Modulation of cholinergic airway reactivity and nitric oxide production by endogenous arginase activity

Abstract

1. Cholinergic airway constriction is functionally antagonized by agonist-induced constitutive nitric oxide synthase (cNOS)-derived nitric oxide (NO). Since cNOS and arginase, which hydrolyzes L-arginine to L-ornithine and urea, use L-arginine as a common substrate, competition between both enzymes for the substrate could be involved in the regulation of cholinergic airway reactivity. Using a perfused guinea-pig tracheal tube preparation, we investigated the modulation of methacholine-induced airway constriction by the recently developed, potent and specific arginase inhibitor N(ω)-hydroxy-nor-L-arginine (nor-NOHA). 2. Intraluminal (IL) administration of nor-NOHA caused a concentration-dependent inhibition of the maximal effect (E(max)) in response to IL methacholine, which was maximal in the presence of 5 μM nor-NOHA (E(max) = 31.2 ± 1.6% of extraluminal (EL) 40 mM KCl-induced constriction versus 51.6 ± 2.1% in controls, P < 0.001). In addition, the pEC50 (-log10 EC50) was slightly but significantly reduced in the presence of 5 μM nor-NOHA. 3. The inhibition of E(max) by 5 μM nor-NOHA was concentration-dependently reversed by the NOS inhibitor N(ω)-nitro-L-arginine methyl ester (L-NAME), reaching an E(max) of 89.4 ± 7.7% in the presence of 0.5 mM L-NAME (P < 0.01). A similar E(max) in the presence of 0.5 mM L-NAME was obtained in control preparations (85.2 ± 9.7%, n.s.). 4. In the presence of excess of exogenously applied L-arginine (5 mM), 5 μM nor-NOHA was ineffective (E(max) = 33.1 ± 5.8 versus 31.1 ± 7.5% in controls, n.s.). 5. The results indicate that endogenous arginase activity potentiates methacholine-induced airway constriction by inhibition of NO production, presumably by competition with cNOS for the common substrate, L-arginine. This finding may represent an important novel regulation mechanism of airway reactivity.