Effect of different ozone concentrations on the neurogenic contraction and relaxation of guinea pig airways

Summary - Prejunctional and postjunctional effects of several ozone (0,) concentrations, including those found in highly polluted cities, were evaluated in guinea pig airways. Animals bred in 0,-free conditions were exposed to air or 0, (0.3,0.6 or 1.2 ppm) during 4 h. and studied 1618 h later. Tracheal and bronchial rings were studied in organ baths. Electrical field stimulation (EFS) (100 V. 2 ms. 10 s) was given at increasing frequencies (0.25-16 Hz). Some tissues received atropine (2 pkl) andlor propranolol (10 jN). Concentration-response curves to carbachol, isoproterenol, nitroprusside, and substance P were constructed. In tracheas, almost all 0, concentrations decreased the relaxation at low EFS frequencies, but had no effect on the propranolol-resistant (I-NANC) relaxation, suggesting that only adrenergic relaxation was affected. This was a prejunctional effect, since 0, did not modify the responses to isoproterenol. Relaxation induced by a nitric oxide (NO) donor, nitroprusside, was not affected by 0,, which agrees with the lack of 0,-effect on i-NANC system. 0, did not modify the EFS-induced e-NANC contraction in atropine-treated bronchi, nor the contraction caused by exogenous substance P. By contrast, in bronchi without atropine, 1.2 ppm 0, increased the e-NANC contraction induced by the highest EFS (16 Hz). 0, increased the maximum responses to carbachol in tracheas (1.2 ppm) and bronchi (0.6 and 1.2 ppm). In conclusion, we found that: a) 0, decreased adrenergic relaxation in guinea pig tracheas at low EFS frequencies through a prejunctional alteration; b) 0, did not modify the i-NANC relaxation in tracheas, at least the NO-mediated; c) 0, added a cholinergic component to the bronchial slow-phase (e-NANC) contraction evoked by EFS; and d) 0, enhanced the cholinergic responses in trachea and bronchi by a postjunctional mechanism. ozone / airway hyperresponsiveness / prejunctional / postjunctional / trachea / bronchus / e-NANC / i-NANC / adrenergic relaxation