Effects of isoetharine on airways resistance, heart rate, and contractions of the soleus muscle of the anaesthetised cat

The actions of the sympathomimetic bronchodilator, isoetharine, were compared with those of laevoisoprenaline, racemic isoprenaline and salbutamol, on the heart and lungs and on contractions of the soleus muscle of cats under chloralose anaesthesia. Isoetharine and salbutamol injected i.v. were approximately equipotent in all tests, and were about 8 times less potent than laevoisoprenaline both in decreasing the tension and degree of fusion of incomplete tetanic contractions of the cat soleus muscle, and in opposing the bronchoconstrictor action of 5-hydroxytryptamine. They were about 22 times less potent than laevoisoprenaline in increasing heart rate. In all tests, racemic isoprenaline was about 2 times less potent than laevoisoprenaline. The results suggest that the configuration of soleus β-receptors is closely similar to that of β-receptors in the lungs, and that the systemic administration of isoetharine might produce muscle tremor as a side-effect in man.