Inhibitors of anion exchanger activity reduce sodium chloride-dependent taurine transport by brush border vesicles

Abstract

The erythrocyte anion exchanger, or band 3 protein, catalyzes the exchange of chloride and bicarbonate in many cells. Anion exchanger activity is predominantly found in the basolateral membrane of the cortical collecting duct. Because proximal tubular renal taurine uptake across the brush border surface is dependent upon sodium and chloride, the effect of inhibitors of anion exchanger activity on renal taurine accumulation by rat renal brush border membrane vesicles was examined. The anion exchanger probes, DIDS (4,4- diisothiosulfonic acid), and NBD-taurine (N-[7-nitrobenz-2-oxa-1,3-diazol-4- yl]-aminoethane sulfonic acid), and NBD-taurine (N-[7-nitrobenz-2-oxa-1,3- diazol-4-yl]-aminoethane sulfonic acid), were used as inhibitors of anion exchanger activity. DIDS, NAP-, and NBD-taurine all markedly inhibit the initial rate of NaCl-dependent accumulation of taurine by brush border membrane vesicles. NAP- and NBD-taurine accumulation is chloride dependent because inhibition was not found when uptake was performed in the presence of NaNO3 in place of chloride. In the presence of maximal inhibition of taurine uptake by NAP- or NBD-taurine, no additional inhibition was evident after incubation with DIDS. On the other hand, when a competitive inhibitor of taurine uptake, β-alanine, was used, additional inhibition of taurine accumulation was found in the presence of NAP- or NBD-taurine (p < .01 respectively). These results suggest some interaction of the anion exchanger and the taurine accumulation process at the apical surface of the proximal tubule. Because NBD-taurine is a fluorescent probe, it may be possible to isolate membrane peptides that bind NBD-taurine and demonstrate fluorescence. Preliminary isoelectric focusing experiments of brush border protein incubated with NBD-taurine show fluorescence localized to several particular fractions. Hence this observation that inhibitors of the anion exchanger system block NaCl-dependent taurine uptake can potentially serve as a means of isolating the taurine transporter protein.