EETs relax airway smooth muscle via an EpDHF effect: BKCa channel activation and hyperpolarization

Epoxyeicosatrienoic acids (EETs) are produced from arachidonic acid via the cytochrome P-450 epoxygenase pathway. EETs are able to modulate smooth mus cle tone by increasing K+ conductance, hence generating hyperpolarization o f the tissues. However, the molecular mechanisms by which EETs induce smoot h muscle relaxation are not fully understood. In the present study, the eff ects of EETs on airway smooth muscle (ASM) were investigated using three el ectrophysiological techniques. 8,9-EET and 14,15-EET induced concentration- dependent relaxations of the ASM precontracted with a muscarinc agonist (ca rbamylcholine chloride), and these relaxations were partly inhibited by 10 nM iberiotoxin (IbTX), a specific large-conductance Ca2+-activated K+ (BKCa ) channel blocker. Moreover, 3 muM 8,9- or 14,15- EET induced hyperpolariza tions of -12 +/- 3.5 and -16 +/- 3 mV, with EC50 values of 0.13 and 0.14 mM , respectively, which were either reversed or blocked on addition of 10 nM IbTX. These results indicate that BKCa channels are involved in hyperpolari zation and participate in the relaxation of ASM. In addition, complementary experiments demonstrated that 8,9- and 14,15-EET activate reconstituted BK Ca channels at low free Ca2+ concentrations without affecting their unitary conductance. These increases in channel activity were IbTX sensitive and c orrelated well with the IbTX-sensitive hyperpolarization and relaxation of ASM. Together these results support the view that, in ASM, the EETs act thr ough an epithelium-derived hyperpolarizing factorlike effect.