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...

The gastric hormone ghrelin and its receptor, growth hormone secretagogue receptor (GHSR), are expressed in pancreas. Here, we report that ghrelin is released from pancreatic islets to regulate glucose-induced insulin release. Plasma concentrations of ghrelin, as well as insulin, were higher in pancreatic veins than in arteries. GHSR antagonist and immunoneutralization of endogenous ghrelin enhanced glucose-induced insulin release from perfused pancreas, whereas exogenous ghrelin suppressed it. GHSR antagonist increased plasma insulin levels in gastrectomized and normal rats to a similar extent. Ghrelin knockout mice displayed enhanced glucose-induced insulin release from isolated islets, whereas islet density, size, insulin content, and insulin mRNA levels were unaltered. Glucose tolerance tests (GTTs) in ghrelin knockout mice showed increased insulin and decreased glucose responses. Treatment with high-fat diet produced glucose intolerance in GTTs in wild-type mice. In ghrelin knockout mice, the high-fat diet-induced glucose intolerance was largely prevented, whereas insulin responses to GTTs were markedly enhanced. These findings demonstrate that ghrelin originating from pancreatic islets is a physiological regulator of glucose-induced insulin release. Antagonism of the ghrelin function can enhance insulin release to meet increased demand for insulin in high-fat diet-induced obesity and thereby normalize glycemic control, which may provide a potential therapeutic application to counteract the progression of type 2 diabetes. Diabetes 55: 3486 -3493, 2006

In a wide variety of cells, various intracellular agents, such as Ca2+, ATP and cyclic nucleotides, regulate ionic conductances of the membrane. In cardiac cells, the intracellular Na+ concentration [( Na+]i) frequently increases when a disturbance occurs in the electrogenic Na-K pump activity or the Na-Ca exchange mechanism. We have investigated a possible role of [Na+]i in controlling ion channels by using a patch-clamp method, and have found a K+ channel that is gated by [Na+]i greater than 20 mM, but not by the intracellular Ca2+ concentration (approximately 10(-4) M). We report here that the channel has a unitary conductance of 207 +/- 19 pS (n = 16) with K+ concentrations of 150 mM outside and 49 mM inside, and shows no detectable voltage-dependent kinetics. The Na+-activated K+ channel represents a novel class of ionic channel.

SUMMARY1. A class of K channels in cardiac muscle is reversibly blocked by intracellular adenosine 5'-triphosphate (ATP). The characteristics of this K channel were studied by recording single-channel currents in ventricular cells isolated enzymatically from guinea-pig heart. The reversal potential of single-channel currents agreed well with the K equilibrium potential. Blockers of other K channels, such as tetraethylammonium and 4-aminopyridine, decreased the mean open time of the channel.2. The chord conductance increased as the 0-24th power of the K concentration on the outer surface of the membrane, and showed a marked inward-going rectification on strong depolarizations. The degree of rectification was larger with increasing Na concentration on the inner side of the membrane.3. The kinetics of the channel were almost voltage independent, but depended on the concentration of intracellular ATP. The conductance of the channel was not affected by ATP.4. When channel kinetics were examined in the presence of ATP, the distribution of open times and closed times was fitted well with a sum of two exponential components. When ATP concentration was increased, the time constants obtained from the open-time histogram decreased and those from the closed-time histogram increased, resulting in a decrease of the open-state probability.5. The channel was blocked by ATP, adenosine 5'-diphosphate,5'-adenylylimidodiphosphate, guanosine 5'-triphosphate and uridine 5'-triphosphate, but not by adenosine 5'-monophosphate, creatine phosphate, creatine or adenosine. Plots of the open-state probability versus the ATP concentration revealed Michaelis-Menten saturation kinetics with strong co-operativity of multiple receptor sites (Hill coefficient 3-4, concentration of half-saturation 05 mM). It was concluded that this K channel has three or four receptor sites selective for triphosphate nucleotide on the inner surface of the membrane, and that the channel is blocked through the binding of agonists to the receptors.

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