Edible marine algae have been highlighted recently as multifunctional foods for maintaining human health. They are rich in minerals, vitamins and dietary fibers. Sargassum horneri is one of the edible brown marine algae and is distributed along the seacoast of Japan, and the slightly boiled alga is used as a savory food in Japan (Preeprame et al. 2001). Interestingly, water extracts of Sargassum horneri has a stimulatory effect on bone formation in rat in vitro (Uchiyama & Yamaguchi, 2002), and shows an antiviral activity against herpes simplex virus type I, human cytomegalovirus and immunodeficiency virus type I (Hoshino et al. 1998; Preeprame et al. 2001).
Herein we tested if the water extracts from Sargassum horneri affects K+ and Cl– transports in isolated rat colonic mucosa. Rats were sacrificed rapidly by stunning and cervical dislocation. Effects of the extracts on the short-circuit current (Isc), the potential difference across the mucosa (Pd) and the tissue conductance (Gt) were examined in isolated rat distal colon mounted on Ussing chamber. Data are shown as means ± S.E.M. Differences between groups were analysed by one-way ANOVA. Comparison between the two groups was made with paired t test.
Two kinds of water-soluble (ethanol-insoluble) extracts (EIS-1 and EIS-2) were prepared from the alga. The non-polysaccharide fraction (EIS-2; 100 µg ml-1) significantly decreased Isc by 7.9 ± 1.6 µA cm-2, and increased Gt by 0.8 ± 0.3 mS cm-2 (n = 4, P < 0.05). The half maximal effect of EIS-2 was obtained at 20 µg ml-1. In contrast, polysaccharide fraction (EIS-1; 100 µg ml-1) had little effects on Isc and Gt (n = 3, P > 0.05). The effect of EIS-2 depended on the presence of Cl– and HCO3– but not K+ in the bathing solution (n = 4-5). Increase in Isc by EIS-2 (100 µg ml-1) was inhibited by 3-[1-(p-chlorobenzyl)-5-(isopropyl)-3-t-butylthioindol-2-yl]-2, 2-dimethyl-propanoic acid sodium (MK-886; 10 µM), a 5-lipoxygenase-activating protein inhibitor, and 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB; 100 µM), a Cl– channel blocker (n = 4). EIS-2 attenuated the prostaglandin E2 (0.5 µM)-increased Isc (n = 5, P < 0.01), and the half maximal effect of EIS-2 was obtained at 50 µg ml-1.
These results suggest that the EIS-2 stimulates Cl– absorption mediated by basolateral leukotrienes-sensitive Cl– channels and apical Cl– /HCO3– exchanger in the rat colonic mucosa.