| 1. |
- Fan, Bo-Guang, et al.
(författare)
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Total parenteral nutrition influences both endocrine and exocrine function of rat pancreas
- 1997
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Ingår i: Pancreas. - 0885-3177. ; 15:2, s. 147-153
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study was to examine the effect of total parenteral nutrition (TPN) on the endocrine and exocine function of the pancreas. Endocrine function was investigated using an intravenous glucose tolerance test (IGTT) in rats with TPN for 7 or 14 days. Exocrine function was evaluated by measuring amylase secretion from isolated acini as well as pancreatic weight, water content, protein, and enzymes after 7 days of TPN. When the TPN rats were compared with the controls, the glucose tolerance curve after an IGTT was unchanged, the basal plasma insulin levels were slightly lower and the insulin secretory response to intravenous glucose was markedly impaired. No differences could be seen between the insulin response after 7 days and that after 14 days of TPN. The weight of pancreas, the total content and concentration of pancreatic protein, and the total amylase content of the pancreas were lower, whereas the total content of both chymotrypsin and trypsin was higher. The concentration of DNA remained intact, whereas the total DNA content decreased. The levels of lipolytic enzymes, except for carboxylesterlipase, were unaffected. After TPN treatment, the insulin secretory response to glucose is impaired, the exocrine pancreas is hypoplastic and the storage pattern of pancreatic exocrine enzymes is altered.
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| 2. |
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| 3. |
- Lundquist, Ingmar, et al.
(författare)
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Islet acid glucan-1,4-alpha-glucosidase: a putative key enzyme in nutrient-stimulated insulin secretion
- 1996
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Ingår i: Endocrinology. - 0013-7227. ; 137:4, s. 1219-1225
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Tidskriftsartikel (refereegranskat)abstract
- Little attention has been paid to a possible relationship between lysosomal function and stimulation of secretory processes in endocrine cells. The last few years it has become increasingly evident that the secretion of insulin from the pancreatic beta-cell is the result of a very complex cascade of events, the details of which are far from elucidated and indeed may include the participation of the lysosomal system. We report here, with a combined in vitro and in vivo approach, that selective inhibition of islet lysosomal glycogenolytic acid glucan-1,4-alpha-glucosidase activity by the long-acting 1-deoxynojirimycin derivative emiglitate induces a profound suppression of nutrient-induced insulin release. In islet homogenate emiglitate strongly and dose-dependently inhibited the activity of acid glucan-1,4-alpha-glucosidase (EC50 approximately 10(-6) M) without affecting other classical lysosomal enzyme activities. The emiglitate-induced inhibition curve for glucose-stimulated insulin secretion from isolated islets was remarkably similar to the inhibition curve for acid glucan-1,4-alpha-glucosidase. Moreover, insulin release stimulated by the nonglucose nutrient secretagogues, leucine, and alpha-ketoisocaproic acid (KIC) was totally suppressed by emiglitate. In contrast, receptor activated insulin secretion induced by the insulinotropic hormone cholecystokinin (CCK-8) was unaffected by the drug. Further, parenteral pretreatment of mice with emiglitate markedly suppressed the insulin secretory response to an iv injection of glucose or KIC, whereas the response to an iv injection of CCK-8 was unaffected. In accordance with this, islets isolated from emiglitate-treated mice showed a reduced activity of acid glucan-1,4-alpha-glucosidase and, moreover, such islets incubated in vitro, secreted less insulin in response to glucose than did control islets. Finally, pretreatment of mice with purified fungal acid glucan-1,4-alpha-glucosidase, enzyme replacement, brought about a markedly increased insulin secretory response after an iv injection of KIC, whereas the insulin response after CCK-8 injection was unaffected. Taken together with previous observations, the present data strongly suggest that islet lysosomal acid alpha-glucosidehydrolases are involved in the multifactorial process of nutrient-induced insulin secretion. The existence of hitherto unresolved and complex interactions between different beta-cell organelles in the insulin secretory processes should be thoroughly reevaluated.
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| 4. |
- Monstein, H J, et al.
