IC50: 0.37 nM for GPCR Apelin-13 is an endogenous ligand of the APJ receptor. The apelin receptor APJ, one of a group of G-proteincoupled receptors (GPCR), have recently been paired with their cognate peptide ligands using ‘‘reverse pharmacology’’, and functional evidence suggests a role for this receptor in the regulation of cardiovascular function, fluid homeostasis, and as a coreceptor for HIV infection. In vitro: Apelin-13 was identified as an endogenous ligand of the APJ receptor, which could activate this G protein-coupled receptor with an EC50 value of 0.37 nM. In addition, the EC50 values for apelin-17 and apelin-36 have been found to be 2.5 and 20 nM, respectively . In vivo: In a previous study, urethane anaesthetised, paralysed and ventilated male SD rats were used to investigate the action of apelin-13 directly microinjected into the nucleus tractus solitarius (NTS) and the rostral ventrolateral medulla (RVLM) on arterial pressure and phrenic nerve activity. Results showed that Apelin-13 microinjections into the NTS led to either apnea or decreased phrenic nerve discharge amplitude by up to 30%. In the RVLM, apelin-13 caused increase in phrenic nerve discharge amplitude depending on the exact site of injection . Clinical trial: Previous clinical study showed that intrabrachial infusions of (Pyr1)apelin-13, acetylcholine, and sodium nitroprusside could cause forearm vasodilatation in patients and control subjects. Systemic infusions of (Pyr1)apelin-13 was able to increase cardiac index and lower mean arterial pressure and peripheral vascular resistance in patients and healthy control subjects but increased heart rate only in control subjects .