Antiplatelet action of losartan involves TXA2 receptor antagonism but not TXA2 synthase inhibition.

Abstract

Various AT1 receptor antagonists including losartan are known to inhibit human platelet activation by antagonising TXA2/PGH2 receptors (TP receptors). Presently, we check a hypothesis that losartan, an imidazole derivative in contrast with valsartan, a non-imidazole compound, may inhibit human platelet activation also through inhibition of TXA2 synthesis. Inhibitory action of losartan (2-n butyl-4-chloro-5-hydroxymethyl-1-beta(2'-(1H-tetrazol-5yl)biphenyl-4-yl)methyl] imidazole), its active metabolite EXP 3174 (2-n-butyl-4-chloro-1-beta (2-(1H-tetrazol-5-yl) biphenyl-4-yl) methyl]imidazole-5-carboxylic acid) and valsartan ((S)-N-valeryl-N-(beta2'-(1H-tetrazol-5-yl)biphenyl-4-yl]methyl]valine), on collagen-induced platelet aggregation and TXA2 generation was compared to effects achieved by each compound on U46619-induced aggregation in aspirinized platelets. Losartan and aspirin inhibited collagen-induced platelet aggregation with approximately the same potency, whereas EXP 3174 and valsartan showed much weaker antiplatelet effects. Interestingly, losartan, EXP 3174 and valsartan displayed similar potencies as inhibitors of U46619-induced aggregation in asprinized platelets as in collagen-induced aggregation in non-aspirinized platelets. None of the above three AT1 antagonists, up to a concentration of 300 microM, did influence collagen-induced TXA2 synthesis in human platelets. In conclusion, antiplatelet effects of AT1 antagonists, irrespective of the presence or absence of non-condensed imidazole in their chemical structure, involve antagonism of TP receptors but not inhibition of TXA2 synthesis in platelets.