Rise In D-glucose Concentration Research Articles

Tritiated Taurine Handling by Isolated Rat Pancreatic Islets

A gating of volume-sensitive anion channels may participate in the depolarization of the plasma membrane caused by high concentrations of D-glucose in insulin-producing B-cells of the endocrine pancreas. The efflux of tritiated taurine from prelabeled cells is currently used to assess changes in the activity of such channels. The handling of [1,2-3H]taurine by isolated rat pancreatic islets was therefore investigated. The net uptake of [1,2-3H]taurine was found to represent a concentration-, time-, and temperature-dependent process. It was progressively increased in the range of D-glucose concentrations between 2.8 and 8.3 mM, but no further increase was observed at 16.7 mM D-glucose. Over 15 min incubation, the efflux of radioactivity from prelabeled islets was inhibited by MK571 (1.0 mM). It was increased in response to hypoosmolarity both in the presence and absence of extracellular Na+. Whether in salt-balanced or Na+-deprived media, the efflux of radioactivity from prelabeled islets increased in response to a rise in D-glucose concentration from 2.8 to 5.6 or 8.3 mM, but decreased when the concentration of the hexose was further increased from 8.3 to 16.7 mM. In perifused islets, however, the radioactive efflux from prelabeled islets was inhibited, in a concentration- related manner, when islets first deprived of D-glucose for 45 min were then exposed to 2.8, 5.6, or 16.7 mM D-glucose. Likewise, in prelabeled and perifused islets first exposed for 45 min to 4.0 mM D-glucose, a later rise in hexose concentration to 8.3 mM failed to affect significantly effluent radioactivity, while an increase in hexose concentration from 4.0 to 16.7 mM inhibited the radioactive outflow. In these perifusion experiments, the rise in D-glucose concentration provoked the expected changes in insulin output. The findings obtained in islets examined immediately after preincubation in the presence of [1,2-3H]taurine are consistent with the presence of volume-sensitive anion channels in islet cells and with a gating of such channels in response to a rise in D-glucose concentration from 2.8 to 5.6-8.3 mM. However, the radioactive fractional outflow rate from prelabeled islets seems to reach its highest value at about 8.3 mM D-glucose, being unexpectedly decreased at a higher concentration (16.7 mM) of the hexose. In conclusion, the pleiotropic effects of D-glucose upon tritiated taurine outflow from prelabeled rat islets, which could conceivably be ascribed to differences in the handling of this amino sulfonic acid by distinct islet cell types, indicates that the present approach is far from optimal to characterize unambiguously the regulation by the hexose of volume-sensitive anion channel activity in insulin-producing islet cells.

Pancreatic Islet Function in ω3 Fatty Acid-Depleted Rats: Glucose Metabolism and Nutrient-Stimulated Insulin Release

In order to gain information on the determinism of the perturbation of fuel homeostasis in situations characterized by a depletion in long-chain polyunsaturated omega3 fatty acids (omega3), the metabolic and hormonal status of omega3-depleted rats (second generation) was examined. When required, these rats were injected intravenously 120 min before sacrifice with a novel medium-chain triglyceride-fish oil emulsion able to provoke a rapid and sustained increase of the omega3 content in cell phospholipids. The measurement of plasma glucose, insulin, phospholipid, triglyceride, and unesterified fatty acid concentration indicated modest insulin resistance in the omega3-depleted rats. The plasma triglyceride and phospholipid concentrations were decreased in the omega3-depleted rats with abnormally low contribution of omega3 in both circulating and pancreatic islet lipids. The protein, insulin, and lipid content of the islets, as well as their intracellular and extracellular spaces, were little affected in the omega3-depleted rats. The metabolism of D-glucose in the islets of omega3-depleted rats was characterized by a lesser increase in D-[5-3H]glucose utilization and D-[U-14C]glucose oxidation in response to a given rise in hexose concentration and an abnormally low ratio between D-glucose oxidation and utilization. These abnormalities could be linked to an increased metabolism of endogenous fatty acids with resulting alteration of glucokinase kinetics. The release of insulin evoked by D-glucose, at a close-to-physiological concentration (8.3 mM), was increased in the omega3-depleted rats, this being considered as consistent with their insulin resistance. Relative to such a release, that evoked by a further rise in D-glucose concentration or by non-glucidic nutrients was abnormally high in omega3-depleted rats, and restored to a normal level after of the intravenous injection of the omega3-rich medium-chain triglyceride-fish oil emulsion. Because the latter procedure failed to correct the perturbation of D-glucose metabolism in the islets of omega3-depleted rats, it is proposed that the anomalies in the secretory behaviour of islets in terms of their response to an increase in hexose concentration or non-nutrient secretagogues is mainly attributable to alteration in K+ and Ca2+ handling, as indeed recently documented in separate experiments.

