Role of BK(Ca) channels and cyclic nucleotides in synergistic relaxation of trachea.

beta-Adrenoceptor agonists, nitric oxide (NO), and NO donors have been shown to mediate their effects through large conductance Ca(2+)-activated K(+) (BK(Ca)) channels. The mechanism of the synergistic effect of the beta(2)-adrenoceptor agonist, salbutamol, and an NO donor, sodium nitroprusside, was studied in guinea pig tracheal preparations. Salbutamol (0.1 nM) and sodium nitroprusside (0.33 microM) alone relaxed the acetyl-beta-methylcholine chloride (methacholine)-contracted preparations only by 0.5% and 28%, respectively, but their combination caused a maximum of 60% relaxation (at 3 min), which stabilized to 40% (at 10 min). Iberiotoxin, a selective inhibitor of the BK(Ca) channels, did not abolish the synergistic effect. 3-isobutyl-1-methylxanthine (IBMX) did not modify relaxation evoked by the drugs. Concentrations of cyclic nucleotides did not correlate with relaxations as a function of time. The mechanism of synergy remains to be clarified. The results show that NO is an important modulator in the relaxation of guinea pig trachea induced by beta(2)-adrenoceptor agonists in vitro.