M. Hara et al., KINETIC-STUDIES ON THE INTERACTION OF NONLABELED ANTAGONISTS WITH THEANGIOTENSIN-II RECEPTOR, European journal of pharmacology. Molecular pharmacology section, 289(2), 1995, pp. 267-273
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.