The stimulus-secretion coupling of glucose-induced insulin release. Does glycolysis control calcium transport in the B-cell?

Editor's Highlight: Therapeutic Concentrations of Antidepressants Inhibit Pancreatic Beta-Cell Function via Mitochondrial Complex Inhibition

Diabetes mellitus risk is increased by prolonged usage of antidepressants (ADs). Although various mechanisms are suggested for their diabetogenic potential, whether a direct effect of ADs on pancreatic β-cells is involved is unclear. We examined this idea for 3 ADs: paroxetine, clomipramine and, with particular emphasis, fluoxetine, on insulin secretion, mitochondrial function, cellular bioenergetics, KATP channel activity, and caspase activity in murine and human cell-line models of pancreatic β-cells. Metabolic assays showed that these ADs decreased the redox, oxidative respiration, and energetic potential of β-cells in a time and concentration dependent manner, even at a concentration of 100 nM, well within the therapeutic window. These effects were related to inhibition of mitochondrial complex I and III. Consistent with impaired mitochondrial function, lactate output was increased and insulin secretion decreased. Neither fluoxetine, antimycin nor rotenone could reactivate KATP channel activity blocked by glucose unlike the mitochondrial uncoupler, FCCP. Chronic, but not acute, AD increased oxidative stress and activated caspases, 3, 8, and 9. A close agreement was found for the rates of oxidative respiration, lactate output and modulation of KATP channel activity in MIN6 cells with those of primary murine cells; data that supports MIN6 as a valid model to study beta-cell bioenergetics. To conclude, paroxetine, clomipramine and fluoxetine were all cytotoxic at therapeutic concentrations on pancreatic beta-cells; an action suggested to arise by inhibition of mitochondrial bioenergetics, oxidative stress and induction of apoptosis. These actions help explain the diabetogenic potential of these ADs in humans.

Pancreatic fate of D-[3H] mannoheptulose

D-Mannoheptulose was recently postulated to be transported into cells by GLUT2. The validity of such an hypothesis was assessed by comparing the uptake of tritiated D-mannoheptulose by pancreatic islets versus pieces of pancreas and, in the latter case, by comparing results obtained in control rats versus animals injected with streptozotocin (STZ). The uptake of D-[3H] mannoheptulose by islets represents a time-related and temperature-sensitive process, inhibited by cytochalasin B and enhanced by D-glucose. The uptake of the tritiated heptose was much lower in pieces of pancreatic tissue and inhibited by D-glucose, at least in the STZ rats. Whether in pieces of pancreas exposed in vitro to D-[3H] mannoheptulose or after intravenous injection of the tritiated heptose, the radioactive content of the pancreatic tissue was lower in STZ rats than in control animals. This contrasted with an unaltered radioactive content of liver and muscle in the STZ rats, at least when treated with insulin. Suitably radiolabelled D-mannoheptulose or an analogue of the heptose could thus conceivably be used for quantification of the endocrine pancreatic mass.

Improved viability and metabolic behavior of hepatocytes after liver storage in the presence of a succinic acid ester

BACKGROUND

Selected esters of succinic acid were recently proposed as novel nutrients to support ATP generation in cells endangered by an imbalance between the formation and breakdown of this adenine nucleotide. In the present study, a new ester, glycerol-1,2,3-trimethylsuccinate, was examined for its potential beneficial effect in the procedures preceding liver transplantation.

METHODS

The viability and metabolic behavior of hepatocytes were examined after perfusion and storage of rat livers for 20 hr at 4 degrees C with a Belzer UW-CSS solution in the absence or presence or 2 mM glycerol-1,2,3-trimethylsuccinate.

RESULTS

Although it failed to affect significantly the release of cellular enzymes during storage and the protein or glycogen content of the liver, and was unable to prevent the storage-induced decrease in both biosynthetic activity and D-[U-14C]glucose incorporation into glycogen in isolated hepatocytes, the ester restored to a close-to-normal value the viability of the hepatocytes and opposed the starvation-like effects of liver storage upon both the conversion of D-[U-14C]glucose to 14CO2 and radioactive amino acids and the de novo generation of 14C-labeled D-glucose from [2-14C]pyruvate.

CONCLUSIONS

Because succinic acid esters are efficiently metabolized in several cell types, the present results suggest that such esters may have a wide field of application in transplantation procedures.

Stimulation by hexose esters of lactate production by rat erythrocytes: insensitivity to 3-O-methyl-D-glucose and inhibition by 2-deoxy-D-glucose and its tetraacetic ester

Selected esters of D-glucose were recently proposed as tools to provide the sugar to cells, whilst bypassing the carrier system for hexose transport across the plasma membrane. In the present study, alpha-D-glucose pentaacetate, beta-D-glucose pentaacetate, alpha-D-mannose pentaacetate and, to a lesser extent, 6-O-acetyl-D-glucose, all tested at a 1.7 mM concentration, were found to increase lactate production above basal value in rat erythrocytes. Over 90 min incubation, the increment in lactate production ranged from about 1.2 (alpha-D-glucose pentaacetate) to 0.6 (6-O-acetyl-D-glucose) micromol/microl of erythrocytes. Little or no change in lactate production was observed in cells exposed to beta-L-glucose pentaacetate, alpha-D-glucose pentaethylsuccinate, alpha-D-galactose pentaacetate or beta-D-galactose pentaacetate. The metabolic response to alpha-D-glucose pentaacetate was resistant to 3-O-methyl-D-glucose (10-80 mM) which suppressed, however, that evoked by D-glucose. D-mannoheptulose (10 mM) virtually failed to affect the response to D-glucose and its pentaacetate ester. On the contrary, 2-deoxy-D-glucose (10.6 mM) inhibited to the same relative extent (55% decrease) lactate production in erythrocytes exposed to either unesterified D-glucose or alpha-D-glucose pentaacetate. The tetraacetic ester of 2-deoxy-D-glucose was more efficient than unesterified 2-deoxy-D-glucose in inhibiting lactate production from alpha-D-glucose pentaacetate. It is proposed that selected esters of saccharides represent useful tools to bypass defects in hexose transport, and to increase their nutritional or therapeutic efficiency.

