Abstract
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 α2 are more potent than α1- or β-agonists. In addition, the effects of adrenalin were completely blocked by yohimbine (α2-antagonists) but not by prazosin (α1-antagonists) or propranolol (β-antagonist). These results indicate the existence of an α2-receptor in the seawater eel intestine. Adrenalin may activate the α2-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 α2-action in the seawater eel intestine and the presence of OH and CH3 on β-carbon and amide, respectively, seems to potentiate the α2-action.
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