Figure 8. Extracellular Ca²⁺ both potentiates and blocks the ACh-activated current in immature IHCs.

A, isolated ACh-activated currents (P7, apical IHCs) obtained by subtracting the control currents from the currents in the presence of 100 μm ACh when a Ca²⁺-free solution or solutions containing different Ca²⁺ concentrations were superfused. All recordings were obtained using 0.9 mm extracellular Mg²⁺. Currents were recorded in response to a series of voltage steps from −144 mV to more positive potentials in 10 mV nominal increments (160 ms in duration) from the holding potential of −84 mV. Dotted lines indicate zero current. Ca²⁺-free (I_hold −145 pA) and 100 μm Ca²⁺ (I_hold−246 pA): C_m 8.6 pF; R_s 1.7 MΩ; g_leak 2.8 nS. 500 μm Ca²⁺ (I_hold −459 pA) and 1 mm Ca²⁺ (I_hold −316 pA): C_m 6.8 pF; R_s 1.5 MΩ; g_leak 1.2 nS. 5 mm Ca²⁺ (I_hold −71 pA): C_m 7.9 pF; R_s 2.0 MΩ; g_leak 1.5 nS. B, average instantaneous I–V curves for the ACh-activated currents in the presence of different Ca²⁺ concentrations including those in A (numbers of cells as in C–E). C, size of instantaneous inward currents measured near −144 mV. Note that the size of the currents in both the Ca²⁺-free solution and the solution containing 5 mm Ca²⁺ was significantly smaller than that obtained in the presence of intermediate Ca²⁺ concentrations. D, size of instantaneous outward current measured around −35 mV. E, reversal potential of the instantaneous ACh-activated current as a function of extracellular Ca²⁺ concentration. Numbers of cells are shown above each column.