Bioenergetic response of pancreatic islets to stimulation by fuel molecules

A Consensus Model of Glucose-Stimulated Insulin Secretion in the Pancreatic β -Cell

The pancreas plays a critical role in maintaining glucose homeostasis through the secretion of hormones from the islets of Langerhans. Glucose-stimulated insulin secretion (GSIS) by the pancreatic β -cell is the main mechanism for reducing elevated plasma glucose. Here we present a systematic modeling workflow for the development of kinetic pathway models using the Systems Biology Markup Language (SBML). Steps include retrieval of information from databases, curation of experimental and clinical data for model calibration and validation, integration of heterogeneous data including absolute and relative measurements, unit normalization, data normalization, and model annotation. An important factor was the reproducibility and exchangeability of the model, which allowed the use of various existing tools. The workflow was applied to construct the first consensus model of GSIS in the pancreatic β -cell based on experimental and clinical data from 39 studies spanning 50 years of pancreatic, islet, and β -cell research in humans, rats, mice, and cell lines. The model consists of detailed glycolysis and equations for insulin secretion coupled to cellular energy state (ATP/ADP ratio). Key findings of our work are that in GSIS there is a glucose-dependent increase in almost all intermediates of glycolysis. This increase in glycolytic metabolites is accompanied by an increase in energy metabolites, especially ATP and NADH. One of the few decreasing metabolites is ADP, which, in combination with the increase in ATP, results in a large increase in ATP/ADP ratios in the β -cell with increasing glucose. Insulin secretion is dependent on ATP/ADP, resulting in glucose-stimulated insulin secretion. The observed glucose-dependent increase in glycolytic intermediates and the resulting change in ATP/ADP ratios and insulin secretion is a robust phenomenon observed across data sets, experimental systems and species. Model predictions of the glucose-dependent response of glycolytic intermediates and insulin secretion are in good agreement with experimental measurements. Our model predicts that factors affecting ATP consumption, ATP formation, hexokinase, phosphofructokinase, and ATP/ADP-dependent insulin secretion have a major effect on GSIS. In conclusion, we have developed and applied a systematic modeling workflow for pathway models that allowed us to gain insight into key mechanisms in GSIS in the pancreatic β -cell.

Identification of Factors Complicating Bioluminescence Imaging

In vivo bioluminescence imaging (BLI) has become a standard, non-invasive imaging modality for following gene expression or the fate of proteins and cells in living animals. Currently, bioluminescent reporters used in laboratories are mostly derivatives of two major luciferase families: ATP-dependent insect luciferases and ATP-independent marine luciferases. Inconsistent results of experiments using different bioluminescent reporters, such as Akaluc and Antareas2, have been reported. Herein, we re-examined the inconsistency in several experimental settings and identified the factors, such as ATP dependency, stability in serum, and molecular sizes of luciferases, that contributed to the observed differences. We expect this study will make the research community aware of these factors and facilitate more accurate interpretation of BLI data by considering the nature of each bioluminescent reporter.

Imaging energy status in live cells with a fluorescent biosensor of the intracellular ATP-to-ADP ratio

The ATP:ADP ratio is a critical parameter of cellular energy status that regulates many metabolic activities. Here we report an optimized genetically-encoded fluorescent biosensor, PercevalHR, that senses the ATP:ADP ratio. PercevalHR is tuned to the range of intracellular ATP:ADP expected in mammalian cells, and it can be used with one- or two-photon microscopy in live samples. We use PercevalHR to visualize activity-dependent changes in ATP:ADP when neurons are exposed to multiple stimuli, demonstrating that it is a sensitive reporter of physiological changes in energy consumption and production. We also use PercevalHR to visualize intracellular ATP:ADP while simultaneously recording currents from ATP-sensitive potassium (K_ATP) channels in single cells, showing that PercevalHR enables the study of coordinated variation in ATP:ADP and K_ATP channel open probability in intact cells. With its ability to monitor changes in cellular energetics within seconds, PercevalHR should be a versatile tool for metabolic research.

Calcium co-regulates oxidative metabolism and ATP synthase-dependent respiration in pancreatic beta cells

Mitochondrial energy metabolism is essential for glucose-induced calcium signaling and, therefore, insulin granule exocytosis in pancreatic beta cells. Calcium signals are sensed by mitochondria acting in concert with mitochondrial substrates for the full activation of the organelle. Here we have studied glucose-induced calcium signaling and energy metabolism in INS-1E insulinoma cells and human islet beta cells. In insulin secreting cells a surprisingly large fraction of total respiration under resting conditions is ATP synthase-independent. We observe that ATP synthase-dependent respiration is markedly increased after glucose stimulation. Glucose also causes a very rapid elevation of oxidative metabolism as was followed by NAD(P)H autofluorescence. However, neither the rate of the glucose-induced increase nor the new steady-state NAD(P)H levels are significantly affected by calcium. Our findings challenge the current view, which has focused mainly on calcium-sensitive dehydrogenases as the target for the activation of mitochondrial energy metabolism. We propose a model of tight calcium-dependent regulation of oxidative metabolism and ATP synthase-dependent respiration in beta cell mitochondria. Coordinated activation of matrix dehydrogenases and respiratory chain activity by calcium allows the respiratory rate to change severalfold with only small or no alterations of the NAD(P)H/NAD(P)(+) ratio.

