Ghrelin Attenuates cAMP-PKA Signaling to Evoke Insulinostatic Cascade in Islet P-Cells

摘要

Ghrelin reportedly restricts insulin release in islet 3-cells via the Ga12 subtype of G-proteins and thereby regulates glucose homeostasis. This study explored whether ghrelin regulates cAMP signaling and whether this regulation induces insulinostatic cascade in islet (3-cells. RESEARCH DESIGN AND METHODS-Insulin release was measured in rat perfused pancreas and isolated islets and cAMP production in isolated islets. Cytosolic cAMP concentrations ([cAMP])) were monitored in mouse MCM6 cells using evanescent-wave fluorescence imaging. In rat single p-cells, cytosolic protein kinase-A activity ([PKA]i) and Ca2+ concentration QCa2+]i) were measured by DR-II and fura-2 microfiuorometry, respectively, and whole cell currents by patch-clamp technique. RESULTS-Ghrelin suppressed glucose (8.3 mmol/L)-induced insulin release in rat perfused pancreas and isolated islets, and these effects of ghrelin were blunted in the presence of cAMP analogs or adenylate cyclase inhibitor. Glucose-induced cAMP production in isolated islets was attenuated by ghrelin and enhanced by ghrelin receptor antagonist and anti-ghrelin antise-rum, which counteract endogenous islet-derived ghrelin. Ghrelin inhibited the glucose-induced [cAMPJi elevation and [PKAJi activation in MIN6 and rat p-cells, respectively. Furthermore, ghrelin potentiated voltage-dependent K+ (Kv) channel currents without altering Ca2+ channel currents and attenuated glucose-induced [Ca2+]i increases in rat p-cells in a PKA-dependent manner. CONCLUSIONS-Ghrelin directly interacts with islet p-cells to attenuate glucose-induced cAMP production and PKA activation, which lead to activation of Kv channels and suppression of glucose-induced [Ca2+]i increase and insulin release.