Fasting-induced impairment of glucose-1,6-bisphosphate synthesis in pancreatic islets

In pancreatic islets removed from rats fasted for 48 hours, the insulin secretory response to glucose is decreased. Although the activity of phosphoglucomutase is unaffected by fasting, the decrease in glucose-stimulated insulin release coincides with a suppression of the glucose-induced increment in both glucose-1,6-P2 content and lactate or pyruvate output. These findings are compatible with a regulatory role of glucose-1,6-P2 in the control of glycolysis in pancreatic islets.     

Fructose 2,6-bisphosphate in human platelets: its possible role in the control of basal and thrombin-stimulated glycolysis

Human platelets contain fructose 2,6-bisphosphate, 6-phosphofructo-l-kinase (ATP: D-Fructose-6-phosphate-1-phosphotransferase, E.C.2.7. 1.11), the rate-limiting enzyme in platelet glycolysis appear to be significantly activated by physiological concentration of the compound, suggesting for fructose 2,6-bisphosphate a key regulatory role in the control of the glycolytic flux. Incubation of human platelets with thrombin results in a parallel and rapid increase of fructose-2,6-bisphosphate levels and glycolytic flux, suggesting that the compound may also be involved in the enhancement of glycolysis elicited by the stimulating agent.     

Effect of vanadate on the glucose-1,6-bisphosphate synthase and glucose-1,6-bisphosphatase activities of phosphoglucomutase

Phosphoglucomutase, in addition to catalyzing the interconversion of glucose 1-P and glucose 6-P, catalyzes both the synthesis of glucose 1,6-P2 from glucose monophosphate and either fructose 1,6-P2 or glycerate 1,3-P2, and the hydrolysis of glucose 1,6-P2. Vanadate inhibits the mutase activity, activates the synthase activities, and does not affect the phosphatase activity. These effects suggest that the "exchange" step postulated for the phosphoglucomutase pathway is specifically inhibited by vanadate.     

Glucose-induced changes in cytosolic ATP content in pancreatic islets

The cytosolic and mitochondrial contents in ATP, ADP and AMP were measured in islets incubated for 45 min at increasing concentrations of D-glucose and then exposed for 20 s to digitonin. The latter treatment failed to affect the total islet ATP/ADP ratio and adenylate charge. D-Glucose caused a much greater increase in cytosolic than mitochondrial ATP/ADP ratio. In the cytosol, a sigmoidal pattern characterized the changes in ATP/ADP ratio at increasing concentrations of D-glucose. These findings are compatible with the view that cytosolic ATP participates in the coupling of metabolic to ionic events in the process of nutrient-induced insulin release.     

Glucose-induced, calcium-mediated protein phosphorylation in intact pancreatic islets

Conditions for studying protein phosphorylation in intact pancreatic islets were developed in order to study the effects of glucose and other effectors. Islets were incubated in Krebs-Ringer bicarbonate buffer containing 5 mM malate and 5 mM pyruvate (metabolic fuels that are not insulin secretagogues) for 150 min to permit incorporation of 32Pi into islet phosphate pools. Glucose or other effectors were then added, and the incubation was terminated after 10 to 30 min. Glucose increased phosphorylation of four islet peptides with molecular weights of 20,000, 33,000, 43,000 and 57,000. The calcium channel blockers, verapamil and D-600, inhibited phosphorylation of each of the four proteins, and trifluoperazine inhibited phosphorylation of the proteins with molecular weights of 20,000 and 57,000. The results indicate that glucose-induced insulin release may be mediated in part by protein phosphorylation, and that calcium may act as an intracellular messenger in coupling the glucose stimulus to the secretory process.     

Effects of epinephrine on glucose-1,6-bisphosphate and carbohydrate metabolism in skin

1. Injection of epinephrine induced in skin a decrease in the level of glucose-1,6-bisphosphate (Glc-1,6-P2), which was accompanied by correlated changes in the activities of several enzymes which are modulated by this regulator. 2. These effects were blocked by the alpha adrenergic blocker phentolamine, in contrast to muscle where the hormone increases Glc-1,6-P2, acting through beta receptors. 3. The changes in the enzymes' activities, as well as in glycogen and lactate content induced by epinephrine, reveal that the hormone causes, in skin, a stimulation of glycogenolysis and glycolysis, as well as an acceleration of pentose phosphate pathway. 4. The reduction in glycogen content induced by epinephrine, was blocked by the beta adrenergic blocker propranolol, whereas the hormone's effects on the other processes were mainly mediated through alpha receptors.     