Protein restriction during lactation alters the autonomic nervous system control on glucose-induced insulin secretion in adult rats

Involvement of autonomic nervous system (ANS) neurotransmitters on insulin secretion in rats submitted to protein malnutrition during lactation was studied. During the first 2/3 of lactation, mothers received a 4% protein diet (LP). Control group received normal diet (23% protein) (NP). After protein restriction, mothers received normal diets. At 81 days rats were submitted to intravenous glucose tolerance tests (ivGTT). Plasma glucose and insulin concentration (PIC) were measured. Glucose-induced insulin secretion (GIIS) was tested in pancreatic islets. Fasting normoglycemia and hypoinsulinemia were observed in LP rats. Glucose intolerance and low PIC in LP group were detected during ivGTT. Acetylcholine (Ach) or blockage of alpha-adrenoceptors induced high PIC increment in LP rats; atropine or stimulation of alpha-adrenoceptors did not change PIC. Insulin secretion of LP rat islets showed low glucose and carbachol responses. Epinephrine-inhibited GIIS in both islet groups. Hypoinsulinemia observed in lactation-malnourished rats might be caused by alterations in GIIS regulation, including ANS modulation.

Capillary LC-MS for high sensitivity metabolomic analysis of single islets of Langerhans

Reversed-phase, packed capillary liquid chromatography interfaced by electrospray ionization to mass spectrometry was explored as an analytical method for determination of metabolites in microscale tissue samples using single islets of Langerhans as a model system. With the use of a 75 microm inner diameter column coupled to a quadrupole ion trap mass spectrometer in full scan mode, detection limits of 0.1-33 fmol were achieved for glycoloytic and tricarboxylic acid cycle metabolites. Reproducible processing of islets for analysis with little loss of metabolites was performed by rapid freezing followed by methanol-water extraction. The method yielded 20 microL of extract of which just 15 nL was injected suggesting the potential for performing multiple assays on the same islet. Approximately 200 presumed metabolites could be detected, of which 22 were identified by matching retention times and MS/MS spectra to standards. Relative standard deviations for peak detection was from 7 to 18% and was unaffected by storage for up to 11 days. The method was used to detect changes in metabolism associated with increasing extracellular islet glucose concentration from 3 to 20 mM yielding results largely consistent with known metabolism of islets. Because most previous studies of islet metabolism have only observed a few compounds at once and require far more tissue, this measurement method represents a significant advance for studies of metabolism of islets and other microscale samples.

Glucose or insulin, but not zinc ions, inhibit glucagon secretion from mouse pancreatic alpha-cells

The mechanisms by which hypoglycemia stimulates glucagon release are still poorly understood. In particular, the relative importance of direct metabolic coupling versus paracrine regulation by beta-cell secretory products is unresolved. Here, we compare the responses to glucose of 1) alpha-cells within the intact mouse islet, 2) dissociated alpha-cells, and 3) clonal alphaTC1-9 cells. Free cytosolic concentrations of ATP ([ATP](c)) or Ca(2+) ([Ca(2+)](c)) were imaged using alpha-cell-targeted firefly luciferase or a green fluorescent protein-based Ca(2+) probe ("pericam"), respectively. Consistent with a direct effect of glucose on alpha-cell oxidative metabolism, an increase in glucose concentration (from 0 or 3 mmol/l to 20 mmol/l) increased [ATP](c) by 7-9% in alpha-cells within the intact islet and by approximately 4% in alphaTC1-9 cells. Moreover, glucose also dose-dependently decreased the frequency of [Ca(2+)](c) oscillations in both dissociated alpha-cells and alphaTC1-9 cells. Although the effects of glucose were mimicked by exogenous insulin, they were preserved when insulin signaling was blocked with wortmannin. Addition of ZnCl(2) slightly increased the frequency of [Ca(2+)](c) oscillations but failed to affect glucagon release from either islets or alphaTC1-9 cells under most conditions. We conclude that glucose and insulin, but not Zn(2+) ions, independently suppress glucagon secretion in the mouse.

Physiological and molecular characteristics of rat hypothalamic ventromedial nucleus glucosensing neurons

To evaluate potential mechanisms for neuronal glucosensing, fura-2 Ca(2+) imaging and single-cell RT-PCR were carried out in dissociated ventromedial hypothalamic nucleus (VMN) neurons. Glucose-excited (GE) neurons increased and glucose-inhibited (GI) neurons decreased intracellular Ca(2+) ([Ca(2+)](i)) oscillations as glucose increased from 0.5 to 2.5 mmol/l. The Kir6.2 subunit mRNA of the ATP-sensitive K(+) channel was expressed in 42% of GE and GI neurons, but only 15% of nonglucosensing (NG) neurons. Glucokinase (GK), the putative glucosensing gatekeeper, was expressed in 64% of GE, 43% of GI, but only 8% of NG neurons and the GK inhibitor alloxan altered [Ca(2+)](i) oscillations in approximately 75% of GK-expressing GE and GI neurons. Insulin receptor and GLUT4 mRNAs were coexpressed in 75% of GE, 60% of GI, and 40% of NG neurons, although there were no statistically significant intergroup differences. Hexokinase-I, GLUT3, and lactate dehydrogenase-A and -B were ubiquitous, whereas GLUT2, monocarboxylate transporters-1 and -2, and leptin receptor and GAD mRNAs were expressed less frequently and without apparent relationship to glucosensing capacity. Thus, although GK may mediate glucosensing in up to 60% of VMN neurons, other regulatory mechanisms are likely to control glucosensing in the remaining ones.

