Guinea pigs have been used as experimental animals for

Guinea pigs have been used as experimental animals for the development of LT modifiers, because their airway smooth muscles respond well to CysLTs. In guinea pigs, CysLT2 receptors are mainly activated by LTC4, whereas LTD4 is known to potently activate CysLT1 receptors (Ito et al., 2008). In humans, on the other hand, LTC4 and LTD4 are reported to have similar binding affinity for human CysLT2 receptors (Nothacker et al., 2000). Because LTC4 is rapidly metabolized to LTD4 by γ-glutamyl transpeptidase (γ-GTP) (Orning and Hammarström, 1980, Synder et al., 1984), LTC4-induced bronchoconstriction in guinea pigs is preferentially mediated via CysLT1 receptors. Thus, in order to make an animal model with asthmatic response mediated via CysLT2 receptors, a number of γ-GTP inhibitors have been used to minimize LTC4 SJ 172550 to LTD4 (Bäck et al., 2001). We have recently reported that pre-treatment with S-hexyl GSH, a synthetic substrate of γ-GTP, in guinea pigs promotes LTC4- or antigen-induced bronchoconstriction and/or airway vascular hyperpermeability via both CysLT1 and CysLT2 receptors (Yonetomi et al., 2015a, Yonetomi et al., 2015b). The purposes of this study is to develop an experimental model of CysLT2 receptor-mediated LTC4-induced lung air-trapping in guinea pigs using S-hexyl GSH, and to clarify the mechanism underlying response to such trapping using montelukast, a CysLT1 receptor antagonist, BayCysLT2RA, a CysLT2 receptor antagonist, and salmeterol, a bronchodilatory adrenergic β2 agonist.
Materials and methods
Results
Discussion In this study, we developed an experimental model of CysLT2 receptor-mediated air-trapping in the lung by exposure to LTC4 in S-hexyl GSH-treated guinea pigs. The CysLT2 receptor-mediated air-trapping involves adrenergic β2 agonist-resistant mechanism. It has been well known that CysLTs are important lipid mediators, which are elevated in large amounts in the blood and lung tissues after allergen challenge in bronchial asthma patients (Dahlén et al., 1983, Hansson et al., 1983, Sasagawa et al., 1994, Wenzel et al., 1990). In addition, we have reported that CysLT2 receptors are expressed in airway tissues isolated from bronchial asthma subjects (Sekioka et al., 2015). Thus, the present experimental results suggest the possibility that CysLTs can cause air-trapping via activation of CysLT2 receptors in the pathogenesis of bronchial asthma of humans. Consistent with the results of our previous report (Yonetomi et al., 2015a), single dose of S-hexyl GSH (60mg/kg) promoted CysLT2 receptor-mediated LTC4-induced bronchoconstriction by inhibition of LTC4 conversion to LTD4 in anesthetized artificially ventilated guinea pigs. However, under these experimental conditions, bronchoconstriction can be considered to reflect increased pressure similar to that induced by a ventilator (Konzett and Rössler, 1940), which is not relevant to expiratory dyspnea, a typical airway obstruction at asthma attack (DeGiorgi and White, 2008). Thus, in the next experiments of this study, conscious, spontaneously breathing guinea pigs were exposed to LTC4 mist, and the increase in sRaw was measured as index of expiratory dyspnea (Pennock et al., 1979). In this model, LTC4 induced CysLT2-mediated increases in sRaw in guinea pigs treated with S-hexyl GSH. In addition to the increase in sRaw, LTC4 enlarged whole lung volume, indicating air-trapping in the lung. Because air-trapping has been suggested to be related to small airway obstruction (Contoli et al., 2012, Sorkness et al., 2008, Hartley et al., 2016), activation of CysLT2 receptors may contribute to small airway obstruction. As far as we know, this is the first report showing that CysLT2 receptors contribute to air-trapping in the lung of guinea pigs.