1 A de®ciency of constitutive nitric oxide synthase (cNOS)-derived nitric oxide (NO), due to reduced availability of L-arginine, importantly contributes to allergen-induced airway hyperresponsiveness (AHR) after the early asthmatic reaction (EAR). Since cNOS and arginase use L-arginine as a common substrate, we hypothesized that increased arginase activity is involved in the allergeninduced NO de®ciency and AHR. 2 Using a guinea-pig model of allergic asthma, we addressed this hypothesis by examining the e ects of the speci®c arginase inhibitor N o -hydroxy-nor-L-arginine (nor-NOHA) on the responsiveness to methacholine of isolated perfused tracheae from unchallenged control animals and from animals 6 h after ovalbumin challenge. Arginase activity in these preparations was investigated by measuring the conversion of L-[ 14 C]arginine to [ 14 C]urea. 3 Airways from allergen-challenged animals showed a 2 fold (P50.001) increase in responsiveness to intraluminal (IL) administration of methacholine compared to controls. A similar hyperresponsiveness (1.8 fold, P50.01) was observed in control airways incubated with the NOS inhibitor N onitro-L-arginine methyl ester (L-NAME, 0.1 mM, IL), while L-NAME had no further e ect on the airways from challenged animals. 4 Remarkably, 5 mM nor-NOHA (IL) normalized the hyperresponsiveness of challenged airways to basal control (P50.001), and this e ect was fully reversed again by 0.1 mM L-NAME (P50.05). Moreover, arginase activity in homogenates of the hyperresponsive airways was 3.5 fold (P50.001) enhanced compared to controls. 5 The results indicate that enhanced arginase activity contributes to allergen-induced de®ciency of cNOS-derived NO and AHR after the EAR, presumably by competition with cNOS for the common substrate, L-arginine. This is the ®rst demonstration that arginase is involved in the pathophysiology of asthma. British Journal of Pharmacology (2002) 136, 391 ± 398 Keywords: Arginase; constitutive nitric oxide synthase; nitric oxide; methacholine; N o -hydroxy-nor-L-arginine; allergic asthma; early asthmatic reaction; airway hyperresponsiveness; tracheal perfusion; guinea-pig Abbreviations: AHR, airway hyperreactivity; cNOS, constitutive nitric oxide synthase; EAR, early asthmatic reaction; EL, extraluminal; E max , maximal e ect; eNOS, endothelial nitric oxide synthase; IL, intraluminal; iNANC, inhibitory nonadrenergic noncholinergic; iNOS, inducible nitric oxide synthase; KH, Krebs-Henseleit; LAR, late asthmatic reaction; L-NAME, N o -nitro-L-arginine methyl ester; L-NMMA, N G -monomethyl-L-arginine; nNOS, neuronal nitric oxide synthase; nor-NOHA, N o -hydroxy-nor-L-arginine; DP, di erential (hydrostatic) pressure; P inlet , (hydrostatic) pressure at the inlet; P outlet , (hydrostatic) pressure at the outlet; pEC 50 , 7log 10 of the concentration causing 50% of the e ect

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 Larginine 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 methacholineinduced airway constriction by the recently developed, potent and speci®c arginase inhibitor N ohydroxy-nor-L-arginine (nor-NOHA). 2 Intraluminal (IL) administration of nor-NOHA caused a concentration-dependent inhibition of the maximal e ect (E max ) in response to IL methacholine, which was maximal in the presence of 5 mM nor-NOHA (E max =31.2+1.6% of extraluminal (EL) 40 mM KCl-induced constriction versus 51.6+2.1% in controls, P50.001). In addition, the pEC 50 (7log 10 EC 50 ) was slightly but signi®cantly reduced in the presence of 5 mM nor-NOHA. 3 The inhibition of E max by 5 mM nor-NOHA was concentration-dependently reversed by the NOS inhibitor N o -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 (P50.01). 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 mM nor-NOHA was ine ective (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 ®nding may represent an important novel regulation mechanism of airway reactivity.