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Glucose metabolism by pancreatic ␤ and ␣ cells is essential for stimulation of insulin secretion and inhibition of glucagon secretion. Studies using rodent islets have suggested that the ATP/ADP ratio serves as second messenger in ␤ cells. This study compared the effects of glucose on glucose oxidation ([U-14 C]glucose) and adenine nucleotides (luminometric method) in purified rat ␣ and ␤ cells. The rate of glucose oxidation at 1 mM glucose was higher in ␤ than ␣ cells (4.5-fold, i.e. ϳ2-fold after normalization for cell size). It was more strongly stimulated by 10 mM glucose in ␤ cells (9-fold) than in ␣ cells (5-fold). At 1 mM glucose, ATP levels were similar in both cell types, which corresponds to an approximately 2-fold higher concentration in ␣ cells (ϳ6.5 mM) than in ␤ cells (ϳ3 mM). In ␤ cells, glucose dose-dependently increased ATP and decreased ADP levels, causing a rise in the ATP/ADP ratio from 2.4 to 11.6 at 1 and 10 mM, respectively. In ␣ cells, glucose did not affect ATP and ADP levels, and the ATP/ADP ratio remained stable around 7.5. In human islets, the ATP/ADP ratio progressively increased between 1 and 10 mM glucose. In duct cells, which often contaminate human islet preparations, an increase in the ATP/ADP ratio sometimes occurred between 1 and 3 mM glucose. In conclusion, the present observations establish that the regulation of glucagon secretion by glucose does not involve changes in ␣ cell adenine nucleotides and further support the role of the ATP/ADP ratio in the control of insulin secretion.Glucose homeostasis is largely regulated in the endocrine pancreas through opposite effects of glucose on insulin and glucagon secretion. Pancreatic ␤ cells are fuel sensors that adjust the rate of insulin secretion to the rate at which they metabolize glucose (reviewed in Refs. 1-3). Two major transduction pathways are involved. The first one uses ATP-sensitive K ϩ channels (K ] i ) then triggers exocytosis of insulin granules (reviewed in Refs. 4 -6). The second pathway, known as the K ϩ -ATP channel-independent pathway, increases the effectiveness of Ca 2ϩ on exocytosis by as yet incompletely elucidated mechanisms (7-9). Much less information is available on how glucose inhibits glucagon secretion from ␣ cells (10). Measurements of glucose metabolism in ␣ cell-rich islets (11) and purified ␣ cells (12) and studies using metabolic inhibitors in whole islets or pancreas (13-15) suggest that the inhibition of glucagon secretion is mediated by glucose metabolism in the ␣ cells.Although early studies reported that glucose increases ATP levels in rodent islets (16, 17), a rise in the ATP/ADP ratio was not a consistent finding (reviewed in Ref. 18). Recently, we demonstrated that glucose causes a large, concentration-dependent increase in the ATP/ADP ratio in mouse islets and that this effect might be involved in the regulation of insulin secretion through both pathways (19,20). However, the changes measured in whole islets might not exactly reflect those occurring in ␤ cells. It is also not known whet...

Pancreatic β-cells express glutamate decarboxylase (GAD), which is responsible for the production and release of γ-aminobutyric acid (GABA). Over a 24-h culture period, total GABA release by purified rat β-cells is eightfold higher than the cellular GABA content and can thus be used as an index of cellular GAD activity. GABA release is 40% reduced by glucose (58 pmol/103 cells at 10 mM glucose vs. 94 pmol at 3 mM glucose, P < 0.05). This suppressive effect of glucose was not observed when glucose metabolism was blocked by mannoheptulose or 2,4-dinitrophenol; it was amplified when ATP-dependent β-cell activities were inhibited by addition of diazoxide, verapamil, or cycloheximide or by reduction of extracellular calcium levels; it was counteracted when β-cell functions were activated by nonmetabolized agents, such as glibenclamide, IBMX, glucagon, or glucacon-like peptide-1 (GLP-1), which are known to stimulate calcium-dependent activities, such as hormone release and calcium-dependent ATPases. These observations suggest that GABA release from β-cells varies with the balance between ATP-producing and ATP-consuming activities in the cells. Less GABA is released in conditions of elevated glucose metabolism, and hence ATP production, but this effect is counteracted by ATP-dependent activities. The notion that increased cytoplasmic ATP levels can suppress GAD activity in β-cells, and hence GABA production and release, is compatible with previous findings on ATP suppression of brain GAD activity.