| 1. |
- Obermüller, Stefanie, et al.
(författare)
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Defective secretion of islet hormones in chromogranin-B deficient mice
- 2010
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Ingår i: PLOS ONE. - : Public Library of Science (PLoS). - 1932-6203. ; 5:1, s. e8936-
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Tidskriftsartikel (refereegranskat)abstract
- Granins are major constituents of dense-core secretory granules in neuroendocrine cells, but their function is still a matter of debate. Work in cell lines has suggested that the most abundant and ubiquitously expressed granins, chromogranin A and B (CgA and CgB), are involved in granulogenesis and protein sorting. Here we report the generation and characterization of mice lacking chromogranin B (CgB-ko), which were viable and fertile. Unlike neuroendocrine tissues, pancreatic islets of these animals lacked compensatory changes in other granins and were therefore analyzed in detail. Stimulated secretion of insulin, glucagon and somatostatin was reduced in CgB-ko islets, in parallel with somewhat impaired glucose clearance and reduced insulin release, but normal insulin sensitivity in vivo. CgB-ko islets lacked specifically the rapid initial phase of stimulated secretion, had elevated basal insulin release, and stored and released twice as much proinsulin as wildtype (wt) islets. Stimulated release of glucagon and somatostatin was reduced as well. Surprisingly, biogenesis, morphology and function of insulin granules were normal, and no differences were found with regard to beta-cell stimulus-secretion coupling. We conclude that CgB is not required for normal insulin granule biogenesis or maintenance in vivo, but is essential for adequate secretion of islet hormones. Consequentially CgB-ko animals display some, but not all, hallmarks of human type-2 diabetes. However, the molecular mechanisms underlying this defect remain to be determined.
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| 2. |
- Salehi, S Albert, et al.
(författare)
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Insulin release transduction mechanism through acid glucan 1,4-alpha-glucosidase activation is Ca2+ regulated
- 1998
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Ingår i: American Journal of Physiology - Endocrinology and Metabolism. - 1522-1555. ; 274:3, s. 459-468
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Tidskriftsartikel (refereegranskat)abstract
- An important signal involved in glucose-stimulated insulin secretion is transduced through the action of a lysosomal acid, glucan 1,4-alpha-glucosidase. We investigated the Ca2+ dependency of this enzyme activity in relation to insulin release. In isolated islets, increased levels of extracellular Ca2+ induced a large increase in acid glucan 1,4-alpha-glucosidase activity accompanied by a similar increase in insulin release at both substimulatory and stimulatory concentrations of glucose. At low glucose the Ca2+ "inflow" blocker nifedipine unexpectedly stimulated enzyme activity without affecting insulin release. However, nifedipine suppressed 45Ca2+ outflow from perifused islets at low glucose and at Ca2+ deficiency when intracellular Ca2+ was mobilized by carbachol. This nifedipine-induced retention of Ca2+ was reflected in increased acid glucan 1,4-alpha-glucosidase activity. Adding different physiological Ca2+ concentrations or nifedipine to islet homogenates did not increase enzyme activity. Neither selective glucan 1,4-alpha-glucosidase inhibition nor the ensuing suppression of glucose-induced insulin release was overcome by a maximal Ca2+ concentration. Hence, Ca(2+)-induced changes in acid glucan 1,4-alpha-glucosidase activity were intimately coupled to similar changes in Ca(2+)-glucose-induced insulin release. Ca2+ did not affect the enzyme itself but presumably activated either glucan 1,4-alpha-glucosidase-containing organelles or closely interconnected messengers.
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| 3. |
- Vieira, Elaine, et al.
