Does the beta 1-adrenoceptor blocking agent celiprolol have alpha 2-adrenoceptor blocking properties?

The beta 1-adrenoceptor blocking agent celiprolol was tested for alpha 2-adrenoceptor blocking properties in different pharmacological models. In radioligand binding studies celiprolol shows a tenfold higher affinity to alpha 2- in comparison to alpha 1-adrenoceptors; however, the affinity of celiprolol to alpha-adrenoceptors is at least 100 times lower than that to beta-adrenoceptors. Celiprolol enhances the inhibitory effect of the alpha 2-adrenoceptor agonist clonidine in the electrically stimulated rat vas deferens in vitro; this celiprolol effect is abolished, but not reversed by propranolol. In pithed rats celiprolol inhibits the tachycardia induced by spinal electrical stimulation; the inhibitory effect of clonidine on the same parameter is augmented by celiprolol, but reversed by rauwolscine. Also in pithed rats celiprolol does not affect the pressor effect of the alpha 2-adrenoceptor agonist B-HT 920. From these results it is concluded that celiprolol does not possess any relevant alpha 2-adrenoceptor blocking properties which could explain the broncho- and vasodilating actions of that substance.