Apelin is also referred as APLN and is a peptide coded by the gene APLN. Apelin is a G protein which is coupled to the APJ receptor and is expressed in some cell type’s surfaces. It is known to be expressed widely in different organs which include the lung, heart, kidney, brain, endothelium, gastrointestinal tract, human plasma and adrenal glands. The peptide Apelin was first discovered by the team of Professor M. Fujino. The gene for Apelin encodes 77 amino acid containing pre-protein. The signal peptide gets cleaved once it is translocated to the endoplasmic reticulum. The presence and expression of the Apelin receptor in the vascular system is known for controlling the blood pressure along with promotion and formation of new blood vessels or angiogenesis.

The activation of receptors which is expressed on the endothelial cell surfaces is due to hypotensive effects of apelin which further releases NO which is a very potent vasodilator.  The release and vasodilation results in smooth muscle relaxation in the walls of the arteries. Apelin is also expressed during embryonic formation of heart and cardiomyocytes. Apelin is also expressed in brain neurons and is involved in intake of food and water. The areas of brain which controls appetite shows presence of Apelin. Receptors of apelin are also expressed in gastrointestinal tract cells. Activation is also involved in controlling the glucose levels in blood and in uptake of glucose. Various tissue specific signalling pathways which are known to regulate various biological functions like cardiovascular function, fluid homeostasis and insulin secretion are activated.