Insulin Release In Pancreatic Islets Research Articles

Concordant glucose induction of glucokinase, glucose usage, and glucose-stimulated insulin release in pancreatic islets maintained in organ culture

Concordant glucose induction of glucokinase, glucose usage, and glucose-stimulated insulin release in pancreatic islets maintained in organ culture

Nicotinamide partially reverses the interleukin-1β inhibition of glucose-induced insulin release in pancreatic islets

Interleukin-1β (IL-1β) is known to inhibit glucose-induced insulin release by pancreatic islets. We studied the effect of nicotinamide, an inhibitor of poly[adenosine diphosphate (ADP)-ribose] synthetase and a free-radical scavenger, on this IL-1β-induced inhibition using rat pancreatic islets. In static experiments, groups of five islets were incubated for 24 hours in culture medium CMRL-1066, with or without 50 U/mL IL-1β, in the presence or absence of nicotinamide (dose range, 0 to 50 mmol/L), and then exposed for 1 hour to either 1.4 or 19.4 mmol/L glucose, 10 mmol/L arginine, or 10 μmol/L glyburide. Basal insulin secretion was 183 ± 32 pg/islet/h (mean ± SE, n = 7) and 176 ± 39 (n = 7) in control islets and in islets exposed to 50 U/mL IL-1β, respectively. Glucose-stimulated insulin secretion was significantly reduced (185 ± 41) in IL-1β-exposed islets in comparison to control islets (2,037 ± 363). In parallel, arginine-stimulated insulin release was inhibited by IL-1β exposure (166 ± 31 pg/islet/h, mean ± SE, n = 3) in comparison to control islets (1,679 ± 307). In contrast, IL-1β exposure did not significantly reduced glyburide-induced insulin secretion (1,516 ± 231 and 1,236 ± 214 in control and IL-1β-exposed islets, respectively; mean ± SE, n = 3). When islets were simultaneously exposed to IL-1β and increasing concentrations of nicotinamide, a dose-dependent recovery of glucose-induced insulin secretion was observed, with the maximum effect at 25 mmol/L nicotinamide (1,007 ± 123, P < .001). Arginine-stimulated insulin release was also partially restored by the addition of 25 mmol/L nicotinamide. To study the phases of insulin release, groups of 100 islets exposed to IL-1β in the presence or absence of nicotinamide (as in static experiments) were perifused for 20 minutes at 37°C in phosphate buffer with glucose 1.4 mmol/L (basal secretion) and then stimulated with glucose 19.4 mmol/L for 35 minutes. In control islets, glucose-stimulated insulin secretion followed the characteristic biphasic pattern: the first peak was 54.9 ± 9.7 and the second one was 70.8 ± 12.5 pg/islet/min (mean ± SE, n = 7). In islets exposed to IL-1β, both phases were greatly reduced (4.9 ± 1.1 and 7.8 ± 2.2, respectively; P < .001). In the presence of 25 mmol/L nicotinamide, glucose-stimulated insulin secretion was partially restored (19.3 ± 3.9, P < .01 in the first phase, and 28.1 ± 6.7, P < .01 in the second phase). Nicotinamide, therefore, is able to partially prevent the effect of IL-1β on blunting both glucose- and arginine-induced insulin secretion.

An inhibitor of sweet taste response modulates glucose-induced phosphoinositide breakdown in rat pancreatic islets.

