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 (Δµj ) 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 Δµj (equal to the
bulk-phase-to-bulk-phase Δµj ) 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.