Model of beta-cell mitochondrial calcium handling and electrical activity. I. Cytoplasmic variables

We continue our development of a kinetic model of bursting electrical activity in the pancreatic beta-cell (J. Keizer and G. Magnus. Biophys. J. 56: 229-242, 1989), including the influence of Ca2+ handling by the mitochondria. Our minimal model of mitochondrial Ca2+ handling [G. Magnus and J. Keizer. Am. J. Physiol. 273 (Cell Physiol. 42): C717-C733, 1997] is expanded to include the D-glucose dependence of the rate of production of mitochondrial reducing equivalents. The Ca2+ dependence of the mitochondrial dehydrogenases, which is also included in the model, plays only a small role in the simulations, since the dehydrogenases appear to be maximally activated when D-glucose concentrations are sufficient to produce bursting. A previous model of ionic currents in the plasma membrane is updated using a recent experimental characterization of the dependence of the conductance of the ATP-sensitive K+ (KATP) current on adenine nucleotides. The resulting whole cell model is complex, involving 12 dynamic variables that couple Ca2+ handling in the cytoplasm and the mitochondria with electrical activity in the plasma and inner mitochondrial membranes. Simulations with the whole cell model give rise to bursting electrical activity similar to that seen in pancreatic islets and clusters of pancreatic beta-cells. The full D-glucose dose response of electrical activity is obtained if the cytosolic rate of ATP hydrolysis is a sigmoidal function of glucose. The simulations give the correct shape, period, and phase of the associated oscillations in cytosolic Ca2+, predict that the conductance of the KATP current oscillates out of phase with electrical activity [as recently observed in ob/ob mice (O. Larsson, H. Kindmark, R. Bränstrom, B. Fredholm, and P.-O. Berggren. Proc. Natl. Acad. Sci. USA 93: 5161-5165, 1996)], and make other novel predictions. In this model, bursting results because Ca2+ uptake into mitochondria during the active phase reduces the mitochondrial inner membrane potential, reducing the rate of production of ATP, which in turn activates the KATP current and repolarizes the plasma membrane.

Kinetics and specificity of human B-cell glucokinase: relevance to hexose-induced insulin release

The present study reevaluates the relevance of human B-cell glucokinase activity to the process of hexose-induced insulin release. Taking into account a phenomenon of positive cooperativity (Hill number: 1.34), the Km of the enzyme for glucose ( < or = 5.1 mM) was lower than the concentration of the hexose required to cause half-maximal stimulation of insulin release in intact islets. Likewise, there were obvious discrepancies between the kinetics of glucose, mannose and fructose phosphorylation by B-cell glucokinase, e.g. in terms of maximal velocity, and the secretory and metabolic responses to these hexoses in intact islets. Glucose 6-phosphate decreased, modestly but significantly, B-cell glucokinase activity, such an inhibitory action being of the non-competitive type. Mannoheptulose caused competitive inhibition of B-cell glucokinase. It is concluded that the intrinsic catalytic properties of B-cell glucokinase cannot fully account for the concentration dependency and sugar specificity of the secretory response to D-glucose or other hexoses in pancreatic islets.

Interference of glycogenolysis with glycolysis in pancreatic islets from glucose-infused rats

When pancreatic islets isolated from rats infused for 48-72 h with a hypertonic solution of D-glucose were incubated for two successive periods of 10 min each, in the presence first of 16.7 mM and then 2.8 mM D-[U-14C]glucose, the total output of L-lactic acid during the second incubation was as high as that recorded during the first incubation, while the specific radioactivity of L-lactic acid dramatically decreased during the second incubation. In islets from normoglycemic rats, however, the total output of L-lactic acid decreased and its specific radioactivity modestly increased as the concentration of D-glucose was lowered from 16.7 to 2.8 mM. Such contrasting results indicate that in the glycogen-rich islets isolated from glucose-infused rats, the fall in extracellular D-glucose concentration was not accompanied by a parallel fall in glycolytic flux, the decreased utilization of exogenous D-[U-14C]glucose coinciding with stimulation of glycogenolysis. This unusual metabolic situation also coincided with a transient and paradoxical stimulation of insulin release in response to the decrease in extracellular D-glucose concentration. It is proposed, therefore, that the interference of glycogenolysis with glycolysis in pancreatic islets from glucose-infused rats participates in the paradoxical changes in insulin output which represent a typical feature of B-cell glucotoxicity.

Acute stimulation of pancreatic islets by inhibitors of lactic acid transport

The transport of L-lactate into rat pancreatic islets and RINm5F insulinoma cells was inhibited by alpha-cyano-4-hydroxycinnamate, alpha-fluorocinnamate, quercetin and by p-chloromercuribenzene-sulphonic acid. The addition of each of these compounds to perifused islets resulted in an immediate, marked stimulation of insulin release. Enhanced insulin secretion was accompanied by a similarly rapid and pronounced increase in the rate of 45Ca2+ efflux from pre-loaded, perifused islets. In general, these stimulatory effects were most pronounced in the presence of a threshold concentration of glucose (5 mM) in the perifusion medium. In islets pre-loaded with 86Rb+, the addition of alpha-fluorocinnamate or quercetin caused a modest diminution in efflux rate whilst enhanced rates of 86Rb+ outflow were apparent in the presence of 5 mM glucose. It is suggested that these inhibitors of lactic acid transport stimulate the beta-cell, at least in part, by increasing the intracellular: extracellular lactate gradient, thereby promoting the electrogenic efflux of endogenous lactate from the cell.

