Epoxyeicosatrienoic acids relax airway smooth muscles and directly activate reconstituted KCa channels

Epoxyeicosatrienoic acids (EETs) relax various smooth muscles by increasing outward K+ movement, but the molecular mode of action of EET regioisomers remains to be clarified. The effects of EETs were investigated on bovine airway smooth muscle tone and on reconstituted Ca2+-activated K+ (K-Ca) channels. 5,6-EET and 11,18-EET induced dose-dependent relaxations of precontracted bronchial spirals. These effects were partly abolished by 10 nM iberiotoxin. Bilayer experiments have shown that 0.1-10 mu M 11,12-EET produced up to fourfold increases in the open probability of K-Ca channels from the cis (extracellular) side by enhancing the mean open time constant and reducing the long closed time constant, without affecting the unitary conductance. EET-induced activations were blocked by 10 nM iberiotoxin. Addition of vehicles or other lipids as well as of GTP and guanosine 5'-O-(3-thiotriphosphate) in the absence of EET had no effect on channel activity. Thus EETs directly activate K-Ca channels from airway smooth muscle through an interaction with the extracellular face of the channel. We propose that EETs could represent candidate molecules as epithelium-derived hyperpolarizing factors.