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Updated: Oct 14, 2026

Quantifying Agonist Activity at G Protein-coupled Receptors
Published on: December 26, 2011
Relative agonist potency as a means of differentiating alpha-adrenoceptors and alpha-adrenergic mechanisms
Abstract:
Relative potencies (ED50), affinities (assessed as dissociation constants) and efficacies (i.e. ability to activate the receptor) of agonists are useful in subclassifying and differentiating alpha-adrenoceptors. The postsynaptic alpha-adrenoceptor of rat aorta is of the alpha 1-subtype, but may differ from the alpha 1-adrenoceptor of rabbit and guinea pig aortae, based on comparison of relative potencies of selected agonists. By evaluating the relative potency of agonists, qualitative and quantitative species differences between alpha 1-adrenoceptors in rat and guinea pig are observed in a variety of test systems in vivo and in vitro. By comparing the relative potencies of aromatic hydroxyl-substituted phenethylamines and imidazolines at alpha 1-adrenoceptors in guinea pig aorta, differences in the ability of these two classes of alpha-adrenoceptor agonists to bind to, and subsequently activate, alpha 1-adrenoceptors have been observed. Evaluating the relative potencies of agonists, when used in conjunction with other techniques, provides a valuable method for classifying alpha-adrenoceptors and for studying alpha-adrenergic mechanisms.
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