Figure 4. Ca²⁺ current inactivation.

A, Ca²⁺ currents recorded from adult apical (middle panel) and basal (bottom panel) gerbil IHCs. Currents were elicited by depolarizing voltage steps (10 mV nominal increments and 600 ms in duration) from −91 mV starting from the holding potential of −81 mV. A schematic representation of the voltage protocol is shown above the current traces. For clarity only a few traces are shown and are not averages. Apical IHC: C_m 10.7 pF; R_s 4.1 MΩ; g_leak 2.5 nS. Basal IHC: C_m 8.3 pF; R_s 4.9 MΩ; g_leak 1.0 nS. B and C, Ca²⁺ tail currents recorded at a membrane potential of −11 mV after a series of depolarizing conditioning steps (600 ms) from −91 mV (see protocol in panel A). Residual capacitative transients have been blanked. Recordings in B and C are from the same apical and basal IHCs shown in A. D and E, average steady-state inactivation curves of I_Ca from adult (D: apical n = 8; basal n = 6) and immature (E: apical n = 12; basal n = 4) gerbil IHCs obtained using tail currents as described in B and C. The shaded areas delineate the resting membrane potential of IHCs recorded in patch clamp experiments. The continuous lines are fits up to the potential of maximum inactivation using eqn (3). Fitting parameters are: adult apical IHCs (D: ^) I_max=−166 pA, I_const= 0.56, V_½=−38.8 mV, S = 7.2 mV; adult basal IHCs (D•) I_max=−148 pA, I_const= 0.43, V_½=−42.9 mV, S = 6.0 mV; immature apical IHCs (E: ▵) I_max=−274 pA, I_const= 0.50, V_½=−39.3 mV, S = 9.3 mV; immature basal IHCs (E: s) I_max=−196 pA, I_const= 0.53, V_½=−34.4 mV, S = 8.7 mV. F, average percentage (left panel), V_½ (middle panel) and slope (right panel) of inactivation obtained from fitting individual inactivation curves using eqn (3) (number of cells shown above the columns).