(författare)
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Glucose inhibits glucagon secretion by a direct effect on mouse pancreatic alpha cells
- 2007
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Ingår i: Diabetologia. - : Springer Science and Business Media LLC. - 0012-186X .- 1432-0428. ; 50:2, s. 370-379
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Tidskriftsartikel (refereegranskat)abstract
- Aims/hypothesis The mechanisms by which glucose regulates glucagon release are poorly understood. The present study aimed to clarify the direct effects of glucose on the glucagon-releasing alpha cells and those effects mediated by paracrine islet factors. Materials and methods Glucagon, insulin and somatostatin release were measured from incubated mouse pancreatic islets and the cytoplasmic Ca2+ concentration ([Ca2+](i)) recorded in isolated mouse alpha cells. Results Glucose inhibited glucagon release with maximal effect at 7 mmol/l. Since this concentration corresponded to threshold stimulation of insulin secretion, it is unlikely that inhibition of glucagon secretion is mediated by beta cell factors. Although somatostatin secretion data seemed consistent with a role of this hormone in glucose-inhibited glucagon release, a somatostatin receptor type 2 antagonist stimulated glucagon release without diminishing the inhibitory effect of glucose. In islets exposed to tolbutamide plus 8 mmol/l K+, glucose inhibited glucagon secretion without stimulating the release of insulin and somatostatin, indicating a direct inhibitory effect on the alpha cells that was independent of ATP-sensitive K+ channels. lucose lowered [Ca2+](i) of individual alpha cells independently of somatostatin and beta cell factors (insulin, Zn2+ and gamma-aminobutyric acid). Glucose suppression of glucagon release was prevented by inhibitors of the sarco(endo)plasmic reticulum Ca2+-ATPase, which abolished the [Ca2+](i)-lowering effect of glucose on isolated alpha cells. Conclusions/interpretation Beta cell factors or somatostatin do not seem to mediate glucose inhibition of glucagon secretion. We instead propose that glucose has a direct inhibitory effect on mouse alpha cells by suppressing a depolarising Ca2+ store-operated current.
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| 4. |
- Salehi, S Albert, et al.
(författare)
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Paradoxical stimulation of glucagon secretion by high glucose concentrations
- 2006
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Ingår i: Diabetes. - : American Diabetes Association. - 0012-1797 .- 1939-327X. ; 55:8, s. 2318-2323
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Tidskriftsartikel (refereegranskat)abstract
- Hypersecretion of glucagon contributes to the dysregulation of glucose homeostasis in diabetes. To clarify the underlying mechanism, glucose-regulated glucagon secretion was studied in mouse pancreatic islets and clonal hamster In-R1-G9 glucagon-releasing cells. Apart from the well-known inhibition of secretion with maximal effect around 7 mmol/l glucose, we discovered that mouse islets showed paradoxical stimulation of glucagon release at 25-30 mmol/l and In-R1-G9 cells at 12-20 mmol/l sugar. Whereas glucagon secretion in the absence of glucose was inhibited by hyperpolarization with diazoxide, this agent tended to further enhance secretion stimulated by high concentrations of the sugar. Because U-shaped dose-response relationships for glucose-regulated glucagon secretion were observed in normal islets and in clonal glucagon-releasing cells, both the inhibitory and stimulatory components probably reflect direct effects on the a-cells. Studies of isolated mouse a-cells indicated that glucose inhibited glucagon secretion by lowering the cytoplasmic Ca2+ concentration. However, stimulation of glucagon release by high glucose concentrations did not require elevation of Ca2+, indicating involvement of novel mechanisms in glucose regulation of glucagon secretion. A U-shaped dose-response relationship for glucose-regulated glucagon secretion may explain why diabetic patients with pronounced hyperglycemia display paradoxical hyperglucagonemia.
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| 5. |
- Meidute, Sandra, et al.
