l-Ornithine stimulates growth hormone release in a manner dependent on the ghrelin system
We found that intraduodenal administration of L-ornithine (L-Orn) stimulates growth hormone (GH) secretion in Wistar rats, and then investigated its mechanism. GH-releasing activity after intraduodenal administration of L-Orn was blocked by [D-Lys3]-GHRP-6, an antagonist of the ghrelin receptor; however, L-Orn (100 μM) has no affinity for the ghrelin receptor, suggesting that the GH-releasing activity of L-Orn is mediated via ghrelin release and activation of the ghrelin receptor. Intraduodenally administered L-Orn increased ghrelin mRNA expression in the duodenum but not in the stomach or hypothalamus. In addition, L-Orn-induced GH-releasing activity was inhibited by propranolol, an antagonist of β-adrenergic receptor, which is known to be coupled to ghrelin release. In conclusion, intraduodenally administered L-Orn stimulates GH secretion through the sympathetic nervous and ghrelin systems.