Effects of intracellular stores and extracellular Ca2+ on Ca2+-activated K+ currents in mature mouse inner hair cells

Ca2+-activated K+ currents were studied in inner hair cells (IHCs) of mature mice. I-K,I-f, the large-conductance Ca2+-activated K+ current (BK) characteristic of mature IHCs, had a fast activation time constant (0.4 ms at - 25 mV at room temperature) and did not inactivate during 170 ms. Its amplitude, measured at - 25 mV, and activation time constant were similar between IHCs in the apical and basal regions of the cochlea. I-K,I-f was selectively-blocked by 30 nm IbTx but was unaffected by superfusion of Ca2+ -free solution, nifedipine or Bay K 8644, excluding the direct involvement of voltage-gated Ca2+ channels in I-K,I-f activation. Increasing the intracellular concentration of the Ca2+ chelator BAPTA from 0.1mM to 30 mM reduced the amplitude of I-K,I-f at - 25 mV and shifted its activation by 37 mV towards more depolarized-potentials. A reduction in the size of I-K,I-f and a depolarizing shift of its activation were also seen when either thapsigargin and caffeine or ryanodine were added intracellularly, suggesting that I-K,I-f is modulated by voltage-dependent release from intracellular Ca2+ stores. Mature IHCs had a small additional Ca2+ -activated K+ current (I-K(Ca)), activated by Ca2+ flowing through L-type Ca2+ channels. This current was still present during superfusion of either IbTx (60 nm) or apamin (300 nm) but was abolished in Cs+ -based intracellialar solution or during superfusion of 5 mm TEA, suggesting the presence of an additional BK-channel type. Current clamp experiments at body temperature show that I-K,I-f, but not I-K(Ca), is essential for fast voltage responses of mature IHCs.