The intestinal unstirred layer: Its surface area and effect on active transport kinetics

Uptake into the intestinal mucosa is determined by the rates of penetration of solute molecules across the unstirred water layer and lipid cell membrane of the mucosal cell. The presence of the unstirred water layer results in artifactually low permeability coefficients for passive transport processes and high K_m values for active transport processes. Appropriate correction for this unstirred layer resistance requires knowledge of the effective surface area of the diffusion barrier in the intestine. Using three separate experimental and mathematical approaches this surface area was found to vary from 1.02 cm² to 14.24 cm² per 100 mg dry weight of rat intestine. These values are very much lower than the 1226 cm² and 696 cm² per 100 mg area of the microvillus membrane in the jejunum and ileum, respectively, and indicates that the effective surface area of the rate-limiting membrane for such highly permeant solutes as long-chain fatty acids is from 1 100 to 1 200 the actual anatomical surface area. These studies also demonstrate that the presence of the unstirred layer introduces major artifacts into the determination of K_m and J_max (maximal transport rate) values for active transport processes.