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Program of Neural and Behavioral Sciences, Department of Physiology and Pharmacology, State University of New York Downstate Medical Center, Brooklyn, New York 11203
Pancreatic islets contain
ionotropic glutamate receptors that can modulate hormone secretion. The
purpose of this study was to determine whether islets express
functional group III metabotropic glutamate (mGlu) receptors. RT-PCR
analysis showed that rat islets express the mGlu8 receptor subtype.
mGlu8 receptor immunoreactivity was primarily displayed by
glucagon-secreting
-cells and intrapancreatic neurons. By
demonstrating the immunoreactivities of both glutamate and the
vesicular glutamate transporter 2 (VGLUT2) in these cells, we
established that
-cells express a glutamatergic phenotype. VGLUT2
was concentrated in the secretory granules of islet cells, suggesting
that glutamate might play a role in the regulation of glucagon
processing. The expression of mGlu8 by glutamatergic cells also
suggests that mGlu8 may function as an autoreceptor to regulate
glutamate release. Pancreatic group III mGlu receptors are functional
because mGlu8 receptor agonists inhibited glucagon release and
forskolin-induced accumulation of cAMP in isolated islets, and
(R,S)-cyclopropyl-4-phosphonophenylglycine, a group III mGlu receptor
antagonist, reduced these effects. Because excess glucagon secretion
causes postprandial hyperglycemia in patients with type 2 diabetes,
group III mGlu receptor agonists could be of value in the treatment of
these patients.
glutamate; glucagon; vesicular glutamate transporter 2; (R,S)-4-phosphonophenylglycine; (S)-3,4-dicarboxyphenylglycine; (R,S)-cyclopropyl-4-phosphonophenylglycine; cyclic adenosine monophosphate
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