Equivalent electrical circuit models and the study of Na transport across epithelia: nonsteady-state current-voltage relations.

After successfully impaling an absorptive cell of rabbit colonic epithelium with a microelectrode, "instantaneous" transepithelial current-voltage relations were determined under control conditions and in the presence of a maximally effective dose of amiloride. The results are interpreted using a very general equivalent electrical circuit model that admits the presence of an apical membrane conductance(s) to an ion(s) other than Na as well as the possibility of an incomplete action of amiloride on the Na entry step. The derived relation between the instantaneous Na current across that barrier conforms closely to the predictions of a simple, charged carrier model. The "reversal potential" of 64 mV is consistent with an intracellular Na activity of 11 nM. Finally, some of the implications of these findings are discussed with respect to 1) the applicability of linear models of active transepithelial Na transport, and 2) the experimental determinations and meanings of the "overall electromotive force" (ENa) and the "overall resistance" (RNa) of active Na transport.