Effects of intravenous propranolol and metoprolol and their interaction with isoprenaline on pulmonary function, heart rate and blood pressure in asthmatics
The effects of propranolol (0.06 mg/kg i.v.), the selectiveβ 1-receptor antagonist metoprolol (0.12 mg/kg i.v.) and a placebo on pulmonary function, heart rate and blood pressure have been compared in asthmatics. The interaction of these drugs with increasing doses of isoprenaline on the same variables was also studied. The twoβ-blockers reduced resting heart rate to the same extent, indicating the same degree of blockade of cardiacβ-receptors. Bothβ-blockers reduced the basal forced expiratory volume in one second (FEV1), and the effect tended to be more pronounced after propranolol. Isoprenaline caused a dose-dependent increase in FEV1 and vital capacity (VC). These effects were almost completely blocked by propranolol, whereas after metoprolol the changes approached that of the placebo. The isoprenaline-induced increase in heart rate and fall in diastolic blood pressure was also inhibited to a considerably greater extent by propranolol than by metoprolol. The results show a selectivity of metoprolol for so-calledβ 1-receptors and indicate that metoprolol may be used in asthmatics provided that it is combined withβ 2-receptor-stimulating drugs.