Hexose metabolism in pancreatic islets. Feedback control of d-glucose oxidation by functional events (original) (raw)
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Molecular and Cellular Endocrinology, 1991
In rat pancreatic islets, a rise in D-@uCOSe concentrations increases the oxidation of hexose-derived acetyl residues relative to glycolytic flux, an effect possibly attributable, in part at least, to the activation of key mitochondrial dehydrogenases by Ca2+ accumulated in the mitochondria of glucose-stimulated islet cells. The effects of non-nutrient insuhnotropic agents upon D-[6-i4C]ghtcose oxidation and ~-[5-~H]glucase utilization were investigated. At an intermediate concentration of D-glucose (6 mM), the oxidation of D-[6-i4C]glucose was unaffected by hypoglycemic sulfonylureas, an organic Ca2+ agonist, a cholinergic agent, forskolin, theophylline and cytochalasin B. At a higher concentration of the hexose (17 mM), however, the i4C02/'H20 production rate was decreased by organic and inorganic Ca2+-antagonists and by ouabain, whilst being increased by NH: (10 mM) and aminooxyacetate. These findings suggest that the preferential stimulation of oxidative events in the Krebs cycle is largely independent of the rate of insulin release, and not merely consequential to the stimulation of Ca2+ inflow into the B-cell. It might be regulated, in a feedback process, by the rate of ATP utilization and, both directly and indirectly, by the mitochondrial redox state. The glucose-induced mitochondrial accumulation of Cat+ and subsequent activation of the Krebs cycle appear to require an increase in both cytosolic Ca2+ activity and ATP availability.
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1996
Mitochondrial NAD ÷, NADH, NADP + and NADPH were measured in dispersed pancreatic islet cells incubated in the absence or presence of D-glucose and then exposed for 20 s to 0.5 mg/ml digitonin. The latter treatment resulted in the full release of lactate dehydrogenase without any detectable loss of glutamate dehydrogenase. The permeabilized cells were separated from the incubation medium by centrifugation through an oil layer and their content in pyridine nucleotides measured by a radioisotopic procedure coupled to the classical cycling technique. Relative to basal value, D-glucose, in concentrations of 2.8 and 16.7 mM, caused a concentration-related increase in both the NADH/NAD + and NADPH/NADP + ratio. These findings provide the first direct evidence for the induction of a more reduced mitochondrial redox state in glucose-stimulated pancreatic islets.
Stimulation by d-glucose of mitochondrial oxidative events in islet cells
Biochemical Journal, 1987
D-Glucose catabolism was examined in rat pancreatic islets and tumoral insulin-producing cells (RINm5F line) exposed to D-[5-3H]glucose, D-[3,4-14C]glucose or D-[6-14C]glucose. In normal islets, a rise in D-glucose concentration from 2.8 to 16.7 mM augmented (i) the transfer of NADH, presumably via the glycerol phosphate shuttle, into the mitochondria, (ii) the decarboxylation of pyruvate and (iii) the oxidation of acetyl residues, even when the flow rates were expressed relative to the total rate of D-glucose utilization. Islet cells are thus organized to favour those mitochondrial oxidative events yielding the major fraction of the energy derived from the catabolism of D-glucose. In tumoral islet cells of the RINm5F line, all three oxidative mitochondrial processes, when expressed relative to the overall rate of D-glucose utilization, occurred at a low rate and failed to increase in response to the rise in D-glucose concentration. These findings emphasize the importance of distal ...
Metabolic Fate of Glucose in Purified Islet Cells
Journal of Biological Chemistry, 1997
Previous studies in rat islets have suggested that anaplerosis plays an important role in the regulation of pancreatic  cell function and growth. However, the relative contribution of islet  cells versus non- 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  cells using specific 14 C-labeled glucose tracers. Between 5 and 20 mM glucose, the oxidative production of CO 2 from [3,4-14 C]glucose represented close to 100% of the total glucose utilization by the cells. Anaplerosis, quantified as the difference between 14 CO 2 production from [3,4-14 C]glucose and [6-14 C]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 (INS-1) cells. All glucose carbon that was not oxidized to CO 2 was recovered from the cells after extraction in trichloroacetic acid. This indirectly indicates that lactate output is minimal in  cells. From the effect of cycloheximide upon the incorporation of 14 C-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- cells (approximately 80% glucagon-producing ␣ cells) exhibited rates of glucose oxidation that were 1 ⁄3 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  cells, as well as a 4-fold lower ratio of lactate dehydrogenase to FADlinked glycerol phosphate dehydrogenase in  cells versus ␣ cells. Finally, glucose caused a dose-dependent suppression of the activity of the pentose phosphate pathway in  cells. In conclusion, rat  cells metabolize glucose essentially via aerobic glycolysis, whereas glycolysis in ␣ cells is largely anaerobic. The results support the view that anaplerosis is an essential pathway implicated in  cell activation by glucose.