Hexose metabolism in pancreatic islets: enzyme-to-enzyme tunnelling of hexose 6-phosphates

The fate of unlabelled D-glucose and D-[2-3H]glucose in pancreatic islets was simulated taking into account experimental values for glycolytic flux, intracellular concentration of D-glucose 6-phosphate and phosphoglucoisomerase activity. The model, which also takes into account the isotopic discrimination in velocity and intramolecular transfer of tritium between D-[2-3H]glucose 6-phosphate and D-[1-3H]fructose 6-phosphate in the reaction catalyzed by phosphoglucoisomerase, revealed that the predicted generation of 3HOH from D-[2-3H]glucose was much higher than the true experimental value. Such a discrepancy is reinforced by the consideration that the generation of 3HOH from D-[2-3H]glucose in islet cells is not solely attributable to the phosphoglucoisomerase-catalyzed detritiation of hexose 6-phosphates metabolized in the glycolytic pathway. In order to reconcile experimental and theoretical values for 3HOH production, it was found necessary to postulate enzyme-to-enzyme tunnelling of hexose 6-phosphates in the hexokinase/phosphoglucoisomerase/phosphofructokinase sequence. It is proposed that such a tunnelling may favour the anomeric specificity of D-glucose metabolism in islet cells, by restricting the anomerization of hexose 6-phosphates.

Perturbation of pancreatic islet function in glucose-infused rats

The secretory behavior of insulin- and glucagon-producing cells was found to be perturbed in isolated perfused pancreases removed from rats infused with hypertonic solutions of glucose for 48 hours. The anomalies included a high basal release of insulin and a paradoxical increase in insulin output and decrease in glucagon release in response to a fall in D-glucose concentration. Likewise, in isolated islets prepared from the glucose-infused rats, L-arginine or theophylline stimulated insulin release at a low ambient concentration of D-glucose, at variance with the situation found in islets removed from normal rats. These secretory perturbations could not be attributed to any obvious defect in either the transport of D-glucose into islet cells or its further utilization and oxidation, but coincided with the abnormal accumulation of glycogen in the B-cell. It is proposed that the latter anomaly may play a role in the altered dynamics of insulin release found in animals or patients with long-term hyperglycemia.

Effects of lactate on pancreatic islets. Lactate efflux as a possible determinant of islet-cell depolarization by glucose

The secretion of insulin from perifused rat pancreatic islets was stimulated by raising the glucose concentration from 5.6 to 20 mM or by exposure to tolbutamide. The addition of sodium lactate (40 mM) to islets perifused in the presence of glucose (5.6 mM) resulted in a small, transient, rise in the rate of secretion. The subsequent removal of lactate, but not glucose or tolbutamide, from the perifusate produced a dramatic potentiation of insulin release. The rate of efflux of 45Ca2+ was also increased when islets were exposed to a high concentration of glucose or lactate or to tolbutamide, and again subsequently upon withdrawal of lactate. Efflux of 86Rb+ was modestly inhibited upon addition of lactate and markedly enhanced by the subsequent withdrawal of lactate from islets. The output of [14C]lactate from islets incubated in the presence of [U-14C]glucose increased linearly with increasing concentrations of glucose (1-25 mM). It is proposed that the activation of islets by the addition or withdrawal of lactate is not due to increased oxidative flux, but occurs as a result of the electrogenic passage of lactate ions across the plasma membrane, resulting in islet-cell depolarization, Ca2+ entry and insulin secretion. The production of lactate via the glycolytic pathway, and the subsequent efflux of lactate from the islet cells with concomitant exchange of H+ for Na+, could be a major determinant of depolarization and hence insulin secretion, in response to glucose.

Prostaglandin E2 and alpha 2 adrenoceptor agonists inhibit the pentose phosphate shunt in pancreatic islets

Glucose utilization in isolated pancreatic islets of the rat was inhibited by prostaglandin (PG) E2 and the alpha 2 adrenoceptor agonist, clonidine, to a similar extent; other prostaglandins did not affect glucose utilization. Islet oxidation of [1-14C]glucose and [6-14C]glucose demonstrated that the pentose phosphate shunt was inhibited by PGE2 and clonidine. Pertussis toxin antagonizes the effects of clonidine and PGE2 on total glucose utilization and pentose phosphate shunt activity. The results suggest that PGE2 and alpha 2 adrenoceptor agonists may regulate glucose metabolism through similar transduction mechanisms, and that a guanine nucleotide binding regulatory (G) protein modulates certain metabolic effects of prostaglandins and adrenergic agonists.

Hexose metabolism in pancreatic islets. Metabolic and secretory responses to D-fructose

D-Fructose (3.3 to 33.0 mmol/liter) caused a concentration-related increase in insulin output from rat islets exposed to D-glucose (3.3 to 7.0 mmol/liter), such an increase not being more marked in mouse islets. The fructose-induced increment in insulin release, relative to that evoked by D-glucose, was two times higher in islets exposed to D-glucose than in islets stimulated by D-mannose, 2-ketoisocaproate, or nonnutrient secretagogs. Likewise, the metabolism of D-fructose in islet cells was significantly different in the absence or presence of D-glucose. Thus, the ketose was largely channeled into the pentose phosphate pathway in glucose-deprived, but not so in glucose-stimulated, islets. In both glucose-deprived and glucose-stimulated islets, however, the magnitude of the secretory response to D-fructose was commensurate with the increase in ATP production attributable to its catabolism. These findings indicate that the metabolic fate of hexoses--and, hence, their insulinotropic capacity--is not ruled solely at the level of their phosphorylation.