Metabolic pathways that mediate inhibition of hypothalamic neurons by glucose

Neurons in the ventromedial hypothalamus mediate some counterregulatory responses to hypoglycemia and 2-deoxyglucose, but the mechanisms that mediate these responses to glucose are unclear. In the present study, ventromedial hypothalamus neurons were identified on the basis of their inhibition by the transition from 5 to 20 mmol/l glucose. Tolbutamide, which activates glucose-stimulated neurons, failed to inhibit or activate glucose-inhibited neurons. Inhibitors of glucose transport and glycolysis, in particular by the glucokinase inhibitor glucosamine, blocked the effect of glucose on glucose-inhibited neurons. Furthermore, the glucose-inhibited neurons were activated by 2-deoxyglucose, which also activates counterregulatory responses. Conversely, glucose-inhibited neurons were inhibited by glycolytic metabolites, including lactate, but not by pyruvate. These data indicate that hypoglycemia induces electrical activity in glucose-inhibited neurons by attenuating glycolysis in those neurons. Thus, counterregulatory failure could be due to relatively enhanced glycolysis in glucose-stimulated neurons during hypoglycemia and attenuation of glycolysis in glucose-inhibited neurons might reverse counterregulatory failure.

Effects of pH on murine insulinoma betaTC3 cells

Confluent monolayer cultures of betaTC3 cells were exposed for 4 h to acidic, neutral, or alkaline pH media. Studies determined the impact of pH on viability, insulin secretion rate, glucose consumption rate, lactate production rate, and ATP content. Cell viability was not affected by exposure to media of different pH (>95% for all groups). Insulin release from cells exposed to acidic media (pH of 6.4) was approximately 75% higher than that from cells exposed to either neutral (pH of 7.1) or alkaline (pH of 7.8) conditions. Conversely, ATP content was significantly reduced in cultures exposed to acidic conditions, although there was no statistical difference between neutral and alkaline conditions. Glucose consumption and lactate production rates increased linearly with increasing pH.

Glucose generates sub-plasma membrane ATP microdomains in single islet beta-cells. Potential role for strategically located mitochondria

Increases in the concentration of free ATP within the islet beta-cell may couple elevations in blood glucose to insulin release by closing ATP-sensitive K+ (KATP) channels and activating Ca2+ influx. Here, we use recombinant targeted luciferases and photon counting imaging to monitor changes in free [ATP] in subdomains of single living MIN6 and primary beta-cells. Resting [ATP] in the cytosol ([ATP]c), in the mitochondrial matrix ([ATP]m), and beneath the plasma membrane ([ATP]pm) were similar ( approximately 1 mM). Elevations in extracellular glucose concentration (3-30 mM) increased free [ATP] in each domain with distinct kinetics. Thus, sustained increases in [ATP]m and [ATP]pm were observed, but only a transient increase in [ATP]c. However, detectable increases in [ATP]c and [ATP]pm, but not [ATP]m, required extracellular Ca2+. Enhancement of glucose-induced Ca2+ influx with high [K+] had little effect on the apparent [ATP]c and [ATP]m increases but augmented the [ATP]pm increase. Underlying these changes, glucose increased the mitochondrial proton motive force, an effect mimicked by high [K+]. These data support a model in which glucose increases [ATP]m both through enhanced substrate supply and by progressive Ca2+-dependent activation of mitochondrial enzymes. This may then lead to a privileged elevation of [ATP]pm, which may be essential for the sustained closure of KATP channels. Luciferase imaging would appear to be a useful new tool for dynamic in vivo imaging of free ATP concentration.

Methylmalonate toxicity in primary neuronal cultures

Several inhibitors of mitochondrial complex II cause neuronal death in vivo and in vitro. The goal of the present work was to characterize in vitro the effects of malonate (a competitive blocker of the complex) which induces neuronal death in a pattern similar to that seen in striatum in Huntington's disease. Exposure of striatal and cortical cultures from embryonic rat brain for 24 h to methylmalonate, a compound which produces malonate intracellularly, led to a dose-dependent cell death. Methylmalonate (10 mM) caused >90% mortality of neurons although cortical cells were unexpectedly more vulnerable. Cell death was attenuated in a medium containing antioxidants. Further characterization revealed that DNA laddering could be detected after 3 h of treatment. Morphological observations (videomicroscopy and Hoechst staining) showed that both necrotic and apoptotic cell death occurred in parallel; apoptosis was more prevalent. A decrease in the ATP/ADP ratio was observed after 3 h of treatment with 10 mM methylmalonate. In striatal cultures it occurred concomitantly with a decline in GABA and a rise in aspartate content and the aspartate/glutamate ratio. Changes in ion concentrations were measured in similar cortical cultures from mouse brain. Neuronal [Na+]i increased while [K+]i and membrane potential decreased after 20 min of continuous incubation in 10 mM methylmalonate. These changes progressed with time, and a rise in [Ca2+]i was also observed after 1 h. The results demonstrate that malonate collapses cellular ion gradients, restoration of which imposes an additional load on the already compromised ATP-generation machinery. An early elevation in [Ca2+]i may trigger an increase in activity of proteases, lipases and endonucleases and production of free radicals and DNA damage which, ultimately, leads to cells death. The data also suggest that maturational and/or extrinsic factors are likely to be critical for the increased vulnerability of striatal neurons to mitochondrial inhibition in vivo.

