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

Summary 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.