Abnormal calcium handling by perifused pancreatic islets from neonatal streptozotocin diabetic model rats

To elucidate the mechanism of impaired insulin release in case of non-insulin-dependent diabetes (NIDDM), we investigated insulin release and 45Ca++ efflux from perifused islets obtained from neonatal streptozotocin diabetic model rats. The model rats were prepared by the intraperitoneal administration of 65 mg/kg streptozotocin (STZ) to neonatal males. Rats treated with STZ did not differ from controls in body weight from 1 week to 16 weeks. The model rats had significant hyperglycemia both in the fasting state and after intraperitoneal administration of 2 g/kg glucose. Although the diameter of the islets from the model rats was not significantly different from that of controls, immunoreactivity to anti-insulin was slightly diminished, and degranulation was slightly observed in B-cells. Insulin content was reduced to 45.6% of the control. Insulin release from the perifused islets of STZ-treated rats responded little to 16.7 mmol/L glucose, but normally to 20 mmol/L arginine in the presence of 5.5 mmol/L glucose. In experiments to test the 45Ca++ efflux from the perifused islets prelabeled with 45Ca++, a rise of 45Ca++ efflux concomitant with the second phase of insulin release from the islets of the model rats was inhibited although a sharp increase of 45Ca++ efflux concomitant with the first phase of insulin release was maintained. 45Ca++ uptake for 30 minutes was reduced in the islets from the model rats in the basal and stimulated state of insulin secretion although the incremental 45Ca++ uptake was similar. It is possible that the abnormal calcium handling in pancreatic B-cells may be one of the causes of defect in insulin release in our model rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic and secretory response of tumoral-insulin producing cells to D-fructose and D-galactose

At variance with normal islet cells, tumoral insulin-producing cells of the RINm5F line were found to display a positive secretory response not solely to D-glucose and D-mannose, but also to D-fructose and D-galactose. All hexoses increased the ATP/ADP ratio, exerted a sparing action upon the oxidation of endogenous nutrients in cells prelabelled with either L-[U-14C]glutamine or [U-14C]palmitate, increased the output of lactic acid and, as judged from data collected in the presence of D-[U-14C]hexoses, underwent oxidation in the RINm5F cells. The secretory response to these four hexoses appeared commensurate with the extent of their metabolic effects.

Can desensitization of the B-cell to D-glucose be simulated in cultured pancreatic islets?

In order to investigate the phenomenon of B-cell desensitization to D-glucose, rat pancreatic islets were cultured for 20-44h in the presence of increasing concentrations of D-glucose in the 5.6 to 27.8 mM range, and then incubated for 30 to 120 min for measurement of secretory, metabolic and ionic variables. After culture in the presence of 5.6 mM D-glucose, the release of insulin evoked by D-glucose (16.7 mM) was less marked than that seen in islets cultured in the presence of 11.1 mM D-glucose. In the latter islets, the secretory response to D-glucose (8.3 mM or more) was still modest, especially over short periods of incubation, but was markedly enhanced by either theophylline or forskolin. The release of insulin evoked by D-glucose in the presence of theophylline was little affected by either Ca2+ concentration of the culture medium or length of culture period (20h vs 44h). The culture-induced alteration in the responsiveness to D-glucose coincided with a smaller relative increase of D-[5(3)H]glucose utilization, D-[U-14C]glucose oxidation or net 45Ca uptake at increasing concentrations of the hexose. It contrasted with a well-preserved secretory response to nonnutrient secretagogues. Although these findings could be interpreted as evidence of B-cell desensitization to D-glucose, the fact that the secretory behavior of the islets was not vastly different whether they were first cultured at physiological (8.3 mM) or higher (11.1 to 27.8 mM) concentrations of D-glucose suggests that this experimental design may not be an optimal model for the functional alteration of the B-cell in hyperglycemic non-insulin-dependent diabetic subjects.

Glucose metabolism in insulin-producing tumoral cells

Homogenates of insulin-producing tumoral cells catalyzed the phosphorylation of glucose, mannose, and fructose. The kinetics of phosphorylation at increasing glucose concentrations, the inhibitory effect of glucose 6-phosphate, and the comparison of results obtained with distinct hexoses indicated the presence of both low-Km hexokinase-like and high-Km enzymatic activities, the results being grossly comparable to those collected in normal pancreatic islets. Relative to protein content, the glucose-phosphorylating enzymatic activity was higher in tumoral than normal islet cells. The activity of other enzymes was either lower (glutamate dehydrogenase), moderately higher (phosphoglucomutase, lactate dehydrogenase) or considerably greater (ornithine decarboxylase) in tumoral than in normal islet cells. In intact tumoral cells, incubated under increasing glucose concentrations, the oxidation of D-[U-14C]glucose and the output of lactic and pyruvic acids reached a close-to-maximal value at 2.8 mM glucose. The ratios for glucose oxidation/utilization and lactate/pyruvate output were much lower in tumoral than in normal islet cells. Although glucose caused a modest increase in insulin output from the tumoral cells, this effect was saturated at a low glucose concentration (2.8 mM) and less marked than that of other secretagogues (e.g., L-leucine, L-ornithine, or forskolin). Thus, despite a close-to-normal enzymatic equipment for glucose phosphorylation, the tumoral cells displayed severe abnormalities in the metabolism and secretory response to this hexose. These findings point to regulatory mechanisms distal to glucose phosphorylation in the control of glucose metabolism in insulin-producing cells.

Glucokinase is not the pancreatic B-cell glucoreceptor

A series of recent experimental findings are reviewed to indicate that glucokinase does not represent the pancreatic B-cell glucoreceptor. Whether in liver, pancreatic islet or insulin-producing tumoral cell homogenates, glucokinase fails to yield a higher reaction velocity with alpha-than beta-D-glucose. At a high glucose concentration (40 mmol/l), when the phosphorylation of glucose by glucokinase is indeed higher with beta- than alpha-D-glucose, no preference for beta-D-glucose is observed in intact islets, as judged from the utilization of D-[5-3H]glucose, production of lactic acid, oxidation of D-[U-14C]glucose, net uptake of 45Ca or release of insulin. The glucose 6-phosphate content of intact islets is higher in the presence of beta- than alpha-D-glucose. At a low glucose concentration (3.3 mmol/l), when the participation of glucokinase to hexose phosphorylation is minimal, alpha-D-glucose is still better metabolized and stimulates both 45Ca net uptake and insulin release more efficiently than beta-D-glucose, despite the fact that hexokinase yields a higher reaction velocity with beta- than alpha-D-glucose. In intact islets, beta-D-glucose is used preferentially to alpha-D-glucose in the pentose cycle pathway as judged from the oxidation of alpha- or beta-D-[1-14C]glucose relative to that of alpha- or beta-D-[6-14C]glucose. In islets removed from fasted rats, the rate of glycolysis is more severely decreased than expected from the repression of glucokinase. The metabolism of glucose in tumoral insulin-producing cells differs, in several respects, from that in normal pancreatic islets, although the pattern of hexokinase and glucokinase activities is similar in these two types of cells. All these observations point to the participation of regulatory sites distal to glucose phosphorylation in the control of glucose metabolism in islet cells.