(författare)
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Cholecystokinin-A and cholecystokinin-B/gastrin receptor mRNA expression in the gastrointestinal tract and pancreas of the rat and man. A polymerase chain reaction study
- 1996
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Ingår i: Scandinavian Journal of Gastroenterology. - : Informa UK Limited. - 1502-7708 .- 0036-5521. ; 31:4, s. 383-390
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Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND: Gastrin and cholecystokinin (CCK) are thought to exert trophic effects on the gastrointestinal tract and pancreas. Two types of receptors have been cloned, CCK-A and CCK-B/ gastrin. We have examined the occurrence of CCK-A and CCK-B receptor mRNA in the brain, digestive tract, pancreas, and kidney of the rat and man by Northern blot and reverse transcribed polymerase chain reaction (RT-PCR). METHODS: Total RNA was isolated from rat tissues and reverse transcribed into cDNA. cDNA from brain, kidney, and pancreas of the rat and man and from human whole stomach were commercially available. Northern blot and a PCR technique based on Taq polymerase-antibody interaction and using CCK-A and CCK-B receptor-specific primers, followed by Southern blot analysis, were the methods used. RESULTS: By means of Northern blots, CCK-A receptor mRNA was detected in rat fundus mucosa and pancreas but not in the remaining GI tract or brain. By means of RT-PCR, CCK-A receptor mRNA was demonstrated in the brain and the mucosa of the fundus, antrum, duodenum, and colon, kidney, pancreas and pancreatic islets. CCK-B receptor mRNA was detected by Northern blot analysis in the brain and the fundus mucosa but not in the rest of the digestive tract and not in the pancreas, pancreatic islets, or kidney. By RT-PCR, expression of CCK-B receptor mRNA could also be detected in antrum mucosa. In man, CCK-A receptor mRNA was detected in the brain, stomach, pancreas, and kidney, whereas CCK-B receptor mRNA was found in the brain, stomach, and pancreas but not in the kidney. Cloning and DNA-sequence analysis of the PCR-amplified rat and human CCK-A and CCK-B receptor DNA fragments, which cover the protein-encoding regions of the intracellular loop C3, showed complete sequence homology as compared with published rat and human sequences. CONCLUSIONS: It appears unlikely that CCK will have effects in the ileum, at least not effects mediated by CCK-A receptors. It also appears unlikely that physiologic concentrations of gastrin in the circulation will promote growth (or exert other effects) in the pancreas, duodenum, ileum, and colon, since CCK-B receptor mRNA is not expressed or is poorly expressed in these tissues.
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| 5. |
- Roth, Bengt, et al.
(författare)
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Biochemical and ultra-structural reactions to parenteral nutrition with two different fat emulsions in rats
- 1998
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Ingår i: Intensive Care Medicine. - : Springer Science and Business Media LLC. - 0342-4642 .- 1432-1238. ; 24:7, s. 716-724
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Tidskriftsartikel (refereegranskat)abstract
- OBJECTIVE: To compare the effects on fat metabolism and Kupffer cell morphology by total parenteral nutrition (TPN) with two different fat emulsions. DESIGN: Thirty-two male Sprague-Dawley rats, divided into three groups, were investigated. Rats fed orally were used as a reference group, and a group of rats receiving TPN with fat emulsions containing pure long-chain triglycerides (LCT) was compared to a group of rats receiving fat emulsions containing both long-chain triglycerides and medium-chain triglycerides (MCT/LCT). The TPN regimens were equicaloric and administered continuously via a jugular catheter for 10 days. INTERVENTIONS: After suffocation, blood of the rats was collected for the determination of serum lipids. Epididymal fat and heart were collected for the analysis of lipoprotein lipase (LPL) activities, and liver specimens were saved for analyses of hepatic triglyceride concentration, as well as activities of hepatic lipase (HL) and lysosomal enzymes. Light and electron microscopy were used for examination of the Kupffer cell reaction. RESULTS: Directly after termination of parenteral feeding, the levels of serum triglycerides and high density lipoprotein (HDL) triglycerides were higher in the MCT/LCT group than in the LCT group, while no differences concerning cholesterol and phospholipid concentrations were found. No significant difference in liver steatosis was found between the two TPN groups. Comparison of the TPN groups showed that the MCT/ LCT group had significantly decreased LPL activity in adipose tissue, while the LCT group had significantly increased LPL activity in the heart. The activity of HL was low in both groups, but significantly lower in the LCT group. Lipid accumulation and an increased number of lysosomes were found in all Kupffer cell when TPN with LCTemulsions was used. Moreover, TPN induced a pronounced increase in various liver lysosomal enzyme activities, but there was no notable difference between LCT and MCT/LCT effects. CONCLUSIONS: Compared to treatment with pure LCTemulsions, treatment with MCT/LCT emulsions evoked weaker biochemical reactions in terms of lower activity of lipoprotein lipase in fat and heart together with higher serum and HDL triglyceride levels. Morphological signs of increased Kupffer cell activity such as the appearance of multiple lysosomes and fat vacuoles in the cytoplasm followed treatment with pure LCT emulsions. However, both TPN groups showed a marked increase in activities of liver lysosomal enzymes.
