Adrenergic regulation of hepatic glycogenolysis in yellow and silver European eel

The European eel (Anguilla anguilla) has a complex life cycle, which includes migrations to and from seawater at different life stages. Yellow eels grow for several years in fresh- or brackish-waters, then metamorphose into silver eels and start a long reproductive migration towards the Sargasso Sea. Eels at this stage are rarely studied, mainly due to the difficulties in their capture at the onset of the migration. However, the regulation of energetic metabolism is an interesting issue, since silver eels migrate for about 5,000 km without eating. Our investigation started from the adrenergic control of glucose metabolism in the liver of yellow and silver eels collected from the coastal lagoon of Comacchio (North Adriatic Sea, Italy). Epinephrine induced glucose release from freshly isolated hepatocytes in a time- and dose-dependent manner. Similar effects were triggered by isoproterenol, and to a lesser extent by noradrenaline. Interestingly, basal glucose levels were significantly lower in silver with respect to yellow eels. These results were correlated with the higher levels of glycogen content in yellow eels, uncommon feature in fish. However, hormone sensitivity was higher in hepatocytes from silver eels. We also found that expression of genes encoding α1 and β2 adrenergic receptors were significantly higher in silver than in yellow eels. These are the first evidence comparing adrenergic regulation of glucose metabolism in yellow and silver eels, and suggest that within a conserved pathway there are significant differences between the two life stages.