Effect of inflammatory mediators on airway nerves and muscle

The neuromuscular mechanisms underlying airway hyperresponsiveness have been reviewed on the basis of studies of the changes induced by ozone inhalation in dogs. In vivo, there is increased, nonspecific airway hyperresponsiveness based on studies of the response to inhaled acetylcholine or histamine. The underlying inflammatory mechanism involves release of LTB₄ and/or other chemotactic agents from epithelial or lumenal cells, ingress of macrophages, neutrophils, and platelets from the blood vessels between the muscle and epithelium, and migration of mast cells into the epithelium. The hyperresponsiveness seems to depend upon the influx of neutrophils and actions of thromboxane A₂ released from the neutrophils. In vitro, there is increased responsiveness to field stimulation of cholinergic nerves and to acetylcholine (not to KCl) in tracheal strips. These effects can be mimicked by a thromboxane A₂ analog (U44619). In the sucrose gap, the TxA₂ analog does not affect the excitatory junction potential, but in low concentration it increases and prolongs a series of fading membrane oscillations closely related to the contractions. We consider these oscillations to reflect ongoing release and/or action of acetylcholine. In high concentrations the analog causes a small depolarization and a tonic contraction, but it does not enhance the sensitivity to acetylcholine. TxA₂ may be acting either presynaptically or postsynaptically or both to produce these effects; however, changes in release of an epithelial-derived relaxing factor do not seem to be involved. We conclude that TxA₂ actions probably underlie hyperresponsiveness developed in vivo and in vitro after ozone inhalation.