Figure 7. EC₅₀ values for block of I_K(IR) by Sr²⁺ over a range of test potentials (−140 to −100 mV).

A, fractional inhibition of peak inward current vs. external Sr²⁺ concentration at −140 mV (▪), −130 mV (•), −120 mV (▴), −110 mV (▾) and −100 mV (♦). I_Con is the peak current in standard bath solution and I_Sr is the current in the presence of 0.05–10 mm Sr²⁺. The voltage dependence of the Sr²⁺ block was well fitted by the logistic equation. The inset shows a plot of EC₅₀ values vs. test potential for the same cells. The data points were well fitted with a linear regression yielding a slope such that an e-fold change in EC₅₀ occurs over a 27 mV range. B, percentage block of peak I_K(IR)vs. the test potentials for 10 mm Ca²⁺ (▪), 5 mm Mg²⁺ (▴), 5 mm Mn²⁺ (▾) and 5 mm Sr²⁺ (•). C, percentage block of steady-state I_K(IR) by 10 mm Ca²⁺ (▪), 5 mm Mg²⁺ (▴), 5 mm Mn²⁺ (▾) and 5 mm Sr²⁺ (•) for the various test potentials shown.