Leukotrienes-mediated effects of water extracts from Sargassum horneri, a marine brown alga, on Cl- absorption in isolated rat colon.

Sargassum horneri is an edible marine brown alga distributed along the seacoast of Japan. Here we examined effects on the water-soluble (ethanol-insoluble) extracts (EIS) from Sargassum horneri on ion transports across the isolated rat colonic mucosa set in Ussing chambers. The nonpolysaccharide fraction of EIS (EIS-2) significantly decreased short-circuit current (Isc) across the mucosa, and increased the tissue conductance (Gt). The half-maximal effect of EIS-2 was obtained at 20 microg/ml. In contrast, the polysaccharide fraction of EIS (EIS-1; 100 microg/ml) had little effect on Isc and Gt. The effect of EIS-2 depended on the presence of Cl- and HCO3- but not K+ in the bathing solution. These results suggest that EIS-2 stimulates Cl)absorption in the colonic mucosa. The EIS-2-induced changes in Isc and Gt were inhibited by 3-(1-[p-chlorobenzyl]-5-[isopropyl]-3-t-butylthioindol-2-yl)-2,2-dimethyl-propanoic acid sodium (MK-886; 10 microM), a 5-lipoxygenase-activating protein inhibitor, and 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB; 100 microM), a Cl- channel blocker. EIS-2 attenuated the prostaglandin E2 (0.5 microM)-increased Isc, and the half-maximal effect of EIS-2 was obtained at 50 microg/ml. The present study suggests that the EIS-2 stimulates Cl- absorption mediated by basolateral leukotriene-sensitive Cl- channels and apical Cl-/HCO3- exchanger in the rat colonic mucosa.