Alpha-adrenoreceptor-related dissociation constants and intrinsic efficacies of stereoisomers of epinephrine.

THe alpha-adrenoreceptor-related dissociation constant and intrinsic efficacy of the stereoisomers of epinephrine were determined on the rabbit aortic strip. (-)-Epinephrine showed higher affinity than (+)-epinephrine or the desoxy analogue epinine. Efficacy of (+)-epinephrine relative to (-)-epinephrine was low (relative efficacy, er = 0.44). Epinephrine had about the same affinity and efficacy as (+)-epinephrine. When the alpha-adrenoreceptors were protected against dibenamine by an equimolar concentration of the isomers, (-)-epinephrine was found to be more effective than (+)-epinephrine or epinine. These results suggest that the stereoisomers of catecholamines differ not only in affinity but also in intrinsic efficacy.