METABOLIC-FATE OF GLUCOSE IN PURIFIED ISLET CELLS - GLUCOSE-REGULATEDANAPLEROSIS IN BETA-CELLS

Previous studies in rat islets have suggested that anaplerosis plays a n important role in the regulation of pancreatic beta cell function an d growth. However, the relative contribution of islet beta cells versu s non-beta cells to glucose-regulated anaplerosis is not known. Furthe r more, the fate of glucose carbon entering the Krebs cycle of islet c ells remains to be determined. The present study has examined the anap lerosis of glucose carbon in purified rat beta cells using specific C- 14-labeled glucose tracers. Between 5 and 20 mM glucose, the oxidative production of CO2 from [3,4-C-14]glucose represented close to 100% of the total glucose utilization by the cells. Anaplerosis, quantified a s the difference between (CO2)-C-14 production from [3,4-C-14]glucose and [6-C-14]glucose, was strongly influenced by glucose, particularly between 5 and 10 mM. The dose dependence of glucose-induced insulin se cretion correlated with the accumulation of citrate and malate in beta (INS-1) cells, All glucose carbon that was not oxidized to CO2 was rec overed from the cells after extraction in trichloroacetic acid. This i ndirectly indicates that lactate output is minimal in beta cells. From the effect of cycloheximide upon the incorporation of C-14-glucose in to the acid-precipitable fraction, it could be calculated that 25% of glucose carbon entering the Krebs cycle via anaplerosis is channeled i nto protein synthesis. In contrast, non-beta cells (approximately 80% glucagon producing alpha cells) exhibited rates of glucose oxidation t hat were 1/3 to 1/6 those of the total glucose utilization and no dete ctable anaplerosis from glucose carbon, This difference between the tw o cell types was associated with a 7-fold higher expression of the ana plerotic enzyme pyruvate carboxylase in beta cells, as well as a 4-fol d lower ratio of lactate dehydrogenase to FAD-linked glycerol phosphat e dehydrogenase in beta cells versus alpha cells. Finally, glucose cau sed a dose-dependent suppression of the activity of the pentose phosph ate pathway in beta cells. In conclusion, rat beta cells metabolize gl ucose essentially via aerobic glycolysis, whereas glycolysis in alpha cells is largely anaerobic. The results support the view that anaplero sis is an essential pathway implicated in beta cell activation by gluc ose.