Angiotensin II Receptor Subtypes in Bovine and Human

Angiotensin II (All) binding sites in bovine and human ventricular myocardium were characterized by a radioreceptor assay. The specific binding of [125l]All to myocardial membranes appeared to be saturable, and it was of high affinity with apparent disso-ciation constants of 1.60 nM (bovine) and 1.09 nM (human). The B,, values were 39.7 fmol/mg protein (bovine) and 6.07 fmol/ mg protein (human). Both losartan (angiotensin type 1 receptor subtype selective antagonist) and PD1 231 77 (the angiotensin type 2 receptor subtype selective antagonist) inhibited specific [‘25l]All binding to bovine myocardium, and their Hill coefficients were less than unity. Nonlinear least-squares regression analysis has suggested the existence of two populations of [1251]AIl binding sites that have high and low affinity for losartan or P01 231 77. The relative proportions of high- and low-affinity site