Anomeric specificity of d-[U- 14C]glucose incorporation into glycogen in rat hemidiaphragms

The anomeric specificity of d-[U- 14C]glucose incorporation into glycogen in rat hemidiaphragms was investigated. For this purpose, the hemidiaphragms were preincubated for 30 min at 37 °C and then incubated for 5 min at the same temperature in the presence of α- or β- d-[U- 14C]glucose. The concentrations of d-glucose (5.6 or 8.8 mM) and insulin (0 or 10 mU/ml) were identical during the preincubation and incubation periods. The incubation medium was prepared in D 2O/H 2O (3:1, v/v) in order to delay the interconversion of the d-glucose anomers. In addition to glycogen labelling, the output of radioactive acidic metabolites was also measured. Insulin caused a preferential stimulation of glycogen labelling relative to glycolysis. Such was not the case in response to a rise in d-glucose concentration. At 5.6 mM d-glucose and whether in the presence or absence of insulin, both glycogen labelling and glycolysis were lower with α- d-glucose than with β- d-glucose suggesting a higher rate of β- d-glucose than α- d-glucose transport across the plasma membrane. A mirror image was found at 8.8 mM d-glucose, especially in the absence of insulin. At this close-to-physiological hexose concentration, insulin lowered the α/β ratio for glycogen labelling. On the contrary, the rise in d-glucose concentration increased such a ratio. Since such a rise is probably little affected by any possible anomeric difference in d-glucose transport across the plasma membrane, the present results strongly suggest that the intracellular factors regulating net glycogen synthesis, as well as glycolytic flux, display obvious preference for α- d-glucose.

Stimulation of proinsulin biosynthesis by K+ in rat pancreatic islets.

Glucose-stimulated proinsulin biosynthesis is regulated mainly at the translational level. This study aims at investigating the possible role of the B-cell K+ content in such a process. In order to increase the islet cells K+ content, rat pancreatic islets exposed to a low D-glucose concentration (e.g., 2.5 mM) were incubated in the presence of 30 or 60 mM K+, as distinct from a control extracellular K+ concentration of 5 mM. Under these conditions, the K+ content of the islets, as judged from the net uptake of 86Rb+ over 60 min incubation, was increased to a level comparable to that otherwise found in the presence of 16.7 mM D-glucose. In the presence of 2.5-4.0 mM D-glucose, the rise in K+ concentration from 5 to 30 and 60 mM caused a progressive increase in the incorporation of L-[4-3H]phenylalanine into both all islet peptides and (pro)insulin. A preferential stimulation of proinsulin biosynthesis was only observed in islets incubated at 60 mM K+ in the presence of 4.0 mM D-glucose. In relative terms, the K+-induced increase in biosynthetic variables was less pronounced, however, than that otherwise evoked by a rise in D-glucose concentration from 2.5 to 4.0 mM to 5.6 or 16.7 mM. These findings may suggest that the effect of D-glucose to increase the K+ content of islet cells represents one modality for coupling a rise in D-glucose concentration to stimulation of proinsulin biosynthesis.

Immediate and delayed effects of D-fructose upon insulin, somatostatin, and glucagon release by the perfused rat pancreas.