(författare)
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Palmitate-induced beta-cell dysfunction is associated with excessive NO pro-duction and is reversed by thiazolidinedione-mediated inhibition of GPR40 transduction mechanisms
- 2008
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Ingår i: PLoS ONE. - : Public Library of Science (PLoS). ; 3:5
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- Background: Type 2 diabetes often displays hyperlipidemia. We examined palmitate effects on pancreatic islet function in relation to FFA receptor GPR40, NO generation, insulin release, and the PPARgama agonistic thiazolidinedione, rosiglitazone. Principal findings: Rosiglitazone suppressed acute palmitate-stimulated GPR40-transduced PI hydrolysis in HEK293 cells and insulin release from MIN6c cells and mouse islets. Culturing islets 24 h with palmitate at 5 mmol/l glucose induced beta-cell iNOS expression as revealed by confocal microscopy and in-creased the activities of ncNOS and iNOS associated with suppression of glucose-stimulated insulin response. Rosiglitazone reversed these effects. The expression of iNOS after high-glucose culturing was unaffected by rosiglitazone. Downregulation of GPR40 by antisense treatment abrogated GPR40 expression and suppressed palmitate-induced iNOS activity and insulin release. Conclusion: We conclude that, in addition to mediating acute FFA-stimulated insulin release, GPR40 is an important regulator of iNOS expression and dysfunctional insulin release during long-term exposure to FFA. The adverse effects of palmitate were counteracted by rosiglitazone at GPR40, suggesting that thiazolidinediones are beneficial for beta-cell function in hyperlipidemic type 2 diabetes.
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| 6. |
- Collins, S. C., et al.
(författare)
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Long-term exposure of mouse pancreatic islets to oleate or palmitate results in reduced glucose-induced somatostatin and oversecretion of glucagon
- 2008
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Ingår i: Diabetologia. - : Springer Science and Business Media LLC. - 1432-0428 .- 0012-186X. ; 51:9, s. 1689-1693
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Tidskriftsartikel (refereegranskat)abstract
- Aims/hypothesis Long-term exposure to NEFAs leads to inhibition of glucose-induced insulin secretion. We tested whether the release of somatostatin and glucagon, the two other major islet hormones, is also affected. Methods Mouse pancreatic islets were cultured for 72 h at 4.5 or 15 mmol/l glucose with or without 0.5 mmol/l oleate or palmitate. The release of glucagon and somatostatin during subsequent 1 h incubations at 1 or 20 mmol/l glucose as well as the islet content of the two hormones were determined. Lipid-induced changes in islet cell ultrastructure were assessed by electron microscopy. Results Culture at 15 mmol/l glucose increased islet glucagon content by similar to 50% relative to that observed following culture at 4.5 mmol/l glucose. Inclusion of oleate or palmitate reduced islet glucagon content by 25% (at 4.5 mmol/l glucose) to 50% (at 15 mmol/l glucose). Long-term exposure to the NEFA increased glucagon secretion at 1 mmol/l glucose by 50% (when islets had been cultured at 15 mmol/l glucose) to 100% (with 4.5 mmol/l glucose in the culture medium) and abolished the inhibitory effect of 20 mmol/l glucose on glucagon secretion. Somatostatin content was unaffected by glucose and lipids, but glucose-induced somatostatin secretion was reduced by similar to 50% following long-term exposure to either of the NEFA, regardless of whether the culture medium contained 4.5 or 15 mmol/l glucose. Ultrastructural evidence of lipid deposition was seen in < 10% of non-beta cells but in > 80% of the beta cells. Conclusions/interpretation Long-term exposure to high glucose and/or NEFA affects the release of somatostatin and glucagon. The effects on glucagon secretion are very pronounced and in type 2 diabetes in vivo may aggravate the hyperglycaemic effects due to lack of insulin.
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| 7. |
- Hellman, Bo, 1930-, et al.
(författare)
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Glucose generates coincident insulin and somatostatin pulses and anti-synchronous glucagon pulses from human pancreatic islets
- 2009
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Ingår i: Endocrinology. - : The Endocrine Society. - 0013-7227 .- 1945-7170. ; 150:12, s. 5334-5340
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Tidskriftsartikel (refereegranskat)abstract
- The kinetics of insulin, glucagon and somatostatin release was studied in human pancreatic islets. Batches of 10-15 islets were perifused and the hormones measured with RIA in 30-sec fractions. Increase of glucose from 3 to 20 mM resulted in a brief pulse of glucagon coinciding with suppression of basal insulin and somatostatin release. There was a subsequent drop of glucagon release concomitant with the appearance of a pronounced pulse of insulin and a slightly delayed pulse of somatostatin. Continued exposure to 20 mM glucose generated pulsatile release of the three hormones with 7- to 8-min periods accounting for 60-70% of the secreted amounts. Glucose caused pronounced stimulation of average insulin and somatostatin release. However, the nadirs between the glucagon pulses were lower than the secretion at 3 mM glucose, resulting in 18% suppression of average release. The repetitive glucagon pulses were antisynchronous to coincident pulses of insulin and somatostatin. The resulting greater than 20-fold variations of the insulin to glucagon ratio might be essential for minute-to-minute regulation of the hepatic glucose production.
