Inflammatory effects of platelet activating factor (PAF) in equine skin

Intradermal administration of PAF (0.001-1 micrograms/site), but not lyso-PAF (10 micrograms/site), in the horse caused an increase in cutaneous vascular permeability which was maximal by 32 min. Responses to PAF and histamine were reduced by coadministration of the histamine 1 receptor antagonist chlorpheniramine, although only the inhibition of histamine-induced responses was dose-related and statistically significant. The cyclo-oxygenase inhibitor indomethacin was without effect on PAF-induced increases in vascular permeability. These findings suggest that the actions of PAF on equine skin microvasculature may be partly due to histamine release but not to prostanoid formation. Coadministration of prostaglandin (PG) E2 enhanced the oedematous responses to both PAF and histamine, although PGE2 failed to exert direct permeability-increasing activity. In addition, and in contrast to PAF and histamine, PGE2 increased cutaneous blood flow and skin surface temperature. PAF, but not lyso-PAF, also caused neutrophil infiltration into the skin which was maximal at 2 h. No significant effects on eosinophil or mononuclear cell numbers were apparent up to 24 h after injection of PAF. These results are consistent with the concept that PAF may be a mediator of inflammatory disorders of the skin in the horse.