Exogenous and endogenous ghrelin counteracts GLP-1 action to stimulate cAMP signaling and insulin secretion in islet β-cells

AbstractWe studied interactive effects of insulinotropic GLP-1 and insulinostatic ghrelin on rat pancreatic islets. GLP-1 potentiated glucose-induced insulin release and cAMP production in isolated islets and [Ca2+]i increases in single β-cells, and these potentiations were attenuated by ghrelin. Ghrelin suppressed [Ca2+]i responses to an adenylate cyclase activator forskolin. Moreover, GLP-1-induced insulin release and cAMP production were markedly enhanced by [d-lys3]-GHRP-6, a ghrelin receptor antagonist, in isolated islets. These results indicate that both exogenous and endogenous islet-derived ghrelin counteracts glucose-dependent GLP-1 action to increase cAMP production, [Ca2+]i and insulin release in islet β-cells, positioning ghrelin as a modulator of insulinotropic GLP-1

Reconstruction-Dependent Recovery from Anorexia and Time-Related Recovery of Regulatory Ghrelin System in Gastrectomized Rats

Gastrectomy reduces food intake and body weight (BW) hampering recovery of physical conditions. It also reduces plasma levels of stomach-derived orexigenic ghrelin. This study explored changes in orexigenic ghrelin system in rats receiving total gastrectomy with Billroth II (B-II) or Roux-en-Y (R-Y) method. Feeding and BW were reduced by gastrectomy and subsequently recovered to a greater extent with R-Y than B-II while plasma ghrelin decreased similarly. At postoperative 12th week, ghrelin contents increased in the duodenum and pancreas, plasma ghrelin levels increased upon fasting, and ghrelin injection promoted feeding but not in earlier periods. In summary, gastrectomized rats partially recover feeding and BW, in a reconstruction-dependent manner. At 12th week, ghrelin is upregulated in extra-stomach tissues, plasma ghrelin levels are physiologically regulated, and orexigenic effect of exogenous ghrelin is restored. This time-related recovery of ghrelin system may provide a strategy for promoting feeding, BW, and thereby physical conditions in gastrectomized patients

A new experimental model of ATP-sensitive K+ channel-independent insulinotropic action of glucose: a permissive role of cAMP for triggering of insulin release from rat pancreatic beta-cells

信州大学博士（医学）・学位論文・平成24年3月31日授与（甲第936号）・武井 真大In pancreatic beta-cells, glucose metabolism leads to closure of ATP sensitive K+ channels (K-ATP channel) and Ca2+ influx, which is regarded as a required step for triggering of insulin release. Here, we demonstrate that glucose triggers rapid insulin release independent from its action on K-ATP channels given the cellular cAMP is elevated. We measured insulin release from rat pancreatic islets by static and perifusion experiments. Changes in cytosolic free Ca2+ concentration ([Ca2+](i)) were monitored using fura-2 loaded rat pancreatic beta-cells. Glucose-induced insulin release was abolished when Ca2+ influx was inhibited by a combination of 250 mu M diazoxide, an opener of K-ATP channel, and 10 mu M nifedipine, a blocker of L-type voltage-dependent Ca2+ channels. However, with both nifedipine and diazoxide, glucose induced a 5-fold increase in insulin release in the presence of 10 mu M forskolin, an activator of adenylyl cyclase. In the presence of diazoxide, nifedipine, and forskolin, 22 mM glucose sharply increased the rate of insulin release within 2 min which peaked at 6 min: this was followed by a further gradual increase in insulin release. In contrast, it lowered [Ca2+](i) with a nadir at 2-3 min followed by a gradual increase in [Ca2+](i). The glucose effect was mimicked by 20 mM alpha-ketoisocaproic acid, a mitochondrial fuel, and it was nullified by 2 mM sodium azide, an inhibitor of mitochondrial electron transport. Cerulenin, an inhibitor of protein acylation, decreased the glucose effect. In conclusion, a rise in [Ca2+](i) through voltage-dependent Ca2+ channels is not mandatory for glucose-induced triggering of insulin release.ArticleENDOCRINE JOURNAL. 60(5):599-607 (2013)journal articl