Site of Action of Platelet-Activating Factor within the Mucosa of Rabbit Distal Colon

1. To determine how platelet-activating factor stimulates colonic anion secretion and increases epithelial permeability, epithelial sheets of rabbit distal colon excluding the submucosal neural plexus were mounted in Ussing chambers. The influence of specific inhibitors and 50 nmol/l platelet-activating factor on short-circuit current and transepithelial resistance was then investigated. 2. Pretreatment with 1 μmol/l indomethacin or 1 μmol/l doxantrazole abolished the biphasic stimulation of the short-circuit current and decrease in transepithelial resistance induced by platelet-activating factor. Addition of 10 μmol/l mepyramine attenuated the early phase and completely inhibited the late phase. Pretreatment with 1 μmol/l ranitidine, 0.1 μmol/l tetrodotoxin, 0.1 μmol/l ritanserin or a 5-lipoxygenase inhibitor (1 μmol/l MK886) had no effect. 3. To assess the influence of platelet-activating factor on epithelial function isolated from lamina propria elements, monolayers were cultured from a human colonic epithelial cell line (T-84). 4. The short-circuit current across monolayers mounted in Ussing chambers stimulated by 10 μmol/l ionomycin could be inhibited by pretreatment with ouabain or frusemide, consistent with the capacity for chloride secretion. Addition of platelet-activating factor (up to 500 nmol/l) had no effect on short-circuit current or transepithelial resistance. Receptor expression was examined with [3H]platelet-activating factor in isolated T-84 and HT-29 cells and found to be absent. 5. The influence of physiological concentrations of platelet-activating factor on colonic epithelial anion secretion and increased permeability in rabbit distal colon is indirect and consistent with mediation by prostaglandins released from mucosal mast cells. Other mediators, including histamine acting through H1 but not H2 receptors, may have a role. 5-Lipoxygenase metabolites or 5-HT2 receptors appear not to be involved and, contrary to previous reports, the influence of platelet-activating factor on colonic epithelial function is independent of the submucosal neural plexus.