title: Ghrelin attenuates cAMP-PKA signaling to evoke insulinostatic cascade in islet β-cells.
abstract: OBJECTIVE
  Ghrelin reportedly restricts insulin release in islet β-cells via the Gα(i2) 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 β-cells.
  
  
  METHODS
  Insulin release was measured in rat perfused pancreas and isolated islets and cAMP production in isolated islets. Cytosolic cAMP concentrations ([cAMP](i)) were monitored in mouse MIN6 cells using evanescent-wave fluorescence imaging. In rat single β-cells, cytosolic protein kinase-A activity ([PKA](i)) and Ca(2+) concentration ([Ca(2+)](i)) were measured by DR-II and fura-2 microfluorometry, 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 antiserum, which counteract endogenous islet-derived ghrelin. Ghrelin inhibited the glucose-induced [cAMP](i) elevation and [PKA](i) activation in MIN6 and rat β-cells, respectively. Furthermore, ghrelin potentiated voltage-dependent K(+) (Kv) channel currents without altering Ca(2+) channel currents and attenuated glucose-induced [Ca(2+)](i) increases in rat β-cells in a PKA-dependent manner.
  
  
  CONCLUSIONS
  Ghrelin directly interacts with islet β-cells to attenuate glucose-induced cAMP production and PKA activation, which lead to activation of Kv channels and suppression of glucose-induced [Ca(2+)](i) increase and insulin release.
authors.affiliation: Division of Integrative Physiology, Department of Physiology, Jichi Medical University School of Medicine, Tochigi, Japan. dezaki@jichi.ac.jp
authors.forename: Katsuya
authors.initials: K
authors.lastname: Dezaki
authors.forename: Boldbaatar
authors.initials: B
authors.lastname: Damdindorj
authors.forename: Hideyuki
authors.initials: H
authors.lastname: Sone
authors.forename: Oleg
authors.initials: O
authors.lastname: Dyachok
authors.forename: Anders
authors.initials: A
authors.lastname: Tengholm
authors.forename: Erik
authors.initials: E
authors.lastname: Gylfe
authors.forename: Tomoyuki
authors.initials: T
authors.lastname: Kurashina
authors.forename: Masashi
authors.initials: M
authors.lastname: Yoshida
authors.forename: Masafumi
authors.initials: M
authors.lastname: Kakei
authors.forename: Toshihiko
authors.initials: T
authors.lastname: Yada
mesh_terms: D000818:Animals; D000242:Cyclic AMP / Q000378:metabolism*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism*; D054439:Ghrelin / Q000378:metabolism* / Q000494:pharmacology; D005947:Glucose / Q000378:metabolism / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D051379:Mice; D051381:Rats; D017208:Rats, Wistar; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology
chemical_list: D054439:Ghrelin; D007328:Insulin; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D005947:Glucose