Distinction beta een surmountable and insurmountable selective AT(1) receptor antagonists by use of CHO-K1 cells expressing human angiotensin II AT(1) receptors

1 CHO-K1 cells that were stably transfected with the gene for the human AT( 1) receptor (CHO-AT(1) cells) were used for pharmacological studies of non- peptide AT(1) receptor antagonists. 2 In the presence of 10 mM LiCl, angiotensin II caused a concentration-depe ndent and long-lasting increase of inositol phosphates accumulation with an EC50 of 3.4 nM. No angiotensin II responses are seen in wild-type CHO-K1 c ells. 3 [H-3]-Angiotensin II bound to cell surface AT(1) receptors (dissociates u nder mild acidic conditions) and is subject to rapid internalization 4 Non-peptide selective AT(1) antagonists inhibited the angiotensin II (0.1 mu M) induced IP accumulation and the binding of [H-3]-angiotensin II (1 n M) with the potency order: candesartan > EXP3174 > irbesartan > losartan. T heir potencies are lower in the presence of bovine serum albumin. 5 Preincubation with the insurmountable antagonist candesartan decreased th e maximal angiotensin II induced inositol phosphate accumulation up to 94% and, concomitantly, decreased the maximal binding capacity of the cell surf ace receptors. These inhibitory effects were half-maximal for 0.6 nM candes artan and were attenuated by simultaneous preincubation with 1 mu M losarta n indicating a syntopic action of both antagonists. 6 Losartan caused a parallel rightward shift of the angiotensin II concentr ation-response curves and did not affect the maximal binding capacity. EXP3 174 (the active metabolite of losartan) and irbesartan showed a mixed-type behavior in both functional and binding studies. 7 Reversal of the inhibitory effect was slower for candesartan as compared with EXP3174 and irbesartan and it was almost instantaneous for losartan, s uggesting that the insurmountable nature of selective AT(1) receptor antago nists in functional studies was related to their long-lasting inhibition.