Endogenous ligand for the apelin receptor (APLNR). Drives internalization of APLNR (By similarity). Apelin-36 dissociates more hardly than (pyroglu)apelin-13 from APLNR (By similarity). Hormone involved in the regulation of cardiac precursor cell movements during gastrulation and heart morphogenesis (By similarity). Has an inhibitory effect on cytokine production in response to T-cell receptor/CD3 cross-linking; the oral intake of apelin in the colostrum and the milk might therefore modulate immune responses in neonates (PubMed:10525157). Plays a role in early coronary blood vessels formation (PubMed:28890073). Mediates myocardial contractility in an ERK1/2-dependent manner (By similarity). May also have a role in the central control of body fluid homeostasis by influencing vasopressin release and drinking behavior (PubMed:11359874). Abundantly secreted in the colostrum. Lower level in milk. Decreases rapidly within several days after parturition in milk, but is still detectable even in commercial milk. Expressed in extraembryonic visceral endoderm and in the primitive streak at 6.5 and 7.5 dpc (PubMed:28854362). Expressed in the anterior visceral yolk sac at 8.25 dpc (PubMed:28854362). Expressed weakly in the embryonic heart at 11.5 dpc (PubMed:26611206). Expressed in the adult heart (PubMed:26611206). Expressed in endothelial cells and cardiomyocytes and weakly expressed in fibroblasts (PubMed:26611206). Not up-regulated following myocardial infarction (MI) (at protein level) (PubMed:26611206). Several active peptides may be produced by proteolytic processing of the peptide precursor. Mice heart of embryos show increased coronary vessel growth at 13.5 dpc and 15.5 dpc (PubMed:28890073). Double knockout mice of APELA and APLN genes exhibit the same penetrance, embryonic lethality and cardiovascular malformations as single APELA knockout mice (PubMed:28854362). Belongs to the apelin family.