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. IK,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. IK,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 IK,f activation. Increasing the intracellular concentration of the Ca2+ chelator BAPTA from 0.1 mM to 30 mM reduced the amplitude of IK,f at –25 mV and shifted its activation by 37 mV towards more depolarized potentials. A reduction in the size of IK,f and a depolarizing shift of its activation were also seen when either thapsigargin and caffeine or ryanodine were added intracellularly, suggesting that IK,f is modulated by voltage-dependent release from intracellular Ca2+ stores. Mature IHCs had a small additional Ca2+-activated K+ current (IK(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 intracellular 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 IK,f, but not IK(Ca), is essential for fast voltage responses of mature IHCs.