We studied the effect of a specific-competitive inhibitor of the sucrose taste response, p-nitrophenyl-D-glucopyranoside (PNP-Glu) on insulin release and phosphoinositide metabolism in rat pancreatic islets. The alpha-anomer, but not the beta-anomer, of PNP-Glu at a concentration of 5 mM inhibited insulin release induced by 10 mM glucose. Islets were labeled by exposure for 2 h to 10 uCi of myo-[2-3H] inositol solution supplemented with 2.8 mM glucose. Forty islets were then incubated in the presence of 10 mM LiCl, 1 mM inositol and 10 mM glucose with or without the anomers of PNP-Glu. [3H] radioactivity in the incubation medium remained significantly greater in the presence of the alpha-anomer of PNP-Glu than in the presence of glucose alone after 5- and 20-min incubation. The inositol monophosphate levels in the islets incubated with glucose alone were increased more than in the islets with alpha-anomer. The beta-anomer of PNP-Glu did not change either glucose-induced insulin release or phosphoinositide breakdown. A patch-clamp study revealed that neither anomer affected the glucose-dependent ATP-sensitive K(+)-channels. These results indicate that the anomeric preference for glucose in insulin release in the pancreatic islets is closely associated with phosphoinositide breakdown.

Pyruvate dehydrogenase and pyruvate carboxylase. Sites of pretranslational regulation by glucose of glucose-induced insulin release in pancreatic islets.

It has been shown previously that glucose-induced insulin release is completely absent in rat pancreatic islets that had been cultured for 1 day at low glucose (1 mM) and that it is restored by culturing islets for a 2nd day at high (20 mM) glucose (MacDonald, M. J., Fahien, L. A., McKenzie, D. I., and Moran, S. M. (1991) Am. J. Physiol. 259, E548-E554). It has been suggested that the incapacitation of glucose's insulinotropism is due to down-regulation of the synthesis of enzymes that process glucose's metabolic signal for insulin release. In the current study, results of metabolic, enzymic, and molecular biologic experiments were each consistent with (an) intramitochondrial site(s) of down-regulation in islets cultured at low glucose. Glucose metabolism was inhibited 80% in islets cultured at 1 mM glucose. The suppression of release of 14CO2 from [6-14C]glucose greater than from [U-14C]glucose greater than [3,4-14C]glucose greater than from [1-14C]glucose in islets cultured at low glucose indicated a mitochondrial site of down-regulation because C-6 of glucose can only be converted to CO2 in the citric acid cycle, whereas C-1 can be released as CO2 in the 6-phosphogluconate dehydrogenase [corrected] reaction, and C-6 of glucose dwells in the citric acid cycle longer than carbons 2-5 of glucose. Since carbons 3 and 4 of glucose can be decarboxylated in the pyruvate dehydrogenase reaction, incomplete suppression of CO2 formation from these carbons is consistent with suppression of pyruvate carboxylation as well as decarboxylation. Formation of 3HOH from [5-3H]glucose was equal in the two groups of islets, indicating that glycolysis as far as phosphoenolpyruvate was intact. This idea was supported by assays which showed that activities of enzymes of the glycolytic pathway between glucokinase/hexokinase and pyruvate kinase were equal in both types of islets. Additional studies indicated that regulation by glucose was at transcription of genes coding for some mitochondrial enzymes. Glucokinase, malic enzyme, and fumarase mRNAs were not affected by glucose, whereas the pyruvate dehydrogenase E1 alpha subunit and pyruvate carboxylase mRNAs were decreased 85-90% in islets cultured at 1 mM glucose. Pyruvate dehydrogenase enzyme activity was decreased to a similar extent in these islets. About 24 h was required for maximal (de)induction of pyruvate dehydrogenase E1 alpha and pyruvate carboxylase mRNAs, and the amounts of transcripts were proportional to the concentrations of glucose between 1 and 20 mM.(ABSTRACT TRUNCATED AT 400 WORDS)

Quinone Reductase Enzyme Activity in Pancreatic Islets*

A water-soluble quinone, coenzyme Q0 (CoQ0), was shown to stimulate insulin release, and dicumarol, an inhibitor of quinone reductase, inhibited glucose-induced insulin release in pancreatic islets. Since this suggested that quinone reductase might play some role in physiological insulin release, this enzyme was characterized in islets. More than 90% of the total activity was located in the cytosol, but the specific enzyme activity was highest in the microsomal fraction. The relative rates of activity with various substrates (CoQ0 approximately equal to durohydroquinone greater than menadione greater than duroquinone greater than CoQ6 = CoQ10 greater than ferricyanide) were similar to those described previously for quinone reductase from liver Dicumarol, chlorpromazine, and T3 were much more potent inhibitors of the enzyme when NADPH was the coenzyme than when NADH was the coenzyme. Dicumarol was the most potent inhibitor. The enzyme was not inhibited by rotenone. Islets ranked second to liver in quinone reductase activity, but the activity in islets was much closer to that found in all other tissues examined. Quinone reductase may play a role in insulin secretion.

