G protein-coupled receptors（GPCRs） represent the largest group of transmembrane proteins responsible for transduction of a diverse array of extracellular signals. Those novel GPCRs do not bind any known endogenous ligand and are described as ＂orphan＂ GPCRs. Apelin,a peptide recently isolated from bovine stomach tissue extracts,has been identified as the endogenous ligand of the human orphan APJ receptor which is a seven-transmembrane receptor related to angiotensin-type 1 receptor. The functions of apelin involves nervous system ,immune sys...
