Sulfate transport in rabbit ileum: characterization of the serosal border anion exchange process.

Abstract

The preceding paper [30] shows that transepithelial ileal SO4 transport involves Na-dependent uptake across the ileal brush border, and Cl-dependent efflux across the serosal border. The present study examines more closely the serosal efflux process. Transepithelial mucosa (m)-to-serosa (s) ands-to-m fluxes (J ms,J sm) across rabbit ileal mucosa were determined under short-circuit conditions. SO4 was present at 0.22mm. In standard Cl, HCO3 Ringer's,J SO4 was 81.3±5.3 (1se) andJ SO4 was 2.5±0.2 nmol cm−2 hr−1 (n=20). Serosal addition of 4-acetamido-4′-isothiocyanostilbene-22′-disulfonate (SITS), 44′-diisothiocyanostilbene-22′-disulfonate (DIDS) or 1-anilino-8-naphthalene-sulfonate (ANS) inhibited SO4 transport, SITS being the most potent. Several other inhibitors of anion exchange in erythrocytes and other cells had no effect on ileal SO4 fluxes. In contrast to its effect on SO4 transport, SITS (500 μm) did not detectably alter Cl transport. Replacement of all Cl, HCO3 and PO4 with gluconate reducedJ SO4 by 70% and increasedJ SO4 by 400%. A small but significantJ net SO4 remained.J SO4 could be increased by addition to the serosal side of Cl, Br, I, NO3 or SO4. The stimulatory effect of all these anions was saturable and SITS-inhibitable. The maximalJ SO4 in the presence of Cl was considerably higher than in the presence of SO4 (73.1 and 42.2 nmol. cm−2 hr−1, respectively;p<0.001). TheK 1/2 value for Cl was 7.4mm, 10-fold higher than that for SO4 (0.7mm). Omitting HCO3 and PO4 had no measurable effects on SO4 fluxes. This study shows that (i) SO4 crosses the serosal border of rabbit ileal mucosa by anion exchange; (ii) the exchange process is inhibited by SITS, DIDS and ANS, but not by several other inhibitors of anion exchange in other systems; (iii) SO4 may exchange for Cl, Br, I, NO3 and SO4 itself, but probably not for HCO3 or PO4; (iv) kinetics of the exchange system suggest there is a greater affinity for SO4 than for Cl, although the maximal rate of exchange is higher in the presence of Cl; and, finally (v) SITS has little or no effect on net Cl transport.