Apelin, the ligand for the endothelial G-protein-coupled receptor, APJ, is a potent angiogenic factor required for normal vascular development of the frog embryo.

Cox CM, D'Agostino SL, Miller MK, Heimark RL, Krieg PA. "Apelin, the ligand for the endothelial G-protein-coupled receptor, APJ, is a potent angiogenic factor required for normal vascular development of the frog embryo." Dev. Biol.. 2006;296(1):177-89.