USE OF MALTOSE HYDROLYSIS MEASUREMENTS TO CHARACTERIZE THE INTERACTION BETWEEN THE AQUEOUS DIFFUSION BARRIER AND THE EPITHELIUM IN THE RAT JEJUNUM

Rates of intestinal absorption and surface hydrolysis are determined b y the interaction of two barriers: poorly stirred fluid adjacent to th e mucosa, and the epithelial cell, These two barriers commonly are mod eled as a fixed, flat layer of epithelium covered by a fixed thickness of unstirred fluid. To more accurately simulate these barriers in a v illous mucosa, maltase activity (measured in vitro) was distributed ov er an anatomically correct model of rat jejunal villi. We then determi ned what interaction of the aqueous and epithelial barriers best predi cted in vivo maltose hydrolysis rates measured over a broad range of i nfusate concentrations. Hydrolysis was accurately predicted by a model in which unstirred fluid extended from 20 mu m over the villous tips throughout the intervillous space. In this model, the depth of diffusi on into the intervillous space is inversely proportional to the effici ency of epithelial handling of the solute. As a result, both the aqueo us barrier and the functional surface area are variables rather than c onstants, Some implications of our findings (relative to the conventio nal model) include: higher predicted V-max, efficient handling of low concentrations of a solute at the villous tips while high concentratio ns must penetrate thick aqueous barriers, and sensitive regulation of transport rates via ease of access to the intervillous space. (J. Clin . Invest. 1996. 97:2308-2315.)