FIGURE 1.

Effect of extracellular cations on the potential profile across a cell plasma membrane. The dye chromophores (red rectangle) is sensing the electric field (blue line) generated by transmembrane potential (V_TM) (A), the difference between the external surface potential (ψ_o) and internal surface potential (ψ_i) (B), and the dipole potential at the outer surface (V_DP(o)) (C). We define all potential differences as potential inside minus potential outside. When the negative charges on the outer surface are neutralized and bound by cations (green oval), the equilibrium values of V_TM is unchanged. However, the ψ_o is reduced, leading to an increase in intramembrane potential across the membrane, although ψ_i is unchanged. The effect of divalent cation binding on dipole potential is relatively small, especially at the concentrations used in this study. Therefore, the change in intramembrane potential, , which is calculated from Eq. 7 , is approximately equal to −Δψ_o.