KINETIC-STUDIES ON THE INTERACTION OF NONLABELED ANTAGONISTS WITH THEANGIOTENSIN-II RECEPTOR

Angiotensin AT(1) receptor antagonists are divided into two types, sur mountable and insurmountable, based on the way they inhibit angiotensi n II-induced vasoconstriction. To elucidate what causes the difference , we studied how antagonists associate with and dissociate from AT(1) receptor sites in rat liver membranes. Three antagonists, ]-4,7-dihydr opyrazolo[1,5a]pyrimidine-3-carboxylic acid (SRL1080227), iphenyl-4-yl ]methyl]-1H-benzimidazole-7-carboxylic acid (CV-11974) and l)biphenyl- 4-yl]methyl]-3H-imidazo-[4,5-b]pyridine (FK739), showed competitive an tagonism when they were added simultaneously with [I-125]angiotensin I I, but CV11974 and SRL1080227 showed apparently noncompetitive antagon ism when membranes were preincubated with each antagonist. The longer the preincubation time with CV11974 or SRL1080227 was, the more effect ively the antagonist inhibited [I-125]angiotensin II binding, while th e inhibition by FK739 did not change with the preincubation time. To e stimate their dissociation rate from the receptor binding site, we stu died [I-125]angiotensin II binding to membranes which had been preincu bated with each antagonist and washed twice. Membranes pretreated with FK739 completely recovered the ability to bind [I-125]angiotensin II with a period of 60 min, while membranes preincubated with CV11974 did not read this level of recovery. [I-125]angiotensin II binding to mem branes preincubated with SRL1080227 increased gradually, but did not r each the control level during the experiment. The kinetic properties o f SRL1080227, CV11974 and FK739 were consistent with their characteris tic modes in inhibiting angiotensin II-induced contraction of isolated rabbit aorta and decreasing blood pressure of spontaneously hypertens ive rats. The present results support the hypothesis that slow dissoci ation of an antagonist from the receptor binding site is involved in i ts apparently noncompetitive behavior in receptor binding and its insu rmountable property in functional studies.