Role for endothelin-1-induced superoxide and peroxynitrite production in rebound pulmonary hypertension associated with inhaled nitric oxide therapy

Our previous studies have demonstrated that inhaled nitric oxide (NO) decre ases nitric oxide synthase (NOS) activity in vivo and that this inhibition is associated with rebound pulmonary hypertension upon acute withdrawal of inhaled NO. We have also demonstrated that inhaled NO elevates plasma endot helin-1 (ET-1) levels and that pretreatment with PD 156707, an ETA receptor antagonist, blocks the rebound hypertension. The objectives of this study were to further elucidate the role of ET-1 in the rebound pulmonary hyperte nsion upon acute withdrawal of inhaled NO. Inhaled NO (40 ppm) delivered to thirteen 4-week-old lambs decreased NOS activity by 36.2% in control lambs (P<0.05), whereas NOS activity was preserved in PD 156707-treated lambs. W hen primary cultures of pulmonary artery smooth muscle cells were exposed t o ET-1, superoxide production increased by 33% (P<0.05). This increase was blocked by a preincubation with PD156707. Furthermore, cotreatment of cells with ET-1 and NO increased peroxynitrite levels by 26% (P<0.05), whereas p reincubation of purified human endothelial nitric oxide synthase (eNOS) pro tein with peroxynitrite generated a nitrated enzyme with 50% activity relat ive to control (P<0.05). Western blot analysis of peripheral lung extracts obtained after 24 hours of inhaled NO revealed a 90% reduction in 3-nitroty rosine residues (P<0.05) in PD156707-treated lambs. The nitration of eNOS w as also reduced by 40% in PD 156707-treated lambs (P<0.05). These data sugg est that the reduction of NOS activity associated with inhaled NO therapy m ay involve ETA receptor-mediated superoxide production. ETA receptor antago nists may prevent rebound pulmonary hypertension by protecting endogenous e NOS activity during inhaled NO therapy.