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Excessive islet NO generation in type 2 diabetic GK rats coincides with abnormal hormone secretion and is counteracted by GLP-1.

Salehi A, Meidute Abaraviciene S, Jimenez-Feltstrom J, Ostenson CG, Efendic S, Lundquist I - PLoS ONE (2008)

Bottom Line: Pharmacological blockade of islet NO production by the NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) greatly improved hormone secretion from GK islets suggesting islet NOS activity being an important target to inactivate for amelioration of islet cell function.The incretin hormone GLP-1, which is used in clinical practice suppressed iNOS and ncNOS expression and activity with almost full restoration of insulin release and partial restoration of glucagon release.The results suggest that abnormally increased NO production within islet cells is a significant player in the pathogenesis of type 2 diabetes being counteracted by GLP-1 through PKA-dependent, nonproteasomal mechanisms.

View Article: PubMed Central - PubMed

Affiliation: Department of Clinical Science, Universitetssjukhuset Malmö Allmäna Sjukhus, Division of Endocrine Pharmacology, Karolinska Institute, Stockholm, Sweden. S_Albert.Salehi@med.lu.se

ABSTRACT

Background: A distinctive feature of type 2 diabetes is inability of insulin-secreting beta-cells to properly respond to elevated glucose eventually leading to beta-cell failure. We have hypothesized that an abnormally increased NO production in the pancreatic islets might be an important factor in the pathogenesis of beta-cell dysfunction.

Principal findings: We show now that islets of type 2 spontaneous diabetes in GK rats display excessive NO generation associated with abnormal iNOS expression in insulin and glucagon cells, increased ncNOS activity, impaired glucose-stimulated insulin release, glucagon hypersecretion, and impaired glucose-induced glucagon suppression. Pharmacological blockade of islet NO production by the NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) greatly improved hormone secretion from GK islets suggesting islet NOS activity being an important target to inactivate for amelioration of islet cell function. The incretin hormone GLP-1, which is used in clinical practice suppressed iNOS and ncNOS expression and activity with almost full restoration of insulin release and partial restoration of glucagon release. GLP-1 suppression of iNOS expression was reversed by PKA inhibition but unaffected by the proteasome inhibitor MG132. Injection of glucose plus GLP-1 in the diabetic rats showed that GLP-1 amplified the insulin response but induced a transient increase and then a poor depression of glucagon.

Conclusion: The results suggest that abnormally increased NO production within islet cells is a significant player in the pathogenesis of type 2 diabetes being counteracted by GLP-1 through PKA-dependent, nonproteasomal mechanisms.

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In vivo action of GLP-1 and glucose.Effect of an iv injection of GLP-1 (10 nmol/kg)+glucose (4.4 mmol/kg)(a–c) or glucose alone (11.1 mmol/kg)(d–f) on the plasma concentrations of insulin, glucagon and glucose in Wistar and GK rats. Values are mean±s.e.m for 8 animals in each group. *P<0.05; ** P<0.01; *** P<0.001.
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pone-0002165-g005: In vivo action of GLP-1 and glucose.Effect of an iv injection of GLP-1 (10 nmol/kg)+glucose (4.4 mmol/kg)(a–c) or glucose alone (11.1 mmol/kg)(d–f) on the plasma concentrations of insulin, glucagon and glucose in Wistar and GK rats. Values are mean±s.e.m for 8 animals in each group. *P<0.05; ** P<0.01; *** P<0.001.

Mentions: Since GK islets apparently hypersecreted glucagon we measured circulating levels of glucagon, insulin and glucose in vivo. In addition we measured islet content of glucagon and insulin. Plasma levels of glucagon and glucose were elevated in GK rats, while insulin levels were not appreciably different. However, calculating the basal insulinogenic index, i.e. dividing circulating insulin concentrations with circulating glucose concentrations, revealed that the insulin response to basal glucose was impaired in GK vs Wistar rats (data not shown). The plasma levels were as follows, Wistar vs GK: Insulin (pmol/l) 100±8 vs 81±6.4 (NS); Glucagon (ng/l) 254.5±10.2 vs 311.5±12.9 (p<0.001); Glucose (mmol/l) 7.3±0.2 vs 11.0±0.2 (p<0.001). There were 28 animals in each group. Islet content of insulin and glucagon at 6 weeks of age was similar in GK and Wistar rats. Insulin content Wistar vs GK was 5.5±0.6 and 5.6±0.9 nmol/mg protein and glucagon content 1.02±0.09 and 0.89±0.09 µg/mg protein (n = 8 in each group). Because the action of GLP-1 is known to be highly glucose-dependent [12], [13] we examined in vivo insulin and glucagon responses to GLP-1 mixed with glucose. Fig. 5a–b shows the effects of an iv injection of this mixture and a control experiment with a high dose of glucose alone (Fig. 5d–e). The insulin response to the combination of GLP-1 and glucose was modestly lower in GK compared with Wistar. However, there was an abnormal transient increase in glucagon response followed by a slow suppression, which did not reach the low levels of glucagon recorded in Wistar. After injection of glucose alone the insulin response in GK rats was abrogated, and the glucagon response showed an initial normal suppression but a marked rebound (Fig. 5d–e). An impaired glucose tolerance curve after glucose alone (Fig. 5f) was still impaired in the GK rat even after addition of GLP-1 (Fig. 5c).


