Application of arachidonic acid (AA) (0.1-4 mg) to the ears of mice produces immediate vasodilatation and erythema (5 min) followed by the abrupt development of edema which is maximal at 40-60 min. The onset of edema coincides with extravasation of protein and leukocytes. After 1 h, the edema begins to wane rapidly and the inflammatory cells leave the tissue so that by 6 h the ears have returned to near normal except for residual erythema. During the period 6-48 h, AA-treated ears show a greatly diminished response with respect to edema and cell infiltrate when AA is applied a second time. Inhibitor studies show that the inflammatory response is due to formation of AA metabolites via both the cyclooxygenase and lipoxygenase pathways. Under appropriate conditions, AA-induced ear edema can be used as a model to screen for compounds showing in vivo lipoxygenase inhibitory activity. Although relatively large doses of AA were applied topically, there was only a modest stimulation of epidermal DNA synthesis and mitotic index with no consequent hyperplasia. Although arachidonic acid is capable of eliciting most aspects of an inflammatory response, the reaction is abrupt in onset and of short duration. Additional factors appear to be required to produce a prolonged inflammatory response with associated tissue destruction, or inflammatory cell activation and immobilization in situ.

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Epidermal strips, free of sebaceous gland and hair follicle contamination, were prepared from mouse tail skin. Epidermal homogenates synthesized prostaglandins and 12-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE) from exogenously added [1-14C]arachidonic acid. The effects of pH, assay time, substrate concentration, and several selective inhibitors upon the lipoxygenase and cyclooxygenase pathways were determined. Ultracentrifugation of the crude homogenate at 105,000 g sedimented both activities, and pellet 12-HETE synthesis increased 2-fold relative to the crude homogenate. Recombination of the 105,000 g pellet and supernatant gave yields of prostaglandins and 12-HETE essentially equivalent to that of crude homogenate. When tested in homogenate with 4.5 microM arachidonic acid, anthralin specifically inhibited 12-HETE production with IC50 of 50.0 microM; no significant effect against cyclooxygenase was observed over the dose range of 2-200 microM. 1,8-Dihydroxy-9,10-anthraquinone (DHAQ) also specifically inhibited 12-HETE synthesis, but the dose response curve was flatter and maximum inhibition was only 55% at 200 microM. 6-Chloro-2,3-dihydroxy-1,4-naphthoquinone (CDNQ), an agent with topical antipsoriatic activity, also inhibited 12-HETE synthesis with an IC50 of 25 microM, but simultaneously stimulated prostaglandin production, up to 2.5-fold at 200 microM. When tested with washed human platelets, anthralin again specifically inhibited 12-HETE production with an IC50 of 10 microM, while DHAQ inhibited lipoxygenase activity by only 40% at 25 microM. When tested in platelets, CDNQ gave 33% inhibition of 12-HETE production at 200 microM, although prostaglandin synthesis was stimulated over the range of 25-200 microM. It is proposed that certain antipsoriatic agents may exert their action through modulation of arachidonic acid metabolism.

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