Introduction: Transient Receptor Potential Cation Channel Subfamily A, Member 1 (TRPA1) is a temperature- and irritant- sensitive cation channel located on sensory nerves in the lung. TRPA1 interacts with Toll- like receptor 7 (TLR7) on sensory nerves to increase intracellular calcium. Since stimulation of TLR7 causes bronchodilation both in vivo in guinea pigs and ex vivo in human trachealis tissue, we postulated that TRPA1 stimulation would similarly lead to bronchodilation.
Methods: Isolated guinea pig tracheas and post mortem human trachealis muscle strips were suspended in Krebs solution in an organ bath and contracted with methacholine (10 pM and 20uM, respectively) or histamine (20uM).
Contractions were measured with a force transducer. Airways were then treated with a TRPA1 agonist, allyl isothiocyanate (AITC). Some airways were pre-treated with the TRPA1 antagonists HCO30031 or AP-18, the beta adrenergic receptor antagonist propranolol, the muscarinic receptor antagonist atropine, the nitric oxide synthase inhibitor L-NG-monomethyl arginine citrate (L-NMMA), or with the non-selective cyclooxygenase inhibitor indomethacin.
The effects of TRPA1 agonists were also evaluated in cultured guinea pig dorsal root ganglia (DRGs) using the intracellular calcium fluorophore Fluo-4-AM (Life).
Finally, TRPA1 expression in lung was characterized using immunofluorescence. Results: TRPA1 stimulation relaxed pre-contracted human and guinea pig airway smooth muscle within minutes of administration. TRPA1-induced relaxation was partially inhibited by L-NMMA and by indomethacin. TRPA1-mediated relaxation did not involve beta or muscarinic receptors. TRPA1 agonists increased intracellular calcium in cultured guinea pig DRGs. Immunostaining confirmed TRPA1 expression on a subset of guinea pig DRG sensory nerves.
Conclusions: TRPA1 stimulation causes rapid relaxation of pre-contracted tracheal smooth muscle, both in guinea pig and human tissue, via production of nitric oxide and prostaglandins. TRPA1’s relaxant effect may be mediated by airway sensory nerves.