Interplay between cytoplasmic Ca2+ and the ATP/ADP ratio: a feedback control mechanism in mouse pancreatic islets

In pancreatic beta cells, the increase in the ATP/ADP ratio that follows a stimulation by glucose is thought to play an important role in the Ca2+-dependent increase in insulin secretion. Here we have investigated the possible interactions between Ca2+ and adenine nucleotides in mouse islets. Measurements of both parameters in the same single islet showed that the rise in the ATP/ADP ratio precedes any rise in the cytoplasmic free-Ca2+ concentration ([Ca2+]i) and is already present during the initial transient lowering of [Ca2+]i produced by the sugar. Blockade of Ca2+ influx with nimodipine did not prevent the concentration-dependent increase in the ATP/ADP ratio produced by glucose and even augmented the ratio at all glucose concentrations which normally stimulate Ca2+ influx. In contrast, stimulation of Ca2+ influx by 30 mM K+ or 100 microM tolbutamide lowered the ATP/ADP ratio. This lowering was of rapid onset and reversibility, sustained and prevented by nimodipine or omission of extracellular Ca2+. It was, however, not attenuated after blockade of secretion by activation of alpha2-adrenoceptors. The difference in islet ATP/ADP ratio during blockade and stimulation of Ca2+ influx was similar to that observed between threshold and submaximal glucose concentrations. The results suggest that the following feedback loop could control the oscillations of membrane potential and [Ca2+]i in beta cells. Glucose metabolism increases the ATP/ADP ratio in a Ca2+-independent manner, which leads to closure of ATP-sensitive K+ channels, depolarization and stimulation of Ca2+ influx. The resulting increase in [Ca2+]i causes a larger consumption than production of ATP, which induces reopening of ATP-sensitive K+ channels and arrest of Ca2+ influx. Upon lowering of [Ca2+]i the ATP/ADP ratio increases again and a new cycle may start.

Role of ATP and Pi in the mechanism of insulin secretion in the mouse insulinoma betaTC3 cell line

Understanding the biochemical events associated with glucose-stimulated insulin secretion by pancreatic beta cells is of importance in gaining insight into both the pathophysiology of diabetes and the development of tissue-engineered bioartificial pancreatic substitutes. We have investigated the effects of glucose concentration on the bioenergetic status and on the metabolic and secretory functions exhibited by mouse insulinoma betaTC3 cells entrapped in calcium alginate/poly-L-lysine/alginate (APA) beads. Cells entrapped in APA beads constitute a possible implantable bioartificial pancreas for the long-term treatment of insulin-dependent diabetes mellitus. Our results show that, in entrapped betaTC3 cells, the oxygen consumption rate and the intracellular nucleotide triphosphate levels are unaffected by a step change in glucose concentration from 16 mM to 0 mM for 4.5 h and then back to 16 mM. The intracellular Pi level and the ammonia production rate were doubled, while insulin secretion was decreased 10-fold, upon switching from 16 mM to 0 mM glucose. The implications of these findings in the context of pancreatic beta cell biochemistry and the mechanism of the 'Fuel Hypothesis' are discussed.

Tight links between adenine and guanine nucleotide pools in mouse pancreatic islets: a study with mycophenolic acid

Glucose metabolism in pancreatic B-cells leads to an increase in the ATP/ADP ratio that might participate in the regulation of insulin secretion. Good correlations have also been observed between guanine nucleotide levels in isolated pancreatic islets and insulin secretion. To assess whether guanine nucleotides have a specific role in stimulus-secretion coupling, their concentration should be modified selectively. This was attempted by culturing mouse islets overnight in the presence of mycophenolic acid (MPA), an inhibitor of GMP synthesis at the level of IMP dehydrogenase. The drug (25-50 microg/ml) did not affect the insulin content but decreased the GTP content of the islets and inhibited insulin secretion during subsequent incubation in the presence of 15 mM glucose. However, MPA also decreased the ATP/ADP ratio in the islets. The addition of guanine to the culture medium (to stimulate the salvage pathway of GTP synthesis) restored normal GTP levels, corrected the ATP/ADP ratio and partly prevented the inhibition of insulin release. In contrast, attempts to stimulate ATP synthesis specifically (by provision of adenine or adenosine) failed to reverse any of the effects of MPA. It is concluded that guanine and adenine nucleotide pools are tightly linked and cannot be specifically affected by MPA in pancreatic islet cells, probably because of the activity of nucleoside diphosphate kinase and because of the role of GTP in several reactions leading to adenine nucleotide generation. Contrary to previous claims, MPA is not an adequate tool for evaluating a specific role of guanine nucleotides in the control of insulin secretion.

Ca(2+)- and GTP-dependent exocytosis in mouse pancreatic beta-cells involves both common and distinct steps

1. The effects of GTP and Ca2+ on secretion from single pancreatic beta-cells were studied using capacitance measurements as an indicator of exocytosis. 2. GTP or GTP gamma S produced a concentration-dependent increase in cell capacitance in the absence of intracellular calcium. There was no effect of cyclic AMP or BAPTA an GTP-induced secretion. 3. In the absence of GTP, the relationship between intracellular calcium concentration and the maximum rate of secretion was fitted by the Hill equation with a slope factor of 2.5 and half-maximal activation at 1.6 microM intracellular Ca2+. Similar values were obtained in the presence of GTP gamma S, suggesting GTP does not alter the sensitivity of the secretory machinery to Ca2+. 4. GDP beta S alone had no effect on cell capacitance but caused a dose-dependent inhibition of exocytosis induced by infusion of either GTP gamma S or Ca2+, suggesting both stimuli involve G-protein activation. GDP beta S was without effect on exocytosis evoked by depolarization-mediated Ca2+ entry. 5. The time course of exocytosis following rapid elevation of GTP gamma S by photolysis of a caged precursor was dependent on the intracellular Ca2+ and cyclic AMP concentrations. 6. Our results are interpreted in terms of a model in which the secretory pathways stimulated by Ca2+ and GTP contain both common and separate parts.

