The insulin response to gastric glucose is reduced in PAC1 and GRP receptor gene deleted mice

Effects of central gastrin-releasing peptide on glucose metabolism

Gastrin-releasing peptide (GRP) mediated signals in the central nervous system (CNS) influence many functions associated with energy metabolism. The purpose of the present study was to investigate the central effect of GRP on glucose metabolism in the male rat. Intracerebroventricular (icv) administration of GRP caused an immediate hyperglycaemia which was sustained till the end of the infusion. The rise in plasma glucose levels was accompanied by an increase in endogenous glucose production (EGP), as well as increases in plasma glucagon and insulin concentrations. Furthermore, no differences in plasma corticosterone levels were noted between control and GRP treated rats. These results demonstrate that central GRP increases plasma glucose levels, probably by stimulating pancreatic glucagon release and concomitantly or subsequently endogenous glucose production.

Characterization of intestinal and pancreatic dysfunction in VPAC1-null mutant mouse

OBJECTIVES

These studies examined the effect of homozygous deletion of vasoactive intestinal peptide receptor type 1 (VPAC1) on development and function of intestines and pancreas.

METHODS

Genetically engineered VPAC1-null mutant mice were monitored for growth, development, and glucose homeostasis. Expression of VPAC1 was examined during embryonic development using VPAC1 promoter-driven β-galactosidase transgenic mice.

RESULTS

Homozygous deletion of VPAC1 resulted in fetal, neonatal, and postweaning death owing to failure to thrive, intestinal obstruction, and hypoglycemia. Histological findings demonstrated disorganized hyperproliferation of intestinal epithelial cells with mucus deposition and bowel wall thickening. The pancreas demonstrated small dysmorphic islets of Langerhans containing α, β, and δ cells. Expression of a VPAC1 promoter-driven transgene was observed in E12.5 and E14.5 intestinal epithelial and pancreatic endocrine cells. Vasoactive intestinal peptide receptor type 1-null mutant animals had lower baseline blood glucose levels compared to both heterozygous and wild-type littermates. Vasoactive intestinal peptide receptor type 1-deficient mice responded to oral glucose challenge with normal rise in blood glucose followed by rapid hypoglycemia and failure to restore baseline glucose levels. Insulin challenge resulted in profound hypoglycemia and inadequate glucose homeostasis in VPAC1-null mutant animals.

CONCLUSIONS

These observations support a role for VPAC1 during embryonic and neonatal development of intestines and endocrine pancreas.

Role of pituitary adenylate cyclase-activating polypeptide in the pancreatic endocrine system

In the pancreatic islets, pituitary adenylate cyclase-activating polypeptide (PACAP) is expressed in beta cells and autonomic nerve terminals; the majority of these nerve terminals are parasympathetic. PACAP binds to three types of G protein-coupled receptors (GPCRs): VPAC1 receptors, VPAC2 receptors, and PAC1 receptors. All these receptor types are expressed in pancreatic islets. PACAP stimulates insulin and glucagon secretion. These actions are achieved in part through increased formation of cAMP after activation of adenylate cyclase and in part through increase in cytosolic calcium, achieved through increase in calcium uptake and release from intracellular calcium stores. Deletion of PAC1 receptors or VPAC2 receptors results in impaired insulin secretion and glucose intolerance. Studies in PAC1 receptor gene deleted mice have suggested that PACAP may be of physiological importance in mediating prandial insulin secretion and in contributing to the glucagon response to hypoglycemia. Animal studies have also suggested that activation of the receptors, in particular VPAC2 receptors, may be used as a therapeutic approach for the treatment of type 2 diabetes. Hence, PACAP is an islet neuropeptide with a potential role in islet physiology and as a basis for development of islet-promoting therapy in type 2 diabetes.