Pentose cycle pathway in normal and tumoral islet cells

Relative to protein content, the activity of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase and the rate of glucose metabolism by the pentose cycle pathway in tumoral insulin-producing cells were similar to or higher than those found in normal rat islets. Hence, the decreased secretory response of tumoral cells to glucose is apparently not attributable to any major anomaly in glucose handling by the hexose monophosphate pathway.

Hexose metabolism in pancreatic islets. Galactose transport, phosphorylation and oxidation

In rat pancreatic islets, the apparent space of distribution of galactose is not different from that of other hexoses. In homogenates of islets or tumoral insulin-producing cells, galactose is phosphorylated at a very low rate relative to either glucose phosphorylation in the same tissues or galactose phosphorylation by liver homogenates. In intact islets, galactose increases modestly the glucose 6-phosphate content and is oxidized at a much lower rate than glucose. Galactose slightly increases insulin output in the presence of a stimulatory concentration of glucose but fails to provoke insulin release in the absence of glucose, whether in islets removed from rats fed a normal or galactose-rich diet. The low rate of galactose oxidation and its poor insulinotropic capacity appear attributable to the weak activity of galactokinase in pancreatic islets.

Hexose metabolism in pancreatic islets: phosphoglycerate 2,3-mutase and enolase activities in rat islets

Rat islet homogenates display both phosphoglycerate 2,3-mutase and enolase activities. When phosphoglycerate 2,3-mutase is activated by 2,3-diphosphoglycerate, the reaction velocity becomes close to that of enolase. The islet content in 2,3-diphosphoglycerate is sufficiently high to allow virtually full activation of phosphoglycerate 2,3-mutase.

Hexose metabolism in pancreatic islets. The phosphorylation of fructose

Fructose, like glucose, rapidly equilibrates across the plasma membrane of pancreatic islet cells, but is poorly metabolized and is a weak insulin secretagogue in rat pancreatic islets. A possible explanation for such a situation was sought by investigating the modality of fructose phosphorylation in islet homogenates. Several findings indicated that the phosphorylation of fructose is catalyzed by hexokinase, but not fructokinase. First, at variance with the situation found in liver homogenates, the phosphorylation of fructose in the islet homogenate was unaffected by K+ and inhibited by glucose, mannose, glucose 6-phosphate or glucose 1,6-bisphosphate. Second, the Km for fructose was much higher in islets than in liver. Third, in islet homogenates the Km and Vmax for fructose were much higher than those for glucose or mannose phosphorylation, at low aldohexose concentrations, in good agreement with the properties of purified hexokinase. In intact islets fructose augmented the islet content in glucose 6-phosphate sufficiently to cause marked inhibition of its own rate of phosphorylation. These findings may account, in part at least, for the low rate of fructose utilization by rat pancreatic islets.

Hexose metabolism in pancreatic islets. Inhibition of hexokinase

In islet homogenates, hexokinase-like activity (Km 0.05 mM; Vmax. 1.5 pmol/min per islet) accounts for the major fraction of glucose phosphorylation. Yet the rate of glycolysis in intact islets incubated at low glucose concentrations (e.g. 1.7 mM) sufficient to saturate hexokinase only represents a minor fraction of the glycolytic rate observed at higher glucose concentrations. This apparent discrepancy between enzymic and metabolic data may be attributable, in part at least, to inhibition of hexokinase in intact islets. Hexokinase, which is present in both islet and purified B-cell homogenates, is indeed inhibited by glucose 6-phosphate (Ki 0.13 mM) and glucose 1,6-bisphosphate (Ki approx. 0.2 mM), but not by fructose 2,6-bisphosphate. In intact islets, the steady-state content of glucose 6-phosphate (0.26-0.79 pmol/islet) and glucose 1,6-bisphosphate (5-48 fmol/islet) increases, in a biphasic manner, at increasing concentrations of extracellular glucose (up to 27.8 mM). From these measurements and the intracellular space of the islets, it was estimated that the rate of glucose phosphorylation as catalysed by hexokinase represents, in intact islets, no more than 12-24% of its value in islet homogenates.

Phosphoglucomutase: its role in the response of pancreatic islets to glucose epimers and anomers

Rat pancreatic islets display phosphoglucomutase activity. The velocity of glucose-1-phosphate conversion to glucose-6-phosphate is increased in a dose-related fashion by glucose-1,6-bisphosphate. The islet homogenate, like purified muscle phosphoglucomutase, also catalyzes the synthesis of glucose-1,6-bisphosphate from glucose-6-phosphate and fructose-1,6-bisphosphate. The rate of the latter reaction is about 10,000 times lower than that of glucose-1-phosphate conversion to glucose-6-phosphate in the presence of glucose-1,6-bisphosphate. D-glucose and D-mannose, but not D-galactose nor D-fructose, markedly increase the islet content in glucose-1,6-bisphosphate. Such a content is twice higher in islets exposed for 5 minutes to alpha-D-glucose than in islets exposed to beta-D-glucose. The process of glucose-1,6-bisphosphate synthesis, as catalyzed by the alpha-stereospecific phosphoglucomutase, may play a role in the metabolic and, hence, secretory responses of the islets to glucose epimers and anomers.

