Epithelial transport of weak electrolytes. Properties of a model of the three compartment system

Abstract

A study has been made of the properties of a theoretical model of a mechanism that has been proposed for the transport of weak electrolytes in several epithelia. The model consists of three aqueous compartments arranged in series, in which adjacent compartments are separated by barriers that may discriminate between the ionized and non-ionized forms of weak electrolytes, and where the pH of the intermediate compartment may be different than that of the bulk phases. Significant asymmetries in weak electrolyte transport may be associated with relatively modest driving forces. The abilities of the barriers to discriminate between the ionized and non-ionized forms of a weak electrolyte are critical parameters of transport in this system, and both the relative, and the individual absolute values of these parameters are important determinants of weak electrolyte transport. The degree of asymmetry in the fluxes of a weak electrolyte varies markedly with the discriminatory properties of the barriers within a range of values that may be appropriate to the barriers found in epithelia. The range of pK a values for which transport may be observed in the system varies with the discriminatory properties of the barriers, and some versions of the system may be effective in the transport of acids of pK a< 1, or of bases of pK a> 13, in addition to the transport of weak electrolytes. The study allows description of two main types of potential biological analogs of the model system, and criteria of tenability of each version are identified.