Figure 6.

Analysis of the concentration- and voltage-dependent reduction in current amplitude by high [Sr²⁺]_i. (A) Current–voltage relationship for different [Sr²⁺]_i. Divalent cation concentrations, 5 μM [Ca²⁺]_i, 100, 200, and 500 μM, and 1, 2, 5, and 10 mM [Sr²⁺]_i, from top to bottom on the right side. Single-channel conductance measured from the amplitude at 50 mV with 5 μM [Ca²⁺]_i (264 pS) was used as a control conductance in further analysis. The fitting curves were drawn according to a modified Woodhull equation with N = 0.74, zd = 0.45, and K _d(F) = 58 mM. See text for details. (B) Hill plot for amplitude reduction. Membrane voltages, 100–30 mV from top to bottom. Hill coefficients (i.e., slopes of the fitting straight lines) are 0.75, 0.73, 0.75, 0.70, 0.66, 0.62, 0.67, and 0.64 from top to bottom. (C) The ratio of the reduction in amplitude, (i_O − i_B)/i_B, is plotted against membrane voltage with a logarithmic ordinate. [Sr²⁺]_i is 10, 5, 2, 1, and 0.5 mM from top to bottom. The slopes of the fitting straight lines are log(e^0.019-fold), log(e^0.018-fold), log(e^0.017-fold), log(e^0.016-fold), and log(e^0.018-fold) mV⁻¹, from the top to bottom. In B and C, data from A at positive voltages and high [Sr²⁺]_i were used.