Uptake of labeled Dand 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

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... and release was inhibited by phlorizin. (d) Countertransport of D-glucose was demonstrated.