Beta-adrenoceptor antagonist activity of 3-methoxyisoprenaline.

1. The beta-adrenoceptor antagonist activity of 3-methoxyisoprenaline, the O-methylated metabolite of isoprenaline, was studied on isolated driven atrial strip and tracheal chain preparations of the guinea-pig and on the hind limb blood flow of the dog.2. On both the atrial strip and tracheal chain preparations the blockade of responses to isoprenaline fulfilled the criteria for simple competitive inhibition.3. 3-Methoxyisoprenaline decreased the vasodilator response to isoprenaline in the dog hind limb, but did not affect the response to noradrenaline.4. 3-Methoxyisoprenaline had about 1/3,700 of the potency of propranolol as a beta-adrenoceptor antagonist on the tracheal chain preparation, 1/1,000 on the atrial strip preparation and less than 1/400 on the hind limb blood flow.5. The antagonist activity of 3-methoxyisoprenaline showed a slight specificity for cardiac beta-adrenoceptors, being 4.3 times more active on guinea-pig atria than on trachea.6. Although 3-methoxyisoprenaline antagonized the actions of isoprenaline in the three preparations, its activity was extremely weak. It is unlikely that the formation of 3-methoxyisoprenaline from isoprenaline, administered therapeutically, could lead to beta-adrenoceptor blockade.