Novel information was recently provided concerning the reciprocal effects of D-glucose and D-fructose upon their respective metabolism in rat pancreatic islets. In the light of such findings, this study aims at comparing the effects of D-glucose and D-fructose on insulin, somatostatin, and glucagon release from the isolated perfused rat pancreas. A rise in D-glucose concentration from 3.3 to 5.0 or 7.3 mM or the administration of D-fructose (17 and 40 mM) in the presence of 3.3 mM D-glucose stimulated insulin release in a concentration-related manner, but failed to affect somatostatin output. The secretion of glucagon was decreased in all cases. The secretory response to L-arginine (5 mM), 25 min after restoring the basal concentration of D-glucose, was more markedly affected, in terms of potentiation of insulin and somatostatin release and reduction of glucagon output, after prior administration of D-fructose than after a prior increase in D-glucose concentration. These findings argue against any major role for a paracrine regulation of hormonal release and, instead, are consistent with a causal link between metabolic and secretory events in the islet cells. Nevertheless, the present results emphasize differences in the response of distinct pancreatic endocrine cell types to the same or distinct hexoses.

Labelling of lipids by D-[1-14C]glucose, D-[6-14C] glucose and D-[3-3H]glucose in pancreatic islets from normal and GK rats.

In pancreatic islets prepared from either normal or GK rats and incubated at either low (2.8 mM) or high (16.7 mM) D-glucose concentration, the labelling of both lipids and their glycerol moiety is higher in the presence of D-[1-14C]glucose than D-[6-14C]glucose. The rise in D-glucose concentration augments the labelling of lipids, the paired 14C/3H ratio found in islets exposed to both D-[1-14C]glucose or D-[6-14C]glucose and D-[3-3H]glucose being even slightly higher at 16.7 mM D-glucose than that found, under otherwise identical conditions, at 2.8 mM D-glucose. Such a paired ratio exceeds unity in islets exposed to D-[1-14C]glucose. The labelling of islet lipids by D-[6-14C]glucose is about 30 times lower than the generation of acidic metabolites from the same tracer. These findings indicate (i) that the labelling of islet lipids accounts for only a minor fraction of D-glucose catabolism in pancreatic islets, (ii) a greater escape to L-glycerol-3-phosphate of glycerone-3-phosphate generated from the C1-C2-C3 moiety of D-glucose than D-glyceraldehyde-3-phosphate produced from the C4-C5-C6 moiety of the hexose, (iii) that only a limited amount of [3-3H]glycerone 3-phosphate generated from D-[3-3H]glucose is detritiated at the triose phosphate isomerase level before being converted to L-glycerol-3-phosphate, and (iv) that a rise in D-glucose concentration results in an increased labelling of islet lipids, this phenomenon being somewhat more pronounced in the case of D-[1-14C]glucose or D-[6-14C]glucose rather than D-[3-3H]glucose.

Altered metabolic priming by D-glucose in pancreatic islets from Goto-Kakizaki rats.

The possible priming by D-glucose of metabolic events in islets from control rats and Goto-Kakizaki rats (GK rats) was investigated by first incubating the islets for 120 min either in the absence of any exogenous nutrient or presence of 16.7 mM D-glucose. The islets were then incubated for a second period of 120 min either at 2.8 mM or 16. 7 mM D-glucose, the hexose being now mixed with tracer amounts of D-[U-14C]glucose and D-[5-3H]glucose. In islets from control rats first incubated in the absence of exogenous nutrient the hierarchy in the 16.7 mM/2.8 mM ratio for metabolic variables was as follows: D-[U-14C]glucose oxidation > D-[5-3H]glucose utilization and D-[U-14C]glucose conversion to amino acids > D-[U-14C]glucose conversion to acidic metabolites. When the islets from control rats were first incubated in the presence of 16.7 mM D-glucose, the preferential stimulation of mitochondrial oxidative events at high hexose concentration, as documented by the increase in the paired ratio between D-[U-14C]glucose oxidation and D-[5-3H]glucose utilization, was further enhanced. The 16.7 mM/2.8 mM ratio for the conversion of D-[U-14C]glucose to amino acids, relative to that for D-[U-14C]glucose conversion to acidic metabolites, was much lower, however, after a first incubation in the presence of D-glucose, rather than in its absence, probably as a result of the progressive exhaustion of endogenous amino acids considered as transamination partners. The major differences between these results and those obtained in islets from GK rats consisted, in the latter animals, in i) higher absolute values for all metabolic fluxes, ii) lower 16.7 mM/ 2.8 mM ratios, iii) lower paired ratio between D-[U-14C]glucose oxidation and D-[5-3H]glucose utilization, and iv) absence of a priming effect of D-glucose (16.7 mM) upon such a paired ratio in the islets incubated at 16.7 mM D-glucose during the second incubation. Taken as a whole, these observations confirm that the preferential stimulation of mitochondrial oxidative events, in response to a rise in D-glucose concentration, is impaired in islets from GK rats and extend this knowledge to the priming action of D-glucose, in high concentration, on the catabolism of the hexose during a subsequent incubation.

