Platelet-activating factor receptor mRNA is localized in eosinophils and epithelial cells in rat small intestine: regulation by dexamethasone and gutflora

Platelet-activating factor (PAF) is a potent mediator involved in bowel inj ury. We investigated PAF receptor transcription and its mRNA localization i n the small intestine of normal (conventionally fed) and germ-free rats, by competitive polymerase chain reaction (PCR) and in situ hybridization. A d ose of PAF (1.5 mu g/kg, i.v.) insufficient to cause gross bowel injury was injected into rats. Some rats were pretreated with dexamethasone (1 mg/kg) . We found: (1) PAF receptor (PAF-R) mRNA localized predominantly in lamina propria eosinophils and in epithelial cells; (2) PAF increased PAF-recepto r signals in the epithelial cells; (3) Dexamethasone depleted eosinophils i n the intestine and markedly decreased PAF-receptor transcripts; the respon se to PAF was also weaker than control rats; (4) Germ-free rats had less PA F-R mRNA than normal rats, and showed a weaker response to PAF than convent ionally fed rats. Thus, we conclude: (1) PAF receptor mRNA is constitutivel y expressed in the epithelium and in lamina propria eosinophils in the inte stine. (2) PAF-R transcription is up-regulated by PAF and gut flora, mostly in the epithelium. (3) PAF-R transcription is down-regulated by glucocorti coids, mainly as a result of eosinophil depletion. These results suggest a functional role for PAF receptors both in host defence and the inflammatory response in the small intestine.