Role of Arginase in Impairing Relaxation of Lung Parenchyma of Hyperoxia-Exposed Neonatal Rats

Abstract

Background: Prolonged exposure of immature lungs to hyperoxia contributes to neonatal lung injury and airway hyperreactivity. We have previously demonstrated that neonatal exposure of rat pups to ≧95% O₂ impairs airway relaxation due to disruption of nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) signaling. Objective: We now hypothesize that these impaired relaxation responses are secondary to hyperoxia-induced upregulation of arginase, which competes with NO synthase for L-arginine. Methods: Rat pups were exposed to moderate neonatal hyperoxia (50% O₂) or room air for 7 days from birth. In additional hyperoxic and room air groups, exogenous L-arginine (300 mg/kg/day i.p.) or arginase inhibitor (Nω-hydroxy-nor-arginine, 30 mg/kg/day i.p.) were administered daily. After 7 days, animals were anesthetized and sacrificed either for preparation of lung parenchymal strips or lung perfusion. Results: In response to electrical field stimulation (EFS), bethanechol-preconstricted lung parenchymal strips from hyperoxic pups exhibited significantly reduced relaxation compared to room air controls. Supplementation of L-arginine or arginase blockade restored hyperoxia-induced impairment of relaxation. Expression of arginase I in airway epithelium was increased in response to hyperoxia but reduced by arginase blockade. Arginase activity was also significantly increased in hyperoxic lungs as compared to room air controls and reduced following arginase blockade. EFS-induced production of NO was decreased in hyperoxia-exposed airway smooth muscle and restored by arginase blockade. Conclusion: These data suggest that NO-cGMP signaling is disrupted in neonatal rat pups exposed to even moderate hyperoxia due to increased arginase activity and consequent decreased bioavailability of the substrate L-arginine. We speculate that supplementation of arginine and/or inhibition of arginase may be a useful therapeutic tool to prevent or treat neonatal lung injury.