O-038 - Impairment of insulin release in pancreatic islets isolated from virus-infected hamsters.

O-038 - Impairment of insulin release in pancreatic islets isolated from virus-infected hamsters.

Insulin release in pancreatic islets by a glycolytic and a krebs cycle intermediate: Contrasting patterns of glyceraldehyde phosphate and succinate

Glyceraldehyde phosphate, a glycolytic intermediate, and succinic acid (as its methyl ester to make it permeable to the cell), a citric acid cycle intermediate, were the only glucose metabolites of many recently tested that stimulated insulin release. The effects of these two “new” insulin secretagogues on several pancreatic islet parameters were compared. Glyceraldehyde phosphate stimulated all of the insulin it released during the first 5 min after islets were exposed to it, and its maximum effect on calcium uptake was observed at 5 min. Monomethyl succinate stimulated insulin release mostly during the last 30 min of a 1-h incubation and its maximum effect on calcium uptake was at 60 min after it was applied to islets. Monomethyl succinate-induced insulin release, but not glyceraldehyde phosphate-induced insulin release, was inhibited by metabolic inhibitors (antimycin A, rotenone, cyanide, FCCP, fluoride, and iodoacetamide). This is consistent with the idea that monomethyl succinate is hydrolyzed to succinate which is metabolized intramitochondrially. The effects of glyceraldehyde suggest that glucose signals the first phase of insulin release by an agonist-like mechanism that originates in the cytosol and requires minimal energy. The effects of monomethyl succinate suggest that the signal for the second phase of glucose-induced insulin release originates in the mitochondrion and requires a large amount of energy.

Mechanisms of protection by glucose against interleukin-1 induced impairment of pancreatic islet insulin release: [formula omitted], Décio L. Eizirik, Eva Strandell, Michael Welsh and Nils Welsh. Department of Medical Cell Biology, Uppsala University, BMC, P.O. Box 571, S-751 23 Uppsala, Sweden

Mechanisms of protection by glucose against interleukin-1 induced impairment of pancreatic islet insulin release: [formula omitted], Décio L. Eizirik, Eva Strandell, Michael Welsh and Nils Welsh. Department of Medical Cell Biology, Uppsala University, BMC, P.O. Box 571, S-751 23 Uppsala, Sweden

Stimulation by cholera toxin of ADP-ribosylation of membrane proteins, adenylate cyclase and insulin release in pancreatic islets.

In rat pancreatic islet membranes exposed to [alpha-32P]NAD, cholera toxin stimulated the labelling of three peptides with Mr close to 22 000, 42 000 and 48 000, respectively. In the islets, the toxin-stimulated ADP-ribosylation of the heavy form of the Ns alpha-subunit predominated over that of the light form, in mirror image of the situation found in the exocrine pancreas. When intact islets were preincubated with cholera toxin, the adenylate cyclase activity of a subcellular particulate fraction was increased. The responsiveness of adenylate cyclase to GTP was also augmented, but that to NaF was decreased. In intact islets, the production of cyclic AMP and the glucose-stimulated release of insulin were also enhanced after pretreatment with cholera toxin. These findings reveal the presence in pancreatic islets of the guanyl nucleotide regulatory protein of adenylate cyclase, with an unusual predominance of the heavy form of the Ns alpha-subunit.