Excessive islet NO generation in type 2 diabetic GK rats coincides with abnormal hormone secretion and is counteracted by GLP-1.

Salehi A, Meidute Abaraviciene S, Jimenez-Feltstrom J, Ostenson CG, Efendic S, Lundquist I - PLoS ONE (2008)

In vivo action of GLP-1 and glucose.Effect of an iv injection of GLP-1 (10 nmol/kg)+glucose (4.4 mmol/kg)(a–c) or glucose alone (11.1 mmol/kg)(d–f) on the plasma concentrations of insulin, glucagon and glucose in Wistar and GK rats. Values are mean±s.e.m for 8 animals in each group. *P<0.05; ** P<0.01; *** P<0.001.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2367446&req=5

pone-0002165-g005: In vivo action of GLP-1 and glucose.Effect of an iv injection of GLP-1 (10 nmol/kg)+glucose (4.4 mmol/kg)(a–c) or glucose alone (11.1 mmol/kg)(d–f) on the plasma concentrations of insulin, glucagon and glucose in Wistar and GK rats. Values are mean±s.e.m for 8 animals in each group. *P<0.05; ** P<0.01; *** P<0.001.
Mentions: Since GK islets apparently hypersecreted glucagon we measured circulating levels of glucagon, insulin and glucose in vivo. In addition we measured islet content of glucagon and insulin. Plasma levels of glucagon and glucose were elevated in GK rats, while insulin levels were not appreciably different. However, calculating the basal insulinogenic index, i.e. dividing circulating insulin concentrations with circulating glucose concentrations, revealed that the insulin response to basal glucose was impaired in GK vs Wistar rats (data not shown). The plasma levels were as follows, Wistar vs GK: Insulin (pmol/l) 100±8 vs 81±6.4 (NS); Glucagon (ng/l) 254.5±10.2 vs 311.5±12.9 (p<0.001); Glucose (mmol/l) 7.3±0.2 vs 11.0±0.2 (p<0.001). There were 28 animals in each group. Islet content of insulin and glucagon at 6 weeks of age was similar in GK and Wistar rats. Insulin content Wistar vs GK was 5.5±0.6 and 5.6±0.9 nmol/mg protein and glucagon content 1.02±0.09 and 0.89±0.09 µg/mg protein (n = 8 in each group). Because the action of GLP-1 is known to be highly glucose-dependent [12], [13] we examined in vivo insulin and glucagon responses to GLP-1 mixed with glucose. Fig. 5a–b shows the effects of an iv injection of this mixture and a control experiment with a high dose of glucose alone (Fig. 5d–e). The insulin response to the combination of GLP-1 and glucose was modestly lower in GK compared with Wistar. However, there was an abnormal transient increase in glucagon response followed by a slow suppression, which did not reach the low levels of glucagon recorded in Wistar. After injection of glucose alone the insulin response in GK rats was abrogated, and the glucagon response showed an initial normal suppression but a marked rebound (Fig. 5d–e). An impaired glucose tolerance curve after glucose alone (Fig. 5f) was still impaired in the GK rat even after addition of GLP-1 (Fig. 5c).

Bottom Line: Pharmacological blockade of islet NO production by the NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) greatly improved hormone secretion from GK islets suggesting islet NOS activity being an important target to inactivate for amelioration of islet cell function.The incretin hormone GLP-1, which is used in clinical practice suppressed iNOS and ncNOS expression and activity with almost full restoration of insulin release and partial restoration of glucagon release.The results suggest that abnormally increased NO production within islet cells is a significant player in the pathogenesis of type 2 diabetes being counteracted by GLP-1 through PKA-dependent, nonproteasomal mechanisms.

View Article: PubMed Central - PubMed

Affiliation: Department of Clinical Science, Universitetssjukhuset Malmö Allmäna Sjukhus, Division of Endocrine Pharmacology, Karolinska Institute, Stockholm, Sweden. S_Albert.Salehi@med.lu.se

ABSTRACT

Background: A distinctive feature of type 2 diabetes is inability of insulin-secreting beta-cells to properly respond to elevated glucose eventually leading to beta-cell failure. We have hypothesized that an abnormally increased NO production in the pancreatic islets might be an important factor in the pathogenesis of beta-cell dysfunction.

Principal findings: We show now that islets of type 2 spontaneous diabetes in GK rats display excessive NO generation associated with abnormal iNOS expression in insulin and glucagon cells, increased ncNOS activity, impaired glucose-stimulated insulin release, glucagon hypersecretion, and impaired glucose-induced glucagon suppression. Pharmacological blockade of islet NO production by the NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) greatly improved hormone secretion from GK islets suggesting islet NOS activity being an important target to inactivate for amelioration of islet cell function. The incretin hormone GLP-1, which is used in clinical practice suppressed iNOS and ncNOS expression and activity with almost full restoration of insulin release and partial restoration of glucagon release. GLP-1 suppression of iNOS expression was reversed by PKA inhibition but unaffected by the proteasome inhibitor MG132. Injection of glucose plus GLP-1 in the diabetic rats showed that GLP-1 amplified the insulin response but induced a transient increase and then a poor depression of glucagon.

Conclusion: The results suggest that abnormally increased NO production within islet cells is a significant player in the pathogenesis of type 2 diabetes being counteracted by GLP-1 through PKA-dependent, nonproteasomal mechanisms.

Show MeSH
Related in: MedlinePlus