Ghrelin, isolated from the human and rat stomach, is the endogenous ligand for the growth hormone (GH) secretagogue receptor, which is expressed in a variety of tissues, including the pancreatic islets. It has been shown that low plasma ghrelin levels correlates with elevated fasting insulin levels and type 2 diabetes. Here we show a physiological role of endogenous ghrelin in the regulation of insulin release and blood glucose in rodents. Acylated ghrelin, the active form of the peptide, was detected in the pancreatic islets. Counteraction of endogenous ghrelin by intraperitoneal injection of specific GH secretagogue receptor antagonists markedly lowered fasting glucose concentrations, attenuated plasma glucose elevation, and enhanced insulin responses during the glucose tolerance test (GTT). Conversely, intraperitoneal exogenous ghrelin GH-independently elevated fasting glucose concentrations, enhanced plasma glucose elevation, and attenuated insulin responses during GTT. Neither GH secretagogue receptor antagonist nor ghrelin affected the profiles of the insulin tolerance test. In isolated islets, GH secretagogue receptor blockade and antiserum against acylated ghrelin markedly enhanced glucose-induced increases in insulin release and intracellular Ca G hrelin, a novel acylated 28-amino acid peptide, was isolated from the human and rat stomach as the endogenous ligand (1) for the growth hormone (GH) secretagogue receptor (2). Circulating ghrelin is produced predominantly in the stomach (3). Ghrelin is a potent stimulator of GH release (1) and feeding (4). In addition, the plasma ghrelin level correlates inversely with obesity (5-7). A recent report of a cohort study has shown that low plasma ghrelin is associated with elevated fasting insulin levels, insulin resistance, and type 2 diabetes (8). These findings suggest that ghrelin could be involved in energy and glucose metabolism, in which insulin plays a crucial role. It has been well documented that systemic administration of ghrelin elevates blood glucose (9 -11). However, the physiological role of endogenous ghrelin in the regulation of blood glucose remains to be established. This question was addressed in the present study by examining the effects of ghrelin receptor blockade on blood glucose and insulin levels in rodents.It has been reported that ghrelin (12-16) and its mRNA (1,12,13,17), as well as GH secretagogue receptor mRNAs (1,12,13,17,18), are expressed in the pancreas and islet cells. The present study aimed to explore a role of endogenous ghrelin in islets in the regulation of insulin release. This was achieved by counteraction of the islet ghrelin with specific GH secretagogue receptor antagonists and antiserum against acylated ghrelin, the active form of the peptide (active ghrelin). Furthermore, although ghrelin reportedly influences insulin release in several different systems (12,19 -21), the signaling mechanisms of ghrelin in islet ␤-cells are unknown. We studied ghrelin's action mechanisms with special attention to the Ca 2ϩ signaling i...

Ghrelin, a novel growth hormone-releasing peptide isolated from human and rat stomach, is a 28 -amino acid peptide with a posttranslational acylation modification that is indispensable for stimulating growth hormone secretion by increasing intracellular Ca 2؉ concentration. It also functions in the regulation of feeding behavior, energy metabolism, and gastric acid secretion and motility. Using two different antibodies against the NH 2 -and COOH-terminal regions of ghrelin, we studied its localization in human and rat pancreas by immunohistochemistry. Ghrelin-immunoreactive cells were identified at the periphery of pancreatic islets in both species. Ghrelin co-localized exclusively with glucagon in rat islets, indicating that it is produced in ␣-cells. We identified ghrelin and des-acyl ghrelin in the rat pancreas using reverse-phase high-performance liquid chromatography combined with two radioimmunoassays. We also detected mRNA encoding ghrelin and its receptor in the rat pancreatic islets. Ghrelin increased the cytosolic free Ca 2؉ concentration in ␤-cells and stimulated insulin secretion when it was added to isolated rat pancreatic islets. These findings indicate that ghrelin may regulate islet function in an endocrine and/or paracrine manner. Diabetes 51:124 -129, 2002 G rowth-hormone secretagogues (GHSs) are small synthetic peptides and nonpeptide molecules that stimulate growth hormone (GH) release from the anterior pituitary through the GHS receptor (GHS-R) (1). GHS-R, a G protein-coupled receptor, promotes calcium release from the endoplasmic reticulum (2,3). Ghrelin, a 28 -amino acid peptide with an n-octanoylated Ser 3, was originally discovered in rat stomach as a cognate endogenous ligand for GHS-R by using an intracellular calcium influx assay on a stable cell line expressing rat GHS-R (4). The Ser 3 n-octanoylation is a unique modification necessary for ghrelin's activity. Ghrelin stimulates GH release when administered intravenously or intracerebroventricularly to rats and when applied directly to rat primary pituitary cells (4 -6). In addition, ghrelin increases food intake and body weight upon intracerebroventricular administration (7,8). Furthermore, intravenously or intracerebroventricularly administered ghrelin stimulates gastric acid secretion by activating the vagal system (9,10). These findings suggest that ghrelin is secreted in response to altered food intake or some other nutritional states and thereby plays a role in the regulation of feeding behavior, energy metabolism, and digestion. Ghrelin is produced primarily in the enteroendocrine cells of rats and humans (4,11,12). Many types of enteroendocrine cells, including insulin-and glucagonproducing cells of the pancreatic islets, develop from endodermal epithelium. Ghrelin, like insulin and glucagon, may be produced in the islets and involved in the regulation of energy metabolism.In the present study, we investigated the cellular source of ghrelin in rat and human pancreas by immunohistochemistry. Ghrelin molecules in rat pancreas we...

When isolated rat pancreatic islets are treated with 16·7 mM glucose, a time-dependent potentiation (TDP) of insulin release occurs that can be detected by subsequent treatment with 50 mM KCl ]i was significantly lowered. Protein acylation is implicated in the underlying mechanism of TDP.