APLN and pulmonary arterial hypertension: Apelin peptides have an emerging role in the adult cardiovascular system1 and in embryonic development of the heart,2 and [Pyr1]apelin-13 is the most abundant endogenous apelin peptide in the human heart.3 Alteration in the apelin system is thought to contribute to the etiology of cardiovascular diseases such as pulmonary arterial hypertension (PAH), a devastating disease with pulmonary vascular remodeling leading to death from right ventricular failure in which a beneficial effect of enhancing apelin receptor signaling has been proposed.1