Effects of high extracellular K + concentrations, diazoxide and/or Ca 2+ deprivation upon d-glucose metabolism in pancreatic islets

A rise in d-glucose concentration may augment insulin release independently of changes in K + conductance or Ca 2+ influx in pancreatic islet cells, the insulinotropic action of the hexose remaining dependent on an increased generation of high-energy phosphates. In the present study, therefore, it was investigated to which extent the procedures currently used to assess the modalities of the secretory response to d-glucose independent of its effect on ATP-sensitive K + channels and Ca 2+ inflow may themselves affect the catabolism of the hexose in isolated rat pancreatic islets. A rise in the extracellular K + concentration from 5 to 30 or 60 mM failed to significantly affect the metabolism of d-glucose. At 90 mM K +, however, the maximal velocity of the glycolytic flux was decreased and the apparent K m for d-glucose lowered, without an obvious alteration of the preferential stimulation of oxidative mitochondrial events in response to a rise in d-glucose concentration. Such a preferential stimulation was abolished, however, either by diazoxide at a low, but not high, K + concentration or by Ca 2+ deprivation, in the absence or presence of diazoxide, at a high K + concentration. It is speculated that these metabolic changes may be attributable, in part at least, to an altered activity of key cytosolic (e.g. pyruvate kinase) and mitochondrial (e.g. FAD-linked glycerophosphate dehydrogenase) enzymes.

Verapamil- and Cadmium-Resistant Stimulation of Calcium Uptake and Insulin Release by d-Glucose in Depolarised Pancreatic Islets Exposed to Diazoxide

The possible activation by d-glucose of voltage-insensitive Ca 2+ channels in rat pancreatic islets was investigated by measuring the release of insulin, as well as the net uptake and efflux of 45Ca, in islets incubated in the presence of 30 mM K +, 0.25 mM diazoxide and either 0.05 mM verapamil or 0.04 mM Cd 2+. In the presence of verapamil, d-glucose (16.7 mM) stimulated, over 90-min incubation, both 45Ca net uptake and insulin release. Likewise, in prelabelled perifused islets, a rise in d-glucose concentration from 2.8 to 16.7 mM augmented both 45Ca outflow and insulin output. When the concentration of the hexose was raised from zero to 8.3 mM, in which case the glucose-induced stimulation of Ca 2+ influx is thought to occur mainly at the intervention of voltage-insensitive Ca 2+ channels, and when Cd 2+ was used to selectively block L-type Ca 2+ channels, a stimulation of both 45Ca efflux and insulin release was again observed in islets perifused, at high extracellular K + concentration, in the presence of diazoxide. These findings support the view that, in depolarised islets in which the ATP-sensitive K + channels are fully activated by diazoxide, the stimulation of insulin release by d-glucose might involve the gating of voltage-insensitive Ca 2+ channels.

Failure of human and mouse leptin to affect insulin, glucagon and somatostatin secretion by the perfused rat pancreas at physiological glucose concentration

In isolated perfused pancreas from normal rats, a rise in d-glucose concentration from 3.3 to 8.3 mM provoked a rapid phasic stimulation of both insulin and somatostatin secretion and rapid fall in glucagon output, these changes being reversed when the concentration of the hexose was brought back to its initial low level. In the presence of 8.3 mM d-glucose, the administration of either human or mouse leptin (10 nM in both cases) for 15 min failed to affect significantly the perfusion pressure and release of the three hormones. It is concluded that leptin does not exert any major immediate and direct effect upon pancreatic insulin, glucagon and somatostatin secretion, at least at the physiological concentration of d-glucose normally found in the plasma of fed rats.