Effects of Enflurane on Release of Insulin by Pancreatic Islets in Vitro

The effects of enflurane upon rate of insulin release from rat pancreatic islets were determined in vitro. A dose-related inhibitory effect of enflurane on glucose-stimulated insulin release was observed with almost complete inhibition being seen when the enflurane concentration in the gas phase was 3.21% (v/v), equivalent to 1.26 mM enflurane in the liquid phase. The onset on enflurane-induced inhibition was rapid, being fully developed within 5 minutes of islet exposure to the agent while the effect was equally rapidly reversed on removal of enflurane from the medium. As glucose metabolism is known to be required for glucose-stimulated insulin secretion to occur, rates of islet metabolism of [U-14C] glucose to 14CO2 were examined but these were unaffected by concentrations of enflurane up to 2.21 mM. To investigate further the metabolic integrity of islets, rates of islet incorporation of L-[4,5-3H] leucine were determined; these too were not significantly altered by concentrations of enflurane up to 1.26 mM. It is concluded that enflurane in the concentration range used clinically leads to a rapid, reversible inhibition of rat pancreatic islet insulin release, which is not attributable to interference with islet glucose metabolism or protein biosynthesis.

Alloxan action on glucose metabolism in cultured fibroblasts. I. Stimulation and inhibition of glucose utilization

The influence of alloxan on mammalian cell glucose metabolism has been investigated using human diploid fibroblastic cells in culture. When cell monolayers were exposed to D-[14C]glucose, the presence of alloxan (0.31-1.87 mM) resulted initially in a dose-dependent enhancement of total cell glucose incorporation. This was observed within 2 min and declined by 6 min. After that time, alloxan inhibited glucose incorporation. When hexose transport was examined directly using glucose analogues, alloxan neither enhanced nor inhibited the uptake of 3-O-methylglucose or 2-deoxyglucose. Alloxan exerted no effect on cell permeability or cell viability. These results suggest that alloxan may directly influence cell glucose metabolism beyond the level of phosphorylation. The dual effect of alloxan on glucose incorporation may be related to the alloxan stimulation and subsequent inhibition of glucose-induced insulin release in pancreatic islets.

Interacting Effects of Sulfonylureas and Glucose on Cyclic AMP Metabolism and Insulin Release in Pancreatic Islets of the Rat

The effects of tolbutamide and glibenclamide on the metabolism of cyclic AMP were investigated in pancreatic islets of the rat. Changes in cyclic AMP were assessed by measuring [(3)H]cyclic AMP after labeling of the islets with [2-(3)H]adenine. In the presence of a nonstimulatory concentration of glucose (3.3 mM), both sulfonylureas caused a rapid increase in islet [(3)H]cyclic AMP, which declined within 5 (tolbutamide) or 10 min (glibenclamide). In the absence of glucose, the glibenclamide effect was shortened, but the initial (1 min) response of [(3)H]-cyclic AMP was unaffected. Glucose could be substituted with d-glyceraldehyde but not pyruvate for prolongation of the glibenclamide response. The effect of glucose withdrawal on the glibenclamide response was reproduced by the addition of d-mannoheptulose to glucose containing media. The [(3)H]cyclic AMP response to glibenclamide was influenced by prior exposure of the islets to glucose, a 30-min preincubation with 27.7 mM glucose, enhancing the response to the sulfonylurea over a subsequent 5-min stimulation period. Sulfonylureas exerted their effects at low but not at high glucose concentrations, i.e., shifted the glucose dose-response curve to the left both for [(3)H]cyclic AMP accumulation and insulin release. On the other hand, increasing concentrations of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, progressively augmented the effects of the drugs. Omission of Ca(++) from the incubation media inhibited both the glucose and the sulfonylurea [(3)H]-cyclic AMP and insulin responses. Epinephrine (1 muM) partially inhibited the [(3)H]cyclic AMP response to both glucose and sulfonylurea, whereas insulin release was completely abolished. It is concluded that the sulfonylurea effects on islet cyclic AMP are intimately related to those of glucose. It is suggested that sulfonylureas exert a major part of their action by facilitating the effect of glucose on the beta-cell adenylate cyclase; the increased cyclic AMP level, in its turn, enhances the secretion rate of insulin.