Biochemical medicine and metabolic biology, 1990
The possible relevance of D-glucose phosphorylation by mitochondria-bound hexokinase to the control of respiration was examined in mitochondria prepared from either tumoral pancreatic islet cells (RINm5F line) or normal rat liver. In both systems, ATP generated by mitochondria exposed to ADP and succinate could serve as a substrate for the phosphorylation of D-glucose. However, after exposure to exogenous ADP in the presence of succinate, only mitochondria isolated from RINm5F cells displayed a sizeable increase in O2 consumption in response to a subsequent administration of D-glucose. In this respect, the discrepancy between mitochondria from islet cells and liver, respectively, was found to be attributable to the much lower hexokinase activity, relative to respiratory rate, in liver than in RINm5F cell mitochondria. It is speculated that the coupling between hexose phosphorylation and respiration in islet cells may prime the mitochondria to generate ATP during the early metabolic ...
Diabetologia, 1990
Smmnary. Pancreatic islets removed from adult rats injected with streptozotocin during the neonatal period display an impaired secretory response to D-glucose and, to a lesser extent, to L-leucine. Despite normal to elevated hexokinase and glucokinase activities in the islets of these glucose-intolerant animals and despite normal mitochondrial binding of the hexokinase isoenzymes, the metabolic response to a high concentration of D-glucose is severely affected, especially in terms of D-[6-14C]glucose oxidation. Thus, the ratio in D-[6-14C]glucose oxidation/D-[5-3H]glucose utilization is much less markedly increased in response to a rise in hexose concentration and, at a high concentration of D-glucose (16.7 mmol/1), less markedly decreased by the absence of Ca 2t and presence of cycloheximide in diabetic than control rats. This metabolic defect contrasts with (1) a close-to-normal or even increased capacity of the islets of diabetic rats to oxidize D-[6-14C]glucose, [2-~4C]pyruvate, L-[U-~4C]glutamine and L-[U-~4C]leucine at low, non-insulinotropic, con-centrations of these substrates; (2) a lesser impairment of the oxidation of L-[U-14C]leucine tested in high concentration (20 mmol/1), the effect of Ca 2+ deprivation upon the latter variable being comparable in diabetic and control rats; (3) an unaltered transamination of either [2-~4C]pyruvate or L-[U-14C]leucine; and (4) a modest perturbation of glycotysis. The most obvious alteration in glycolysis consists in a lesser increase of the glycolytic flux in response to a rise of D-glucose concentration in diabetic than control rats, this coinciding with an apparent decrease in affinity of glucokinase for the hexose. It is speculated that the preferential impairment of the metabolic and secretory response to D-glucose may be mainly attributable to an altered coupling between calcium accumulation and the stimulation of oxidative events in Beta-cell mitochondria of diabetic rats.
Glucose Metabolism in Goto-Kakizaki Rat Islets*
Endocrinology, 1998
Islets from Goto-Kakizaki (GK) rats from our colony, despite marked impairment of glucose-induced insulin release, used glucose and produced CO2 at a rate 3 times that of islets from control Wistar rats. Almost all glucose used was accounted for in CO2 and lactate production. The percentages of glucose carbon used collected in CO2 and lactate were similar for control and GK islets. GK islets also oxidized 40% more acetate and leucine to CO2 than did control islets. The fraction of carbon leaving the Krebs cycle relative to CO2 production was the same in GK and control islets. The capacities of mitochondria from GK islets to generate ATP from glutamate and malate were similar and that to generate ATP from succinate and rotenone was somewhat less from GK islets. The reason for the enhanced utilization of substrates by islets of the GK rat is not apparent. In conclusion, there is no decrease in islet glucose utilization, glucose oxidation, Krebs cycle function, or the electron transpor...