Inactivating and non-activating delayed rectifier K+ currents in hair cells of frog crista ampullaris

The possible presence of different types of delayed rectifier K+ current (I(K)) was studied in vestibular hair cells of frog semicircular canals. Experiments were performed in thin slice preparations of the whole crista ampullaris and recordings were made using the whole-cell patch-clamp technique. We found that an apparent homogeneous I(K), isolated from the other K+ currents, could be pharmacologically separated into two complementary components: a capsaicin-sensitive current (I(Kc)) and a barium-sensitive current (I(K,b)). I(K,c) was recruited at potentials more positive than -60 mV and showed a slow activation having a time constant (tau(a)) ranging on average from 12 ms at 40 mV to 32 ms at -20 mV. This current inactivated slowly with two voltage-independent time constants (ta(d1) and tau(d2) were 300 ms and 4 s respectively) and more than 80% of the channels were in an inactivated state at the cell resting potential. I(K,b) was also recruited at potentials more positive than -60 mV, but in contrast to I(K,c), it activated more rapidly (tau(a) ranged on average from 1 ms at 40 mV to 4.5 ms at -20 mV) and it did not exhibit any inactivation process. Current clamp experiments revealed that I(K,b), at variance with I(K,c), contributes to the cell resting potential and represents the main repolarizing current when sensory cells are depolarized from rest. I(K,c) could have a role in hair cells when they are depolarized after hyperpolarizing stimuli, a condition that removes channel inactivation.