[125I]-(Pyr1)Apelin-13 binds with high affinity and selectivity to human cardiac tissue. Apelin is the endogenous ligand for the G-protein-coupled APJ receptor. It is produced as a pre-proprotein of 77 amino acids. Upon signal peptide cleavage, the proprotein of 55 amino acids may generate several active fragments: Apelin-36 (36 aa corresponding to the sequence 42-77), Apelin-17 (17 aa corresponding to the sequence 61-77) and Apelin-13 (13 aa corresponding to the sequence 65-77). Apelin-13 may undergo a pyroglutamylation at its N-terminal glutamine residue.