Epinephrine enantiomers: affinity, efficacy and potency relationships in rat smooth muscle tissues

The purpose of this study was to examine the relationship between affinity and efficacy for the epinephrine enantiomers in rat tissues. Potency and was measured for (-)- and (+)-epinephrine in aorta, longitudinal muscle of portal vein, spleen and vas deferens. After partial alpha-adrenoreceptor inactivation with phenoxybenzamine, dissociation constant was estimated for each enantiomer and used to estimate efficacy. Whereas potency, affinity and efficacy vary between tissues, there is a constant relationship between (-)- and (+)-enantiomer; the enantiomeric ratio is constant for both potency and affinity. Efficacy is similar for the enantiomers in each tissue. The potency difference between epinephrine enantiomers at alpha-adrenoreceptors in rat tissues is due to differences in affinity rather than efficacy.