Role of calcium-activated potassium channels in the relaxation of tracheal smooth muscles by forskolin.

1. The role of calcium-activated potassium (KCa) channels in bronchodilation produced by a direct adenylyl cyclase activator, forskolin, was investigated. The involvement of intracellular cyclic AMP (cAMP) in the process was also examined. 2. The isometric tension records from guinea-pig tracheal smooth muscles indicated that application of charybdotoxin (ChTX), a selective inhibitor of large conductance KCa channels, led to a suppression of the relaxant effect of forskolin in the precontracted tissue by carbachol (CCh). However, the inhibitory action by ChTX had a much greater effect on the relaxation caused by isoproterenol than by forskolin. 3. In contrast to the effect of ChTX, glybenclamide, a cromakalim-sensitive K+ channel inhibitor and apamin, a small conductance KCa channel blocker, had no effects on the bronchodilation produced by forskolin. 4. The effects of forskolin and nifedipine on tone produced by high K+ was compared. Concentration-inhibition curves in guinea-pig trachealis precontracted by 20 mmol/L K+ solution were similar for forskolin and nifedipine. Conversely, relaxation by forskolin was significantly diminished when tissues were contracted with 40 mmol/L K+ solution, whereas nifedipine relaxations were unaffected. 5. A single channel record from a cell-attached patch in a porcine tracheal myocyte demonstrated that forskolin stimulates reversibly KCa channels without affecting the unitary amplitude. 6. The results are consistent with forskolin-induced relaxation occurring at least in part through the opening of ChTX-sensitive KCa channels, by means of a cAMP-dependent channel modulation.(ABSTRACT TRUNCATED AT 250 WORDS)