EFFECTS OF CHLORIDE TRANSPORT INHIBITORS ON INTESTINAL FLUID AND ION-TRANSPORT IN-VIVO AND IN-VITRO

The hypothesis that intestinal fluid secretion is driven by Cl- has be en tested by investigating the effects of NPPB (5-nitro-2-(3-phenyl-pr opylamino)-benzoate), a blocker of Cl- channels in nephrons, and the l oop diuretic bumetanide, an inhibitor of the Na+, K+, 2Cl--co-transpor ter. Both NPPB (IC50 (inhibitory concentration) approximately 100 muM) and bumetanide (IC50 approximately 2 muM) inhibited stimulated short- circuit current (I(sc)) in monolayers of a colonic cell line T84. NPPB also inhibited Cl-36- uptake by these cells, indicating that NPPB act s as a Cl- channel blocker in the T84 cells. NPPB (300 mum) and bumeta nide (10 mum) abolished both stimulated I(sc) and Cl- secretion in iso lated rat colonic mucosa. As judged by autoradiography, [H-3]NPPB was found both in the crypts and at the surface after exposure of either s ide to the compound. In line with these results, NPPB and bumetanide r educed stimulated fluid secretion in everted colon sacs from the rat. In the anaesthetized rat model, neither bumetanide nor NPPB affected t he net fluid transport. After luminal administration of [H-3]NPPB to t he rat, radioactivity was found mainly in the villus tips, whereas no labelling was found in the crypts. NPPB was bound to plasma protein (9 9%), and the inhibitory effects of both NPPB and bumetanide on I(sc) i n T84 cells and fluid secretion in the colonic sacs decreased in the p resence of albumin, again indicating that the compounds might not reac h the in vivo target, or that the mechanism for fluid secretion in viv o may not be explained solely by the secretion of Cl-.