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| 6. |
- Salehi, S Albert, et al.
(författare)
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Dysfunction of the islet lysosomal system conveys impairment of glucose-induced insulin release in the diabetic GK rat
- 1999
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Ingår i: Endocrinology. - 0013-7227. ; 140:7, s. 3045-3053
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Tidskriftsartikel (refereegranskat)abstract
- Accumulated evidence links an important signal involved in glucose-stimulated insulin release to the activation of the islet lysosomal glycogenolytic enzyme acid glucan-1,4-alpha-glucosidase. We have analyzed the function of the lysosomal system/lysosomal enzyme activities in pancreatic islets of young (6-8 weeks), spontaneously diabetic, GK (Goto-Kakizaki) rats and Wistar control rats in relation to glucose-induced insulin release. The insulin secretory response to glucose was markedly impaired in the GK rat, but was restored by the adenylate cyclase activator forskolin. Islet activities of classical lysosomal enzymes, e.g.. acid phosphatase, N-acetyl-beta-D-glucosaminidase, beta-glucuronidase, and cathepsin D, were reduced by 20-35% in the GK rat compared with those in Wistar controls. In contrast, the activities of the lysosomal alpha-glucosidehydrolases, i.e.. acid glucan-1,4-alpha-glucosidase and acid alpha-glucosidase, were increased by 40-50%. Neutral alpha-glucosidase (endoplasmic reticulum) was unaffected. Comparative analysis of liver tissue showed that lysosomal enzyme activities were of the same magnitude in GK and Wistar rats. Notably, in Wistar rats, the activities of acid glucan-1,4-alpha-glucosidase and acid alpha-glucosidase were approximately 15-fold higher in islets than in liver. Other lysosomal enzymes did not display such a difference. Normalization of glycemia in GK rats by phlorizin administered for 9 days did not influence either the lysosomal alpha-glucosidehydrolase activities or other lysosomal enzyme activities in GK islets. Finally, the pseudotetrasaccharide acarbose, which accumulates in the lysosomal system, inhibited acid glucan-1,4-alpha-glucosidase activity in parallel with its inhibitory action on glucose-induced insulin release in intact Wistar islets, whereas no effect was recorded for either parameter in intact GK islets. In contrast, acarbose inhibited the enzyme activity equally in islet homogenates from both GK and Wistar rats, showing that the catalytic activity of the enzyme itself in disrupted cells was unaffected. We propose that dysfunction of the islet lysosomal/vacuolar system is an important defect impairing the transduction mechanisms for glucose-induced insulin release in the GK rat.
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| 7. |
- Salehi, S Albert, et al.
