Ion currents through pores. The roles of diffusion and external access steps in determining the currents through narrow pores

External access steps, which may include restricted aqueous diffusion, are introduced into a kinetic model for ion transport through narrow pores. The conductance-concentration relation and the concentration dependence of the biionic permeability are calculated using two alternative assumptions: (a) access to the mouth of the pore is allowed only when no ion is within the lumen or at either mouth; (b) ions remain at the mouth only very transiently. With either assumption the concentration dependence of the fluxes is the same as in previous treatments in which all steps in access were lumped into a single process. Also as before, the biionic permeability ratio is independent of concentration so long as the lumen is never doubly occupied. For narrow pores, such as those formed by gramicidin A, the slowest external portion of the access process must occur close to the pore's mouth, and thus the region an ion must occupy to gain access is small. As a consequence, the probability of finding an ion within this region is also small. On this basis, it is argued that the second assumption is appropriate for these pores. The kinetic equations that result are identical to those used by Urban, B., S.B. Hladky, and D.A. Haydon (1980, Biochim. Biophys. Acta. 602:331–354)