Influence of dietary fiber and intraluminal pressure on absorption and pre-epithelial diffusion resistance (unstirred layer) in rat jejunum in situ.

The appearance rates of antipyrine, benzoic acid, benzylamine, urea, and alpha-methyl-D-glucoside (MG) in jejunal venous blood of anesthetized rats were measured with and without dietary fibers methylcellulose, carboxymethylcellulose sodium, guaran, and sodium alginate in the luminal solution. Raising the concentration of methylcellulose from 0 to 17.5 g/l resulted in an exponential increase in the viscosity of the solution to 98 cSt, a linear decrease of the diffusion coefficient for antipyrine by 28%, and an increase in antipyrine absorption in the perfused jejunal segment by 23%. The simultaneous increase in intraluminal pressure and radius resulted in a linear relation between absorption rate and apparent mucosal surface area. Similar results were obtained by raising intraluminal pressure directly using a carbohydrate-free perfusion solution. In the perfused rat jejunum, the effect of increased pre-epithelial diffusion resistance (i.e. reduced diffusion coefficient and lengthened diffusion distance) induced by methylcellulose on absorption was overcome by the effect of the enlarged apparent mucosal surface area. Preperfusion of a substrate-free, guaran containing solution followed by perfusion with a guaran-free solution containing antipyrine and MG retarded the increase in the appearance rate of these substrates due to the additional viscous guaran layer left after preperfusion. Constant distension of the intestinal wall was achieved by injecting 0.5 ml of the solution into a closed jejunal segment. Addition of the carbohydrates to the injection solution (approx. 100 cSt viscosity) resulted in a 3% to 20% reduction in the diffusion coefficients and in the absorption of antipyrine, benzoic acid, and MG. Diffusion coefficients for urea and benzylamine were reduced by 5% to 12%; absorption varied in the range of the control (-22% to +43%). Model analysis revealed that, in the closed jejunal segment of the rat, the limiting step in the absorption process of antipyrine, benzoic acid, and MG was pre-epithelial diffusion resistance; the reduction of absorption, therefore, corresponded roughly to that of the diffusion coefficient. In the case of urea and benzylamine, pre-epithelial diffusion resistance was only 20% of the total permeation resistance: the influence of the polymers on absorption, therefore, was not always significant.