Uptake of labeled d- and l-glucose has been shown to occur with highly purified brush border membranes from the epithelial cells of rat small intestine using a Millipore filtration technique. An intact glucose carrier system in the isolated membranes was demonstrated as evidenced by the following. (a) d-Glucose was taken up and released faster than l-glucose. (b) Sodium ions increased initial rate and extent of d-glucose uptake 3- to 5-fold; no other cation showed this effect. (c) d-Glucose uptake and release was inhibited by phlorizin. (d) Countertransport of d-glucose was demonstrated. (e) d- and l-glucose reached the same level of uptake after prolonged incubation. (f) Uptake of labeled d-glucose was inhibited by higher concentrations of unlabeled d-galactose and vice versa.

Glucose uptake by membrane vesicles represented entry into an intravesicular aqueous space rather than binding to the membrane. Exposure of the membrane to increasing cellobiose concentrations led to osmotic shrinkage of the intravesicular space and decreased glucose uptake. Infinite medium osmolarity and therefore zero intravesicular space resulted in no glucose uptake. Sodium in the medium (but not in the intravesicular spaces) stimulated d-glucose transport.

It is concluded that isolated brush border membranes of intestinal epithelial cells retain the glucose carrier system. The reported findings are consistent with the concept that (a) glucose transport across the brush border membrane represents “facilitated diffusion”; (b) the glucose carrier is dependent on sodium ions; and (c) high extracellular, but not intracellular sodium concentrations lead to increased glucose transport.