ROLE OF CALCIUM-ACTIVATED POTASSIUM CHANNELS IN THE RELAXATION OF TRACHEAL SMOOTH MUSCLES BY FORSKOLIN

1. The role of calcium-activated potassium (K-Ca) 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 K-Ca 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 K-Ca 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 K-Ca 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 K-Ca channels, by means of a cAMP-dependent channel modulation. The lesser effect of ChTX on forskolin compared with isoproterenol-induced relaxations suggests a tighter coupling between relaxation and channel opening by beta-adrenergic receptor stimulation, and is consistent with cAMP-independent mechanisms of receptor-channel coupling.