Evidence that alteration of charge modifies proximal tubular shunt pathway permselectivity

Experiments were performed to test the hypothesis that membrane charge is an important determinant of paracellular pathway ion permselectivity in the proximal tubule. Net negative charge in or around the paracellular pathway should favor cation permeability; net positive charge should favor anion permeability. Therefore compounds such as amiloride and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS), capable of changing net membrane charge, should predictably change the diffusive permselectivity of the paracellular pathway to anions and cations. In the first group of experiments amiloride, a compound capable of increasing net positive membrane charge, inhibited cation and enhanced anion diffusive permeability. In a second group of experiments, SITS, a compound capable of increasing net negative membrane charge, inhibited anion and enhanced cation diffusive permeability. The effects of amiloride and SITS were symmetrical; the lumen-to-bath and the bath-to-lumen diffusion potentials were not significantly different in magnitude. In addition these effects were completely and rapidly reversible. Our results suggest that amiloride increases net positive charge, and SITS increases net negative charge within the paracellular pathway. The most likely site for the actions of SITS and amiloride is the tight junction because the effects of the inhibitors were symmetrical. Both compounds act at low concentrations and reversibly such that removal of the inhibitor rapidly reverses its effects. We propose, on the basis of the ease with which these alterations in charge and thus paracellular pathway permselectivity occurred, that the permselectivity of this pathway may not be fixed and constant for any given proximal tubular segment. In fact, permselectivity may vary and thus serve as an important physiological control mechanism for proximal tubular solute and water reabsorption.