The metabolic effects of ghrelin on muscle, adipose and liver.

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Ghrelin is a gastric peptide best known for its orexigenic (appetite-stimulating) effects. Plasma ghrelin concentrations rise sharply prior to entrained mealtimes and decline following the consumption of food. Ghrelin exists as two isoforms, acylated (AG) and deacylated (DAG) ghrelin. AG is considered the bioactive form in terms of orexigenic actions, although DAG has recently been demonstrated to have metabolic effects on peripheral tissues. AG is a ligand for the growth hormone secretagogue receptor subtype 1a (GHS-R1a) and initiates growth hormone (GH) release. A number of studies suggest that ghrelin infusion impairs the clearance of glucose from the blood; however, these studies utilize a wide range of dosages which are often nonphysiological, and are unable to distinguish between the direct effects of ghrelin vs. those due to the secondary increase in GH. It has been postulated that ghrelin is part of an important mechanism to prevent extreme hypoglycemia during a prolonged fast by stimulating GH release and thereby facilitating lipolysis and the preservation of blood glucose. Surprisingly, the