Uptake of 22-carbon fatty acids into rat retina and brain

Rat retina accumulates high levels of 22-carbon (C22) polyunsaturated fatty acids (PUFA), especially docosahexaenoic acid (DHA, 22:6 omega 3), in rod outer segment (ROS) phospholipids (PL). However, plasma, the source of retina lipids, is enriched in 20-carbon (C20) fatty acids instead of C22 PUFA. This suggests that the retina has a mechanism(s) for selective uptake of C22 PUFA from the blood. It is not known if the selective uptake is specific for the carbon number alone, or if the number of double bonds is also important. To address this question, the following study was carried out using erucic acid (22:1 omega 9) as a metabolic marker molecule. Albino rats were raised from birth on a diet containing 10% (by weight) of either rapeseed oil (43% 22:1 omega 9) or blended canola oil (0.4% 22:1 omega 9). At 4 months of age, plasma, liver, adrenal gland, brain and retina were collected, lipids were extracted, and fatty acids were determined. In those rats fed rapeseed oil, 22:1 omega 9 was incorporated into the lipids of plasma (2.3%), liver (0.6%), and adrenal gland (17.6%), indicating that this fatty acid was absorbed, transported, and metabolized by the rats. However, 22:1 omega 9 was not incorporated into the lipids of retinal ROS or brain. Our results suggest that both the carbon number and degree of unsaturation are important determinants in the selective uptake of C22 fatty acids from plasma into both the brain and the retina.