The stimulus-secretion coupling of glucose-induced insulin release. Environmental influences on L-glutamine oxidation in pancreatic islets

L-Glutamine at a near-physiological concentration (1.0mM) was rapidly taken up and metabolized in rat pancreatic islets. The rate of glutamine deamidation much exceeded that of glutamate conversion into 2-oxoglutarate, the latter conversion being mediated mainly by transamination reactions. The production of 14CO2 from L-[U-14C]glutamine, which reflected the generation of ATP through the metabolism of exogenous glutamine, appeared to be regulated by the redox state of nicotinamide nucleotides and the ATP content of the islet cells. The influence of environmental factors on glutamine oxidation was examined in order to identify ATP-requiring processes. Glutamine oxidation was decreased in the absence of extracellular Ca2+, under conditions aiming at inhibition of the (Na+ + NA+)-dependent ATPase and, provided that glucose was present in the incubation medium, by cycloheximide. These findings were interpreted to suggest that the handling of Ca2+ by the islet cells, the active transport of univalent cations and the biosynthesis of proinsulin represent three major ATP-consuming processes in this fuel-sensor organ.

The stimulus-secretion coupling of glucose-induced insulin release. Thiol: disulfide balance in pancreatic islets

An increase in the production rate of reduced pyridine nucleotides is currently considered as a coupling factor between metabolic and distal events in the process of glucose-stimulated insulin release. The possible participation in such a coupling of thiol: disulfide interchanges was investigated in rat pancreatic islets. NADPH-dependent glutathione reductase and glutathione-cystine transhydrogenase activities were present in islet homogenates, whereas no glutathione peroxidase activity could be detected. In intact islets, glucose (16.7 mM) augmented both the GSH/GSSG ratio (from a basal value of 6.7 +/- 0.6 to 8.4 +/- 0.4) and the tissue content of sulphydryl groups (from a basal value of 119 +/- 7 to 170 +/- 9 pmol/microgram protein). The latter effect was mimicked by D-glyceraldehyde, 2-ketoisocaproate, anoxia and KCN; it failed to be reproduced by L-glucose or D-fructose, was unaffected by theophylline, and was inhibited by D-mannoheptulose, iodoacetate, menadione, cytochalasin B and the absence of extracellular Ca2+. These findings support the view that a glucose-induced reduction of disulphide bridges to sulphydryl groups participates in the stimulus-secretion coupling of nutrient-induced insulin release.

Stimulation of insulin release by L-glutamine

Under normal environmental conditions, L-glutamine is well oxidized but fails to stimulate insulin release in rat pancreatic islets. However, a marked stimulation of insulin release by L-glutamine, without alteration in its oxidation rate occurs when the intracellular pH of the islet cells is decreased and/or when theophylline is added to the incubation medium.

Regulation of 86Rb outflow from pancreatic islets. IV. Effect of cyclic AMP, dibutyryl-cyclic AMP and theophylline

Theophylline (1.4 mM), cyclic AMP (1.0 mM) and dibutyryl-cyclic AMP (0.5 mM) decreased 86Rb fractional outflow rate from pancreatic islets perifused in the absence of glucose. In the presence of glucose (16.7 mM), however, the same drugs provoked a modest increase in 86Rb fractional outflow rate. The increase in 86Rb outflow evoked by theophylline in the presence of glucose was suppressed by quinine, suggesting that it may result from an increase in cytosolic Ca2+ concentration. It is proposed that changes in the cyclic AMP content of islet cells may participate in the regulation of K+ conductance by insulin secretagogues.

Regulation of calcium fluxes in pancreatic islets: two calcium movements' dissociated response to glucose

At normal extracellular 40Ca concentration (1.0--1.5 mM), D-glucose initially reduces and subsequently increases 45Ca efflux from prelabeled rat pancreatic islets, whether in the absence or presence of 20 mM Ca-EGTA. Thus, the initial fall cannot be attributed to a reuptake of effluent 45Ca. Both the fall and rise in effluent 45Ca represent sustained and rapidly reversible phenomena partially masked by one another. The dose-action relationship for the initial fall in 45Ca efflux (Km = 4.2 mM) differs vastly from that characterizing the secondary rise in 45Ca efflux (Km = 9.1 mM). Mannoheptulose, iodoacetate, and low temperature inhibit both the early fall and secondary rise in effluent radioactivity normally evoked by glucose. D-Glucose (8.3 mM) and D-glyceraldehyde (10 mM) exert comparable effects on 45Ca efflux. The initial fall and later increase in 45Ca efflux evoked by glucose, although dependent on the integrity of glucose metabolism, correspond to two distinct processes in the rapid regulation of Ca handling by the islet cells.

Beta-cell recognition of stereoisomers of D-glucose

The ability of all eight D-aldohexose steroisomers to stimulate insulin release and biosynthesis was compared with their ability to serve as a metabolic substrate for isolated islets of Langerhans as judged by formation of lactate. Insulin release and synthesis were stimulated by glucose or mannose but not by allose, altrose, gulose, idose, galactose or talose. No potentiary effects of allose, altrose, gulose, idose, or talose were found on insulin release in the presence of 4 mmol/l glucose nor did these sugars inhibit insulin release in the presence of 20 mmol/l glucose. Lactate formation was increased above values found in the absence of added substrate by 20 mmol/l D-glucose or mannose, but not by allose, altrose, gulose, galactose or talose. The results support the substrate-site hypothesis for the recognition of sugars as stimuli of insulin release and synthesis.

