A Specific Binding Site for Angiotensin II(3-8), Angiotensin IV, in Rabbit Cardiac Fibroblasts

This study demonstrates the existence of a high affinity binding site on rabbit cardiac fibroblasts of the hexapeptide (3-8) fragment of angiotensin II (AngIV). 125I-AngIV binding is saturable, reversible and distinct from angiotensin II (AngII) receptors. At 37 degrees C equilibrium of 125I-AngIV binding is reached within 2 h. AngIV displaces 125I-AngIV bound to cultured rabbit cardiac fibroblasts whereas AngII receptor-specific ligands (Sar1, Ile8-AngII, Dup753, CGP42112A) do not. Scatchard plot analysis revealed that 125I-AngIV binds to a single class of sites with Kd = 4.87 +/- 0.11 x 10(-9) mol/l, Bmax = 371 +/- 8.3 fmol/mg protein and a Hill coefficient of 0.92. In the presence of the non-hydrolyzable GTP analog GTPgammaS 125I-AngIV binding in rabbit cardiac fibroblasts was not markedly affected, whereas binding of 125I-(Sar1,IIe8)-AngII is reduced. The role of AngIV in the heart and in particular in cardiac fibroblasts is unknown, and the putative interaction of AngIV with AngII needs further characterization.