title: L-citrulline supplementation reverses the impaired airway relaxation in neonatal rats exposed to hyperoxia.
abstract: BACKGROUND
  Hyperoxia is shown to impair airway relaxation via limiting L-arginine bioavailability to nitric oxide synthase (NOS) and reducing NO production as a consequence. L-arginine can also be synthesized by L-citrulline recycling. The role of L-citrulline supplementation was investigated in the reversing of hyperoxia-induced impaired relaxation of rat tracheal smooth muscle (TSM).
  
  
  METHODS
  Electrical field stimulation (EFS, 2-20 V)-induced relaxation was measured under in vitro conditions in preconstricted tracheal preparations obtained from 12 day old rat pups exposed to room air or hyperoxia (>95% oxygen) for 7 days supplemented with L-citrulline or saline (in vitro or in vivo). The role of the L-citrulline/L-arginine cycle under basal conditions was studied by incubation of preparations in the presence of argininosuccinate synthase (ASS) inhibitor [α-methyl-D, L-aspartate, 1 mM] or argininosuccinate lyase inhibitor (ASL) succinate (1 mM) and/or NOS inhibitor [Nω-nitro-L-arginine methyl ester; 100 μM] with respect to the presence or absence of L-citrulline (2 mM).
  
  
  RESULTS
  Hyperoxia impaired the EFS-induced relaxation of TSM as compared to room air control (p < 0.001; 0.5 ± 0.1% at 2 V to 50.6 ± 5.7% at 20 V in hyperoxic group: 0.7 ± 0.2 at 2 V to 80.0 ± 5.6% at 20 V in room air group). Inhibition of ASS or ASL, and L-citrulline supplementation did not affect relaxation responses under basal conditions. However, inhibition of NOS significantly reduced relaxation responses (p < 0.001), which were restored to control level by L-citrulline. L-citrulline supplementation in vivo and in vitro also reversed the hyperoxia-impaired relaxation. The differences were significant (p <0.001; 0.8 ± 0.3% at 2 V to 47.1 ± 4.1% at 20 V without L-citrulline; 0.9 ± 0.3% at 2 V to 68.2 ± 4.8% at 20 V with L-citrulline). Inhibition of ASS or ASL prevented this effect of L-citrulline.
  
  
  CONCLUSIONS
  The results indicate the presence of an L-citrulline/L-arginine cycle in the airways of rat pups. L-citrulline recycling does not play a major role under basal conditions in airways, but it has an important role under conditions of substrate limitations to NOS as a source of L-arginine, and L-citrulline supplementation reverses the impaired relaxation of airways under hyperoxic conditions.
authors.affiliation: Department of Pharmacy-Biology, Faculty of Medicine, University of Prishtina, St, Martyrs' Boulevard n,n,, Prishtina, 10000, Kosovo, Macedonia. ramadan.sopi@uni-pr.edu
authors.forename: Ramadan B
authors.initials: RB
authors.lastname: Sopi
authors.forename: Syed I A
authors.initials: SI
authors.lastname: Zaidi
authors.forename: Mitko
authors.initials: M
authors.lastname: Mladenov
authors.forename: Hazbije
authors.initials: H
authors.lastname: Sahiti
authors.forename: Zahide
authors.initials: Z
authors.lastname: Istrefi
authors.forename: Icko
authors.initials: I
authors.lastname: Gjorgoski
authors.forename: Azem
authors.initials: A
authors.lastname: Lajçi
authors.forename: Muharrem
authors.initials: M
authors.lastname: Jakupaj
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D002956:Citrulline / Q000008:administration & dosage*; D019587:Dietary Supplements; D018496:Hyperoxia / Q000188:drug therapy* / Q000503:physiopathology; D009126:Muscle Relaxation / Q000187:drug effects* / Q000502:physiology; D009130:Muscle, Smooth / Q000187:drug effects / Q000502:physiology; D009924:Organ Culture Techniques; D011897:Random Allocation; D051381:Rats; D017207:Rats, Sprague-Dawley; D014132:Trachea / Q000187:drug effects* / Q000502:physiology
chemical_list: D002956:Citrulline