Effect of 2-deoxyglucose on [32P] phosphate and insulin release from perifused rat pancreatic islets

The effect of 2-deoxyglucose on glucose mediated insulin and [32P]phosphate release was studied by perifusion of isolated rat pancreatic islets. When islets were perifused with media containing 2.8 mmol/l glucose and 20 mmol/l 2-deoxyglucose for 60 minutes and then exposed to media containing 8.3 or 16.7 mmol/l glucose and 20 mmol/l 2-deoxyglucose for the next 15 minutes, insulin release at either glucose concentration was prompt but blunted. Similarly, islets preincubated (90 min) with [32P] orthophosphate, then perifused with 20 mmol/l 2-deoxyglucose for 75 min and stimulated by either 8.3 or 16.7 mmol/l glucose for the final 15 minutes or 2-deoxyglucose exposure demonstrated obtundation of [32P]phosphate release. Perifusion of islets with 20 mmol/l 2-deoxyglucose alone induced no heightened 32P efflux. These studies suggest that 2-deoxyglucose affects initial events in stimulus-secretion coupling of glucose mediated insulin release.

Insulin release: the fuel hypothesis

The immediate and direct regulation of insulin release by circulating nutrients, especially glucose, is thought to be mediated in the pancreatic B-cell by a sequence of metabolic, ionic, and motile events. On the basis of previous work, it is assumed that the process by which glucose is recognized as an insulinotropic agent entirely depends on the metabolic changes evoked by the sugar in the islet cells. Several factors are considered as possible candidates for the coupling between these metabolic changes and subsequent ionic events such as altered phosphate, chloride, sodium, potassium, and calcium handling. It is acknowledged that changes in the concentrations of glycolytic intermediates and cyclic nucleotides (adenosine- or guanosine-3', 5'-cyclic monophosphate), or both, could play a modulatory role upon stimulated insulin release. However, the initiation of insulin release seems to depend on the generation of two essential coupling factors: H+ and reduced pyridine nucleotides. The changes in H+ fluxes may account for the glucose-induced decrease in K+ and Ca2+ fractional outflow rate, all three parameters displaying hyperbolic-like dose-response curves with half-maximal values at noninsulinotropic glucose concentrations. The changes in NAD(P)H concentration may account for a glucose-induced Ca2+--Ca2+ exchange process due to a change in affinity of a native ionophoretic system. The dose-response curves for these parameters yield a sigmoidal pattern analogous to that which depicts the rate of insulin release at increasing glucose concentrations. It is proposed that such a coupling between metabolic and cationic events is operative in response to other insulinotropic nutrients and that its time course may be relevant to the phasic aspect of insulin release. Thus, the nutrient-induced release of insulin (and possibly other pancreatic hormones), which is essential for the regulation of fuel homeostasis, would depend on the capacity of circulating nutrients to act as a fuel in the islet cells. This concept raises a question as to the existence and nature of feedback mechanisms regulating the metabolic fluxes in the islet cells as a function of their energy expenditure.

The stimulus-secretion coupling of glucose-induced insulin release XXVI. Are the secretory and fuel functions of glucose dissociable by iodoacetate?

Iodoacetate inhibits glyceraldehyde-3-phosphate dehydrogenase activity in pancreatic islets and causes a time- and dose-related inhibition of glucose oxidation and lactate output by the islets. High concentrations of the drug (0.3 mM or more) fail to affect Ba2+-induced insulin secretion but inhibit glucose-stimulated proinsulin biosynthesis, 45Ca net uptake and insulin release. A mixture of fumarate, glutamate, and pyruvate, the oxidation of which is only partially reduced by iodoacetate, fails to protect the B-cell against the inhibitory effect of the drug. These findings are compatible with the view that glycolysis plays an essential role in the process of glucose-induced insulin release. At low concentrations of iodoacetate (up to 0.2 mM), the reduction in glucose metabolism coincides with a partial inhibition of proinsulin biosynthesis. However, the expected reduction in 45Ca net uptake and subsequent insulin release is masked by a concomitant facilitating action of iodoacetate, possibly due to interference with native ionophoretic processes. It is concluded that iodoacetate is not an adequate tool to dissociate, if they are dissociable, the fuel and secretory functions of glucose.

Effects of phosphoenolpyruvate, other glycolytic intermediates and methylxanthines on calcium uptake by a mitochondrial fraction from rat pancreatic islets

45Ca2+-accumulation by a mitochondrial fraction from isolated rat pancreatic islets was stronly stimulated by ATP. The ATP-dependent uptake was inhibited by phosphoenolpyruvate in a dose-dependent manner over a wide variety of conditions. Inhibition by phosphoenolpyruvate was non-completitive with respect to calcium, competitive with respect to magnesium, and antagonised by high Mg-ATP2- concentrations; fructose 1,6-diphosphate also decreased 45Ca2+-uptake. Other glucose metabolites were either less effective or ineffective in diminishing mitochondrial 45Ca2+-accumulation. The ATP-dependent uptake was also inhibited by xanthine derivatives (caffeine and 3-isobutyl-1-methylaxanthine) which potentiate the effects of glucose on insulin secretion. Cyclic AMP had no effect. It is thought that the rate of insulin secretion is a function of the cytosolic calcium concentration in the B-cell. These data show that phosphoenolpyruvate, fructose 1,6-diphosphate and methylxanthines might influence exocytosis by direct effects on mitochondrial calcium accumulation, and thus the intracellular distribution of calcium.

The stimulus-secretion coupling of glucose-induced insulin release. Cationic and secretory effects of menadione in the endocrine pancreas

1. Menadione (2-methyl-1,4-naphthoquinone) inhibits insulin release evoked in the rat endocrine pancreas by glucose or glyceraldehyde, but fails to affect the secretory response to Ca2+, Ba2+, theophylline or gliclazide. The inhibitory effect of menadione upon glucose-induced insulin release is a dose-related, rapid and reversible phenomenon, menadione and glucose acting apparently as competitive antagonists. Menadione affects both the early and late phase of the secretory response to glucose. Menadione also antagonizes in a dose-related fashion the ability of glucose to reduce 86Rb efflux, to provoke 86Rb accumulation, to cause biphasic changes in 45 Ca efflux and to stimulate 45 Ca net uptake in pancreatic islets. 2. It is concluded that menadione impairs the insulinotropic action of glucose and other nutrients by impeding the remodelling of cationic fluxes normally provoked by these secretagogues in islet cells. Menadione, however, does not affect the capacity of divalent cations to activate the effector system which controls the release of secretory granules. Menadione may therefore represent a valuable tool to elucidate the mechanism by which glucose normally modifies the movement of cations in the pancreatic B-cell.

