Adrenergic receptors and signaling in yellow and silver European eel hepatocytes

European eels spend up to twenty years in fresh or brackish water as “yellow” eels. Then, part of the population change into migrant “silver” eels, moving towards the spawning grounds at the Sargasso sea. Migrating eels do not feed, and show higher levels of cortisol, causing lipolysis in muscle and liver and higher free fatty acids into the blood. Further hormones change their levels and modulate metabolism in migrating eels, including insulin, IGFs, leptin, and ghrelin. Our investigation compared for the first time catecholamine regulation of liver metabolism in yellow and silver eels. Expression of α1- and β2-adrenergic receptor (AR) mRNAs in eel hepatocytes was assessed by qRT-PCR. Levels of both α1- and β2-AR mRNAs were significantly higher in hepatocytes from silver than from yellow eels, with a 5-fold and a 2-fold increase, respectively. As a result, the mean ratio α1-AR/β2-AR transcripts was significantly higher in silver than in yellow eel. In a first approach, the β2-AR coupled signal transduction was investigated. Epinephrine dose-dependently increased cAMP levels in eel hepatocytes, an effect counteracted by propranolol. Under the same stimuli, the extent of cAMP increase was similar at the two life-stages; differently, the induced glucose release was significantly higher in silver than in yellow eels. Since α1-AR mediated pathway concurs to modulate liver glycogenolysis, further studies will evaluate the contribution of calcium signaling in silver eel response.