TRPA1-Agonists: Novel bronchodilators?

      TRPA1 is a cation channel of the transient receptor potential channel family that is predominantly expressed in sensory C-fibers and activated by a wide variety of environmental irritants and endogenous inflammatory mediators relevant for asthma. Activation increases [Ca2+]i, induces inward currents and action potential discharge in lung-innervating C-fibers in vitro and elicits central reflexes in vivo. However, it remained unclear if TRPA1-dependent central reflex activity is associated with changes in lung function. To examine our hypothesis that TRPA1 activation in C-fibers induces bronchoconstriction, we performed head-out-bodyplethysmography in conscious mice and measured the midexpiratory tidal flow (EF50) during inhalation of increasing concentrations of cinnamaldehyde (CA), a TRPA1 agonist. Unexpectedly, CA induced a dose-dependent increase of EF50, characteristic for bronchodilation. Organ bath experiments on explanted, preconstricted tracheal rings to elucidate the nature of bronchodilation revealed that the CA effect was independent from central reflexes and could be mimicked by a variety of electrophilic and non-electrophilic TRPA1 agonists, including arcrolein, AITC, 2-APB, thymol and carvacrol. Surprisingly, the bronchodilatory effect of CA was increased by pre-treatment with TRPA1-antagonists (HC-030031, AP-18, Ruthenium-Red) as well as in tracheas of TRPA1-KO mice, indicating that CA causes a TRPA1-mediated bronchoconstriction which is superimposed by a TRPA1-independent bronchodilation. Since a 5-day organotypic culture of the tracheas was associated with an increased bronchodilation, we conclude that the TRPA1-dependent bronchoconstriction involves sensory neurons. However, the nature of the TRPA1-independent bronchodilation remains unclear since RP-67580, iberiotoxin, indomethacin, tetrodotoxin, propranolol, Nω-nitro-L-arginine and triphenylphosphine-oxide were ineffective. We conclude that activation of TRPA1 in C-fibers induces bronchoconstriction superimposed by a bronchodilatory effect of unknown etiology.
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