RP 49356 and cromakalim relax airway smooth muscle in vitro by opening a sulphonylurea-sensitive K+ channel: a comparison with nifedipine

RP 49356 is a novel compound which relaxes airway smooth muscle in vitro. Like cromakalim, RP 49356 reduced contractility in guinea pig isolated trachealis under basal conditions or when challenged with low (less than 20 mM) but not high K+. These effects were antagonized by the sulphonylureas glibenclamide and glipizide. This spectrum of action is typical of the class of compounds known as potassium channel openers (KCOs). Unlike RP 49356 and cromakalim, nifedipine had no effect on basal tone, relaxed tissues contracted with low or high K+ and was not antagonized by the sulphonylureas. These data suggest that the KCOs are not acting directly at the voltage-gated Ca++ channel in this tissue. RP 49356 and cromakalim were similar to nifedipine by being more potent at relaxing tissues precontracted with carbachol or histamine (spasmolytic effects) than they were at preventing initiation of the response to these spasmogens (antispasmogenic effects). Because the maintained phase of contraction in airway smooth muscle may be associated with some Ca++ influx, the data presented here suggests that, like nifedipine, the KCOs are more active smooth muscle relaxants under conditions of Ca++ influx. In summary, RP 49356, like cromakalim, is a compound which relaxes airway smooth muscle in vitro by opening a sulphonylurea-sensitive K+ channel which may be similar to the ATP-sensitive K+ channel found in other tissues.