ON THE COUPLING OF MEMBRANE DIGESTION WITH INTESTINAL-ABSORPTION OF SUGARS AND AMINO-ACIDS

An hypothesis is advanced to account for the large paracellular component of absorption of nutrients by the small intestine. High concentrations of hexoses and amino acids in the immediate vicinity of transporters and cell junctions are generated by membrane-bound saccharases and peptidases. After saturation of membrane carriers, the concentrations of monomers in the microenvironment increase until paracellular plus transcellular absorption equals rates of formation. During hydrolysis of maltose, the concentration of glucose at which rates of absorption and formation are equal is 200-300 mM, and at these concentrations paracellular transport accounts for 60-80% of total absorption. Transcellular concentrative transport provides the force for osmotic flow and at the same time triggers contraction of perijunctional actomyosin, thereby increasing transjunctional coefficients of osmotic flow and solvent drag. Paracellular absorption requires no oxidative energy other than that required to maintain the transjunctional concentration gradient and to energize contraction of cytoskeletal elements controlling junctional permeability and functional surface of lateral membranes. Almost all glucose generated by membrane hydrolysis is absorbed, but some may diffuse from high concentration in the microenvironment to the lumen thus accounting for the small amounts of free glucose found in macrosamples of chyme. Unstirred layers adjacent to the brush border retard backdiffusion to lumen, thus maintaining high concentrations for passive paracellular absorption. Coupling of membrane digestion with paracellular transport provides almost perfect matching of absorption to digestive loads because transport by solvent drag is proportional to concentration at cell junctions and this in turn is proportional to rate of hydrolysis. Adaptive changes of membrane-bound hydrolases and transporters induced by dietary changes, lactation, or development can be interpreted in terms of coupling between membrane digestion, paracellular and transcellular mechanisms rather than transcellular transport alone.