(författare)
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Gastrectomy induces impaired insulin and glucagon secretion: evidence for a gastro-insular axis in mice
- 1999
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Ingår i: Journal of Physiology. - 1469-7793. ; 514:2, s. 579-591
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Tidskriftsartikel (refereegranskat)abstract
- 1. Mice were subjected to gastrectomy (GX) or food deprivation (24 h). The release of insulin and glucagon in response to different secretagogues was monitored in vivo and in isolated islets 3-4 weeks after surgery. 2. GX animals responded to glucose with an impaired glucose tolerance and a poor increase in plasma insulin. Islets from GX or food-deprived mice displayed impaired insulin release to high glucose and enhanced glucagon release at low glucose. 3. After GX the insulinogenic index, Delta insulin (microU ml-1)/Delta glucose (mg ml-1), was suppressed by 65% after oral glucose and by 59% after i.v. glucose. The integrated insulin response after oral glucose was reduced by 90% in GX mice. After i.v. glucose the reduction was 67%. 4. Carbachol-induced insulin release in vivo was reduced after food deprivation and exaggerated after GX. Carbachol-stimulated glucagon secretion was suppressed after GX and after food deprivation. A similar pattern was found in vitro. 5. Cyclic AMP activation (by the phosphodiesterase inhibitor isobutylmethylxanthine or the adenylate cyclase stimulator forskolin) induced a greater insulin response in GX or food-deprived mice than in sham-operated, fed mice. A similar pattern was found in vitro. The glucagon response was enhanced in vitro but not in vivo. 6. Crude extracts of rat oxyntic mucosa enhanced basal as well as glucose-induced insulin release from isolated islets, whereas glucagon release was markedly inhibited. The effects were dose dependent, the inhibition of glucagon release being achieved at lower concentrations than the potentiation of glucose-induced insulin release. The active principle was inactivated by incubation with trypsin or leucine aminopeptidase. 7. The data suggest that a circulating agent, probably a peptide, from gastric oxyntic mucosa stimulates glucose-induced insulin secretion. It also suppresses glucagon secretion. The GX-evoked impairment of the insulin (and glucagon) response to glucose is partly compensated for by an enhanced insulin response to cholinergic and/or cyclic AMP activation.
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| 8. |
- 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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| 9. |
- Salehi, S Albert, et al.
(författare)
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Islet constitutive nitric oxide synthase: biochemical determination and regulatory function
- 1996
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Ingår i: American Journal of Physiology: Cell Physiology. - 1522-1563. ; 270:6 Pt 1, s. 1634-1641
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Tidskriftsartikel (refereegranskat)abstract
- Recent immunohistochemical findings suggested that a constitutive nitric oxide synthase (cNOS) resides in endocrine pancreas. Here we provide direct biochemical evidence for the presence of cNOS activity in isolated islets. The regulating influence of this nitric oxide synthase (NOS) activity for islet hormone release was also investigated. We observed that cNOS activity could be quantitated in islet homogenates by monitoring the formation of L-citrulline from L-arginine using an Amprep CBA cation-exhange minicolumn before derivatization with o-phthaldialdehyde and subsequent high-performance liquid chromatography analysis. The islet NOS was dependent on both Ca2+ and calmodulin and suppressed by the NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME). This effect was enantiomerically specific. Islet insulin release induced by a mixture of L-arginine and glucose was enhanced by L-NAME, whereas L-arginine-induced glucagon release was inhibited. The effect of L-NAME on insulin release was dose dependently potentiated by increasing glucose concentrations, suggesting that glucose is an important regulator of islet NO production. Complementary in vivo studies showed similar results, i.e., the insulin secretory response to a mixture of glucose and L-arginine was extremely enhanced by pretreatment with L-NAME, whereas L-arginine-stimulated glucagon response was suppressed. Finally, in isolated islets, the intracellular nitric oxide (NO) donor hydroxylamine suppressed insulin release and increased glucagon release. In summary, the islets of Langerhans contain a constitutive, Ca2+/calmodulin-dependent isoform of NOS. Islet NO suppressed insulin but enhanced glucagon secretion. The data also suggest a negative feedback by NO on glucose-induced insulin release. The islet NO system is a novel and important regulatory factor in insulin and glucagon secretion.
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| 10. |
- Salehi, S Albert, et al.
