Endogenous ligand for the apelin receptor (APLNR) (PubMed:11336787, PubMed:11359874, PubMed:26611206). Drives internalization of the apelin receptor (PubMed:11359874). Apelin-36 dissociates more hardly than (pyroglu)apelin-13 from APLNR (PubMed:11336787). 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 (By similarity). Plays a role in early coronary blood vessels formation (By similarity). Mediates myocardial contractility in an ERK1/2-dependent manner (PubMed:26611206). May also have a role in the central control of body fluid homeostasis by influencing vasopressin release and drinking behavior (PubMed:10617103, PubMed:11359874). Abundantly secreted in the colostrum (By similarity). Lower level in milk (PubMed:10525157). Decreases rapidly within several days after parturition in milk, but is still detectable even in commercial milk (By similarity). Expressed in the lung, testis, ovary, uterus and mammary gland (PubMed:11336787). Expressed in neurons in the thalamic paraventricular and hypothalamic supraoptic nuclei (PubMed:11359874). The lung, testis and uterus mainly contain a large form that looks like apelin-36, whereas the mammary gland seems to contain 2 forms of apelin, a large form close to apelin-36 and a small form close to apelin-13 (at protein level) (PubMed:11336787). Widely expressed in the adult, with highest levels in the mammary gland of lactating animals, very high levels in the lung, intermediate levels in the spinal cord, ovary, adipose tissue, brain (neuronal cell bodies and fibers in the supraoptic and the paraventricular nuclei), heart and testis, and lowest levels in the pituitary gland, kidney, stomach, uterus and pancreas (PubMed:10525157, PubMed:10617103, PubMed:11336787). Highly expressed in neonatal tissues. In the adult, reaches a maximal level around parturition. During pregnancy and lactation (PubMed:10525157). Several active peptides may be produced by proteolytic processing of the peptide precursor. Belongs to the apelin family.