Abstract

Data are presented describing a new angiotensin binding site in rabbit and guinea pig heart, distinct from AT1 and AT2, that demonstrates high specificity and affinity for the hexapeptide fragment angiotensin II(3-8), which will be referred to here as angiotensin IV (AIV). Equilibrium binding in rabbit heart membranes was achieved in 2 hr at 37 degrees C and produced a calculated kinetic KD of .174 +/- .018 nM. Saturation equilibrium binding data for rabbit and guinea pig heart were best fit to a one-site model with Hill coefficients near unity. Guinea pig membranes exhibited a KD = 1.33 +/- .02 nM and a Bmax = 144 +/- 19 fmol/mg protein, and rabbit heart membranes had a KD = 1.70 +/- .50 nM and a Bmax = 731 +/- 163 fmol/mg protein. The binding site showed a high specificity for AIV, although it exhibited low affinity for angiotensin II, angiotensin III, Sar1,Ile8-angiotensin II, DuP 753, CGP42112A and PD123177. A large number of nonangiotensin-related peptides were unable to compete effectively for 125I-AIV binding. Deletions made from the C-terminal end of AIV caused a decrease in affinity: AIV > AII(3-7) > AII(3-6) > AII(3-5). Extension of the C-terminal end of AIV corresponding to the amino acids of human angiotensinogen caused little change in affinity. GTP gamma S had no effect on binding, suggesting non-G protein linkage. Binding was widely distributed throughout the heart; it was observed on cardiocytes and blood vessels as well as in the epicardium and the endocardium.