Interaction between the partially insurmountable antagonist valsartan and human recombinant angiotensin II type 1 receptors

The interaction between the AT(1) receptor-selective antagonist valsartan, and its human receptor, was investigated by direct radioligand binding as w ell as by its inhibition of angiotensin TI induced inositol phosphate accum ulation in CHO cells expressing human recombinant AT(1) receptors. Specific binding of [H-3]-valsartan rapidly reached equilibrium at 37 degreesC. It was saturable and occurred to a homogeneous class of sites with a K-D of 0. 88 +/- 0.076. It was inhibited by other AT(1) receptor antagonists with the same potency order as previously described for the binding of [H-3]-angiot ensin TI and [H-3]-candesartan to human AT(1) receptors (i.e. candesartan g reater than or equal to EXP3174> valsartan congruent to irbesartan congruen t to angiotensin II > losartan). When valsartan and angiotensin II were app lied simultaneously to the CHO-AT(1) cells. the antagonist caused a rightwa rd shift of the angiotensin II concentration-response curve. Hence, valsart an interacts with the AT(1) receptor in a manner that is competitive with a ngiotensin II. Pre-incubation of the cells with 0.5. 5 and 50 nM valsartan caused an additional, concentration-dependent, up to 55 % decline of the ma ximal response. The partial nature of this insurmountable inhibition by val sartan was confirmed by biphasic antagonist concentration inhibition curves . These data reflect the co-existence of a fast reversible/surmountable as well as a tight binding/insurmountable valsartan-receptor complex. In agree ment, pre-incubation of the CHO-AT(1) cells with 5 and 50 nM valsartan prod uced a partial inhibition of the angiotensin TI induced increase of the foe intracellular calcium concentration. [H-3]-Valsartan dissociated from its receptors with a half-life of 17 min. In functional recovery experiments wi th valsartan-pre-treated cells, the angiotensin II-mediated response was ha lf-maximally restored within approximately 30 min. These kinetic data sugge st that the insurmountable inhibition by valsartan is related to its relati vely slow dissociation from the human AT(1) receptors. (C) 2000 Editions sc ientifiques ct medicales Elsevier SAS.