Platelet-activating factor contributes to immune cell and oxidant- mediated intestinal secretion

The sensitivity of the Ussing-chambered rat colon to stimulation of Cl^- secretion (as measured by the change in short-circuit current) by exogenous platelet-activating factor (PAF) was increased significantly by washing the colon in vitro with Ringer's solution containing fatty acid-free albumin. When the wash solution was extracted with chloroform/methanol and the lipid extract was added back to Ussing-chambered colons, inhibition of PAF- stimulated short-circuit current was observed, whereas short-circuit current responses to bradykinin or vasoactive intestinal peptide were not affected. Hypoxia appears to be an important trigger for the down-regulation of the PAF response. These data suggest that hypoxia releases PAF or an endogenous lipid PAF inhibitor that desensitizes PAF receptors on colonic epithelial or mucosal cells. The short-circuit current response of rabbit colon to the chemotactic peptide formyl-methionyl-leucyl-phenylalanine was not inhibited by any PAF antagonist devoid of cyclooxygenase inhibitory activity but was strongly inhibited by indomethacin. In contrast, anti-IgE- or H₂O₂- stimulated short-circuit current in rat colon was inhibited by specific PAF antagonists, and this inhibition was additive with indomethacin. Both anti- IgE and H₂O₂ significantly increased PAF production by rat colon. These data suggest that PAF plays an important role in oxidant (H₂O₂)- and anti- IgE-mediated colonic Cl^- secretion but not in Cl^- secretion mediated by formyl-methionyl-leucyl-phenylalanine-stimulated phagocytes.