CATECHOLAMINE RECEPTOR IN THE SEAWATER EEL INTESTINE

Dose-response relationships of catecholamines on seawater eel intestinal ion transport were obtained; the potency order being (-)adrenalin > (-)noradrenalin = clonidine > (+/-)noradrenalin (racemic form of noradrenalin) > (-) phenylephrine > dopamine > (+/-)isoproterenol, indicating that alpha2-agonists are more potent than alpha1- or beta-agonists. In addition, the effects of adrenalin were completely blocked by yohimbine (alpha2-antagonist) but not by prazosin (alpha1-antagonist) or propranolol (beta-antagonist). These results indicate the existence of an alpha2-receptor in the seawater eel intestine. Adrenalin may activate the alpha2-receptor physiologically, since adrenalin is the most potent stimulant and is the predominant catecholamine in American eel plasma (Hathaway and Epple 1989). Presumably ion and water absorption across the seawater eel intestine will be maintained by adrenalin. From the structure and the action of various agents used in the present study, structure-activity relationships of catecholamines are considered: hydroxyl groups on the benzene ring (catechol) seem to be essential for the alpha2-action in the seawater eel intestine and the presence of OH and CH3 on beta-carbon and amide, respectively, seems to potentiate the alpha2-action.