STUDIES IN CARBOHYDRATE METABOLISM

Inhibition of glucose transport into brain by phlorizin, phloretin and glucose analogues

An indicator dilution technique with 22Na+ as the intravascular marker was used to measure unidirectional transport of D-[6-3H]glucose from blood into the isolated, perfused dog brain. 18 compounds which are structurally related to glucose were tested for their ability to inhibit glucose transport. The data suggest that no single hydroxyl group is absolutely required for glucose transport, but rather that glucose binding to the carrier probably occurs through hydrogen bonding at several sites (hydroxyls on carbons 1, 3, 4 and 6). In addition, alpha-D-glucose has higher affinity for the carrier than does beta-D-glucose. A separate series of experiments demonstrated that phlorizin and phloretin are competitive inhibitors of glucose transport into brain; however, phloretin is partially competitive and inhibits at lower concentrations than does phlorizin. Inhibition by phlorizin and phloretin is mutually competitive, indicating that these compounds compete for binding to the glucose carrier. Comparison with the results reported in the literature for similar studies using the human erythrocyte demonstrates a fundamental similarity between glucose transport systems in the blood-brain barrier and erythrocyte.

Anomeric specificity of glucose-stimulated insulin release: evidence for a glucoreceptor?

The effects on insulin secretion of alpha and beta anomers of D-glucose were studied in the in vitro perfused rat pancreas. Both phases of insulin release showed consistent stereospecificity for alpha-glucose; this specificity indicates an action of glucose independent of intracellular glucose metabolism.

Insulin secretion by anomers of d-glucose

Isolated rat islets were incubated for 5 minutes in the media containing either the alpha or beta anomer of D-glucose (2 milligrams per milliliter). The amounts of secreted insulin and changes of anomers ratio were concomitantly determined. In spite of rapid mutarotation, significantly greater stimulation of insulin secretion was observed by alpha-D-glucose as compared with beta-D-glucose.

Beta cell protection to alloxan necrosis by anomers of D-glucose

Various concentrations of either the alpha or beta anomers of D-glucose were injected into fasted rats prior to a diabetogenic dose of alloxan. Plasma concentrations of glucose were measured 24 hours later. There was a significantly greater protection of the pancreatic beta cells by the alpha anomer of D-glucose as compared to the beta anomer, which was evidenced by concentrations of glucose in the plasma, and morphology of beta cells.