Determination of the Intrinsic Efficacies of β2 -adrenergic Agonists

β-adrenergic agonists have been traditionally classified as strong or weak. Attempts to express their effectiveness in quantitative terms has led to the concepts of potency, which designates the concentration range over which the agonist becomes effective, and the intrinsic activity, which designates the maximal effect produced by agonist at saturating concentrations. In the present review we describe developments in which the molecular effects of the common β-adrenergic agonists on their cognate receptors can be related to their effectiveness. This approach is based on the activation/inactivation cycle of G proteins. It has been formalized so that the effectiveness (that is the efficacy) of each individual β-adrenergic agonist can be expressed as a single numerical value. The agonists may, therefore, be listed is order of efficacy. For the β-adrenergic agonists for which there is accurate data the order is : epinephrine > fenoterol≈procaterol > albuterol≈zinterol≈terbutaline > dobutamine > tulobuterol > ephedrine. The formal model of β-adrenergic agonism also allows a novel approach to the question of agonist specificity and a more rational appraisal of which drugs might be most useful for particular purposes.