PDE5A and pulmonary arterial hypertension: Apelin receptor mediated vasodilatation is predominantly nitric oxide dependent in humans (Japp et al., 2008; 2010), suggesting treatment with apelin agonists could also work synergistically with sGC stimulators such as riociguat, or PDE5 inhibitors such as sildenafil and tadalafil, that are currently used to treat PAH patients.