Energy-Dependent Intracellular Translocation of Glucokinase in Rat Pancreatic Islets

It was recently reported that hyperglycemia provokes a rapid and sustained translocation of glucokinase in rat pancreatic B-cells, and it was speculated that this may be associated with enhancement of its catalytic activity, as possibly attributable to the mitochondrial binding of the enzyme. In the present work, the activities of both hexokinase and glucokinase were measured in particulated and cytosolic subcellular fractions prepared from islets first incubated for 60 min either in the absence of exogenous nutrient or in the presence ofd-glucose, tested at both low (2.8 mmol/L) and high (16.7 mmol/L) concentrations. The relative contribution of the cytosolic domain to the total activity of glucokinase recovered in the two subcellular fractions was higher in islets deprived of exogenous nutrient than in islets first exposed to 2.8 or 16.7 mmol/Ld-glucose, the results obtained at each of the latter two hexose concentrations being comparable to one another. The subcellular distribution of hexokinase, however, was not significantly different in islets deprived ofd-glucose or exposed to the hexose. These findings are interpreted as indicative of an energy-dependent translocation of glucokinase in the B-cell, distinct from the redistribution of the enzyme occurring in response to a rise ind-glucose concentration above its physiological value.

GLP-1 receptors in golden Syrian hamster islets: identification and functional characterization.

This study aims at the identification and functional characterization of glucagon-like peptide 1 (7-36) amide (GLP-1) receptor in islets from Golden Syrian hamsters. Using a polyclonal antibody against rat GLP-1 receptors, Western blotting of the islet proteins revealed two major bands of 44 and 70 kDa, similar to those found in rat islets, RINm5F cells, and HIT-T15 cells. In Northern blots, transcripts of 2.7, 3.6 and 3.7 kb were observed in rat islets and RINm5F cells after hybridization with rat GLP-1 receptor cDNA probes of either 21 9 bp or 1.5 kb. Such was not the case in either hamster islets or HIT-T15 cells. However, a single 3.6-kb transcript was observed in the latter two cases when a human GLP-1 receptor cDNA probe of 1.6 kb was used for hybridization. In the isolated perfused pancreas of Golden Syrian hamsters, a rise in D-glucose concentration from 3.3 to 8.3 mM caused a biphasic stimulation of insulin release, which was further increased by either GLP-1 or glucagon (10(-9)M each). The enhancing action of GLP-1 on glucose-stimulated insulin secretion was much more marked than that of glucagon. The rise in D-glucose concentration decreased by 46+/-4% the release of glucagon, but GLP-1 failed to exert any obvious effect on glucagon secretion in the presence of 8.3 mM D-glucose. These results indicate that GLP-1 receptors are expressed in islets of Golden Syrian hamsters with an extracellular part possessing the same immunoreactivity as the rat islet GLP-1 receptors. The expression of the mRNA for the GLP-1 receptor differs, however, from that found in rat or human islets.

Cationic Events Stimulated by D-Glucose in Depolarized Islet Cells

Pancreatic islets prelabelled with either 86Rb or 45Ca were exposed to a rise in D-glucose concentration from 2.8 to 16.7 mM whilst perifused in the presence of 2 μM glibenclamide, 30 mM extracellular K + and both 30 mM K + and 250 μM diazoxide. In all three situations, the rise in glucose concentration provoked a dramatic increase in insulin output, despite unchanged or even increased efflux of 86Rb from the prelabelled islets. Also in all three situations, glucose sharply decreased effluent radioactivity from islets prelabelled with 45Ca but perifused in the absence of extracellular Ca 2+, whilst augmenting 45Ca efflux, to a variable extent, from islets perifused at normal extracellular Ca 2+ concentration (1.0 mM). It is proposed, therefore, that the insulinotropic action of D-glucose in depolarized islets, and presumably also under normal conditions, may involve the gating of voltage-insensitive Ca 2+ channels.

