Leukotrienes-Mediated Effects of Water Extracts from Sargassum horneri, a Marine Brown Alga, on Cl^- Absorption in Isolated Rat Colon

Sargassum homeri 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 μg/ml. In contrast, the polysaccharide fraction of EIS (EIS-1; 100 μg/ml) had little effect on I_SC and G_t. 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 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; 10μM), a 5-lipoxygenase-activating protein inhibitor, and 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB; 100 μM), a Cl^- channel blocker. EIS-2 attenuated the prostaglandin E₂ (0.5 μM)-increased I_SC, and the half-maximal effect of EIS-2 was obtained at 50 μg/ml. The present study suggests that the EIS-2 stimulates Cl^- absorption mediated by basolateral leukotriene-sensitive Cl^- channels and apical Cl^-/HCO ₃^- exchanger in the rat colonic mucosa.