MECHANISM OF COLONIC PERMEATION OF INULIN - IS RAT COLON MORE PERMEABLE THAN SMALL-INTESTINE

Background/Aims: Colonic epithelium is considered to be relatively tig ht. The colonic ''pore'' diameter is 6 Angstrom; therefore, colonic ep ithelium has generally been considered to be impermeable to hydrophili c probes with a cross-sectional diameter of >6 Angstrom. This study ex amined whether rat colon is permeable to inulin, a large hydrophilic m acromolecule having a molecular weight of 5000 g/mol and a cross-secti onal diameter of 15 Angstrom (hydration diameter, 20 Angstrom). Method s: The colonic permeation of inulin (10 mu mol/L) in vivo was investig ated by perfusion of rat colonic segments. Results: There was signific ant colonic permeation of inulin, but tissue retention of inulin was l ow. The net colonic flux of inulin was strongly dependent on net water flux, showing a strong solvent drag effect. Addition of 16,16-dimethy l prostaglandin E(2) decreased water flux with a corresponding decreas e in inulin flux; this process seemed to be mediated by 5'-cyclic aden osine monophosphate because both the phosphodiesterase inhibitor amino phyiline and dibutyryl adenosine 5'-cyclic adenosine monophosphate dec reased water and inulin flux in a parallel manner. Chenodeoxycholic an d taurocholic acids decreased net mucosal-to-serosal water flux but in creased inulin flux. The net colonic permeation rate of inulin was hig her than the small intestinal permeation rate. Conclusions: Rat colon is permeable to inulin. The higher net colonic permeability may be cau sed by differences in mucosal surface, permselectivity, solvent drag e ffect, and differences in net water fluxes of the colon and small inte stine.