Inducible nitric oxide synthase inhibition attenuates lung tissue responsiveness and remodeling in a model of chronic pulmonary inflammation in guinea pigs

Abstract

We evaluated the influence of iNOS-derived NO on the mechanics, inflammatory, and remodeling process in peripheral lung parenchyma of guinea pigs with chronic pulmonary allergic inflammation. Animals treated or not with 1400 W were submitted to seven exposures of ovalbumin in increasing doses. Seventy-two hours after the 7th inhalation, lung strips were suspended in a Krebs organ bath, and tissue resistance and elastance measured at baseline and after ovalbumin challenge. The strips were submitted to histopathological measurements. The ovalbumin-exposed animals showed increased maximal responses of resistance and elastance (p < 0.05), eosinophils counting (p < 0.001), iNOS-positive cells (p < 0.001), collagen and elastic fiber deposition (p < 0.05), actin density (p < 0.05) and 8-iso-PGF2α expression (p < 0.001) in alveolar septa compared to saline-exposed ones. Ovalbumin-exposed animals treated with 1400 W had a significant reduction in lung functional and histopathological findings (p < 0.05). We showed that iNOS-specific inhibition attenuates lung parenchyma constriction, inflammation, and remodeling, suggesting NO-participation in the modulation of the oxidative stress pathway.

Introduction

Although asthma is defined as a chronic airway disease, recent investigations have emphasized the role of lung parenchyma alterations in its pathophysiology. In patients with severe asthma particularly the alterations in small airways as well as in lung tissue contribute to the functional impairment (Kraft, 1999, Mauad et al., 2004). It has been shown that peripheral lung parenchyma of asthmatic patients presents eosinophilic inflammation, overexpression of IL-5 and extracellular matrix remodeling (Bousquet et al., 2000, Mauad et al., 2004, De Magalhães et al., 2005). These alterations have also been reproduced in experimental animal models of chronic allergic pulmonary inflammation (Rocco et al., 2001, Xisto et al., 2005, Lanças et al., 2006).

Different inflammatory mediators and modulators released in the inflammatory milieu, such as nitric oxide (NO), are responsible for many of the structural and functional lung alterations observed in asthmatic patients and in animal models of chronic pulmonary allergic inflammation (Bousquet et al., 1992, Boulet et al., 1995, Meurs et al., 2000, Holgate, 2001). Nitric oxide derived either from constitutive isoforms (nNOS and eNOS) or from other NO-adduct molecules (nitrosothiols) is able to modulate bronchomotor and vascular tone. On the other hand, NO derived from inducible isoenzyme (iNOS) seems to be mainly involved in the immunomodulation (Belvisi et al., 1991, Koarai et al., 2002, Ricciardollo, 2003, Prado et al., 2005a).

iNOS has been found in several types of inflammatory cells, such as eosinophils, neutrophils and macrophages, as well as in respiratory epithelial cells, and the large production of NO from this isoenzyme is related to the amplification of the inflammatory reaction (Ricciardollo et al., 2004, Prado et al., 2005a, Prado et al., 2006). In addition, considering that the interaction between NO and superoxide leads the peroxynitrite formation and generation of isoprostanes, as 8-iso-PGF2α, some authors suggested that oxidative stress pathway could also be involved in the modulation of lung tissue constriction (Ricciardollo et al., 2006b, Jourdan et al., 1997). It is important to note that the isoprostanes are a biomarker of oxidative stress.

Therefore, NO derived from iNOS contributes to airway constriction, inflammation and remodeling (Eynott et al., 2002, Bhandari et al., 2002, Prado et al., 2006), and it has been proposed that specific inhibition of iNOS-derived NO as a future therapeutic strategy for inflammatory diseases such as asthma, sepsis and acute lung inflammation (Garvey et al., 1997, De Boer et al., 1999, Koarai et al., 2002, Ricciardollo et al., 2006a).

Considering that the nitrergic nerve density seems progressively reduced throughout the bronchial tree (Dupuy et al., 1992), NO modulation in asthma pathophysiology has been intensively investigated in airways (Belvisi et al., 1991, Prado et al., 2005a). The alterations in tissue resistance in response to agonists may be determined by the heterogeneity of airway constriction (Lutchen et al., 1996a, Lutchen et al., 1996b), but increases in tissue R may also reflect a direct role of parenchyma tissue in the overall lung response.

Lung parenchyma strips represent the distal units of lung tissue and offer a better assessment of pure tissue properties. For this reason, studies using this technique have been performed to evaluate the mechanical and pharmacological properties of lung periphery (Nagase et al., 1995, Rocco et al., 2001, Lanças et al., 2006).

In conclusion, a plausible hypothesis is that iNOS-derived NO modulates the lung parenchyma alterations induced by chronic ovalbumin exposures in guinea pigs. We investigated the effects of the highly selective inhibitor of iNOS activity (N-(3(Aminomethyl) Benzyl) Acetamidine, 1400-W), in lung tissue mechanics, inflammation and extracellular matrix remodeling in a well-established model of chronic pulmonary allergic inflammation.