On the quantitation of an agonist with dual but opposing components of action: application to vascular endothelial relaxation.

In this communication we show that the same principle that underlies the use of the isobolograph for assessing agonist interactions also leads to a method for analyzing the opposing effects of a single agonist. This is the principle of dose equivalence whose application is illustrated here and applied to the endothelium-dependent relaxing component of two putative vasoconstrictor peptides. These studies, employing angiotensin II and endothelin-1, were conducted with isolated preparations of rat aorta that were measured for agonist-induced isometric tension development in both endothelial-denuded and -intact vessels. The dose-effect relation of the relaxing component of each agonist, which should not be calculated from simple effect subtraction, was derived by the method described here.