(författare)
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Modulation of islet G-proteins, alpha-glucosidehydrolase inhibition and insulin release stimulated by various secretagogues
- 1996
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Ingår i: Bioscience Reports. - 0144-8463. ; 16:1, s. 23-34
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Tidskriftsartikel (refereegranskat)abstract
- Guanine nucleotide-binding proteins (G-proteins) are known to act as important modulators of insulin release from the islets of Langerhans. We have recently found that the deoxynojirimycin-derivative emiglitate, a recognized inhibitor of intestinal alpha-glucosidehydrolase activity, is a powerful inhibitor of glucose-induced insulin release. With the use of isolated mouse islets the present investigation was performed in a primary attempt to elucidate whether this inhibitory mechanism in some way was linked to the beta-cell G-protein system. Treatment of freshly isolated islets with pertussis toxin (PTX), which is known to inactivate the G (i)-proteins, abolished the inhibitory effect of the alpha(2)-adrenoceptor agonist clonidine on insulin release stimulated by the phosphodiesterase inhibitor IBMX in the presence of the protein kinase C activator TPA and even changed it into an increase. Emiglitate did not display any inhibitory action on insulin release induced by these secretagogues. Similarly, clonidine-induced inhibition of glucose stimulated insulin release was reversed by PTX. However, PTX did not influence the suppressive action of emiglitate on glucose-induced insulin secretion. In contrast, the adenylate cyclase activator forskolin totally abolished the inhibitory effect of emiglitate, but not that of the glucose analogue mannoheptulose, on glucose-induced insulin release. Moreover, the stimulatory effect of forskolin and cholera toxin (CTX) (activator of G (s)-proteins) on the secretion of insulin was markedly enhanced in the presence of emiglitate. In conclusion, our results suggest that the inhibitory effect of emiglitate on glucose-induced insulin release is not directly related to the G(s)-proteins, but most likely exerted solely through the selective suppression of lysosomal aglucosidehydrolase activity, a step in between the proximal and the distal G(i)-proteins, in glucose induced stimulus-secretion mechanisms. Our data also suggests that the inhibitory action of emiglitate on glucose stimulated insulin release can be compensated for by an increased sensitivity of the cyclic AMP-protein kinase A pathway. Hence, emiglitate might indirectly elicit an increased activity of the G(s)-proteins to facilitate the secretory process.
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| 11. |
- Salehi, S Albert, et al.
(författare)
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Signal transduction in islet hormone release: interaction of nitric oxide with basal and nutrient-induced hormone responses
- 1998
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Ingår i: Cellular Signalling. - 1873-3913. ; 10:9, s. 645-651
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Tidskriftsartikel (refereegranskat)abstract
- We examined the relation between the islet NO system and islet hormone secretion induced by either the non-glucose nutrient alpha-ketoisocaproic acid (KIC) or, in some experiments, glucose. KIC dose dependently stimulated insulin but inhibited glucagon secretion. In a medium devoid of any nutrient, the NO synthase (NOS)-inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) induced an increase in basal insulin release but a decrease in glucagon release. These effects were evident also in K+-depolarised islets. KIC-induced insulin release was increased by L-NAME. This increase was abolished in K+-depolarised islets. In contrast, glucose- induced insulin release was potentiated by L-NAME after K+ depolarisation. The intracellular NO donor hydroxylamine dose dependently inhibited KIC-stimulated insulin release and reversed KIC-induced suppression of glucagon release. Our data suggest that islet hormone secretion in a medium devoid of nutrients is greatly affected by the islet NO system, whereas KIC-induced secretion is little affected. Glucose-induced insulin release, however, is accompanied by increased NOS activity, the NOS-activating signal being derived from the glycolytic-pentose shunt part of glucose metabolism. The observed NO effects on islet hormone release can proceed independently of membrane-depolarisation events.
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| 12. |
- Salehi, S Albert, et al.
(författare)
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The nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester potentiates insulin secretion stimulated by glucose and L-arginine independently of its action on ATP-sensitive K+ channels
- 1998
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Ingår i: Bioscience Reports. - 0144-8463. ; 18:1, s. 19-28
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Tidskriftsartikel (refereegranskat)abstract
- The nature of the action of the nitric oxide synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME) on hormone release from isolated islets was investigated. We found that glucose-induced insulin release was potentiated by L-NAME in the absence or presence of diazoxide, a potent K+ATP channel opener, as well as in the presence of diazoxide plus a depolarizing concentration of K+. At a low, physiological glucose concentration L-NAME did not influence insulin secretion induced by K+ but inhibited glucagon secretion. L-arginine-induced insulin release was potentiated by L-NAME. This potentiation was observed also in the presence of K+ plus diazoxide. Further, glucagon release induced by L-arginine as well as by L-arginine plus K+ and diazoxide was suppressed by L-NAME. The results strongly suggest that the L-NAME-induced potentiation of insulin secretion in response to glucose or L-arginine as well as the inhibitory effects on glucagon secretion are largely mediated by L-NAME directly suppressing islet NOS activity. Hence NO apparently affects insulin and glucagon secretion independently of membrane depolarization events.
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| 13. |
- Salehi, S Albert, et al.
