Figure 5. IHC Ca²⁺-influx shows normal amplitude but a mild depolarized shift in voltage-dependence.

(A) IV-relationship of the whole-cell Ca²⁺-current in RBE^WT/WT (black, n = 21 cells, N = 8) and RBE^KO/KO (red, n = 21 cells, N = 8) IHCs show comparable (sign-inverted) current amplitudes (A_i, p=0.62, t-test) and density (A_ii, p=0.37, t-test.). Mean (line) ± S.E.M. (shaded areas) are displayed as for (C, D). The protocol, consisting of 20 ms steps of 5 mV from −82 to +63 mV, as well as exemplary resulting currents, aredisplayed in the left. Box plots show 10, 25, 50, 75 and 90^th percentiles with individual data points overlaid, means are shown as crosses, as for (C, D). (B) Activation time constants (mean ± S.E.M.) of Ca²⁺-currents at different potentials were obtained by fitting a power exponential equation to the first 5 ms of the current traces, revealing no differences between conditions. (C) Fractional activation of the whole-cell Ca²⁺-current derived from the IV-relationships (A) was fitted to a Boltzmann function. (Ci) Box plots of the voltage for half-maximal activation V_h and V_h-estimates of individual IHCs show a depolarized shift of the fractional activation of the Ca_v1.3 Ca²⁺-channels in the RBE^KO/KO IHCs (p=0.029, t-test). (Cii) Box plots of the voltage-sensitivity or slope factor k and k-estimates of individual IHCs illustrate comparable voltage sensitivity between both conditions (p=0.67, t-test). (D) Average peak-normalized Ca²⁺-currents resulting from 200 ms depolarizations to −14 mV. We observe an enhanced inactivation in ribbonless IHCs, quantified as a reduced residual Ca²⁺-current (inset). (n = 10 cells, N = 5 for RBE^KO/KO and n = 11 cells, N = 7 in the RBE^WT/WT; p=0.017, Mann-Whitney-Wilcoxon test).