Abstract
Vertebrate retina is highly enriched in 22-carbon polyunsaturated fatty acids (PUFAs), especially docosahexaenoic acid (DHA, 22:6n-3), which is supplied by the plasma. To study the transport and uptake of 22:6n-3 into the retina, young Long Evans rats were fasted overnight and given an oral dose of palm oil containing [4,5- 3H] 22:6n-3 and [9,10- 3H] 18:1n-9 (oleic acid). At 1, 6, 12, 24, and 48 hr postgavage, blood, liver, retinal pigment epithelium (RPE) and retina were collected, and plasma lipoprotein fractions were obtained by density-gradient ultracentrifugation. The distribution of radioactivity among the different lipoproteins (dpm%) showed that both fatty acids were mainly associated with VLDL 1 hr after gavage. For 22:6n-3, the majority of radioactivity was associated with LDL at 6 hr postgavage. By 12 hr post administration, the majority of radioactivity was in HDL 2 which retained around 40% until the 48-hr time point. For 18:1n-9, a different profile was observed. By 6 hr postgavage, approximately 30% of the radioactivity was found in HDL 2, but the percentage of radioactivity decreased to approximately 20% thereafter. The haft-life of 22:6n-3 was significantly longer than for 18:1n-9 ( P < 0·0005) in plasma, liver, and the lipoprotein fractions. The specific activities of 22:6n-3 and 18:1n-9 in the plasma continuously declined from 1 hr to 48 hr postgavage, while liver showed a peak of specific activity at 6 hr, then gradually declined. The specific activity of 22:6n-3 peaked at 12 hr in RPE, but continued to increase in ROS for up to 48 hr. Our data suggest that: both 22:6n-3 and 18:1n-9 are transported in the circulation by conventional lipoprotein pathways. After intestinal absorption, they are first associated with VLDL (major amount) and soluble proteins (small amount). They are later incorporated into LDL and subsequently into HDL 2. We also conclude that 22:6n-3 is cleared from the blood more slowly than 18:1n-9, and retinal 22:6n-3 is supplied from circulation through the RPE to the ROS.
Published Version
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