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Islet constitutive nitric oxide synthase: biochemical determination and regulatory function
A Salehi1, M Carlberg, R Henningson
1Department of Pharmacology, University of Lund, Sweden.
The American Journal of Physiology
|June 1, 1996
Summary
The islets of Langerhans contain nitric oxide synthase (NOS), an enzyme regulating hormone release. This enzyme suppresses insulin secretion while enhancing glucagon secretion, suggesting a novel regulatory role in glucose homeostasis.
Area of Science:
- Endocrinology
- Molecular Biology
- Biochemistry
Background:
- Immunohistochemical studies suggested constitutive nitric oxide synthase (cNOS) presence in the endocrine pancreas.
- Direct biochemical evidence for cNOS activity in isolated islets was lacking.
Purpose of the Study:
- To provide direct biochemical evidence for cNOS activity in pancreatic islets.
- To investigate the role of NOS activity in regulating islet hormone release (insulin and glucagon).
Main Methods:
- Quantification of cNOS activity in islet homogenates via L-citrulline formation assay using HPLC.
- Inhibition studies using NG-nitro-L-arginine methyl ester (L-NAME).
- In vivo studies and experiments with nitric oxide (NO) donors.
Main Results:
- Direct biochemical evidence confirmed Ca2+/calmodulin-dependent cNOS activity in isolated islets, inhibited by L-NAME.
- L-NAME enhanced glucose- and L-arginine-induced insulin release but inhibited L-arginine-stimulated glucagon release.
- Nitric oxide (NO) donors suppressed insulin and increased glucagon release in isolated islets.
- Glucose potentiated L-NAME's effect on insulin release, indicating glucose regulation of islet NO production.
Conclusions:
- The islets of Langerhans possess a constitutive, Ca2+/calmodulin-dependent NOS isoform.
- Islet NO negatively regulates insulin secretion and positively regulates glucagon secretion.
- The islet NO system represents a novel regulatory mechanism for insulin and glucagon secretion and glucose homeostasis.