Pharmacology of 4-hydroxypropranolol, a metabolite of propranolol.

1. 4-Hydroxypropranolol, a metabolite produced after oral administration of propranolol, has been shown to be a beta-adrenoceptor blocking drug. It is of similar potency to propranolol in antagonizing the effects of isoprenaline on heart rate and blood pressure in cats and against isoprenaline protection of guinea-pigs from bronchospasm. It is not cardioselective.2. In rats depleted of catecholamine 4-hydroxypropranolol produced an increase in heart rate, suggesting that it has intrinsic sympathomimetic activity.3. In anaesthetized dogs 4-hydroxypropranolol produced a decrease in heart rate and dP/dt and an increase in A-V conduction time at doses within the range 0.09-1.25 mg/kg. These effects are a result of beta-adrenoceptor blockade. In dogs depleted of catecholamines these same doses produced an increase in heart rate and dP/dt and a decrease in A-V conduction time. These responses were antagonized by propranolol, and were due to the intrinsic sympathomimetic activity of the compound. At higher doses (5.25 and 13.25 mg/kg) a further dose dependent decrease in heart rate and dP/dt and an increase in A-V conduction time occurred. This trend was also seen in animals depleted of catecholamines. These changes represent membrane stabilizing activity of 4-hydroxypropranolol.4. 4-Hydroxypropranolol is a potent beta-adrenoceptor blocking drug with both intrinsic sympathomimetic activity and membrane stabilizing activity.