Interaction of orally administered metoprolol, practolol and propranolol with isoprenaline in asthmatics

Abstract

In a single-blind study eight patients with asthma received on five different days, in randomized order, doses of: propranolol 40 mg, metoprolol 50 mg and 100 mg (a new selective beta1-blocking agent with no intrinsic activity), practolol 200 mg or placebo. The beta1-blocking potency of the selective drugs at these dose levels was higher than or equal to that of propranolol. The effects on heart rate, blood pressure, ventilatory capacity (FEV1) and skeletal muscle tremor were studied at rest and during infusion of increasing doses of isoprenaline. There was a slight decrease in FEV1 after propranolol and both doses of metoprolol. After the placebo, isoprenaline produced a dose-dependent increase in heart rate, systolic blood pressure, FEV1 and tremor, and lowered the diastolic blood pressure. Propranolol almost completely blocked the effects of all doses of isoprenaline on heart rate, FEV1 and tremor, and, to a lesser extent, any change in systolic and diastolic pressure. The two doses of metoprolol and practolol did not inhibit the effect of isoprenaline on FEV1. The effect of metoprolol and practolol on the isoprenaline-induced increase of heart rate indicated their selectivity for beta1-receptors. The increase in tremor during infusion of isoprenaline was blocked by propranolol, but not by the selective beta1-blockers, which implies that the induction of tremor was a beta2-effect. The effect of isoprenaline on FEV1 was not inhibited by selective beta1-blocking agents, and so, when combined with beta2-stimulators, they may be given to asthmatic patients.