Influence of distension on absorption and villous structure in rat jejunum

In urethan-anesthetized rats the appearance rates of urea (U), antipyrine (A), and alpha-methyl-D-glucoside (MG) in the venous blood of perfused jejunal segment were measured in the undistended state and after elevation of the intraluminal pressure up to 10 cmH2O. Serosal and inner cylindrical surface area of the jejunal segment were enlarged by maximally 100 and 150%, respectively. The absorption rates, however, increased only by 34 (U), 28 (A), and 26% (MG). The increase of the supravillous diffusion resistance contributed only partially to this effect. The "cylindrical" permeability coefficient (Pcyl, average permeability coefficient related to inner cylindrical area, neglecting villous structure) decreased by 39, 57, and 50%, respectively. Due to circular stretching, broad intervillous spaces were formed that covered finally approximately 40% of the mucosal surface area. The additional intervillous diffusion resistance in these spaces was more effective than the absorption through the lateral surface of the villi. Thus the overall permeability of the mucosa, i.e., Pcyl, was reduced. Calculations based on a simplified model of the mucosa confirmed the experimental results.