[125I]-(Pyr1)Apelin-13 is a novel radioligand for localizing the APJ orphan receptor in human and rat tissues with evidence for a vasoconstrictor role in man

1 We have determined the binding characteristics of [I-125]-(Pyr(1))Apelin-13, a putative ligand for the APT orphan receptor in human cardiovascular and rat tissue and investigated the functional properties of (Pyr(1))Apelin-13 in human saphenous vein. 2 The binding of [I-125]-(Pyr(1))Apelin-13 to sections of human heart tissue was time dependent and rapid at 23 degreesC. Data were fitted to a single site model with an association rate constant (k(obs)) of 0.115 min(-1). [I-125]-(Pyr(1))Apelin-13 also dissociated from a single site with a dissociation rate constant of 0.0105 min(-1). 3 In saturation binding experiments [I-125]-(Pyr(1))Apelin-13 bound to human left ventricle with a K-D value of 0.35 +/- 0.08 nM, B-max of 4.3 +/- 0.9 fmol mg(-1) protein with a Hill slope of 0.97 +/- 0.04 and to the right atria with a K-D of 0.33 +/- 0.09 nM, B-max of 3.1 +/- 0.6 fmol mg(-1) protein and a Hill slope of 0.93 +/- 0.05. 4 [I-125]-(Pyr(1))Apelin-13 binding sites were localized using autoradiography to human cardiovascular tissue, including coronary artery, aorta and saphenous vein grafts. In rat tissue a high density of receptors were localized to the molecular layer of the rat cerebellum, rat lung, rat heart and low levels in the rat kidney cortex. 5 (Pyr(1))Apelin-13 potently contracted human saphenous vein with a pD(2) value of 8.4 +/- 0.2 (n = 8). The maximum response elicited by the peptide was 22.6 +/- 6% of 100 mM KCl. 6 We provide the first evidence of APJ receptor expression, relative densities and functional properties of (Pyr(1))Apelin-13 in human cardiovascular tissue.