THE α‐ AND β‐ADRENOCEPTOR BLOCKING POTENCIES OF LABETALOL AND ITS INDIVIDUAL STEREOISOMERS IN ANAESTHETIZED DOGS AND IN ISOLATED TISSUES

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Abstract
1 The antagonist potencies of labetalol and each of its four stereoisomers have been compared at α1-, β1- and β2-adrenoceptors in anaesthetized dogs and in isolated tissues. 2 The RR stereoisomer is a potent, non-selective antagonist at β-adrenoceptors but has only weak α1-adrenoceptor blocking activity. 3 The SR stereoisomer was the most potent antagonist at α1-adrenoceptors, and it also had similar potency as an antagonist at β-adrenoceptors. 4 The α- and β-adrenoceptor blocking profile of the RS stereoisomer is intermediate between that of the RR and SR, but the SS stereoisomer is a relatively weak antagonist at both α- and β-adrenoceptors. 5 It is concluded that, although most of the α1-adrenoceptor blocking activity of labetalol is attributable to the SR stereoisomer and nearly all of its β-adrenoceptor blocking activity resides in the RR stereoisomer, each of the stereoisomers contributes to the overall pharmacological profile of labetalol.