Figure 4. BK_Ca channels in intact IHCs are sensitive to a global increase in [Ca²⁺]_i.

A, activation of BK_Ca currents measured in whole-cell mode in IHCs under various conditions interfering with Ca²⁺ extrusion. V_h values were obtained from Boltzmann fits to the activation curves obtained as in Fig. 1C and plotted against the time after establishment of whole-cell access. V_h remained constant under control conditions (▵; n = 6 IHCs) but shifted to more negative potentials when 50 μm carboxy-eosin was included in the pipette solution ( ; n1= 5). Reduction of the intracellular Ca²⁺ buffer EGTA to 0.5 mm (□; n = 5) and replacement of external Na⁺ by NMDG⁺ (•; n = 6) further accelerated the leftward shift. Symbols show pooled data for each condition. Cells usually deteriorated when reaching a V_h of around −100 mV, precluding the observation of the BK_Ca activation curve beyond this value. B, voltage protocol for measuring BK_Ca currents with a depolarizing prepulse to −44 mV to allow for a prolonged inward current through Cav channels prior to each 5-ms test voltage step in nominal 10-mV increments. Intervals between successive sweeps were 55 ms. C, BK_Ca current response to the voltage protocol in B under control conditions with 1.3 mm[Ca²⁺]_ex. D, BK_Ca current response from the same cell as in C after replacement of Ca²⁺ by Sr²⁺. Note that the amplitude of the BK_Ca current increased during the prepulse with each voltage step. Consequently, currents during the short test voltage step were larger and were activated already at more negative potentials. The first sweep is shown in light grey; subsequent sweeps are shown in increasingly darker grey. E, BK_Ca current response as in C after removal of extracellular Sr²⁺.