Cromakalim inhibits transmitter acetylcholine release in rat trachea by an action on epithelial cells and a diffusible factor.

The present study was undertaken to investigate further the effects of the potassium channel opening drug cromakalim on the release of transmitter acetylcholine from cholinergic nerves of rat isolated trachea by using two tracheal preparations superfused in series. In all experiments, the lower chamber contained an epithelium-denuded preparation which had been incubated with [3H]-choline to incorporate [3H]-acetylcholine into cholinergic transmitter stores, whereas the upper chamber contained an unlabelled, epithelium-intact or epithelium-denuded preparation. When the upper chamber contained an epithelium-intact tracheal preparation, cromakalim (1-100 micro M) significantly reduced the stimulation-induced (S-I) efflux of [3H]-acetylcholine from the radiolabelled, epithelium-denuded tracheal preparation in the lower flow chamber. In contrast, when the upper flow chamber contained an epithelium-denuded preparation, cromakalim (10 micro M) was without effect on the S-I efflux. Glibenclamide (1 micro M), an ATP-sensitive potassium channel blocker, was without effect on the S-I efflux when the upper chamber contained an unlabelled, epithelium-intact tracheal preparation. However, glibenclamide (1 micro M) prevented the inhibition of the S-I efflux by cromakalim (10 micro M). When the upper chamber contained an epithelium-intact tracheal preparation and the lower chamber contained an epithelium-denuded tracheal preparation, cromakalim (10 micro M), when infused through the side-arm of the T-piece, such that only the lower radiolabelled epithelium-denuded tracheal preparation was exposed to the drug, was without effect on the S-I efflux. The findings of the present study have provided evidence of an inhibitory action of the potassium channel opener cromakalim on transmitter acetylcholine release in rat trachea which is dependent on the functional integrity of the tracheal epithelium. The findings suggest that cromakalim may inhibit transmitter acetylcholine release by opening ATP-sensitive potassium channels, probably, on cells in the epithelial layer to release a putative epithelial factor, which in turn acts prejunctionally to mediate the inhibitory effect of cromakalim.