A possible role for glucose metabolites in the regulation of inositol-1,4,5-trisphosphate 5-phosphomonoesterase activity in pancreatic islets

Rat pancreatic islets demonstrate inositol-1,4,5-trisphosphate 5-phosphomonoesterase activity which is 3 times higher than that in the exocrine pancreas. This enzyme has several features in common with the erythrocyte and hepatocyte enzymes: it is located primarily in the plasma membrane, it has a similar Km for inositol trisphosphate (IP3) (16 microM), and it requires Mg2+. The activity of the islet enzyme is inhibited by several diphosphorylated glucose metabolites: 2,3-bisphosphoglycerate, fructose 1,6-bisphosphate, fructose 2,6-bisphosphate, and glucose 1,6-bisphosphate. Monophosphorylated and unphosphorylated metabolites have little or no effect on its activity. Several reports show that stimulation of islets with glucose raises the concentrations of various glucose metabolites including fructose 1,6-bisphosphate, glucose 1,6-bisphosphate, and 2,3-bisphosphoglycerate to concentrations that are in the range that inhibit the islet inositol-1,4,5-trisphosphate 5-phosphomonoesterase. Other reports show that IP3 mobilizes calcium when added to permeabilized insulin-secreting cells. It is possible that the increase in cytosolic calcium known to occur during glucose-induced insulin secretion may be sustained in part by higher IP3 levels resulting from the inhibition of inositol-1,4,5-trisphosphate 5-phosphomonoesterase by some of the diphosphorylated glucose metabolites.     

Glucose-induced mobilisation of intracellular Ca2+ in depolarised pancreatic islets

Perifused rat pancreatic islets, prelabelled with ⁴⁵Ca, were exposed for 90 min to a medium containing 30 mM K⁺, 0.25 mM diazoxide and 0.5 mM EGTA, but deprived of CaCl₂. Either verapamil (0.05 mM) or Cd²⁺ (0.05 mM) were also present in the perifusate. Under these conditions a rise in D-glucose concentrations from either 2.8 to 16.7 mM or zero to 8.3 mM increased both ⁴⁵Ca outflow and insulin release, after an initial and transient decrease in effluent radioactivity. These findings suggest that, in islets depolarised by exposure to a high extracellular concentration of K⁺, D-glucose provokes an intracellular redistribution of Ca²⁺ ions and subsequent stimulation of insulin release. The functional response to D-glucose is apparently not attributable to either the closing of ATP-sensitive K⁺ channels, which were actually activated by diazoxide, or stimulation of Ca²⁺ influx, which was prevented by the absence of extracellular Ca²⁺. The present experimental design thus reveals a novel component of the glucose-induced remodelling of Ca²⁺ fluxes in islet cells. Such an effect might also be operative under physiological conditions, when the hexose leads to depolarisation of the islet B-cells.     

Hexose metabolism in pancreatic islets: The glucose-6-phosphatase riddle

Summary Glucose-6-phosphatase activity was measured in rat liver or pancreatic islet crude homogenates and microsomes. The data recorded in the liver were comparable to those reported in prior studies. However, in the islets, the hydrolysis of D-glucose 6-phosphate by disrupted microsomes represented, when expressed relative to the protein content, less than 2% of the value recorded in liver microsomes. Moreover, no phosphotransferase activity was detected in the islets. These findings impose reservation on both the presence of glucose-6-phosphatase in rat islets and its participation to stimulus-secretion coupling.     

Enzyme-to-enzyme channelling in the early steps of glycolysis in rat pancreatic islets

The metabolism of D-glucose displays anomeric specificity in rat pancreatic islets. The aim of the present report is to investigate whether such a situation implies enzyme-to-enzyme tunnelling of metabolites in the early steps of glycolysis. For such a purpose, the modelling of alpha- and beta-D-glucose catabolism, itself based on available information concerning both the utilisation of these two anomers and the intrinsic properties of phosphoglucoisomerase, was first examined. According to a theoretical model with enzyme-to-enzyme channelling, the generation of 3HOH from D-[2-3H]glucose should be higher in islets exposed to beta-D-glucose rather than alpha-D-glucose, whilst the opposite situation should prevail in the case of D-[5-3H]glucose conversion to 3HOH. Experimental data collected in rat islets incubated for 60 min at 4 degrees C in the presence of either alpha- or beta-D-glucose mixed with tracer amounts of either alpha- or beta-D-[2- 3H]glucose and alpha- or beta-D-[5-3H]glucose indicate that the beta/alpha ratio for D-[2-3H]glucose conversion to 3HOH is indeed higher than the beta/alpha ratio for D-[5-3H]glucose conversion to 3HOH. These findings are consistent with the postulated enzyme-to-enzyme tunnelling of glycolytic intermediates between hexokinase isoenzyme(s), phosphoglucoisomerase and, possibly, phosphofructokinase.     