Regulation of pancreatic beta-cell mitochondrial metabolism: influence of Ca2+, substrate and ADP

To gain insight into the regulation of pancreatic beta-cell mitochondrial metabolism, the direct effects on respiration of different mitochondrial substrates, variations in the ATP/ADP ratio and free Ca2+ were examined using isolated mitochondria and permeabilized clonal pancreatic beta-cells (HIT). Respiration from pyruvate was high and not influenced by Ca2+ in State 3 or under various redox states and fixed values of the ATP/ADP ratio; nevertheless, high Ca2+ elevated pyridine nucleotide fluorescence, indicating activation of pyruvate dehydrogenase by Ca2+. Furthermore, in the presence of pyruvate, elevated Ca2+ stimulated CO2 production from pyruvate, increased citrate production and efflux from the mitochondria and inhibited CO2 production from palmitate. The latter observation suggests that beta-cell fatty acid oxidation is not regulated exclusively by malonyl-CoA but also by the mitochondrial redox state. alpha-Glycerophosphate (alpha-GP) oxidation was Ca(2+)-dependent with a half-maximal rate observed at around 300 nM Ca2+. We have recently demonstrated that increases in respiration precede increases in Ca2+ in glucose-stimulated clonal pancreatic beta-cells (HIT), indicating that Ca2+ is not responsible for the initial stimulation of respiration [Civelek, Deeney, Kubik, Schultz, Tornheim and Corkey (1996) Biochem. J. 315, 1015-1019]. It is suggested that respiration is stimulated by increased substrate (alpha-GP and pyruvate) supply together with oscillatory increases in ADP [Nilsson, Schultz, Berggren, Corkey and Tornheim (1996) Biochem. J. 314, 91-94]. The rise in Ca2+, which in itself may not significantly increase net respiration, could have the important functions of (1) activating the alpha-GP shuttle, to maintain an oxidized cytosol and high glycolytic flux; (2) activating pyruvate dehydrogenase, and indirectly pyruvate carboxylase, to sustain production of citrate and hence the putative signal coupling factors, malonyl-CoA and acyl-CoA; and (3) increasing mitochondrial redox state to implement the switch from fatty acid to pyruvate oxidation.

Concentration dependence and time course of the effects of glucose on adenine and guanine nucleotides in mouse pancreatic islets

Changes in the ATP:ADP ratio in pancreatic B cells may participate in the regulation of insulin secretion by glucose. Here, we have investigated the possible role of guanine nucleotides. Mouse islets were incubated in a control medium (when K+-ATP channels are the major site of regulation) or in a high K+ medium (when glucose modulates the effectiveness of cytosolic Ca2+ on exocytosis). Glucose induced a concentration-dependent (0-20 m) increase in GTP and a decrease in GDP in both types of medium, thus causing a progressive rise of the GTP:GDP ratio. ATP and ADP levels were 4-5-fold higher but varied in a similar way as those of guanine nucleotides. Insulin secretion was inversely correlated with ADP and GDP levels and positively correlated with the ATP:ADP and GTP:GDP ratios between 6 and 20 m glucose in control medium and between 0 and 20 m glucose in high K+ medium. The increases in the GTP:GDP and ATP:ADP ratios induced by a rise of glucose were faster than the decreases induced by a fall in glucose, but the changes of both ratios were again parallel. In conclusion, glucose causes large, concentration-dependent changes in guanine as well as in adenine nucleotides in islet cells. This raises the possibility that both participate in the regulation of nutrient-induced insulin secretion.

Limitation of glycolysis by hexokinase in rat brain synaptosomes during intense ion pumping

Incubation of rat brain synaptosomes under conditions of either increased energy utilization (addition of Na+ channel opener, veratridine, or ionophores, monensin and nigericin) or inhibition of oxidative phosphorylation (addition of rotenone), or a combination thereof, decreased [ATP], increased [ADP] and stimulated glycolysis. The rates of lactate generation were linear over a 15-min interval in the presence of rotenone alone but decreased in the other two conditions. During the first 5 min, the amount of lactate formed with veratridine, monensin or nigericin was as high or higher than with rotenone, but it was lower in the last 10 min. With a combination of one of the stimulators of ion movements and rotenone the rate of glycolysis was always markedly lower than with each compound added singly. The stimulated rates of lactate formation correlated positively with the synaptosomal content of [ATP]. After 15 min, [ATP] was 0.9-1.0 nmol/mg with rotenone, 0.5-0.9 nmol/mg with veratridine (or ionophores), and <0.3 nmol/mg with a combination of the two. Under the conditions used, calcium did not affect glycolytic activity directly. The Lineweaver-Burk plot of the rate of lactate formation against [ATP] yielded a straight line with a Km for ATP of about 0.1 mM, which is very similar to the Km for this nucleotide of brain hexokinase bound to mitochondria. In C6 cells glycolytic rate measured with a combination of an ionophore and rotenone was higher than with each of these compounds added singly while [ATP] never declined below about 9 nmol/mg prot. It is concluded that in synaptosomes, the high rate of energy utilization required for intense ion movement decreases [ATP] to a level that limits hexokinase activity kinetically. This may contribute to a reduction in the rate of glycolysis and hence energy production in brain hypoxia and ischemia.

