The G protein-coupled receptor GPR54 (also designated KISS1) is activated by cleavage products of the KiSS1 protein, the kisspeptins (KP), to act as a molecular switch for puberty. Additionally, KP are potent inhibitors of tumor metastasis and play a role in placentation, both processes involving angiogenesis. Our aim was to investigate whether GPR54 and KP are expressed within normal and diseased human vasculature and what their functional role may be. RT-PCR screening of human blood vessels revealed a discrete localization of GPR54 mRNA in smooth muscle of vessels with the same developmental origins, aorta, coronary artery, and umbilical vein, a pattern confirmed by immunocytochemistry and radioligand binding. Novel ligand [(125)I]KP-13 exhibited saturable and high-affinity binding in aorta smooth muscle sections (dissociation constant K(D) = 0.2 +/- 0.03 nM), and using confocal microscopy, we found colocalization of receptor and peptide to vascular endothelial cells and to the atherosclerotic plaque of coronary artery. RIA detected 13.04 +/- 2.94 and 20.50 +/- 5.00 fmol/g KP in human coronary artery and aorta, respectively. KP-10, KP-13, and KP-54 acted as vasoconstrictors with comparable potency and efficacy in isolated rings of coronary artery (negative logarithm of the EC(50) and maximal response, respectively, as follows: KP-10, 7.89 +/- 0.24 and 33.7 +/- 17.0; KP-13, 8.66 +/- 0.88 and 35.1 +/- 7.9; KP-54, 8.86 +/- 1.11 and 25.7 +/- 5.5) and umbilical vein (negative logarithm of the EC(50) and maximal response, respectively, as follows: KP-10, 8.44 +/- 022 and 24.3 +/- 3.7; KP-13, 8.43 +/- 0.88 and 28.4 +/- 8.6; KP-54, 8.93 +/- 0.39 and 36.9 +/- 5.2). In conclusion, we have detected expression of both peptide and receptor in aorta, coronary artery, and umbilical vein and have shown for the first time that the KP are vasoconstrictors in humans, suggesting a previously undescribed role for GPR54 and KP in the cardiovascular system.