Ion fluxes considered in terms of membrane-surface electrical potentials

Ions transported through plasma membranes encounter electrical charges, and associated electrical potentials, at the membrane surfaces. The ionic composition of the tissue-bathing medium influences both the surface charge density and the surface electrical potential. Changes in surface electrical potential may affect ion transport by altering two components of the chemical potential difference (Delta muj) of an ion through the membrane. First, the surface activity of the transported ion will change because of electrostatic attraction or repulsion. Second, the surface-to-surface transmembrane potential difference will change. (This is different from the bulk-phase-to-bulk-phase transmembrane potential difference measured with microelectrodes.) These changes in the components of the chemical potential may change the flux of an ion through the membrane even if the surface-to-surface Delta muj (equal to the bulk-phase-to-bulk-phase Delta muj) remains constant. The Goldman-Hodgkin-Katz (GHK) flux equation does not take into account these surface-potential effects. The equation has been modified to incorporate surface potentials computed by a Gouy-Chapman-Stern model and surface ion activities computed by Nernst equations. The modified equation (despite several additional deficiencies of the GHK model) successfully predicts many transport phenomena not predicted by the standard GHK equation. Thus electrostatic effects may account for saturation, cis- and trans-inhibition, rectification, voltage gating, shifts in voltage optima, and other phenomena also attributable to other mechanisms.