Method of identifying therapies for pulmonary hypertension

The present invention is directed to a method of screening for a therapeutic agent useful for treating pulmonary hypertension comprising: contacting an erythrocyte with a candidate therapeutic agent; and detecting a presence or absence of erythrocyte-derived adenosine triphosphate, wherein a greater erythrocyte-derived adenosine triphosphate level indicates the candidate therapeutic agent has greater activity in treating pulmonary hypertension. Additionally, the present invention is directed to methods of treating pulmonary arterial hypertension by stimulating ATP release from erythrocytes through co-administration to a subject in need thereof an amount of a PDE5 inhibitor compound, and an amount of a prostacyclin compound.