Apelin-13 is a bioactive neuropeptide processed from the pre-proprotein apelin that is active in the nervous system. It is an endogenous ligand of the receptor APJ (EC50=0.37nM). Plays a role in cardiovascular regulation, fluid homeostasis, hypertension, and insulin sensitivity. May undergo pyroglutamylation, and may block HIV-1 and HIV-2 entry into cells expressing the APJ receptor. Compared to apelin-36, blocks HIV entry poorly.