Functional characterisation and application of an ex vivo perfusion-superfusion system in murine airways.

INTRODUCTION: The aim of this study was to develop two dynamic ex vivo airway explant systems, a perfusion-superfusion system and a ventilation-superfusion system, for the study of toxic airborne substances, such as the prevalent smoke constituent acrolein. METHODS: Mouse isolated tracheal segments were perfused with physiological media or ventilated with humidified air at 37°C to mimic dynamic flow conditions, and superfused with media over the exterior surface. At selected time points, the histological and functional integrity of segments was evaluated. The perfusion-superfusion system was subsequently used to examine mucin secretory responses elicited by acrolein in airways in which mucous metaplasia had been induced with lipopolysaccharide (LPS; 1µgml-1) prior to 24h of media perfusion, followed by stimulation with acrolein or ATP for 15min. Epithelial mucin levels were determined by quantitative analysis of periodic acid-Schiff's reagent (PAS)-stained sections. RESULTS: Epithelial morphology was successfully preserved in the perfusion-superfusion and ventilation-superfusion systems for at least 24h and up to 18h, respectively. At these time points, the contractile and relaxation responses of perfused and ventilated tracheal segments to carbachol, the neuropeptide substance P, and the prostanoid PGE2 were also preserved. Using the perfusion-superfusion system, acute exposure to acrolein caused a dose-dependent reduction in the levels of PAS-positive mucin stores induced by LPS, consistent with mucin secretion. DISCUSSION: Both the perfusion-superfusion and ventilation-superfusion systems successfully preserved the viability of mouse isolated tracheal segments on a histological and functional level, and the perfusion-superfusion system was used to characterise the mucin secretory responses elicited by acrolein. Thus, this system may be a useful model through which to conduct further toxicological studies in mammalian airways.

Influenza A infection attenuates relaxation responses of mouse tracheal smooth muscle evoked by acrolein.

The airway epithelium is an important source of relaxant mediators, and damage to the epithelium caused by respiratory tract viruses may contribute to airway hyperreactivity. The aim of this study was to determine whether influenza A-induced epithelial damage would modulate relaxation responses evoked by acrolein, a toxic and prevalent component of smoke. Male BALB/c mice were inoculated intranasally with influenza A/PR-8/34 (VIRUS-infected) or allantoic fluid (SHAM-infected). On day 4 post-inoculation, isometric tension recording studies were conducted on carbachol pre-contracted tracheal segments isolated from VIRUS and SHAM mice. Relaxant responses to acrolein (30 µM) were markedly smaller in VIRUS segments compared to SHAM segments (2 ± 1% relaxation vs. 28 ± 5%, n=14, p<0.01). Similarly, relaxation responses of VIRUS segments to the neuropeptide substance P (SP) were greatly attenuated (1 ± 1% vs. 47 ± 6% evoked by 1 nM SP, n=14, p<0.001). Consistent with epithelial damage, PGE2 release in response to both acrolein and SP were reduced in VIRUS segments (>35% reduction, n=6, p<0.01), as determined using ELISA. In contrast, exogenous PGE2 was 2.8-fold more potent in VIRUS relative to SHAM segments (-log EC50 7.82 ± 0.14 vs. 7.38 ± 0.05, n=7, p<0.01) whilst responses of VIRUS segments to the ß-adrenoceptor agonist isoprenaline were similar to SHAM segments. In conclusion, relaxation responses evoked by acrolein were profoundly diminished in tracheal segments isolated from influenza A-infected mice. The mechanism through which influenza A infection attenuates this response appears to involve reduced production of PGE2 in response to SP due to epithelial cell loss, and may provide insight into the airway hyperreactivity observed with influenza A infection.

Acrolein relaxes mouse isolated tracheal smooth muscle via a TRPA1-dependent mechanism.

Airway sensory C-fibres express TRPA1 channels which have recently been identified as a key chemosensory receptor for acrolein, a toxic and highly prevalent component of smoke. TRPA1 likely plays an intermediary role in eliciting a range of effects induced by acrolein including cough and neurogenic inflammation. Currently, it is not known whether acrolein-induced activation of TRPA1 produces other airway effects including relaxation of mouse airway smooth muscle. The aims of this study were to examine the effects of acrolein on airway smooth muscle tone in mouse isolated trachea, and to characterise the cellular and molecular mechanisms underpinning the effects of acrolein. Isometric tension recording studies were conducted on mouse isolated tracheal segments to characterise acrolein-induced relaxation responses. Release of the relaxant PGE2 was measured by EIA to examine its role in the response. Use of selective antagonists/inhibitors permitted pharmacological characterisation of the molecular and cellular mechanisms underlying this relaxation response. Acrolein induced dose-dependent relaxation responses in mouse isolated tracheal segments. Importantly, these relaxation responses were significantly inhibited by the TRPA1 antagonists AP-18 and HC-030031, an NK1 receptor antagonist RP-67580, and the EP2 receptor antagonist PF-04418948, whilst completely abolished by the non-selective COX inhibitor indomethacin. Acrolein also caused rapid PGE2 release which was suppressed by HC-030031. In summary, acrolein induced a novel bronchodilator response in mouse airways. Pharmacologic studies indicate that acrolein-induced relaxation likely involves interplay between TRPA1-expressing airway sensory C-fibres, NK1 receptor-expressing epithelial cells, and EP2-receptor expressing airway smooth muscle cells.