Nonlinear Intestinal Absorption of (1→3)-β-D-Glucan Caused by Absorptive and Secretory Transporting System

The mechanism of the nonlinear concentration dependence of intestinal absorption of (1 -> 3)-beta-D-glucan was studied using in situ rat intestinal perfusion, as well as the in vitro Ussing-type chamber method mounted with rat intestinal tissue. The intestinal absorption rate constant of a (1 -> 3)-beta-D-glucan, laminaran, evaluated by the loop method increased significantly with increasing concentration of laminaran up to 0.5 mu M in a nonlinear fashion and tended to decrease at higher concentrations. Mucosal-to-serosal directed permeation of the laminaran across rat ileal sheets evaluated by the in vitro Ussing-type chamber method also decreased in a dose-dependent fashion. Serosal-to-mucosal directed permeation decreased in a concentration-dependent manner. In addition, the serosal-to-mucosal flux was reduced in the presence of metabolic inhibitor, 2,4-di-nitrophenol. These results suggest that laminaran is secreted into the intestinal lumen predominantly by the efflux transporting system. We conclude that intestinal transport of (1 -> 3)-beta-D-glucan involves specialized transporter or something similar in both absorptive and secretory directions, and complex nonlinear intestinal absorption characteristics can be ascribed to the participation of multiple transport mechanism.