Evidence for association of an ATP-stimulatable Ca(2+)-independent phospholipase A2 from pancreatic islets and HIT insulinoma cells with a phosphofructokinase-like protein

Glucose-induced insulin secretion from pancreatic islets requires metabolism of glucose within islet beta-cells, and ATP has attracted interest as a messenger of glucose metabolism within beta-cells. Glucose-induced insulin secretion from islets and HIT insulinoma cells is accompanied by activation of an ATP-stimulatable Ca(2+)-independent phospholipase A2 (ASCI-PLA2) enzyme, the catalytic activity of which resides in a 40 kDa protein. An analogous PLA2 enzyme in myocardium was recently found to consist of a complex of a 40 kDa catalytic protein with a tetramer of an isoform of the glycolytic enzyme phosphofructokinase (PFK). Association of the PFK isoform with the myocardial PLA2 catalytic protein was found to confer ATP sensitivity onto the enzyme complex. Here we demonstrate that the majority of HIT cell and islet ASCI-PLA2 catalytic activity elutes from a gel filtration column in a region corresponding to 400 kDa, suggesting that the 40 kDa beta-cell ASCI-PLA2 catalytic protein exists as part of a larger molecular mass complex. Islet and HIT cell ASCI-PLA2 activities were immunoprecipitated by antibodies directed against PFK, and the immunoprecipitates contained 40 and 85 kDa proteins which correspond to the molecular masses of the PLA2 catalytic protein and of a PFK monomer, respectively. Islet and HIT cell ASCI-PLA2 activities were selectively and reversibly adsorbed to affinity matrices containing immobilized PFK but not to similar matrices containing immobilized transferrin or bovine serum albumin. Addition of free PFK prevented binding of HIT cell ASCI-PLA2 activity to immobilized PFK matrices and promoted desorption of activity previously bound to such matrices. These results suggest that beta-cell ASCI-PLA2, like the myocardial enzyme, exists as a complex comprised of a catalytic protein and a PFK-like protein and raise the possibility that the ASCI-PLA2 complex may represent a component of the beta-cell glucose sensor, which links glycolysis, phospholipid hydrolysis, and membrane electrochemical events involved in glucose-induced insulin secretion.

Evidence of a role for GTP in the potentiation of Ca(2+)-induced insulin secretion by glucose in intact rat islets

Glucose initiates insulin secretion by closing K(+)-ATP channels, leading to Ca2+ influx (E1); it also potentiates Ca(2+)-induced secretion (E2) when the K(+)-ATP channel is kept open using diazoxide and depolarizing concentrations of K+ are provided. To examine the roles of purine nucleotides in E2, we compared the effects of glucose to those of the mitochondrial fuel monomethylsuccinate. Either agonist could induce E2 accompanied by significant increases in ATP, ATP/ADP ratio, and GTP/GDP ratio; GTP increased significantly only with glucose. Mycophenolic acid (MPA), an inhibitor of cytosolic GTP synthesis, markedly inhibited glucose-induced E2 (either in perifusions or in static incubations) and decreased GTP and the GTP/GDP ratio, but did not alter the ATP/ADP ratio. Provision of guanine (but not adenine) reversed these changes pari passu. In contrast, MPA had no effect on succinate-induced E2, despite generally similar changes in nucleotides. A similar lack of effect of MPA on E2 was seen with a second mitochondrial fuel, alpha-ketoisocaproic acid (KIC). However, in the absence of diazoxide and K+, MPA blunted the secretory effects of either glucose, succinate, or KIC. These studies suggest that GTP plays a role in both glucose and succinate or KIC-induced insulin secretion at a step dependent on mitochondrial metabolism and the K(+)-ATP channel. In addition to mitochondrial effects, glucose appears to have extramitochondrial effects important to its potentiation of Ca(2+)-induced insulin secretion that are also dependent on GTP.

Regulation of the glutamate dehydrogenase activity in rat islets of Langerhans and its consequence on insulin release

Kinetic properties of glutamate dehydrogenase (GDH) and the effects on its activity of several putative modulators were examined in mitochondrial extracts of rat pancreatic islets. In the presence of 40 mmol/L NH4Cl and 0.1 mmol/L NADH, stepwise elevation of the 2-oxoglutarate concentration from 0.005 to 0.05 mmol/L increased glutamate formation, whereas further increases led to a progressive decrease of the reaction velocity. Adenosine diphosphate (ADP) at 0.1 mmol/L partially and at 1 mmol/L completely reversed the inhibitory effect of 2-oxoglutarate. The sensitivity to activation by either ADP or leucine was dependent on 2-oxoglutarate concentrations. At higher concentrations of the latter, greater amounts of the activators were needed to attain maximal effect. In the absence of allosteric activators, sulfate or phosphate at 20 mmol/L partially released the inhibitory effect of 2-oxoglutarate levels and increased the maximal velocity (Vmax) for the reaction. In the presence of 0.1 mmol/L ADP, both anions prevented the inhibition by higher concentrations of 2-oxoglutarate, whereas with 1 mmol/L ADP their only effect was a slight increase in the Vmax. Mg2+ and naturally occurring polyamines decreased glutamate formation in a dose-dependent manner; with 0.1 mmol/L ADP, inhibition was seen at all 2-oxoglutarate concentrations studied, whereas with 1 mmol/L ADP, it was noticeable at substrate concentrations higher than 0.5 mmol/L. This inhibitory effect on GDH activity was partially attenuated by sulfate. Addition of either 2 mmol/L spermidine or extra magnesium (final 2.5 or 5 mmol/L) to the perifusion buffer markedly attenuated the insulin release elicited by alpha-ketoisocaproate. It is suggested that naturally occurring polyamines, magnesium, and phosphate act as physiological modulators of GDH activity in pancreatic beta cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral alanine transport and alanine aminotransferase reaction: alanine as a source of neuronal glutamate

