1. 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 α-adrenoceptors.

2. The postsynaptic α-adrenoceptor of rat aorta is of the α1-subtype, but may differ from the α1-adrenoceptor of rabbit and guinea pig aortae, based on comparison of relative potencies of selected agonists.

3. By evaluating the relative potency of agonists, qualitative and quantitative species differences between α1-adrenoceptors in rat and guinea pig are observed in a variety of test systems in vivo and in vitro.

4. By comparing the relative potencies of aromatic hydroxyl-substituted phenethylamines and imidazolines at α1-adrenoceptors in guinea pig aorta, differences in the ability of these two classes of α-adrenoceptor agonists to bind to, and subsequently activate, α1-adrenoceptors have been observed.

5. Evaluating the relative potencies of agonists, when used in conjunction with other techniques, provides a valuable method for classifying α-adrenoceptors and for studying α-adrenergic mechanisms.

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