Epidemiological studies have suggested that environmental exposure to Dibutyl phthalate (DBP) as an indoor air pollutant, found in flooring, cosmetics and food packaging, is linked with worsening or development of asthma. It is thought that due to its high lipophilicity, DBP can accumulate in tissues, including the lung and may contribute to symptoms in this way. Though the potential cellular mechanisms behind DBP’s action are not entirely clear, DBP has been shown to activate some of the Transient Receptor Potential (TRP) family of ion channels, some of which are expressed on airway sensory nerves. Activation of these nerves can lead to respiratory reflexes which are exacerbated in asthma, such as cough and bronchospasm. The aim of this thesis was to investigate whether exposure to DBP can activate airway sensory nerves and identify possible mechanisms behind this. Additionally, this thesis aimed to investigate whether any potential nerve activation would worsen asthma symptoms in an animal model.
Using an in vivo model of single afferent airway nerve firing in anaesthetised guinea-pigs, aerosolised administration of DBP caused firing of airway Aδ- and C-fibres. Furthermore, using in vitro nerve depolarisation and calcium imaging assays showed that DBP could activate the whole nerve trunk as well as induce calcium flux in airway terminating jugular and nodose neurons. With the use of pharmacological tools and tissues from genetically modified mice, DBP’s action on nerves/neurons was shown to be mediated through activation of TRPA1, TRPV1 and TRPV4, which are all known to activate sensory nerves and lead to respiratory reflexes such as cough and bronchospasm. Interestingly, in vivo single afferent nerve assays showed that activation of airway C-fibres was through activation of TRPA1 and TRPV1, whereas TRPV4 was responsible for DBP-induced activation of airway Aδ-fibres.
Further in vitro investigation using pharmacological tools showed that DBP’s action on TRPA1 was due to mitochondrial-derived oxidative stress, mediated by the aryl hydrocarbon receptor. Additionally, DBP-induced activation of TRPV1 was shown to be mediated through prostanoid products (e.g. PGE2), and DBP-induced activation of TRPV4 involved the subsequent release of ATP. Finally, exposure to DBP in an allergic model of asthma in the rat showed a trend to exacerbated responses in increases in PenH and BALF eosinophils, though this was not significantly different.
Data in this thesis have shown that exposure to DBP can activate airway sensory nerves and identified novel mechanisms by which DBP can do this. During this thesis, it was not possible to conclude that exposure to DBP exacerbated a disease model, though further investigation (e.g. in a different model or looking at the effect of DBP on cough) may show how DBP may worsen or trigger asthma symptoms.