Comparative study of chronotropic and inotropic responses to 5-(-hydroxy-2-isopropylaminobutyl)-8-hydroxycarbostyril hydrochloride hemihydrade (procaterol), salbutamol, noradrenaline and isoprenaline in the dog heart.

1. The cardiac chronotropic and inotropic responses to 5-(1-hydroxy-2-isopropylaminobutyl)-8-hydroxycarbostyril hydrochloride hemihydrade (procaterol) and salbutamol have been compared to noradrenaline and isoprenaline in isolated blood-perfused canine atrial and ventricular preparations. 2. All four compounds induced dose-related positive chronotropic and inotropic effects, but different individual response patterns were observed. 3. Procaterol and salbutamol were partial agonists compared to noradrenaline and isoprenaline for causing chronotropic and inotropic responses. The order of efficacy for the adrenoceptors mediating both chronotropic and inotropic responses was isoprenaline, noradrenaline, salbutamol and procaterol. The slopes of the salbutamol dose-response curves were flatter than those for isoprenaline and noradrenaline; the slopes of the procaterol dose-response curves were flatter than those for salbutamol. 4. Because of these differences, the order of activity depended upon the level of response chosen for the comparison. When doses producing small chronotropic and inotropic responses were compared, the order of activity was isoprenaline, noradrenaline, procaterol and salbutamol; whereas with doses producing large responses the order was isoprenaline, noradrenaline, salbutamol and procaterol. 5. Procaterol was longer acting than salbutamol, and salbutamol was longer acting than noradrenaline.