Circulatory catecholamines in the eel: origins and functions

While the three catecholamines (CAs) dopamine (DA), norepinephrine (NE) and epinephrine (E), are wide-spread in tissues of the American eel (Anguilla rostrata), the bulk of these CAs in the systemic blood originates from chromaffin cells in the wall of the posterior cardinal veins. In addition, the brain and unidentified structures in the opisthonephric kidney also release appreciable quantities of CAs. The functional realms attributed to systematically circulating CAs in teleosts comprise cardiovascular, respiratory, osmoregulatory, metabolic and endocrinotropic actions. In the eel, cardiovascular and respiratory effects are well established. However, we were unable to prove a physiological role of the CAs in osmoregulation. On the other hand, the eel is the only species among five vertebrates of greatly varying phylogenetic position (the others: hagfish, lamprey, rat, human) in which physiological doses of E were hyperglycemic. As in lamprey and rat, DA and NE are released in the eel by physiological doses of E. In addition, DA and NE also release the respective other two CAs. The physiological significance of the catecholaminotropic (CA-tropic) interactions remains to be established; however, the CA-tropic effect of E does not require the presence of the brain or ‘preganglionic’ nerve cells. In the eel, mild stress causes an immediate ‘unorthodox’ drop of plasma CAs, while stronger stress is followed by the expected increase of plasma CAs.