Modulation of cholinergic airway reactivity and nitric oxide production byendogenous arginase activity

1 Cholinergic airway constriction is functionally antagonized by agonist-in duced constitutive nitric oxide synthase (cNOS)-derived nitric oxide (NO). Since cNOS and arginase, which hydrolyzes L-arginine to L-ornithine and ure a, use L-arginine as a common substrate, competition between both enzymes f or the substrate could be involved in the regulation of cholinergic airway reactivity. Using a perfused guinea-pig tracheal tube preparation, we inves tigated the modulation of methacholine-induced airway constriction by the r ecently developed, potent and specific arginase inhibitor N-omega-hydroxy-n or-L-arginine (nor-NOHA). 2 Intraluminal (IL) administration of nor-NOHA caused a concentration-depen dent inhibition of the maximal effect (E-max) in response to IL methacholin e, which was maximal in the presence of 5 mu M 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 pEC(50) (-log(10) EC50) was slightly but significantly reduced in the presence of 5 mu M nor-NOHA. 3 The inhibition of E-max by 5 mu M nor-NOHA was concentration-dependently reversed by the NOS inhibitor N-omega-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<0.0 1). A similar E-max in the presence of 0.5 mM L-NAME was obtained in contro l preparations (85.2+/-9.7%, n.s.). 4 In the presence of excess of exogenously applied L-arginine (5 mM), 5 mu M nor-NOHA was ineffective (E-max = 33.1 +/- 5.8 versus 31.1 +/- 7.5% in co ntrols, n.s.). 5 The results indicate that endogenous arginase activity potentiates methac holine-induced airway constriction by inhibition of NO production, presumab ly by competition with cNOS for the common substrate, L-arginine. This find ing may represent an important novel regulation mechanism of airway reactiv ity.