D-Glucose Metabolism in BRIN-BD11 Islet Cells

A novel insulin-secreting cell line, BRIN-BD11, was recently established following electrofusion of RINm5F cells with NEDH rat pancreatic islet cells. In the present study,d-glucose metabolism was compared in BRIN-BD11 and RINm5F cells. The concentration dependency ofd-[5-3H]glucose utilization displayed a comparable pattern in the two cell lines, but the absolute values were lower in BRIN-BD11 than RINm5F cells. Except in the case ofd-[1-14C]glucose, the ratio between14C labeledd-glucose oxidation andd-[5-3H]glucose utilization was higher, however, in BRIN-BD11 than RINm5F cells. Moreover, BRIN-BD11 cells were less affected than RINm5F cells by a rise ind-glucose concentration, in terms of the inhibitory action of the hexose upon oxidative variables, such as oxidative glycolysis, pyruvate decarboxylation, and oxidation of glucose-derived acetyl residues in the Krebs cycle. The total energy yield fromd-glucose catabolism appeared similar, however, in BRIN-BD11 and RINm5F cells. These findings extend the knowledge that BRIN-BD11 cells display an improved metabolic and secretory behavior, when considering the difference otherwise found between normal and tumoral islet cells.

D-Glucose Metabolism in Cross-Linked Pancreatic Islets

D-glucose metabolism was investigated in pancreatic islets that underwent cross-linking of intracellular proteins by dimethyl suberimidate. Both the utilization of D-[5-3H]glucose and oxidation of D-[U-14C]glucose were much more severely affected at high (16.7 mM) than at low (2.8 mM) concentration of the hexose, when comparing cross-linked to control islets. The preferential stimulation of D-[U-14C]glucose oxidation relative to D-[5-3H]glucose utilization, resulting from an increase in hexose concentration, was not abolished, however, in cross-linked islets. Likewise, the activation of phosphofructokinase in glucose-stimulated islets did not appear to be impaired in cross-linked islets, the islet content in tritiated acidic metabolites generated from D-[5-3H]glucose failing to be increased in cross-linked islets. It is proposed, therefore, that the impaired responsiveness of cross-linked islets to a rise in D-glucose concentration might be attributable to an altered regulation of glucokinase, as possibly resulting from a missing interaction of the enzyme with GLUT-2.

Dual effect of formycin a upon the hydrolysis of phosphoinositides in perifused pancreatic islets

Formycin A (1.0 mM) caused a rapid, sustained and rapidly reversible inhibition of effluent radioactivity in rat pancreatic islets prelabelled with myo-[2- 3H]inositol and perifused in the presence of 8.3 mM d-glucose. This coincided with a progressive decrease in islet ATP content and transient inhibition of insulin release. Thereafter, however, formycin A increased glucose-induced insulin release. Moreover, in islets that were preincubated with myo-[2- 3H]inositol and then exposed during perifusion to a rise in d-glucose concentration from 2.8 to 16.7 mM, the release of insulin and 3H fractional outflow rate at both the low and high hexose concentrations were much higher when both the preincubation and perifusion were conducted in the presence, rather than absence, of formycin A. It is concluded that formycin A first inhibits and later enhances both the hydrolysis of phosphoinositides and release of insulin, these effects being possibly related to changes in the islet cell content of adenosine and/or formycin A triphosphates.

Metabolic, ionic, and secretory response to D-glucose in islets from rats with acquired or inherited non-insulin-dependent diabetes.

The metabolic, ionic, and secretory response to D-glucose was investigated in islets of adult rats either injected with streptozotocin during the neonatal period (STZ rats) or presenting with inherited diabetes (GK rats). At a high concentration of D-glucose (16.7 mM), the ATP/ADP ratio was lower in islets from STZ and GK than control rats. This coincided with an impaired response of perifused islets to a rise in D-glucose concentration in terms of stimulation of insulin release, suppression of effluent radioactivity from islets prelabeled with [2-3H]adenosine, reduction in 86Rb efflux, and induction of a phosphate flush in islets prelabeled with 32P(i). The ratio in either D-[5-3H]glucose utilization or D-[2-14C]glucose oxidation at high/low hexose concentration, as well as the paired ratio between D-[2-14C]glucose oxidation and D-[5-3H]glucose utilization in islets incubated at a high concentration of the hexose, was also lower in STZ and GK rats than in control rats. Such was not the case, however, from the oxidation of [2-14C]pyruvate. Instead, the latter 2-keto acid, when tested at a 5.0 mM concentration, improved more efficiently the overall oxidative response of the islets to a rise in D-glucose concentration in STZ and GK rats than in control animals. It is proposed, therefore, that in both STZ and GK rats, the B-cell secretory defect is primarily attributable to an anomaly in oxidative glycolysis. In islets exposed to a high concentration of D-glucose, this metabolic deficiency results in impaired ATP generation, altered closing of ATP-responsive K+ channels, and, hence, diminished insulin output.

