Fig. 4. The pulmonary vasodilation produced by sildenafil (50 mg) is abolished by concomitant infusion of l-NAME. PHTN = pulmonary hypertension induced by U46619 infusion. Open circles = the group of animals not receiving l-NAME infusion (n = 7, data from protocol 2); closed circles = the group of animals receiving l-NAME infusion (n = 6). *P < 0.01 versus group without l-NAME infusion; †P < 0.001 versus PHTN. Data are expressed as mean ± SE.