Direct modulation of tracheal Cl--channel activity by 5,6- and 11,12-EET.

Using microelectrode potential measurements, we tested the involvement of Cl- conductances in the hyperpolarization induced by 5,6- and 11,12-epoxyeicosatrienoic acid (EET) in airway smooth muscle (ASM) cells. 5,6-EET and 11,12-EET (0.75 μM) caused -5.4 ± 1.1- and -3.34 ± 0.95-mV hyperpolarizations, respectively, of rabbit tracheal cells (from a resting membrane potential of -53.25 ± 0.44 mV), with significant residual repolarizations remaining after the Ca2+-activated K+ channels had been blocked by 10 nM iberiotoxin. In bilayer reconstitution experiments, we demonstrated that the EETs directly inhibit a Ca2+-insensitive Cl- channel from bovine ASM; 1 μM 5,6-EET and 1.5 μM 11,12-EET lowered the unitary current amplitude by 40 ( n = 6 experiments) and 44.7% ( n = 4 experiments), respectively. Concentration-dependent decreases in channel open probability were observed, with estimated IC50 values of 0.26 μM for 5,6- and 1.15 μM for 11,12-EET. Furthermore, pharmacomechanical tension measurements showed that both regioisomers induced significant bronchorelaxations in epithelium-denuded ASM strips. These results suggest that 5,6- and 11,12-EET can act in ASM as epithelium-derived hyperpolarizing factors.