Long-term regulation of pancreatic B-cell responsiveness to d-glucose by food availability, feeding schedule, and diet composition

The immediate metabolic, cationic, and secretory response of the insulin-producing B-cell to d-glucose is regulated, in a delayed or long-term manner, by nutritional factors such as food availability, feeding schedule, or diet composition. The B-cell keeps the memory of these nutritional manipulations so that the corresponding changes in its responsiveness to d-glucose can be documented in vitro in isolated pancreatic islets. The results of experiments conducted in starved rats, in animals exposed to an altered feeding schedule, and in rats given free access to a high-carbohydrate, high-protein, or high-lipid, as distinct from balanced, diet all suggest that a sufficient prandial hyperglycemia is essential for maintenance of an optimal metabolic and secretory behavior of the islet B-cell in response to a rise in d-glucose concentration.

Hexose metabolism in pancreatic islets. Ca(2+)-dependent activation of the glycerol phosphate shuttle by nutrient secretagogues.

A method is proposed for the measurement of the flux through the glycerol phosphate shuttle in pancreatic islets. Such a flux is taken as the ratio between the production of 3HOH and the specific radioactivity of L-[2-3H]glycerophosphate in islets exposed to [2-3H]glycerol. D-Glucose and non-glucidic nutrient secretagogues, such as 2-ketoisocaproate and 2-aminobicyclo[2,2,1]heptane-2-carboxylate, stimulate, in a Ca(2+)-dependent manner, circulation in the glycerol phosphate shuttle. The shuttle flux is commensurate with the fraction of pyruvate generation which is not coupled with L-lactate production. These findings support the view that a rise in D-glucose concentration leads to activation of the FAD-linked mitochondrial glycerophosphate dehydrogenase through an increase in cytosolic Ca2+ concentration.

Study of hexose transport, glycerol phosphate shuttle and Krebs cycle in islets of adult rats injected with streptozotocin during the neonatal period

At 3–4°C, the transport of 3-O- methyl- d - glucose (30 mM) was severely impaired in islets prepared from adult rats injected with streptozotocin during the neonatal period. However, at 37°C, the first and second phase of glucose-stimulated insulin release were decreased to the same relative extent in perifused islets of diabetic, as compared to control, animals. Moreover, the time-related increase in the oxidative response of the islets to 16.7 mM d-glucose was less pronounced in diabetic than control rats. The activity of the mitochondrial FAD-linked glycerophosphate dehydrogenase in islet homogenates of diabetic rats only represented one-fifth of that found in control rats, whereas the activity of the cytosolic NAD-glycerophosphate dehydrogenase was comparable in both types of rats. This coincided with the fact that a rise in d-glucose concentration from 2.8 to 16.7 mM failed to increase significantly l-[2- 3H]glycerol conversion to 3HOH in islets from diabetic rats, in contrast to the situation found in control animals. The activity of 2-ketoglutarate dehydrogenase in islet homogenates when expressed per μg protein was not different in control and diabetic rats. Likewise, the ratio between d-[6− 14C]glucose oxidation and d-[3,4- 14C]glucose oxidation and the capacity of either a non-metabolized analog of l-leucine or 3-phenylpyruvate to preferentially stimulated d-[6− 14C]glucose oxidation relative to d-[5- 3H]glucose utilization were both unaffected in islets from diabetic rats. These findings argue against the existence of a primary defect in the Krebs cycle of diabetic rats. It is proposed that, despite an obvious alteration of the hexose transport system in the islet cells of diabetic rats, the preferential impairment of the B-cell secretory response to d-glucose, as distinct from other secretagogues, in this model of non-insulin-dependent diabetes is mainly attributable to the low activity of FAD-linked glycerophosphate dehydrogenase, resulting in a decreased metabolic flow through the glycerol phosphate shuttle and a reduced rate of aerobic glycolysis.