Metabolic Fate of Glucose in Purified Islet Cells

Abstract

Previous studies in rat islets have suggested that anaplerosis plays an important role in the regulation of pancreatic β cell function and growth. However, the relative contribution of islet β cells versus non-β cells to glucose-regulated anaplerosis is not known. Furthermore, the fate of glucose carbon entering the Krebs cycle of islet cells remains to be determined. The present study has examined the anaplerosis of glucose carbon in purified rat β cells using specific 14C-labeled glucose tracers. Between 5 and 20 mM glucose, the oxidative production of CO2 from [3,4-14C]glucose represented close to 100% of the total glucose utilization by the cells. Anaplerosis, quantified as the difference between 14CO2 production from [3,4- 14C]glucose and [6-14C]glucose, was strongly influenced by glucose, particularly between 5 and 10 mM. The dose dependence of glucose-induced insulin secretion correlated with the accumulation of citrate and malate in β(INS-1) cells. All glucose carbon that was not oxidized to CO2 was recovered from the cells after extraction in trichloroacetic acid. This indirectly indicates that lactate output is minimal in β cells. From the effect of cycloheximide upon the incorporation of 14C-glucose into the acid-precipitable fraction, it could be calculated that 25% of glucose carbon entering the Krebs cycle via anaplerosis is channeled into protein synthesis. In contrast, non-β cells (approximately 80% glucagon-producing α cells) exhibited rates of glucose oxidation that were 3/4 to 1/6 those of the total glucose utilization and no detectable anaplerosis from glucose carbon. This difference between the two cell types was associated with a 7-fold higher expression of the anaplerotic enzyme pyruvate carboxylase in β cells, as well as a 4-fold lower ratio of lactate dehydrogenase to FAD-linked glycerol phosphate dehydrogenase in β cells versus α cells. Finally, glucose caused a dose-dependent suppression of the activity of the pentose phosphate pathway in β cells. In conclusion, rat β cells metabolize glucose essentially via aerobic glycolysis, whereas glycolysis in α cells is largely anaerobic. The results support the view that anaplerosis is an essential pathway implicated in β cell activation by glucose.