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 (I-SC) across the mucosa, and increased the tissue conductance (G(t)). The half-maximal effect of EIS-2 was obtained at 20 mug/ml. In contrast, the polysaccharide fraction of EIS (EIS-1; 100 mug/ml) had little effect on I-SC.,c and Gt. The effect of EIS-2 depended on the presence of CI- and HCO3- but not K+ in the bathing solution. These results suggest that EIS-2 stimulates CI- absorption in the colonic mucosa. The EIS-2-induced changes in I-SC and G(t) were inhibited by 3-(1-[p-chlorobenzyl]-5-[isopropyl]-3-t-butylthioindol-2-yl) -2,2-dimethyl-propanoic acid sodium (MK-886; 10muM), a 5-lipoxygenase-activating protein inhibitor, and 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB; 100 muM), a Cl- channel blocker. EIS-2 attenuated the prostaglandin E-2 (0.51 muM)-increased I-SC, and the half-maximal effect of EIS-2 was obtained at 50 mug/ml. The present study suggests that the EIS-2 stimulates Cl- absorption mediated by basolateral leukotriene-sensitive CI- channels and apical CI-/HCO3- exchanger in the rat colonic mucosa. [The Japanese Journal of Physiology 54: 71-77, 2004].