Rapidly inactivating and non-inactivating calcium-activated potassium currents in frog saccular hair cells

Using a semi-intact epithelial preparation we examined the Ca2+-activated K+ (K-Ca) currents of frog (Rana pipiens) saccular hair cells. After blocking voltage-dependent K+ (K-V) currents with 4-aminopyridine (4-AP) an outward current containing inactivating (I-transient) and noninactivating (I-steady) components remained. 2. The contribution of each varied greatly from cell to cell, with I-transient contributing from 14 to 90% of the total outward current. Inactivation of I-transient was rapid (tau approximate to 2-3 ms) and occurred within the physiological range of membrane potentials(V-1/2 = -63 mV). Recovery from inactivation was also rapid (tau approximate to 10 ms). 3. Suppression of both I-transient and I-steady by depolarizations that approached the Ca2+ equilibrium potential and by treatments that blocked Ca2+ influx (application Ca2+-free saline or Cd2+), suggest both are Ca2+ dependent. Both were blocked by iberiotoxin, a specific blocker of large-conductance K-Ca channels (BK), but not by apamin, a specific blocker of small-conductance K-Ca channels. 4. Ensemble-variance analysis showed that I-transient and I-steady flow through two distinct populations of channels, both of which have a large single-channel conductance (similar to 100 pS in non-symmetrical conditions). Together, these data indicate that both I-transient and I-steady are carried through BK channels, one of which undergoes rapid inactivation while the other does not. 5. Inactivation of I-transient could be removed by extracellular papain and could later be restored by intracellular application of the 'ball' domain of the auxiliary subunit (beta2) thought to mediate BK channel inactivation in rat chromaffin cells. We hypothesize that I-transient results from the association of a similar beta subunit with some of the BK channels and that papain removes inactivation by cleaving extracellular sites required for this association.