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| 8. |
- Salehi, S Albert, et al.
(författare)
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Absence of adenosine A(1) receptors unmasks pulses of insulin release and prolongs those of glucagon and somatostatin
- 2009
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Ingår i: Life Sciences. - : Elsevier BV. - 0024-3205 .- 1879-0631. ; 85:11-12, s. 470-476
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Tidskriftsartikel (refereegranskat)abstract
- Aims: Extracellular ATP modulates pulsatile release of insulin, glucagon and somatostatin by activating P2Y(1) receptors. The present study examines if adenosine via A(1) receptors (A(1)R) interferes with pulsatile islet hormone release. Main methods: Pancreas was perfused in mice expressing or lacking the A(1) receptor and the hormones measured with radioimmunoassay. Cytoplasmic Ca2+ was recorded in isolated beta-cells using the fura-2 indicator. Key findings: Addition of 10 mu M adenosine removed the Ca2+ transients supposed to coordinate the insulin release pulses. This effect of adenosine was counteracted by 100 nM of the A(1)R antagonist DPCPX. In situ perfusion of the pancreas indicated two phases of islet hormone release when glucose was raised from 3.3 to 16.7 mM. The first phase was characterized by a brief dip followed by a peak which was more pronounced for insulin and somatostatin than for glucagon. The second phase was markedly affected by knock out of A(1)R. The wild-type AIR (+/+) mice, usually lacked statistically verified insulin pulses but generated antisynchronous glucagon and somatostatin pulses with half-widths of 4 min. In the A(1)R (-/-) mice time-average release of insulin during the second phase was almost three times higher than in the controls and 30% of the hormone was released as distinct pulses with half-widths of 3 min. The absence of the AIR receptor resulted in 50% prolongation of the pulse cycles of glucagon and somatostatin and loss of their antisynchronous relationship. Significance: The A(1)R receptor is important both for the amplitude (insulin) and duration (glucagon and somatostatin) of islet hormone pulses. (C) 2009 Elsevier Inc. All rights reserved.
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| 9. |
- Ekelund, Mats, et al.
(författare)
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Selective induction of inducible nitric oxide synthase in pancreatic islet of rat after an intravenous glucose or intralipid challenge.
- 2006
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Ingår i: Nutrition. - : Elsevier BV. - 1873-1244 .- 0899-9007. ; 22:2006 Apr 22, s. 652-660
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Tidskriftsartikel (refereegranskat)abstract
- Objective: Constant exposure of pancreatic islets to high levels of glucose or free fatty acids can lead to irreversible beta-cell dysfunction, a process referred to as glucotoxicity or lipotoxicity, respectively. In this context a role for nitric oxide generated by pancreatic islet has been suggested. The present investigation examined whether the route of glucose administration, i.e., given orally (OG) or infused intravenously (IVG), could have any effect on the expression and activity of inducible nitric oxide synthase (iNOS) in pancreatic islets. Methods: Rats were infused with glucose (50%) or Intralipid intravenously for 24 h or given glucose orally. A freely fed control group (FF) was also included. At 24 h rats were killed and blood samples were drawn for analysis of plasma insulin, glucagon, and glucose. Pancreatic islets were harvested from each animal and investigated for the occurrence of iNOS by the use of confocal microscopy, western blot, and high-performance liquid chromatographic analysis. The effect of intravenously infused glucose was then compared with the effect of an intravenous infusion of Intralipid (IL). Results: Plasma insulin levels were markedly decreased after 24 h of infusion of glucose (IVG group) or Intralipid (IL group) compared with the FF or OG group. Plasma glucagon and glucose levels were markedly increased in the IVG group, whereas both parameters were decreased in the IL group. No significant differences in plasma insulin, glucagon, or glucose were found between the OG and FF groups. Immunocytochemical (confocal microscopy), western blot, and biochemical (high-performance liquid chromatographic) analyses showed that a sustained increase in plasma level of glucose or free fatty acids by an intravenous infusion of either nutrient for 24 h resulted in a marked expression and activity of iNOS in pancreatic islets. No sign of iNOS expression could, however, be detected in the islets of FF control or OG rats. Conclusion: The data suggest that impaired beta-cell function found after 24 It of an intravenous infusion of glucose or Intralipid might be mediated, at least in part, by the induction of iNOS in pancreatic islets. This may subsequently result in an exclusive production of nitric oxide, which is deleterious for beta-cells. (C) 2006 Elsevier Inc. All rights reserved.
