Digestive and absorptive functions along dog small intestine: comparative distributions in relation to biochemical and morphological parameters.

The digestive (hydrolytic enzymes) and absorptive (sugar and amino acid transport) functions of dog small intestine have been evaluated in different segments and analysed in relation to morphometric and biochemical parameters. The dog small intestine is a cylinder of decreasing diameter in which the underlying mucosa thins down from duodenum to ileum, though maintaining its cellular homogeneity as revealed by measuring the mucosal weight, the total DNA and protein content and the protein content of the brush border membrane. Sucrase, gamma-glutamyltranspeptidase, leucylnaphthylamidase and alkaline phosphatase specific activities, measured both in homogenates of the mucosa and purified brush border membrane fractions, were found distributed along proximo-distal gradients of activity. However, different patterns were obtained which are specific for the enzyme considered. Kinetic parameters, Vmax and Km, were estimated for sucrase and alkaline phosphatase in purified brush border membrane fractions. It appeared that Vmax correlated well with the observed distribution of catalytic sites along the small intestine. Sugar (glucose) and amino acid (alanine and leucine) transport capacities were also distributed according to specific proximo-distal gradients but passive and facilitated diffusions were not affected. Only the active, Na+ -dependent component of transport was sensitive to position along the small intestine and we postulated that this adaptation should involve variations in carrier densities. It is therefore concluded that absorbo-digestive functions are intrinsic characteristics of the brush border membrane which are regulated according to the position along the small intestine.