The levels of exhaled nitric oxide (NO) are increased in subjects with asthma. Although NO may relax central airways in certain animal models, its role in asthmatic airway inflammation remains unclear. This study evaluated the effect of endogenous and exogenous NO in antigen-induced contractions of the Guinea Pig Lung Parenchyma (GPLP). The mediators of this particular contraction response (Wikström et al, 1994) are closely similar to those established for the anaphylactic contraction of human airways in vitro (Björck et al, 1993) as well as in vivo (Roquet et al, 1997). Four weeks prior to experiments, male Dunkin Hartley Guinea-Pigs (weight 300-350 g) were sensitized to ovalbumin (OVA) by i.p. and s.c. injections with 10 mg OVA. On the experimental day, the animals were sacrificed by an overdose of CO₂. The lungs were removed en bloc and the GPLP preparation was cut from the peripheral margins of the lung lobe. The GPLP was set up in 5 mL organ baths containing Tyrode's solution kept at 37°C, and gassed with 6.5 % CO₂ in O₂ Isometric responses were recorded at a resting tension of 2.5 mN using the IOX data acquisition (EMKA, Paris, France). Drugs were given 15 min before challenge with OVA. Contractions induced by cumulative challenge with OVA (10-10,000 µg/L) were expressed as percent (means ± SEM) of the terminal maximal contraction induced by the combination of histamine (1 mM), Ach (1 mM) and KCl (50 mM). Statistical analysis was performed using a two-way analysis of variances (ANOVA) test followed by Tukey's test (multiple comparisons). A p value of less than 0.05 was considered significant.

Endogenous NO: Challenge with OVA induced concentration-dependent contractions of the GPLP. Pretreatment with the NO synthesis inhibitor N-nitro-L-arginine (L-NOARG; 100 µM) shifted the concentration-response relation for OVA to the left. At 1000 µg/L, the response to OVA was 66±4.5% (n=5) of maximal contraction in the controls and 83±4.6% (n=5) in the presence of L-NOARG (P<0.05). When L-NOARG was given together with the substrate L-arginine (100 µM), the response reversed to the control level (63±4.0%, n=5; P>0.05 vs control). The enhancement of the OVA response induced by L-NOARG was mimicked by three other inhibitors of endogenous NO synthesis: L-NAME 100 µM (n=5), L-NMMA 100 µM (n=5) and 1400W 1 µM (n=5).

Exogenous NO: In another experiment, the concentration-response relation for OVA was dose-dependently shifted to the right by addition of the NO-donor NCX-2057 (3-(4-Hydroxy-3-methoxyphenyl)-2-propenoic acid) 4-(nitrooxy)butyl ester) 1-100µ M. The effect of NCX-2057 was significant at 10 µM and 100 µM with depression of the response to OVA 1000 µg/L from 60±3.0% (n=6) in controls to 33±6.6%(n=5) in the presence of NCX-2057 100 µM.

The findings add to the indications that NO has a protective effect in antigen-induced bronchoconstriction. It remains to establish if this is due to a direct effect on the airway smooth muscle, or caused by inhibition of the release of bronchoconstrictive mediators from inflammatory cells, or is explained by both mechanisms.

Björck T, Dahlén S-E. (1993) Pulm Pharm Ther 6:87-96.
Roquet A et al. (1997) AJRCCM. 155:1856-1863.
Wikström-Jonsson E, Dahlén S-E. (1994) JPET 271:615-623.