Light Elicits the Release of Docosahexaenoic Acid from Membrane Phospholipids in the Rat Retina In Vitro

Docosahexaenoic acid is the major polyunsaturated fatty acid in photoreceptor membrane phospholipids and is thought to be necessary for photoreceptor function. Docosahexaenoic acid may be metabolized to docosanoids or, by retroconversion, to eicosapentaenoic acid followed by lipoxygenation and synthesis of n-3 metabolites. In this study we investigated the time- and illuminance-dependent release of docosahexaenoic acid from photoreceptor phospholipids in the rat retina in vitro and the effects of the phospholipase A 2inhibitor, quinacrine, on this release. Isolated rat retinae were incubated in oxygenated Ringer–Bicarbonate–Glucose–Medium and labelled with [ 3H]docosahexaenoic acid for 180 min in darkness. The incorporation of [ 3H]docosahexaenoic acid into retinal phospholipids was monitored by thin-layer chromatography. The release of [ 3H]docosahexaenoic acid was determined under illuminances of 100, 500, 1000, 5000 and 10000 lx for 10 min, illuminance durations of 0.25, 2, 5 and 10 min at 10000 lx, and with the addition of 10 and 100 μmol quinacrine to the incubation medium at 10000 lx for 10 min. Our results demonstrate a release of docosahexaenoic acid from retinal phospholipids that is finely tuned by light levels and exposure duration. The kinetics of the time dependent docosahexaenoic acid release and the effects of quinacrine suggest that this release is mediated in part by activation of phospholipase A 2. The light-elicited docosahexaenoic acid release may serve as a protective measure against formation of prostaglandins by inhibiting cyclooxygenase and by promoting the synthesis of less potent leukotrienes of the 5-series via retroconversion to eicosapentaenoic acid and 5-lipoxygenation.