Alanine transport and the role of alanine amino-transferase in the synthesis and consumption of glutamate were investigated in the preparation of rat brain synaptosomes. Alanine was accumulated rapidly via both the high- and low-affinity uptake systems. The high-affinity transport was dependent on the sodium concentration gradient and membrane electrical potential, which suggests a cotransport with Na+. Rapid accumulation of the Na(+)-alanine complex by synaptosomes stimulated activity of the Na+/K+ pump and increased energy utilization; this, in turn, activated the ATP-producing pathways, glycolysis and oxidative phosphorylation. Accumulation of Na+ also caused a small depolarization of the plasma membrane, a rise in [Ca2+]i, and a release of glutamate. Intra-synaptosomal metabolism of alanine via alanine amino-transferase, as estimated from measurements of N fluxes from labeled precursors, was much slower than the rate of alanine uptake, even in the presence of added oxoacids. The velocity of [15N]alanine formation from [15N]glutamine was seven to eight times higher than the rate of [15N]-glutamate generation from [15N]alanine. It is concluded that (a) overloading of nerve endings with alanine could be deleterious to neuronal function because it increases release of glutamate; (b) the activity of synaptosomal alanine aminotransferase is much slower than that of glutaminase and hence unlikely to play a major role in maintaining [glutamate] during neuronal activity; and (c) alanine amino-transferase might serve as a source of glutamate during recovery from ischemia/hypoxia when the alanine concentration rises and that of glutamate falls.

Small elevations of glucose concentration redirect and amplify the synthesis of guanosine 5'-triphosphate in rat islets

Recent studies suggest a permissive requirement for guanosine 5'-triphosphate (GTP) in insulin release, based on the use of GTP synthesis inhibitors (such as myocophenolic acid) acting at inosine monophosphate (IMP) dehydrogenase; herein, we examine the glucose dependency of GTP synthesis. Mycophenolic acid inhibited insulin secretion equally well after islet culture at 7.8 or 11.1 mM glucose (51% inhibition) but its effect was dramatically attenuated when provided at < or = 6.4 mM glucose (13% inhibition; P < 0.001). These observations were explicable by a stimulation of islet GTP synthesis derived from IMP since, at high glucose: (a) total GTP content was augmented; (b) a greater decrement in GTP (1.75 vs. 1.05 pmol/islet) was induced by mycophenolic acid; and (c) a smaller "pool" of residual GTP persisted after drug treatment. Glucose also accelerated GTP synthesis from exogenous guanine ("salvage" pathway) and increased content of a pyrimidine, uridine 5'-triphosphate (UTP), suggesting that glucose augments production of a common regulatory intermediate (probably 5-phosphoribosyl-1-pyrophosphate). Pathway-specific radiolabeling studies confirmed that glucose tripled both salvage and de novo synthesis of nucleotides. We conclude that steep changes in the biosynthesis of cytosolic pools of GTP occur at modest changes in glucose concentrations, a finding which may have relevance to the adaptive (patho) physiologic responses of islets to changes in ambient glucose levels.

Mechanisms by which glucose can control insulin release independently from its action on adenosine triphosphate-sensitive K+ channels in mouse B cells

Glucose stimulation of insulin release involves closure of ATP-sensitive K+ channels (K(+)-ATP channels), depolarization, and Ca2+ influx in B cells. However, by using diazoxide to open K(+)-ATP channels, and 30 mM K to depolarize the membrane, we could demonstrate that another mechanism exists, by which glucose can control insulin release independently from changes in K(+)-ATP channel activity and in membrane potential (Gembal et al. 1992. J. Clin. Invest. 89:1288-1295). A similar approach was followed here to investigate, with mouse islets, the nature of this newly identified mechanism. The membrane potential-independent increase in insulin release produced by glucose required metabolism of the sugar and was mimicked by other metabolized secretagogues. It also required elevated levels of cytoplasmic Cai2+, but was not due to further changes in Cai2+. It could not be ascribed to acceleration of phosphoinositide metabolism, or to activation of protein kinases A or C. Thus, glucose did not increase inositol phosphate levels and hardly affected cAMP levels. Moreover, increasing inositol phosphates by vasopressin or cAMP by forskolin, and activating protein kinase C by phorbol esters did not mimic the action of glucose on release, and down-regulation of protein kinase C did not prevent these effects. On the other hand, it correlated with an increase in the ATP/ADP ratio in islet cells. We suggest that the membrane potential-independent control of insulin release exerted by glucose involves changes in the energy state of B cells.

Interleukin-1 beta effects on cyclic GMP and cyclic AMP in cultured rat islets of Langerhans-arginine-dependence and relationship to insulin secretion

When islets were cultured with interleukin-1 beta (1 or 100 pmol/l) for 12 h in arginine-containing medium, cyclic GMP levels were increased 1.6- and 4.5-fold respectively. The arginine analogue, N-omega-nitro-L-arginine methyl ester, which blocks nitric oxide formation and partially reverses inhibition of insulin secretion by 100 pmol/l interleukin-1 beta, largely, but not completely, blocked generation of cyclic GMP. Treatment of islets with 100 pmol/l interleukin-1 beta for 12 h significantly decreased islet cyclic AMP generation in the absence of isobutylmethylxanthine (from 13.1 +/- 0.7 to 9.3 +/- 0.8 fmol/micrograms islet protein), this fall was arginine-dependent and may have resulted from an effect on a cyclic AMP phosphodiesterase, since it was masked if isobutylmethylxanthine was present. Isobutylmethylxanthine (0.4 mmol/l) reduced the inhibitory potency of interleukin-1 beta in 15 h slightly but significantly from 80.5 to 59.0%. The morpholinosydnonimine SIN-1, which is a nitric oxide donor, inhibited insulin secretion, raised islet cyclic GMP and lowered cyclic AMP; its effects were similar to those of interleukin-1 beta. However, 6-anilinoquinoline-5,8-quinone, [LY83583 (1-10 mumol/l)], inhibited insulin secretion, and significantly decreased cyclic GMP while 8-bromocyclic GMP stimulated insulin secretion. Both low- and high-dose interleukin-1 beta treatment give a large arginine-dependent and a small, yet significant, arginine-independent increase in cyclic GMP. The inhibitory effect of SIN-1 or interleukin-1 beta on insulin secretion seems to depend to a small extent on decreased islet cyclic AMP, though sustained increases in nitric oxide or depleted islet GTP may directly affect the secretory process.

