Comparative studies with the enantiomers of the glycol metabolite of propranolol and their effects on the cardiac β-adrenoceptor

The two enantiomers ((R)- and (S)-) of propranolol glycol, a metabolite of propranolol, have been synthesized, and their effects upon the beta-adrenoceptor studied by two methods. The ability of these compounds to antagonize the inotropic actions of isoprenaline was examined on spontaneously beating rat atrial preparations. Also, the effects of these enantiomers upon the binding of [3H]dihydroalprenolol to beta-receptors in rat cardiac ventricular membranes was studied. Experiments with the atria indicated that the (S)-glycol was a reversible competitive antagonist of isoprenaline with a potency approximately one thousand times lower than that of (+/-)-propranolol. In contrast, the (R)-glycol appeared to act as an irreversible antagonist, producing complex dose-response curves. The effects of these compounds to cause displacement of alprenolol binding were consistent with the organ bath data. The interaction of the (S)-glycol with the beta-receptor binding site was reversible (Ki of 27.6 +/- 4.2 microM) but less potent than that of (+/-)-propranolol (Ki of 0.99 +/- 0.07 nM). On the other hand, pretreatment of ventricular membranes with the (R)-glycol, followed by extensive washing techniques, resulted in alprenolol binding which did not regain control values, providing further evidence for an irreversible effect upon the beta-receptor. The possible significance of these pharmacological actions of the two enantiomers is discussed in terms of the in vivo metabolic pathways for propranolol.