Role of calcium and EPAC in norepinephrine-induced ghrelin secretion

Ghrelin is an orexigenic hormone secreted principally from a distinct population of gastric endocrine cells. Molecular mechanisms regulating ghrelin secretion are mostly unknown. Recently, norepinephrine (NE) was shown to enhance ghrelin release by binding to1-Adrenergic receptors on ghrelin cells. Here,weuseanimmortalized stomach-derived ghrelin cell line to further characterize the intracellular signaling pathways involved in NE-induced ghrelin secretion, with a focus on the roles of Ca²⁺ and cAMP. Several voltage-gated Ca²⁺ channel (VGCC) family members were found by quantitative PCR to be expressed by ghrelin cells. Nifedipine, a selective L-type VGCC blocker, suppressed both basal and NE-stimulated ghrelin secretion.NEinduced elevation of cytosolic Ca²⁺levels both in the presence and absence of extracellular Ca²⁺. Ca²⁺-sensing synaptotagmins Syt7 and Syt9 were also highly expressed in ghrelin cell lines, suggesting that they too help mediate ghrelin secretion. Raising cAMP with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine also stimulated ghrelin secretion, although such a cAMP-mediated effect likely does not involve protein kinase A, given the absence of a modulatory response to a highly selective protein kinase A inhibitor. However, pharmacological inhibition of another target of cAMP, exchange protein-Activated by cAMP (EPAC), did attenuate both basal and NE-inducedghrelin secretion,whereasanEPACagonistenhancedbasal ghrelin secretion.Weconclude that constitutive ghrelin secretion is primarily regulated by Ca²⁺ influx through L-type VGCCs and that NE stimulates ghrelin secretion predominantly through release of intracellular Ca²⁺. Furthermore, cAMP and its downstream activation of EPAC are required for the normal ghrelin secretory response to NE.