PERMEATION IN IONIC CHANNELS - A STATISTICAL RATE THEORY APPROACH

A novel way to model permeation through ionic channels is formulated. Our method does not require that equilibrium exists in the channel or at the channel interfaces. In addition, the potential profile does not need to be specified and the assumption of constant field across the membrane does not need to be made. Our formulation relies on statistic al rate theory for its development and uses a form of the electrochemi cal potential which assumes that the ions are in solution. We show tha t the conductance and the degree of nonlinearity are dependent on the relative equilibrium exchange rates in the channel and at the interfac es. Nonlinear current-voltage plots can be obtained in symmetric solut ions as well as a nonunity exponent for the Ussing flux ratio. Due to the dependence of the partition coefficient on solubility, it is highl y unlikely that the intracellular and extracellular partition coeffici ents are the same. A manifestation of unequal partition coefficients i s a current reversal at a membrane voltage that is different from the Nernst potential of the current-carrying ionic species.