K+ channel openers produce epithelium-dependent relaxation of the guinea-pig trachea

Cromakalim-induced membrane current in guinea-pig tracheal smooth muscle cells

The characteristics of the cromakalim-induced membrane current were examined in single tracheal myocytes of the guinea-pig under voltage-clamp conditions. When K(+) concentrations in the pipette and bathing solutions were approximately 140 mM, cromakalim activated a membrane current (I(crom)) which was inward at -60 mV and reversed at -2 mV. I(crom) was blocked by 10 microM glibenclamide and potentiated when the ATP concentration in the pipette solution was decreased. The K(d) and Hill coefficient of glibenclamide for I(crom) block were 200 nM and 1.05, respectively. Application of the tyrosine kinase inhibitors, genistein and alpha-cyano-3-ethoxy-4-hydroxy-5-phenylthiomethylcinnamamid (ST638), reduced I(crom) in a concentration-dependent manner. Daidzein, which does not inhibit tyrosine kinase, was about 10 times less effective than genistein. Herbimycin A had no effect on I(crom). Internal application of these inhibitors from the pipette did not affect I(crom). In conclusion, cromakalim is a potent activator of the ATP-sensitive K(+) channel (K(ATP) channel) in guinea-pig tracheal myocytes. The inhibition of I(crom) by genistein and ST638 may be due to the direct block of the channel from outside.

Role of Ca(2+)-activated K+ channel in epithelium-dependent relaxation of human bronchial smooth muscle

1. To elucidate whether K+ channels play a role in the action of epithelium-dependent bronchodilatation, we studied responses in human bronchial strips in the presence of indomethacin and NG-nitro-L-arginine methylester under isometric conditions, in vitro. 2. Mechanical removal of the epithelium increased the contractile responses to acetylcholine; the pD2 values increased from 5.0 +/- 0.2 to 5.9 +/- 0.3 (P < 0.001). This potentiation was abolished by iberiotoxin but not by apamin or glibenclamide. 3. In cascade bioassay, application of the bathing medium from dispersed, bronchial epithelial cells to epithelium-denuded bronchial strips decreased acetylcholine-induced contraction by 44 +/- 6%. This effect was reduced to 10 +/- 3% (P < 0.01) when the epithelial cells were pretreated with iberiotoxin, and to 4 +/- 1% (P < 0.001) when the epithelial cells were incubated with Ca(2+)-free medium containing [1,2-bis(2) aminophenoxy] ethane N,N,N',N'-tetraacetic acid-acetomethoxy ester. 4. In contrast, the bronchodilator effect of the medium bathing epithelial cells was not altered by the direct addition of iberiotoxin to epithelium-denuded tissues. 5. These results suggest that the Ca(2+)-activated K+ channel may play a role in the synthesis and/or release of smooth muscle relaxing factor, which is neither nitric oxide nor a cyclo-oxygenase product, from airway epithelial cells.