The Changes in Adenine Nucleotides Measured in Glucose-stimulated Rodent Islets Occur in β Cells but Not in α Cells and Are Also Observed in Human Islets

Abstract

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...

The Common Single Nucleotide Polymorphism E23K in KIR6.2 Sensitizes Pancreatic β-Cell ATP-Sensitive Potassium Channels Toward Activation Through Nucleoside Diphosphates

“…Actually, in concurrence with the conclusion that insulin secretion might be controlled by the cytosolic ATP/ADP ratio (22), there is evidence in favor of both sites. First, in resting pancreatic ␤-cells, estimates of the cytosolic concentration of ATP range from 0.6 to 3 mmol/l (19,(22)(23)(24) and those of ADP, from 0.3 to 1.2 mmol/l (19,22). Consistent with previous reports (25), our experiments show modulation of channel activity if at 300 mol/l GDP the ATP concentration varies ϳ1 mmol/l (Fig.…”

Inhibition of the malate–aspartate shuttle in mouse pancreatic islets abolishes glucagon secretion without affecting insulin secretion

“…Notably, the mitochondrial substrate pyruvate was more efficient than glucose in provoking glucagon secretion from the αTC1-6 cells; this coincided with a minor, albeit significant, increase in the OCR also observed in the αTC1-6 cells. These results confirm findings from primary islet cells in which glucose-provoked ATP [22,47] and FADH 2 production [48] have been shown to be lower in the α-cell than in the β-cell. Overall, our findings of less efficient coupling of glycolysis and TCA-cycle metabolism in the αTC1-6 cell line support other studies in which mitochondrial metabolism has been shown to be significantly more glucose-responsive in the β-cell [22,23,49].…”

Molecular basis of Kir6.2 mutations associated with neonatal diabetes or neonatal diabetes plus neurological features

“…3B). Because [ATP] i is unlikely to fall below 0.1 mM in beta cells, even when extracellular glucose is low (12), the difference in wild-type and mutant K ATP current at this (and higher) ATP concentration probably explains why the R201C and R201H mutations result in neonatal diabetes. Accumulating evidence implicates small changes in K ATP current with impaired insulin secretion (11).…”