Platelet-activating factor stimulates secretion of respiratory glycoconjugate from human airways in culture

Platelet-activating factor (PAF) is a naturally occurring phospholipid that acts as a potent mediator of inflammation and bronchoconstriction. Since mucus secretion accompanies many pulmonary-allergic reactions, we examined the effect of PAF on respiratory glycoconjugate (RGC) release from human airways in vitro. PAF, in concentrations of 5 to 100 ng/ml, induced a specific, dose-dependent release of [³H]RGC from human airways in vitro (range of 15% to 120% increase above control, p < 0.001; n = 8). Time-course studies revealed that RGC release reached its peak level by 60 minutes, and by the end of 4 hours, decreased almost to the baseline level, suggesting a stimulatory effect on secretion rather than synthesis. PAF analog, RO 19-3704, which is a PAF-receptor antagonist, inhibited RGC secretion mediated by PAF in a dose-dependent manner with an inhibitory concentration of 50% of 70 ng/ml. BW 755C and nordihydroguaiaretic acid, but not indomethacin, inhibited RGC release by PAF. LY 171883, a specific leukotriene D₄-receptor antagonist, totally inhibited the release of RGC by PAF. Similar results were observed with FPL 55712. PAF-treated airways generated peptidoleukotrienes significantly above control airways in association with enhanced RGC secretion. This enhanced effect on RGC secretion was specifically and significantly blocked by LY 171883. Atropine (10^-5 mol/L) augmented the secretagogue activity of PAF, whereas dexamethasone (10^-5 mol/L) inhibited it. These data indicate that PAF is a significant RGC secretagogue that affects the RGC-secreting cells via the lipoxygenase pathway of arachidonic acid metabolism and the generation of leukotrienes in the airways, but not through the cholinergic receptors on the secretory cells.