Identification and characterization of angiotensin II receptors in cardiac sarcolemma

We have used [¹²⁵I] angiotensin II to investigate the presence of specific angiotensin II receptors in beef heart sarcolemmal membranes. The observed binding is saturable, reversible and specific. The apparent equilibrium dissociation constant is 2.23 ± 0.15 ($\text{x}$ ± SEM) and the maximal number of binding sites per mg membrane protein is 32.8 ± 5.4 fmol ($\text{x}$ ± SEM). The specific binding is 80–100% of the total [¹²⁵I] angiotensin II bound and is directly proportional to membrane protein concentration over the range of 33–173 μg protein per ml. Angiotensin II and its antagonists competed for binding in a potency order of (agent, K_i): angiotensin II, 0.9nM > Sar¹ Ala³, 7 nM > Sar¹-Ile³, 51 nM > Sar¹-Leu³, 427nM > angiotensin I, 1709 nM. The ability to characterize and quantify these receptors should now provide a method for investigating the mechanisms underlying the effects of angiotensin II on myocardial tissues.