Effect of Lithium on Pancreatic Islet Insulin Release*

Lithium exerts an inhibitory effect on glucose and amino acid-induced insulin release. The inhibitory effect of Li+ persists even in subsequent Li+-free conditions, indicating an only slowly reversible effect. Lithium fails to inhibit glucagon-induced insulin release. The exact mechanism of the lithium effect is as yet undetermined, but interference with calcium flux and/or microtubular function is an attractive hypothesis. The inhibitory effect of lithium on insulin release cannot be reversed by alteration in ionic (Ca++, Mg++, K+) concentrations in the incubation media. Studies involving the effect of low sodium on insulin release in which lithium had been used as an osmotic replacement for sodium must be carefully reassessed because of these findings.

Cyclic AMP metabolism and insulin release in pancreatic islets of the rat Effects of agents which alter microtubular function

In order to investigate the relationship between microtubular function, insulin release and islet cyclic AMP metabolism, the effects of 2H 2O, colchicine and vincristine were studied in rat islets prelabeled with [ 3H]adenine. Glucose-induced insulin secretion and efflux of cyclic [ 3H]AMP was markedly inhibited by 8–50% 2H 2O. At a higher concentration (75%), deuterated water still suppressed insulin release, while the inhibition of nucleotide release was abolished. Glucose-induced intra-islet cyclic [ 3H]AMP accumulation was augmented by 2H 2O progressively with time. With 75% 2H 2O, although efflux of cyclic AMP was no more inhibited, intra-islet accumulation of the nucleotide was still enhanced. The cyclic AMP efflux induced by cholera toxin, or a high concentration of 3-isobutyl-1-methylxanthine was suppressed and the intra-islet nucleotide accumulation was enhanced by 2H 2O. The latter effect tended to be less pronounced than when glucose was the stimulator. All the effects of 2H 2O on glocuse-stimulated islets were mimicked by incubating the tissue in H 2O at 28°C. Colchicine and vincristine had no significant effect on glucose-induced insulin release, and did not enhance the intra-islet cyclic [ 3H]AMP response; efflux of the nucleotide was, however, significantly inhibited. This pattern of response was shared with probenecid. Preincubation of islets with colchicine did not influence the subsequent effects of 2H 2O on insulin release and cyclic AMP metabolism. It is concluded that: (1) enhancement of intra-islet cyclic AMP accumulation by 2H 2O is not due to inhibition of the nucleotide efflux; (2) the effects on cyclic AMP metabolism described here are not exclusive for microtubular affecting agents and do not seem to be related to the microtubular system of the islet.

Calcium Ion Fluxes and Insulin Release in Pancreatic Islets

Conference Article| August 01 1977 Calcium Ion Fluxes and Insulin Release in Pancreatic Islets WILLY J. MALAISSE WILLY J. MALAISSE 1Laboratory of Experimental Medicine, Brussels University School of Medicine, Brussels, Belgium Search for other works by this author on: This Site PubMed Google Scholar Biochem Soc Trans (1977) 5 (4): 872–875. https://doi.org/10.1042/bst0050872 Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Facebook Twitter LinkedIn MailTo Cite Icon Cite Get Permissions Citation WILLY J. MALAISSE; Calcium Ion Fluxes and Insulin Release in Pancreatic Islets. Biochem Soc Trans 1 August 1977; 5 (4): 872–875. doi: https://doi.org/10.1042/bst0050872 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAll JournalsBiochemical Society Transactions Search Advanced Search This content is only available as a PDF. © 1977 Biochemical Society1977 Article PDF first page preview Close Modal You do not currently have access to this content.

Effects of D-glyceraldehyde and D-glucose on the insulin release of pancreatic islets isolated from the newborn rat

Effects of D-glyceraldehyde and D-glucose on the insulin release of pancreatic islets isolated from the newborn rat