Anomeric dissociation between glucokinase activity and glycolysis in pancreatic islets

In pancreatic islet homogenates incubated in the presence of a high glucose concentration (40 mM), the beta-anomer of D-glucose is phosphorylated at a higher rate than the alpha-anomer, whether in the absence or presence of exogenous glucose 6-phosphate. However, in intact islets also exposed to 40 mM D-glucose, the production of 3H2O from D-[5-3H] glucose, the oxidation of D-[U-14C] glucose and the glucose-induced increment in either lactate production or 45Ca net uptake, as well as the release of insulin from isolated perfused pancreases, are not higher with beta- than alpha-D-glucose. It is concluded that the rate of glucose utilization by islet cells is not regulated solely by the activity of hexokinase and/or glucokinase.     

Complementarity in the regulation of phosphoglucomutase, phosphofructokinase and hexokinase; the role of glucose 1,6-bisphosphate.

ATP and citrate, the well known inhibitors of phosphofructokinase (ATP: D-fructose 6-phosphate 1-phosphotransferase, EC 2.7.1.11), were found to inhibit the activities of the multiple forms of phosphoglucomutase (alpha-D-glucose 1,6-bisphosphate: alpha-D-glucose 1-phosphate phosphotransferase, EC 2.7.5.1) from rat muscle and adipose tissue. This inhibition could be reversed by an increase in the glucose 1,6-bisphosphate (Glc-1,6-P2) concentration. Other known activators (deinhibitors) of phosphofructokinase, viz. cyclic AMP, AMP, ADP or Pi, had no direct deinhibitory action on the ATP or citrate inhibited multiple phosphoglucomutases. Cyclic AMP and AMP, could however lead indirectly to deinhibition of the phosphoglucomutases, by activating phosphofructokinase which catalyzes the ATP-dependent phosphorylation of glucose 1-phosphate to form Glc-1,6-P2, the la-ter then released the multiple phosphoglucomutases from ATP or citrate inhibition. The Glc-1,6-P2 was also found to exert a selective inhibitory effect on hexokinase (ATP: D-hexose 6-phosphotransferase, EC 2.7.1.1) type II, the predominant form in skeletal muscle. This selective inhibition by Glc-1,6-P2 was demonstrated on the multiple hexokinases which were resolved by cellogel electrophoresis or isolated by chromatography on DEAE-cellulose. Based on the in vitro studies it is suggested that during periods of highly active epinephrine-induced glycogenolysis in muscle, the Glc-1,6-P2, produced by the cyclic AMP-stimulated reaction of phosphofructokinase with glucose 1-phosphate, will release the phosphoglucomutases from ATP or citrate inhibition, and will depress the activity of muscle type II hexokinase.     

Glucose-induced insulin mRNA accumulation is impaired in islets from neonatal streptozotocin-treated rats.

According to the glucose toxicity hypothesis, hyperglycemia contributes to defective beta-cell function in type 2, non-insulin-dependent diabetes mellitus. This concept is supported by substantial data in rodent models of diabetes. However, the ability of glucose to stimulate the accumulation of insulin mRNA, a critical feature of normal beta-cell physiology, has not been investigated in in vivo models of chronic hyperglycemia. The aim of this study was to determine whether glucose-induced insulin mRNA accumulation is impaired in the neonatal streptozotocin-treated rat (n0-STZ rat), a model of non-obese, non-insulin-dependent diabetes mellitus. Islets of Langerhans isolated from n0-STZ and control rats were cultured for 24 h in the presence of 2.8 or 16.7 mmol/L glucose, and insulin mRNA levels were measured by Northern analysis. Insulin mRNA levels were increased more than twofold by glucose in control islets. In contrast, no significant effect of glucose was found on insulin mRNA levels in n0-STZ islets. We conclude that insulin gene regulation by glucose is impaired in n0-STZ rat islets.     

Calcium and glycolysis mediate multiple bursting modes in pancreatic islets

Pancreatic islets of Langerhans produce bursts of electrical activity when exposed to stimulatory glucose levels. These bursts often have a regular repeating pattern, with a period of 10-60 s. In some cases, however, the bursts are episodic, clustered into bursts of bursts, which we call compound bursting. Consistent with this are recordings of free Ca2+ concentration, oxygen consumption, mitochondrial membrane potential, and intraislet glucose levels that exhibit very slow oscillations, with faster oscillations superimposed. We describe a new mathematical model of the pancreatic beta-cell that can account for these multimodal patterns. The model includes the feedback of cytosolic Ca2+ onto ion channels that can account for bursting, and a metabolic subsystem that is capable of producing slow oscillations driven by oscillations in glycolysis. This slow rhythm is responsible for the slow mode of compound bursting in the model. We also show that it is possible for glycolytic oscillations alone to drive a very slow form of bursting, which we call "glycolytic bursting." Finally, the model predicts that there is bistability between stationary and oscillatory glycolysis for a range of parameter values. We provide experimental support for this model prediction. Overall, the model can account for a diversity of islet behaviors described in the literature over the past 20 years.