Mathematical modeling of transport in structured tissues: corneal epithelium.

A computer model has been constructed to describe multi-ionic transport across an epithelium consisting of a transcellular pathway in parallel with a shunt. The model is parameterized to simulate rabbit corneal epithelium. Calculations show that when this tissue is stimulated by epinephrine at open circuit, there is a flux of electrolyte across it into the tears. The magnitude of the flux is sensitive to paracellular resistance, which also affects the potentials across the mucosal cell border and entire epithelium. The electrolyte extrusion rate of the tissue is found to be near zero in the absence of stimulation. More generally, it is shown that for epithelia whose structure is at least as complex as that used here, the active transport rate of ions exceeds their net flux at short circuit; tracer fluxes in one direction at two polarizations cannot be used to identify active transport or significant tracer interactions; and tracer measurements yield ionic permeabilities only at short circuit and only if the ion is not actively transported.