Apelin is a recently identified endogenous ligand for the G-protein-coupled APJ receptor. It is widely expressed in various organs such as the heart, lung, kidney, adipose tissue, gastrointestinal tract, brain, adrenal glands, endothelium, and human plasma. Apelin acts as a mediator of the cardiovascular control, including for blood pressure and blood flow. It is one of the most potent stimulators of cardiac contractility yet identified, and plays a role in cardiac tissue remodeling. Apelin levels are increased in left ventricles of patients with chronic heart failure.

It is also an adipocyte endocrine secretion, making adipocytic and circulating levels of apelin higher in obesity. Insulin exerts a positive action on adipocyte apelin production. In addition, apelin regulates fluid homeostasis. It is important in the hypothalamic regulation of food and water intake, and pituitary hormone release.