Decomposition of experimental I-V curves for the total current into the components carried by Ca²⁺ and by all other ions, assuming an ohmic behavior of all currents. In both A and B, E_Ca²⁺ = 86 mV, calculated using the Nernst equations with [Ca²⁺]_o = 100 mM and [Ca²⁺]_i = 0.1 mM (i.e., 0.1% of the main cation, a reasonable upper bound for the value). Total current (i_total; solid lines) has a slope conductance of 30.6 pS (as in Fig. 15 of Elenes et al., 2009). (A) With [Ca²⁺]_o = 100 mM and [Na⁺]_o = 150 mM, i_total has a reversal potential of +18 mV and an amplitude of −0.6 pA at 0 mV (Elenes et al., 2009). The pure Ca²⁺ current (i_Ca²⁺; dotted line) is the line that passes through this experimental point and is 0 at E_Ca²⁺, which yields a slope conductance of 7 pS. The lumped current carried by all other ions (i_Na⁺_−Mg²⁺; dashed line) has been calculated by subtraction (i_total− i_Ca²⁺) and reverts at 0 mV as expected. (B) When NaCl is not included into the external solution, we can estimate that [Na⁺]_o = 0.1 mM and the Nernst equation yields E_Na⁺_−Mg²⁺ = −180 mV (neglecting Mg²⁺ for the sake of simplicity). We assumed that i_total has a reversal potential of −10 mV (Villarroel and Sakmann, 1996). i_Ca²⁺ was calculated imposing i_Ca²⁺ = i_total at −180 mV and i_Ca²⁺ = 0 at 86 mV, and has a slope conductance of 20 pS. i_Na⁺_−Mg²⁺ reverts at E_Na⁺_−Mg²⁺ = −180 mV.