The effect of membrane heterogeneity on the predictability of fluxes, with application to the cornea

Abstract

Many phenomenological treatments of biological membrane transport are based on the assumption that the membrane consists of a single class of passive transport paths; i.e. that the membrane is simple. Simplicity is assumed both in the measurement of the membrane's transport coefficients and in the use of these coefficients to predict membrane fluxes. Such a procedure will in general lead to an error in the prediction of solute flux across parallel arrays. The error depends on the distribution of the reflection coefficients of the parallel paths and upper and lower bounds on it are given in terms of conventionally measured transport coefficients. The frictional representation of electrolyte transport across membranes possessing metabolic pumps is generalized to take this structure effect into account. The flux error resulting from the neglect of membrane heteroreflectivity is essentially the same for nonelectrolytes and electrolytes, irrespective of whether phenomenological or frictional membrane transport properties are used. It is shown that some information about transport structure can be obtained from global measurements made without regard for the organization of pathways across the membrane. The values of the measured transport coefficients of the corneal epithelium and endothelium imply that these cell layers are heteroreflective. Analysis of corneal transport, taking structure effects into account, shows that the corneal thickness may be intrinsically insensitive to tear tonicity, by a mechanism which may be of more general homeostatic significance.