Endothelial dysfunction is a core pathophysiologic process in pulmonary arterial hypertension (PAH). We developed PulmoBind (PB), a novel imaging biomarker of the pulmonary vascular endothelium. 99mTechnetium (99mTc)-labelled PB binds to adrenomedullin receptors (AM1) densely expressed in the endothelium of alveolar capillaries. We evaluated the effect of sildenafil on AM1 receptors activity using 99mTc-PB. PAH was induced in rats using the Sugen/hypoxia model and after 3 weeks, animals were allocated to sildenafil (25 or 100 mg/kg/day) for 4 weeks. 99mTc-PB uptake kinetics was assessed by single-photon emission computed tomography. PAH caused right ventricular (RV) hypertrophy that was decreased by low and high sildenafil doses. Sildenafil low and high dose also improved RV function measured from the tricuspid annulus plane systolic excursion. Mean integrated pulmonary uptake of 99mTc-PB was reduced in PAH (508% · min ± 37, p < 0.05) compared to controls (630% · min ± 30), but unchanged by sildenafil at low and high doses. Lung tissue expressions of the AM1 receptor components were reduced in PAH and also unaffected by sildenafil. In experimental angio-proliferative PAH, sildenafil improves RV dysfunction and remodeling, but does not modify pulmonary vascular endothelium dysfunction assessed by the adrenomedullin receptor ligand 99mTc-PB.