Angiotensin II type 1 receptor antagonists: Why do some of them produce insurmountable inhibition?

Chinese hamster ovary (CHO) cells expressing human recombinant angiotensin II type 1 (AT1) receptors offer a useful experimental system in which antagonist binding and inhibition of AT-induced inositol mono-, bis-, and trisphosphate accumulation can be measured under identical experimental conditions. The major conclusions of the current work are: All investigated AT1 antagonists are competitive with respect to AT. They bind to a common or overlapping binding site on the receptor in a mutually exclusive way. Reduction of the maximal angiotensin II response, i.e. insurmountable inhibition, is observed only when the cells are preincubated with candesartan, EXP3174, or irbesartan and is strictly related to the dissociation rate of the antagonist–receptor complex. On the other hand, inhibition by losartan is fully surmountable by AT, and its dissociation is very rapid. With respect to the binding kinetics, the antagonist–receptor complex can adopt a fast and a slow reversible state. The equilibrium between both states, which is dependent upon the nature of the antagonists, determines the extent of insurmountable inhibition. Consequently, the dissociation rate of the different antagonists correlates with the amount of insurmountable inhibition. In addition to the relatively slow dissociation of candesartan, reassociation to the receptor, which is measurable in CHO-AT1 cells, likely contributes to its long-lasting blood pressure lowering effect in vivo.