(författare)
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The pseudotetrasaccharide acarbose inhibits pancreatic islet glucan-1,4-alpha-glucosidase activity in parallel with a suppressive action on glucose-induced insulin release
- 1995
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Ingår i: Diabetes. - : American Diabetes Association. - 1939-327X .- 0012-1797. ; 44:7, s. 830-836
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Tidskriftsartikel (refereegranskat)abstract
- The pseudotetrasaccharide acarbose, previously known as a potent inhibitor of intestinal alpha-glucoside hydrolases, was investigated with regard to its influence on islet lysosomal enzyme activities and the insulin secretory processes. We observed that acarbose was a potent inhibitor of mouse islet lysosomal acid glucan-1,4-alpha-glucosidase activity, EC50 approximately 5 mumol/l, as well as of acid alpha-glucosidase activity. In contrast, acarbose did not influence other lysosomal enzyme activities such as acid phosphatase and N-acetyl-beta-D-glucosaminidase. Neutral alpha-glucosidase (endoplasmic reticulum) was only moderately inhibited in homogenate and was unaffected in intact islets. Incubation of isolated mouse islets with acarbose revealed that the pseudotetrasaccharide was a strong inhibitor of glucose-induced insulin secretion, EC50 approximately 500 nmol/l, and a significant inhibition was already observed at a concentration of acarbose as low as 100 nmol/l. The acarbose analogue maltotetrose did not influence either glucose-induced insulin release or islet lysosomal enzyme activities. Further, acarbose as well as two other alpha-glucoside hydrolase inhibitors, the deoxynojirimycin derivatives miglitol and emiglitate, did not affect islet glucose oxidation at low or high glucose levels. Acarbose also inhibited insulin release induced by the sulfonylurea glibenclamide, whereas insulin secretion stimulated by the cholinergic muscarinic agonist carbachol or the phosphodiesterase inhibitor isobutylmethylxanthine was unaffected by the drug. Moreover, complementary in vivo experiments showed that pretreatment of mice with acarbose to allow for endocytosis of the compound markedly suppressed the insulin secretory response to an intravenous glucose load.(ABSTRACT TRUNCATED AT 250 WORDS)
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| 14. |
- Åkesson, Bengt, et al.
(författare)
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Islet constitutive nitric oxide synthase and glucose regulation of insulin release in mice
- 1999
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Ingår i: Journal of Endocrinology. - : Bioscientifica. - 1479-6805 .- 0022-0795. ; 163:1, s. 39-48
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Tidskriftsartikel (refereegranskat)abstract
- We have studied, by a combined in vitro and in vivo approach, the relation between the inhibitory action of N(G)-nitro-l-arginine methyl ester (L-NAME), a selective inhibitor of nitric oxide synthase (NOS), on the activity of islet constitutive NOS (cNOS) and glucose regulation of islet hormone release in mice. The cNOS activity in islets incubated in vitro at 20 mM glucose was not appreciably affected by 0.05 or 0.5 mM L-NAME, but was greatly suppressed (-60%) by 5 mM L-NAME. Similarly, glucose-stimulated insulin release was unaffected by the lower concentrations of L-NAME but greatly enhanced in the presence of 5 mM of the NOS inhibitor. In incubated islets inhibition of cNOS activity resulted in a modestly enhanced insulin release in the absence of glucose, did not display any effect at physiological or subphysiological glucose concentrations, but resulted in a markedly potentiated insulin release at hyperglycaemic glucose concentrations. In the absence of glucose, glucagon secretion was suppressed by L-NAME. The dynamics of glucose-induced insulin release and (45)Ca(2+) efflux from perifused islets revealed that L-NAME caused an immediate potentiation of insulin release, and a slight increase in (45)Ca(2+) efflux. In islets depolarized with 30 mM K(+) in the presence of the K(+)(ATP) channel opener, diazoxide, L-NAME still greatly potentiated glucose-induced insulin release. Finally, an i.v. injection of glucose to mice pretreated with L-NAME was followed by a markedly potentiated insulin response, and an improved glucose tolerance. In accordance, islets isolated directly ex vivo after L-NAME injection displayed a markedly reduced cNOS activity. In conclusion, we have shown here, for the first time, that biochemically verified suppression of islet cNOS activity, induced by the NOS inhibitor L-NAME, is accompanied by a marked potentiation of glucose-stimulated insulin release both in vitro and in vivo. The major action of NO to inhibit glucose-induced insulin release is probably not primarily linked to changes in Ca(2+) fluxes and is exerted mainly independently of membrane depolarization events.
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