Apelin-13

Apelin-13 binds selectively to the APJ receptor, a G-protein-coupled receptor coupled primarily to Gi proteins. This coupling inhibits adenylyl cyclase while simultaneously activating PI3K/Akt and MAPK/ERK pathways. The result is cardioprotection, improved contractility, and reduced oxidative stress in cardiac tissue.

In the vasculature, APJ activation triggers eNOS phosphorylation and nitric oxide production, causing endothelium-dependent vasodilation. In the heart, apelin increases myocardial contractility (positive inotropy) without inducing hypertrophy or harmful calcium overload, distinguishing it from classical inotropes. Apelin also opposes angiotensin II vasoconstriction by promoting its degradation via ACE2.

Apelin levels decline with age and obesity, while restoration normalizes glucose metabolism and reduces adiposity in animal models. Apelin enhances insulin sensitivity through AMPK activation in skeletal muscle and suppresses appetite via hypothalamic APJ receptors. Circulating apelin is now considered a biomarker for cardiovascular and metabolic aging.

Intravenous Apelin-13 infusion in heart failure patients improved cardiac output, reduced filling pressures, and decreased systemic vascular resistance in Phase I/II studies without adverse hemodynamic effects. Plasma apelin levels inversely correlate with heart failure severity, supporting its therapeutic potential.

Phase II trials evaluated apelin analogs in pulmonary arterial hypertension, showing reduced pulmonary vascular resistance. The short half-life of native Apelin-13 has prompted development of longer-acting analogs currently in clinical trials.

Mouse studies show exogenous apelin restores muscle function in aged animals, improves glucose tolerance, and reduces adipose inflammation. These findings position the apelin system as a potential therapeutic target for metabolic frailty associated with aging.