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1 Department of Physiology, University of British Columbia, Vancouver, British Columbia, Canada, V6T 1Z3; and 2 Department of Metabolism and Clinical Nutrition, Graduate School of Medicine, Kyoto University, Kyoto 606-8507, Japan
The
incretins glucose-dependent insulinotropic polypeptide (GIP) and
glucagon-like peptide-1 (GLP-1) are gut hormones that act via the
enteroinsular axis to potentiate insulin secretion from the pancreas in
a glucose-dependent manner. Both GLP-1 receptor and GIP receptor
knockout mice (GLP-1R
/
and
GIPR
/
, respectively) have been generated to
investigate the physiological importance of this axis. Although reduced
GIP action is a component of type 2 diabetes, GIPR-deficient mice
exhibit only moderately impaired glucose tolerance. The present study
was directed at investigating possible compensatory mechanisms that
take place within the enteroinsular axis in the absence of GIP action.
Although serum total GLP-1 levels in GIPR knockout mice were unaltered, insulin responses to GLP-1 from pancreas perfusions and static islet
incubations were significantly greater (40-60%) in
GIPR
/
than in wild-type
(GIPR+/+) mice. Furthermore, GLP-1-induced cAMP
production was also elevated twofold in the islets of the knockout
animals. Pancreatic insulin content and gene expression were reduced in
GIPR
/
mice compared with
GIPR+/+ mice. Paradoxically, immunocytochemical
studies showed a significant increase in
-cell area in the GIPR-null
mice but with less intense staining for insulin. In conclusion,
GIPR
/
mice exhibit altered islet structure
and topography and increased islet sensitivity to GLP-1 despite a
decrease in pancreatic insulin content and gene expression.
insulin; GLP-1; pancreas perfusion; cAMP; incretin
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