Cromakalim inhibits electrically-evoked [3H]acetylcholine release from a tube-preparation of the rat isolated trachea by an epithelium-dependent mechanism.

Rat isolated tracheae were labelled by incubation with [3H]choline to measure the tritium efflux elicited by electrical stimulation of the extrinsic parasympathetic nerves in vitro. Stimulated tritium efflux reflects the neuronal release of newly synthesized acetylcholine; the effects of potassium channel openers on the stimulated tritium efflux were investigated. In tracheae opened longitudinally neither cromakalim nor its 3S,4R-enantiomer, BRL 38227, reduced the stimulated tritium efflux, whereas in intact tube-preparations cromakalim (0.01-1 mumol/l) mediated a concentration-dependent inhibition. The inhibitory effect of 1 mumol/l cromakalim was prevented by 0.1 mumol/l glibenclamide. Likewise, BRL 38227 (0.01 and 0.1 mumol/l) inhibited the stimulated tritium efflux, but the inhibitory effect vanished at high concentrations (1 and 10 mumol/l). The 3R,4S-enantiomer of cromakalim, BRL 38226 (0.1, 1 and 10 mumol/l), on its own did not significantly inhibit the stimulated tritium efflux, but a combination of both enantiomers (0.5 or 1 mumol/l of each) produced an inhibition similar to that caused by 1 mumol/l cromakalim. In epithelium-denuded tube-preparations neither cromakalim nor BRL 38227 reduced the stimulated tritium efflux. The mucosal/submucosal microenvironment is better preserved in intact tube-preparations than in longitudinally-opened tracheae which are cut along their whole length so that the luminal surface is exposed directly to the surrounding medium. The present experiments show an neuronal inhibitory effect of cromakalim which is mediated by an epithelium-dependent mechanism.