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| 10. |
- Salehi, S Albert, et al.
(författare)
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Total parenteral nutrition modulates hormone release by stimulating expression and activity of inducible nitric oxide synthase in rat pancreatic islets
- 2001
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Ingår i: Endocrine. - 1355-008X. ; 16:2, s. 97-104
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Tidskriftsartikel (refereegranskat)abstract
- The expression and activities of constitutive nitric oxide synthase (cNOS) and inducible nitric oxide synthase (iNOS) in relation to insulin and glucagon secretory mechanisms were investigated in islets isolated from rats subjected to total parenteral nutrition (TPN) for 10 d. TPN is known to result in significantly increased levels of plasma lipids during the infusion time. In comparison with islets from freely fed control rats, islets taken from TPN rats at d 10 displayed a marked decrease in glucose-stimulated insulin release (4.65 +/- 0.45 ng/[islet x h] vs 10.25 +/- 0.65 for controls) (p < 0.001) accompanied by a strong iNOS activity (18.3 +/- 1.1 pmol of NO/[min x mg of protein]) and a modestly reduced cNOS activity (11.3 +/- 3.2 pmol of NO/[min x mg of protein] vs 17.7 +/- 1.7 for controls) (p < 0.01). Similarly, Western blots showed the expression of iNOS protein as well as a significant reduction in cNOS protein in islets from TPN-treated rats. The enhanced NO production, which is known to inhibit glucose-stimulated insulin release, was manifested as a strong increase in the cyclic guanosine 5'-monophosphate content in the islets of TPN-treated rats (1586 +/- 40 amol/islet vs 695 +/- 64 [p < 0.001] for controls). Moreover, the content of cyclic adenosine monophosphate (cAMP) was greatly increased in the TPN islets (80.4 +/- 2.1 fmol/islet vs 42.6 +/- 2.6 [p < 0.001] for controls). The decrease in glucose-stimulated insulin release was associated with an increase in the activity of the secretory pathway regulated by the cAMP system in the islets of TPN-treated rats, since the release of insulin stimulated by the phosphodiesterase inhibitor isobutylmethylxanthine was greatly increased both in vivo after iv injection and after in vitro incubation of isolated islets. By contrast, the release of glucagon was clearly reduced in islets taken from TPN-treated rats (33.5 +/- 1.5 pg/[islet x h] vs 45.5 +/- 2.2 for controls) (p < 0.01) when islets were incubated at low glucose (1.0 mmol/L). The data show that long-term TPN treatment in rats brings about impairment of glucose-stimulated insulin release, that might be explained by iNOS expression and a marked iNOS-derived NO production in the beta-cells. The release of glucagon, on the other hand, is probably decreased by a direct "nutrient effect" of the enhanced plasma lipids. The results also suggest that the islets of TPN-treated rats have developed compensatory insulin secretory mechanisms by increasing the activity of their beta-cell cAMP system.
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