Contribution of Cl(-)-OH- exchange to electroneutral NaCl absorption in rat distal colon

Abstract

Neutral NaCl absorption is the predominant Na+ absorptive process in rat distal colon. Whether this neutral NaCl absorptive process represents Na(+)-Cl- cotransport or dual ion exchanges of Na(+)-H+ and Cl(-)-OH- has been uncertain. Recent studies using rat colonic brush-border membrane vesicles (BBMVs) have described a Na(+)-H+ exchange mechanism and have proposed that net NaCl absorption occurs via a dual ion exchange process. To date, however, an anion exchanger on the colonic apical membrane has not been identified. To determine whether a Cl(-)-OH- exchange process is present, 36Cl uptake was evaluated across rat distal colonic BBMVs. A pH gradient (7.7 in/5.5 out) stimulated active Cl- uptake. Cl- uptake was not significantly affected by the presence of a valinomycin-induced K+ diffusion potential (inside positive), suggesting that a Cl- conductive pathway is not present in these membranes. The pH gradient-stimulated Cl- uptake was a saturable function of the Cl- concentration with a Km of 14.3 +/- 5.0 mM and a Vmax of 20.4 +/- 5.5 nmol.mg protein-1.30 s-1 and was almost completely inhibited by 1 mM concentrations of SITS and DIDS, inhibitors of anion exchange processes in other epithelia. Inward gradients of Na+, K+, or Na+ and K+ did not further stimulate initial Cl- uptake, suggesting that coupling of Na+ and Cl- movement does not occur by a cotransport mechanism. Thus a Cl(-)-OH- exchange process is present in rat distal colonic BBMVs.(ABSTRACT TRUNCATED AT 250 WORDS)