K+ current (IIR). In both regions, hair cells showed a rapidly activating Ca(2+)-dependent outward K+ current (IK(Ca)) that rapidly inactivated to reach a steady-state level during 150-ms test pulses. 4. IA activated close to -60 mV and was inhibited by 12 mM 4-aminopyridine (4-AP). The time course of this current showed time to peak values of 3-4 ms at 0 mV. Inactivation was fast and almost voltage-independent. The decay time constant was approximately 35 ms at 0 mV. 5. IK was recruited close to -60 mV and activated slowly, reaching peak values in approximately 100 ms at 0 mV. It showed no evidence of inactivation during 150-ms test pulses and it was insensitive to 4-AP. 6. IIR activated at membrane potentials more negative than -90 mV and was blocked by exposure to 6 mM Cs+ or to a K(+)-free medium. This current showed an outward relaxation at potentials more negative than -140 mV, an effect that disappeared after exposure to a Na(+)-free medium. 7. IK(Ca) was recruited close to -40 mV and was inhibited by exposure to a Ca(2+)-free external medium or to 0.5 mM Cd2+. The time to peak of this current was approximately 3 ms at 0 mV and inactivation was very fast and almost independent from the membrane potential. The decay time constant was approximately 4 ms at 0 mV. 8. IK and IA were prominent in hair cells from the peripheral regions, whereas IK accounted for most of the membrane conductance in cells from the central region. The contribution of IK(Ca) was comparable in cells from both epithelial regions.