DIRECT MODULATION OF TRACHEAL CL--CHANNEL ACTIVITY BY 5,6-EET AND 11,12-EET

Using microelectrode potential measurements, we tested the involvement of Cl- conductances in the hyperpolarization induced by 5,6- and 11,1 2- epoxyeicosatrienoic acid (EET) in airway smooth muscle (ASM) cells. 5,6-EET and 11,18-EET (0.75 mu M) caused -5.4 +/- 1.1- and -3.34 +/- 0.95-mV hyperpolarizations, respectively, of rabbit tracheal cells (fi om a resting membrane potential of -53.25 +/- 0.44 mV), with signific ant residual repolarizations remaining after the Ca2+-activated K+ cha nnels had been blocked by 10 nM iberiotoxin. In bilayer reconstitution experiments, we demonstrated that the EETs directly inhibit a Ca2+-in sensitive Cl- channel from bovine ASM; 1 mu M 5,6-EET and 1.5 mu M 11, 18-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 IC 50 values of 0.26 mu M for 5,6- and 1.15 mu M for 11,18-EET. Furthermo re, pharmacomechanical tension measurements showed that both regioisom ers induced significant bronchorelaxations in epithelium-denuded ASM s trips. These results suggest that 5,6- and 11,18-EET can act in ASM as epithelium-derived hyperpolarizing factors.