Secretin, glucagon, gastric inhibitory polypeptide, parathyroid hormone, and related peptides in the regulation of the hy
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Secretin, glucagon, gastric inhibitory polypeptide (GIP), and parathyroid hormone (PTH) belong, together with vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase (AC)-activating polypeptide, to a family of peptides (the VIP-secretin-glucagon family), which also includes growth hormone-releasing hormone and exendins. All the members of this peptide family possess a remarkable amino-acid sequence homology, and bind to G-protein-coupled receptors, whose signaling mechanism primarily involves AC/protein kinase A and phospholipase C/protein kinase C cascades. VIP and pituitary AC-activating polypeptide play a role in the regulation of the hypothalamus-pituitary-adrenal (HPA) axis, and in this review we survey findings that also other members of the VIP-secretin-glucagon family may have the same function. Secretin and secretin receptors are expressed in the hypothalamus and pituitary gland, and secretin inhibits adrenocorticotropic hormone (ACTH) release. No evidence is available for the presence of secretin receptors in adrenal glands, but secretin selectively depresses the glucocorticoid response to ACTH of dispersed zona fasciculata-reticularis (ZF/R) cells. Glucagon and glucagon-like peptide-1 are contained in the hypothalamus, and all the components of the HPA axis are provided with glucagon and glucagons-like-1 receptors. These peptides exert a short-term inhibitory effect on stress-induced pituitary ACTH release and depress the ZF/R cell response to ACTH by inhibiting the AC/protein kinase A cascade; they also stimulate hypothalamic arginine-vasopressin release. GIP receptors are present in the ZF/R of the normal adrenals, and are particularly abundant in some types of adrenocortical adenomas and hyperplasias. GIP, through the activation of the AC/protein kinase A cascade, evokes a sizeable glucocorticoid secretagogue effect, leading to the identification of a food/GIP-dependent Cushing's syndrome. PTH and PTH-related protein are expressed in the hypothalamus and pituitary gland, and PTH and PTH-related protein receptors in all the components of the HPA axis. Both peptides enhance ACTH and arginine-vasopressin release, as well as stimulate aldosterone and glucocorticoid secretion of dispersed zona glomerulosa and ZF/R cells, respectively. The involvement of growth hormone-releasing hormone and exendins in the functional regulation of the HPA axis has not yet been extensively investigated.
Nitric oxide (NO) is one of the important biologic mediators in regulation of gastrointestinal (GI) functions, but the influence of NO on the release of secretin and cholecystokinin (CCK) and exocrine pancreatic secretion has not been adequately investigated in the rat. The aim of this study was to determine the role of NO on endogenous and exogenous secretin- or CCK-stimulated pancreatic exocrine secretion both in anesthetized and conscious rats. Experiments were carried out in four different groups of rats with duodenal pancreatobiliary cannulas and jugular vein catheters. Group 1: During duodenal infusion of 0.05N HCl or 15% casein (pH 7.0), N-nitro-L-arginine (NNA), an inhibitor of NO-synthase in graded doses (2.5, 5, 10 mg/kg/h), was infused intravenously. Group 2: One hour after starting intravenous secretin at 5 pmol/kg/h or intravenous CCK-8 at 0.06 microg/kg/h, NNA in graded doses was administered intravenously. Group 3: In conscious rats, NNA (5 mg/kg/h) was given intravenously for 1 hour after a meal. Group 4: L-Arginine at 100 mg/kg/h was infused intravenously during the period of NNA (5 mg/kg/h) infusion in groups 1, 2, and 3. Pancreatic juice was collected at 30-minute intervals to measure volume, as well as output of bicarbonate and protein. At the end of the experiment, plasma secretin, vasoactive intestinal polypeptide (VIP) and CCK levels were determined by radioimmunoassay (RIA). NNA dose dependently inhibited the pancreatic secretion of fluid and bicarbonate stimulated by duodenal acidification, exogenous secretin, and a meal. NNA dose dependently inhibited the pancreatic secretion of protein stimulated by duodenal infusion of casein, exogenous CCK, and a meal. L-Arginine significantly reversed the NNA-induced inhibition of pancreatic secretion in all experiments. NNA did not alter significantly the plasma levels of secretin, VIP, and CCK. Our results indicated that endogenous NO plays a significant role in the regulation of pancreatic exocrine secretion stimulated by secretin and CCK. However, NO does not influence the release of secretin, VIP, or CCK in the rat.