Role of the l-citrulline/l-arginine cycle in iNANC nerve-mediated nitric oxide production and airway smooth muscle relaxation in allergic asthma

Nitric oxide synthase (NOS) converts l-arginine into nitric oxide (NO) and l-citrulline. In NO-producing cells, l-citrulline can be recycled to l-arginine in a two-step reaction involving argininosuccinate synthase (ASS) and -lyase (ASL). In guinea pig trachea, l-arginine is a limiting factor in neuronal nNOS-mediated airway smooth muscle relaxation upon inhibitory nonadrenergic noncholinergic (iNANC) nerve stimulation. Moreover, in a guinea pig model of asthma iNANC nerve-induced NO production and airway smooth muscle relaxation are impaired after the allergen-induced early asthmatic reaction, due to limitation of l-arginine. Using guinea pig tracheal preparations, we now investigated whether (i) the l-citrulline/l-arginine cycle is active in airway iNANC nerves and (ii) the NO deficiency after the early asthmatic reaction involves impaired l-citrulline recycling. Electrical field stimulation-induced relaxation was measured in tracheal open-rings precontracted with histamine. l-citrulline as well as the ASL inhibitor succinate did not affect electrical field stimulation-induced relaxation under basal conditions. However, reduced relaxation induced by a submaximal concentration of the NOS inhibitor Nω-nitro-l-arginine was restored by l-citrulline, which was prevented by the additional presence of succinate or the ASS inhibitor α-methyl-d,l-aspartate. Remarkably, the impaired iNANC relaxation after the early asthmatic reaction was restored by l-citrulline. In conclusion, the l-citrulline/l-arginine cycle is operative in guinea pig iNANC nerves in the airways and may be effective under conditions of low l-arginine utilization by nNOS (caused by NOS inhibitors), and during reduced l-arginine availability after allergen challenge. Enzymatic dysfunction in the l-citrulline/l-arginine cycle appears not to be involved in the l-arginine limitation and reduced iNANC activity after the early asthmatic reaction.