Evidence that the efficacy (intrinsic activity) of fenoterol is higher than that of salbutamol on beta-adrenoceptors in guinea-pig trachea.

The functional antagonism of carbachol by fenoterol and salbutamol (beta-adrenoceptor agonists) has been used to demonstrate that the efficacy (intrinsic activity) of fenoterol was about twice that of salbutamol on guinea-pig trachea. The mean maximum shifts of the carbachol concentration--response lines by fenoterol and salbutamol were (log units) 1.07 +/- 0.07 (n = 5) and 0.64 +/- 0.07 (n = 5) respectively. This difference in their efficacies could be demonstrated as differences in maximum relaxation on tracheal preparations contracted with carbachol, although this was dependent on the concentration of carbachol used. beta-Adrenoceptor blocking properties of salbutamol (1 mM) but not fenoterol (1 mM) could be demonstrated on trachea in that salbutamol, but not fenoterol, antagonised the shift in the carbachol concentration--response line produced by isoprenaline. The implications of these findings in relation to the use of fenoterol and salbutamol as bronchodilators is discussed.