A-type outward currents were studied in sensory hair cells isolated from the semicircular canals (SCC) of the leopard frog (Rana pipiens) with whole-cell voltage- and current-clamping techniques. 2. There appear to be two classes of A-type outward-conducting potassium channels based on steady-state, kinetic, pharmacological parameters, and reversal potential. 3. The two classes of A-type currents differ in their steady-state inactivation properties as well as in the kinetics of inactivation. The steady-state inactivation properties are such that a significant portion of the fast channels are available from near the resting potential. 4. The inactivating channels studied do not appear to be calcium dependent. 5. The A-channels in hair cells appear to subserve functions that are analogous to IA functions in neurons, that is, modulating spike latency and Q (the oscillatory damping function). The A-currents appear to temporally limit the hair cell voltage response to a current injection.