Glutamate production in islets of Langerhans: properties of phosphate-activated glutaminase

Homogenates of rat pancreas, pancreatic islets, and HIT-T15 cells (a clonal line derived from B cells) catalyzed the breakdown of glutamine to glutamate. This activity was markedly stimulated by the addition of orthophosphate and was much greater in homogenates from islets and the B-cell-derived clonal cell line than in those from whole pancreas. Islet glutaminase was half-maximally stimulated with 40 mmol/L phosphate. Kinetic analyses of the rates of glutamine hydrolysis showed that the Vmax for the reaction increased with the increase in phosphate concentration, whereas the Km for glutamine (2.6 +/- 0.2 mmol/L) was unaltered. The pH optimum for enzyme activity was 8.0 to 8.5 at all phosphate concentrations studied. Glutamine breakdown was enhanced by adenosine triphosphate ([ATP] approximately 100% at 10 mmol/L) and citrate (approximately 30% at 10 mmol/L), but it was unaffected by malate, 2-oxoglutarate, lactate, and ammonia. Glutamate significantly inhibited glutamine hydrolysis. Freshly isolated islets had a low content of both glutamate and glutamine. After culturing for 1 hour in an amino acid-containing medium, the concentrations of glutamine and glutamate increased. Subsequent perifusion without amino acids caused a loss of glutamine and a concomitant increase in glutamate level. Perifusion with 1 mmol/L glutamine led to an increase in both internal glutamine and glutamate. The addition to the perifusion medium of either 10 mmol/L glutamine, 10 mmol/L orthophosphate, or both substantially enhanced insulin release evoked by 10 mmol/L leucine.(ABSTRACT TRUNCATED AT 250 WORDS)

Energetic metabolism of glucose, mannose and galactose in glucose-starved rat insulinoma cells anchored on microcarrier beads. A phosphorus-31 NMR study

Insulin-secreting cells (RINm5F) have successfully been grown on a large scale on poly-L-lysine coated-polystyrene microcarriers, providing a high cell number in a restricted volume under conditions that respect the metabolic integrity of these anchorage-dependent cells. The energetic metabolism of the perfused cells has been followed non-invasively by phosphorus-31 nuclear magnetic resonance spectroscopy. Glucose starvation induced a rapid decrease in nucleoside triphosphates (mainly ATP) pools, correlated with an increase in Pi level. The initial ATP level was rapidly recovered when the cells were refed with glucose or with mannose, but not with galactose, even after 2 h of perfusion. These differential effects of hexoses on energetic metabolism might be related to their various insulin-release actions on tumor islet cells.

Relationships between energy level and insulin secretion in isolated rat islets of Langerhans. Manipulation of [ATP]/[ADP][Pi] by 2-deoxy-D-glucose

Perifusion of islets with nominally phosphate-free buffer containing increasing concentrations of 2-deoxy-D-glucose (2.5 to 10 mM) produced increments in high alpha-ketoisocaproic acid-induced secretion of insulin beyond those observed in the absence of the sugar analogue. 3-O-methyl-D-glucose, a poorly metabolized sugar, was without effect. Insulin release evoked by 40 mM KCl was not altered by 2-deoxyglucose. The concentration of intracellular inorganic phosphate was lower in islets perifused with 2-deoxyglucose and declined to a lower level after addition of 20 mM alpha-ketoisocaproic acid. The enhancement of alpha-ketoisocaproic acid-induced hormone secretion by 2-deoxyglucose was not seen in islets perifused with medium containing 1.5 mM phosphate; instead a small inhibition was observed. It is postulated that conditions which lower intracellular [Pi] facilitate, either directly or indirectly, hormone release although the mechanism of this effect remains to be elucidated.

Relationships between energy level and insulin secretion in isolated rat islets of Langerhans. A study at various pH values

To define better the role of [ATP]/[ADP] in insulin release from pancreatic islets, changes in the adenine nucleotide ratios elicited by alterations in external pH were correlated with the secretion profiles produced by administration of two metabolic secretagogues, 16 mM glucose and 10 mM alpha-ketoisocaproic acid. Experiments were carried out in buffers with and without bicarbonate, in the pH range 6.5-7.7. Insulin release was dependent on pHe irrespective of the secretagogue used. Secretion profiles for alpha-ketoisocaproic acid were the same both with and without bicarbonate; the release was decreased below pH 7.1 but maintained at 7.4-7.7. The same pattern was seen with glucose in media buffered with Hepes. With bicarbonate present, secretion caused by high glucose showed a bell-shaped dependence on [H+], with reductions at the acid and alkaline sides of pH 7.1-7.4. [ATP] and [ADP] were higher when Hepes was the buffer, at all pH values studied. The [ATP]/[ADP] declined with increasing pH under both basal and stimulated conditions; the values were always larger after stimulation although at pH 7.7 with bicarbonate present and glucose as the stimulant the difference was very small. It is concluded that: (i) the [ATP]/[ADP] in pancreatic islets is markedly dependent on pHe; (ii) there is no straight-forward correlation between either [ATP] or the absolute value for [ATP]/[ADP] and insulin secretion; and (iii) a rise in [ATP]/[ADP] is necessary for glucose-stimulated insulin release although it is not always the rate-determining event.