Application of membrane potential equations to tight epithelia

It is shown that equations developed to analyze the contributions of secondary active transport processes to symmetrical cells (Gordon, L.G.M., Macknight, A.D.C., 1991, J. Membrane Biol. 120:139-152) can be used, with minor modifications, to analyze the steady-state membrane potential in epithelia under the unique situation of short circuiting. Only under such conditions is there a single intracellular potential relative to both the mucosal and serosal media. The equations are investigated in relation to a model tight epithelium--the toad urinary bladder. It is shown that the properties of the membrane transport pathways are such that the intracellular potential under short-circuit conditions must be more negative than often reported. Given measurements of membrane potential and of voltage-divider ratio, it is possible to use the equations to estimate the absolute values of the membrane permeabilities and conductances under short-circuit conditions.