Pulmonary Hypertension Drug Therapies Target Endothelial Cell Derived Pulmonary Vasodilator Pathways. Nitric oxide (NO) and prostacyclin (PGI2) signaling pathways regulate pulmonary vascular tone in the developing lung. ATP, Adenosine triphosphate; cAMP, cyclic AMP; cGMP, cyclic GMP; COX-1, cyclooxygenase 1; GTP, guanosine triphosphate; NOS, nitric oxide synthase; PDE3, phosphodiesterase 3; PDE5, phosphodiesterase 5; PBF, pulmonary blood flow; PGIS, prostacyclin synthase. (Created with BioRender.com)