Figure 5. Ca²⁺ channel inactivation is reduced by barium.

A, inward currents recorded during the superfusion of 5 mm Ca²⁺ or 5 mm Ba²⁺ from a P50 apical IHC in response to a 600 ms voltage step to −11 mV from a holding potential of −81 mV. Fits through the data were obtained using a double exponential equation. The fitting parameters are: 5 mm Ca²⁺τ_fast 19 ms, τ_slow 299 ms; 5 mm Ba²⁺τ_fast 86 ms, τ_slow 855 ms. Cell properties were: C_m 11.9 pF, R_s 6.3 MΩ, g_leak 1.7 nS. B, average I–V curves of peak inward currents in 5 mm Ca²⁺ or 5 mm Ba²⁺ from the same 6 IHCs (2 apical and 4 basal IHCs, P50). Continuous lines are fits using eqn (1). Fitting parameters are: 5 mm Ca²⁺g_max 6.0 nS, V_rev 27 mV, V_½−26.8 mV, S 7.6 mV; 5 mm Ba²⁺g_max 6.8 nS, V_rev 25 mV, V_½−29.2 mV, S 6.8 mV. C and D, inward 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 Fig. 4A–C). Residual capacitative transients have been blanked. Recordings in C and D are from the same IHC shown in A. E, average inactivation curves obtained by plotting the peak inward tail currents described in panels C and D (in 5 mm Ca²⁺ and 5 mm Ba²⁺). The continuous lines are fits up to the potential of maximum inactivation using eqn (3). Fitting parameters are: 5 mm Ca²⁺I_max=−274 pA, I_const= 0.57, V_½=−23.3 mV, S = 9.3 mV; 5 mm Ba²⁺I_max=−249 pA, I_const= 0.81, V_½=−21.7 mV, S = 10.1 mV. F, average percentage of current inactivation obtained from fitting individual inactivation curves using eqn (3). For comparisons, the value obtained in 1.3 mm Ca²⁺ (from Fig. 4F) is also shown. Number of cells is shown above the columns.