DOCOSAHEXAENOIC ACID IS TAKEN UP BY THE INNER SEGMENT OF FROG PHOTORECEPTORS LEADING TO AN ACTIVE SYNTHESIS OF DOCOSAHEXAENOYL-INOSITOL LIPIDS - SIMILARITIES IN METABOLISM IN-VIVO AND IN-VITRO

Retinal uptake and metabolism of docosahexaenoic acid (DHA) was studie d in vivo in frogs 1, 2, and 6 hours after dorsal lymph sac injections of [H-3]-DHA (50 mu Ci/g). Light microscope autoradiography and bioch emical techniques were used to compare the profiles of cellular uptake and lipid labeling with those obtained from 6 hour [3H]-DHA retinal i ncubations (final DHA concentration, 0.11 and 25 mu M). Light microsco pe autoradiography demonstrated that rod photoreceptor ellipsoids and synaptic terminals preferentially labeled both in vivo and in vitro co nditions. Also, the cytoplasm and oil droplets of retinal pigment epit helial cells became very heavily labeled after 6 hours of in vivo labe ling. Phosphatidic acid showed the highest labeling in one hour, while other phospholipids accumulated label throughout the 6 hours. At that time point, most label was recovered in phosphatidyl-ethanolamine (37 %), phosphatidylcholine (27%), and phosphatidylinositol (16%), the lat ter displaying 1.6-fold higher labeling than phosphatidylserine. The p rofile of labeled lipids was similar to that obtained in vitro when th e concentration of DHA was in the nanomolar range. Our results suggest that de novo lipid synthesis is a major route for esterification of [ H-3]-DHA into retinal lipids, giving rise to an early and rapid labeli ng of DHA-phosphatidylinositol, both in vivo and in vitro, when DHA is present at low concentrations. Furthermore, the profile of labeled re tinal cells under in vivo conditions closley resembles in vitro DHA la beling.