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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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(a) NOS activities and hormone secretion in islets incubated at low glucose. Islet NO production from ncNOS, iNOS and total NOS as well as insulin and glucagon release from islets of Wistar or GK rats incubated at 3.3 mmol/l glucose in the absence (open bars) and presence (dark bars) of 100 nmol/l GLP-1. Values are mean±s.e.m for 5–9 batches of islets at each point. *P<0.05; ** P<0.01; *** P<0.001. (b) Representative examples of Western blots of iNOS and ncNOS protein in the absence and presence of GLP-1 are shown.
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pone-0002165-g002: (a) NOS activities and hormone secretion in islets incubated at low glucose. Islet NO production from ncNOS, iNOS and total NOS as well as insulin and glucagon release from islets of Wistar or GK rats incubated at 3.3 mmol/l glucose in the absence (open bars) and presence (dark bars) of 100 nmol/l GLP-1. Values are mean±s.e.m for 5–9 batches of islets at each point. *P<0.05; ** P<0.01; *** P<0.001. (b) Representative examples of Western blots of iNOS and ncNOS protein in the absence and presence of GLP-1 are shown.

Mentions: Since not only pharmacological blockade [5], [6], [8], [10], [11] but also cyclic AMP stimulating agents are known to suppress islet NO production in healthy animals[3], [7], we tested possible beneficial effects of GLP-1 on islet NOS activities in the GK rat. Fig. 2 a, b and Table 2a show the effect of GLP-1 on islet NOS activities and protein expression (Western blot) as well as insulin and glucagon secretion in islets from GK and Wistar rats incubated at low glucose (3.3 mmol/l). We used a concentration of 100 nmol/l of GLP-1, having maximal stimulating effect on glucose-induced insulin release in isolated rat islets [3]. Total NO generation was markedly increased in GK islets (Fig. 2a). This was mainly due to iNOS activity. No significant iNOS expression and activity was detectable in Wistar islets (Fig. 2a, b). ncNOS activity was modestly upregulated in GK islets (Fig. 2a). GLP-1 induced pronounced suppression of iNOS expression and activity in GK islets and suppressed ncNOS activity in both types of islets (Fig. 2a, b and Table 2a). Basal insulin secretion in GK and Wistar islets was similar at low glucose and GLP-1 had no effect (Fig. 2a). Glucagon secretion was impressively increased in GK islets vs Wistar islets (33.2±2.4 pg/islet per h vs 19.8±1.7 pg/islet per h; p<0.01) (Fig. 2a). GLP-1 suppressed glucagon secretion to 17.2±1.3 pg/islet per h in GK islets and to 12.8±1.1 pg/islet per h in Wistar controls. Notably, GK islets still hypersecreted glucagon after GLP-1 treatment (Fig. 2b). The densitometric analysis showed that GLP-1 induced a pronounced suppression of both ncNOS and iNOS expression in GK islets. A marked suppression of ncNOS expression was found also in Wistar islets, while no iNOS expression could be detected (Table 2a).


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)

(a) NOS activities and hormone secretion in islets incubated at low glucose. Islet NO production from ncNOS, iNOS and total NOS as well as insulin and glucagon release from islets of Wistar or GK rats incubated at 3.3 mmol/l glucose in the absence (open bars) and presence (dark bars) of 100 nmol/l GLP-1. Values are mean±s.e.m for 5–9 batches of islets at each point. *P<0.05; ** P<0.01; *** P<0.001. (b) Representative examples of Western blots of iNOS and ncNOS protein in the absence and presence of GLP-1 are shown.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2367446&req=5

pone-0002165-g002: (a) NOS activities and hormone secretion in islets incubated at low glucose. Islet NO production from ncNOS, iNOS and total NOS as well as insulin and glucagon release from islets of Wistar or GK rats incubated at 3.3 mmol/l glucose in the absence (open bars) and presence (dark bars) of 100 nmol/l GLP-1. Values are mean±s.e.m for 5–9 batches of islets at each point. *P<0.05; ** P<0.01; *** P<0.001. (b) Representative examples of Western blots of iNOS and ncNOS protein in the absence and presence of GLP-1 are shown.
Mentions: Since not only pharmacological blockade [5], [6], [8], [10], [11] but also cyclic AMP stimulating agents are known to suppress islet NO production in healthy animals[3], [7], we tested possible beneficial effects of GLP-1 on islet NOS activities in the GK rat. Fig. 2 a, b and Table 2a show the effect of GLP-1 on islet NOS activities and protein expression (Western blot) as well as insulin and glucagon secretion in islets from GK and Wistar rats incubated at low glucose (3.3 mmol/l). We used a concentration of 100 nmol/l of GLP-1, having maximal stimulating effect on glucose-induced insulin release in isolated rat islets [3]. Total NO generation was markedly increased in GK islets (Fig. 2a). This was mainly due to iNOS activity. No significant iNOS expression and activity was detectable in Wistar islets (Fig. 2a, b). ncNOS activity was modestly upregulated in GK islets (Fig. 2a). GLP-1 induced pronounced suppression of iNOS expression and activity in GK islets and suppressed ncNOS activity in both types of islets (Fig. 2a, b and Table 2a). Basal insulin secretion in GK and Wistar islets was similar at low glucose and GLP-1 had no effect (Fig. 2a). Glucagon secretion was impressively increased in GK islets vs Wistar islets (33.2±2.4 pg/islet per h vs 19.8±1.7 pg/islet per h; p<0.01) (Fig. 2a). GLP-1 suppressed glucagon secretion to 17.2±1.3 pg/islet per h in GK islets and to 12.8±1.1 pg/islet per h in Wistar controls. Notably, GK islets still hypersecreted glucagon after GLP-1 treatment (Fig. 2b). The densitometric analysis showed that GLP-1 induced a pronounced suppression of both ncNOS and iNOS expression in GK islets. A marked suppression of ncNOS expression was found also in Wistar islets, while no iNOS expression could be detected (Table 2a).

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