Potassium currents in the hair cells of vestibular epithelium: position-dependent expression of two types of A channels

The complement of voltage-dependent K+ currents was investigated in hair cells of the frog crista ampullaris. The currents were recorded in transversal slices of the peripheral, intermediate and central regions of the crista by applying the whole-cell variant of the patch clamp technique to cells located at different positions in the slices. Voltage-clamp recordings confirmed that cells located in each region have a distinctive complement of K+ channels. A close investigation of the currents in each region revealed that the complement of K+ channels in intermediate and central regions showed no variations among cells, while peripheral hair cells differed in the expression of two classes of A-type currents. These currents showed different kinetics of inactivation as well as steady-state inactivation properties. We called these currents fast IA and slow IA based on their inactivation speed. The magnitude of both currents per unit membrane area exhibited a significant gradient along the transversal axis of the peripheral regions. Fast IA magnitude was maximal in cells located in the external zone of the crista slice and decreased gradually to become very small in the median zone (centre) of the section, while slow IA gradient of magnitude was opposite. A-type currents appear to act as a transient buffer that opposes hair cell depolarisation induced by positive current injections. However, fast IA is partially active at the cell resting potential, while slow IA can be recruited only following large hyperpolarisations. Thus, two types of A currents are differentially distributed in vestibular hair cells and have different roles in shaping the receptor potential.