To examine the role of cyclic 3', 5'-guanosine monophosphate (cGMP) in airway responsiveness the effects of substances known to interfere with nitric oxide (NO) or cGMP were investigated on guinea pig airways. Using a perfused organ bath system, it was possible to apply the chemicals from either the serosal or the mucosal side independently. In addition, levels of intracellular cGMP were determined in tissues after various treatments. Sodium nitroprusside (a donor of NO), zaprinast (a specific inhibitor of cGMP phosphodiesterase) and 8-bromo-cGMP (8-Br-cGMP) caused a concentration-dependent relaxation of guinea pig trachea. These results indicate that cGMP is an important second messenger mediating tracheal relaxations. The above mentioned drugs caused a more profound relaxation when applied to the serosal side compared to the mucosal side, suggesting a barrier function of the epithelial layer. Incubation on the mucosal side of the tissues with 100 microM pyrogallol (a generator of superoxide that may inactivate NO) increased the contractile response to histamine at concentrations 0.3-3.2 microM (P < 0.05). Treatment of the preparations with 1 mM cystamine (an inactivator of guanylate cyclase) caused a 5-fold increase in the sensitivity to histamine (P < 0.05), indicating the involvement of the NO/cGMP pathway in the development of airway hyperresponsiveness. Incubation of the tissues with 100 microM histamine elevated the intracellular cGMP levels 10-fold; this effect was completely prevented by incubation of the tissues with methylene blue (a potent inactivator of guanylate cyclase). Mucosal incubation of the tracheal tubes with 10 microM methylene blue induced an 8-fold increase in sensitivity to histamine (P < 0.01) and the Emax was slightly increased. 25 min after instillation of 0.4 mumol methylene blue into the airways of anaesthetized guinea pigs, the lung resistance in response to histamine was elevated up to 395 +/- 82% (P < 0.001). The present study revealed that inactivation of NO or guanylate cyclase enhances the histamine-induced contractions of guinea pig tracheas. Therefore, it is suggested that the NO/cGMP pathway may be implicated in the pathogenesis of airway hyperresponsiveness and that drugs which enhance cGMP levels in airway smooth muscle may be of significance in the treatment of airway obstruction and enhanced reactivity.