Apelin (65-76), human is derived from the apelin peptide which acts as a ligand for the apelin receptor (APJ) G protein coupled receptor and is a substrate for angiotensin converting enzyme 2. Preprapelin, encoded for by APLN located on Xq25-26.1, is cleaved to form either apelin 36 or apelin-17, 12 and 13. As a member of the adipokine hormone family, which are involved in processes such as vascular homeostasis and angiogenesis, the apelin is secreted from adipose tissue.

Apelin has been found to be expressed in the spinal cord and the human brain and when performing immunohistochemistry it was observed that apelin-17 is significantly expressed in the human heart, brain, lungs and endothelial cells.

Both apelin and the apelin receptor are widely distributed around the body thus apelin has been found to be associated with cardiovascular diseases, obesity, diabetes and cancer. Studies exploring myocardial infarction showed there to be greater apelin mRNA expression during human heart failure compared to in healthy tissue. Apelin protects against heart failure due to, the pyroglutamyl form of apelin, [Pyr1]apelin-13 playing a role in decreasing infarct size of myocardial infarctions. Furthermore in rats with hypertension, the expression of apelin and APJ was decreased.