Arachidonic acid metabolites and airway epithelium-dependent relaxant factor

Exogenous arachidonic acid (10(-8) to 10(-4) M) contracted epithelium-free guinea pig tracheal strips. Intact tracheal strips were contracted slightly by low concentrations of arachidonic acid (10(-8) to 10(-5) M), but higher concentrations relaxed them. In contrast, when tracheal strips were precontracted with histamine or carbachol, exogenous arachidonic acid had no effect on epithelium-free preparations but induced concentration-dependent (10(-8) to 10(-4) M) relaxation of intact tracheal strips. The effects of arachidonic acid both in epithelium-free and epithelium-containing trachea were blocked by either indomethacin (10(-6) M) or aspirin (10(-4) M). Studies on the effects of exogenous arachidonic acid, performed with a "sandwich protocol," demonstrated that the postulated airway epithelium-dependent relaxant factor released by an intact tracheal strip relaxes an adjacent epithelium-free strip in the same organ bath. This relaxation is antagonized by indomethacin suggesting the involvement of a cyclooxygenase product in this phenomenon. Comparison of concentration-response curves for contractile agonists in epithelium-free preparations and in one containing epithelium suggests the mobilization of airway epithelium-dependent relaxant factor by histamine but not by carbachol. The effects of cyclooxygenase and lipoxygenase inhibitors indicated that both relaxant and contractile arachidonic acid metabolites are generated by epithelial and nonepithelial cells alike in response to contractile agonists.