Dependence of Ca²⁺ channel unitary current on voltage. Experiments were performed as in Fig 4 except that Vp was varied from −40 to +40 mV. In the cell-attached configuration, unitary current i is related to (Vm-Vp) according to the Goldman-Hodgkin-Katz equation.43 Vm was measured to be −10 mV in human erythroblasts.22 In human RBCs, it should be recalled that P_Cl is much higher than P_K or P_Na and that V_m is essentially identical to the Cl⁻ equilibrium potential of −9 mV.28 Thus, absence of external Na⁺ or K⁺ in these experiments would be expected to have little effect on V_m . (A) Current tracings at (Vm-Vp) of −30 to +30 mV from a representative cell. (B) Scatter plot of i vs (Vm-Vp). Dotted line is a linear regression line42 from which the slope conductance and reversal potential were estimated to be 3.2 pS and +72 mV, respectively.