In vitro evaluation of a series of sympathomimetic amines and the beta-adrenergic blocking properties of cyclopentamine

A series of sympathomimetic amines of varying structure was studied on the isolated rabbit ileum and uterus in the presence of phentolamine. Norepinephrine was the most potent and cyclopentamine the least potent of the drugs giving responses. The structure required for high activity was discussed. It was demonstrated that cyclopentamine was a very weak alpha-adrenergic receptor stimulant and a weak beta-adrenergic receptor blocker. This drug also potentiated the auto-inhibition produced by high doses of isoproterenol and the effect lasted as long as the autoinhibition persisted. It was suggested that the blockade by both drugs was the result of a direct action on the beta receptors. Isoproterenol also produced an auto-inhibition of its own responses on rabbit intestine which was gradually reversible with time.