The stimulus-secretion coupling of glucose-induced insulin release. Metabolic effects of menadione in isolated islets

Pancreatic islets contain an enzyme system which catalyzes the donation of hydrogen from NAD(P)H to menadione (2-methyl-1,4-naphthoquinone). In high concentrations (20 to 50 micrometer), menadione, in addition to lowering the concentration of reduced pyridine nucleotides in the islets, also impairs glycolysis and glucose oxidation, decreases ATP concentration, and inhibits proinsulin biosynthesis. However, at a 10 micrometer concentration, menadione fails to affect the concentration of adenine nucleotides, the utilization of glucose, the production of lactate and pyruvate, the oxidation of [6-14C]glucose and the synthesis of proinsulin; whereas the metabolism of glucose through the pentose shunt is markedly increased. The sole inhibitory effect of menadione 10 micrometer upon metabolic parameters is to reduce the concentration of both NADH and NADPH, such an effect being noticed in islets exposed to glucose 11.1 mM but not in those incubated at a higher glucose level (27.8 mM). Since, in the presence of glucose 11.1 mM, menadione 10 micrometer also severely decreases glucose-stimulated45 calcium net uptake and subsequent insulin release, it is concluded that the availability of reduced pyridine nucleotides may play an essential role in the secretory sequence by coupling metabolic to cationic events. Thus, when insulinotropic nutrients are oxidized in the B-cell, the increased availability of reduced pyridine nucleotides could modify the affinity for cations of native ionophoretic systems, eventually leading to the accumulation of calcium up to a level sufficient to trigger insulin release.

Somatostatin- and epinephrine-induced modifications of 45Ca++ fluxes and insulin release in rat pancreatic islets maintained in tissue culture

The effects of somatostatin and epinephrine have been studied with regard to glucose-induced insulin release and (45)Ca(++) uptake by rat pancreatic islets after 2 days in tissue culture and with regard to (45)Ca(++) efflux from islets loaded with the radio-isotope during the 2 days of culture. (45)Ca(++) uptake, measured simultaneously with insulin release, was linear with time for 5 min. (45)Ca(++) efflux and insulin release were also measured simultaneously from perifused islets. Glucose (16.7 mM) markedly stimulated insulin release and (45)Ca(++) uptake. Somatostatin inhibited the stimulation of insulin release by glucose in a concentration-related manner (1-1,000 ng/ml) but was without effect on the glucose-induced stimulation of (45)Ca(++) uptake. Similarly, under perifusion conditions, both phases of insulin release were inhibited by somatostatin while no effect was observed on the pattern of (45)Ca(++) efflux after glucose.Epinephrine, in contrast to somatostatin, caused a concentration-dependent inhibition of the stimulation of both insulin release and (45)Ca(++) uptake by glucose. Both phases of insulin release were inhibited by epinephrine and marked inhibition could be observed with no change in the characteristic glucose-evoked pattern of (45)Ca(++) efflux (e.g., with 10 nM epinephrine). The inhibitory effect of epinephrine on (45)Ca(++) uptake and insulin release appeared to be mediated via an alpha-adrenergic mechanism, since is was abolished in the presence of phentolamine. Somatostatin inhibits insulin release without any detectable effect upon the handling of calcium by the islets. In contrast, inhibition of insulin release by epinephrine is accompanied by a partial inhibition of glucose-induced Ca(++) uptake.

The stimulus-secretion coupling of glucose-induced insulin release. Insulin release due to glycogenolysis in glucose-deprived islets

1. When pancreatic islets are preincubated for 20h in the presence of glucose (83.3mM) and thereafter transferred to a glucose-free medium, theophylline (1.4mM) provokes a dramatic stimulation of insulin release. This phenomenon does not occur when the islets are preincubated for either 20h at low glucose concentration (5.6mM) or only 30 min at the high glucose concentration (83.3mM). 2. The insulinotropic action of theophylline cannot be attributed to contamination of the islets with exogenous glucose and is not suppressed by mannoheptulose. 3. The secretory response to theophylline is an immediate phenomenon, but disappears after 60min of exposure to the drug. 4. The release of insulin evoked by theophylline is abolished in calcium-depleted media containing EGTA. Theophylline enhances the net uptake of 45Ca by the islets. 5. Glycogen accumulates in the islets during the preincubation period, as judged by both ultrastructural and biochemical criteria. Theophylline significantly increases the rate of glycogenolysis during the final incubation in the glucose-free medium. 6. The theophylline-induced increase in glycogenolysis coincides with a higher rate of both lactate output and oxidation of endogenous 14C-labelled substrates. 7. These data suggest that stimulation of glycolysis from endogenous stores of glycogen is sufficient to provoke insulin release even in glucose-deprived islets, as if the binding of extracellular glucose to hypothetical plasma-membrane glucoreceptors is not an essential feature of the stimulus-secretion coupling process.

The insulinotropic action of D-erythrose

D-erythrose (5.0 to 20.0 mM) stimulates insulin release. This insulinotropic action of erythrose displays several features in common with that of glucose. First, erythrose (20 mM) causes a shift to the left of the sigmoidal curve relating the secretory rate to the glucose eoncentration, but fails to enhance the maximal response to glucose. Second, the secretory response to erythrose occurs as an early peak followed by a phase of sustained release. Third, erythrose increases the output of lactate from the islets. Last, erythrose inhibits the efflux of 45calcium and favours its accumulation in isolated islets. It is suggested that, whether in response to glucose or erythrose, an increase in glycolytic flux may represent the key process involved in the identification of the secretagogue, a subsequent remodeling of calcium fluxes being apparently responsible for the activation of the insulin-releasing system.