APJ, which was initially identified as a gene with closest homology to the angiotensin II type 1 receptor, is a seven transmembrane G protein-coupled receptor. Apelin is an endogenous ligand of the APJ originally isolated from bovine stomach extracts. There are several isoforms of apelin. Apelin preproproteins contain 77 amino-acid residues, which can be cleaved to form shorter bioactive isoforms, including apelin-36, apelin-17, apelin-13, apelin-12, and so on. Apelin/APJ receptor is extensively distributed in the central nervous system and peripheral tissues. Apelin/APJ system involves in a w...