Adult male unanesthetized rats, reared on a diet enriched in both a-linolenic acid (a-LNA) and docosahexaenoic acid (DHA), were infused intravenously for 5 min with [1-14 C]a-LNA. Timed arterial samples were collected until the animals were killed at 5 min and the brain was removed after microwaving. Plasma and brain lipid concentrations and radioactivities were measured. Within plasma lipids, > 99% of radioactivity was in the form of unchanged [1-14 C]a-LNA.Eighty-six per cent of brain radioactivity at 5 min was present as b-oxidation products, whereas the remainder was mainly in 'stable' phospholipid or triglyceride as a-LNA or DHA.Equations derived from kinetic modeling demonstrated that unesterified unlabeled a-LNA rapidly enters brain from plasma, but that its incorporation into brain phospholipid and triglyceride, as in the form of synthesized DHA, is £ 0.2% of the amount that enters the brain. Thus, in rats fed a diet containing large amounts of both a-LNA and DHA, the a-LNA that enters brain from plasma largely undergoes b-oxidation, and is not an appreciable source of DHA within brain phospholipids.

We quantified incorporation rates of plasmaderived a-linolenic acid (a-LNA, 18:3n-3) into "stable" liver lipids and the conversion rate of a-LNA to docosahexaenoic acid (DHA, 22:6n-3) in male rats fed, after weaning, an n-3 PUFA-adequate diet (4.6% a-LNA, no DHA) or an n-3 PUFAdeficient diet (0.2% a-LNA, no DHA) for 15 weeks. Unanesthetized rats were infused intravenously with [1-14 C]a-LNA, and arterial plasma was sampled until the liver was microwaved at 5 min. Unlabeled a-LNA and DHA concentrations in arterial plasma and liver were reduced .90% by deprivation, whereas unlabeled arachidonic acid (20:4n-6) and docosapentaenoic acid (22:5n-6) concentrations were increased. Deprivation did not change a-LNA incorporation coefficients into stable liver lipids but increased synthesis-incorporation coefficients of DHA from a-LNA by 6.6-, 8.4-, and 2.3-fold in triacylglycerol, phospholipid, and cholesteryl ester, repectively. Assuming that synthesized-incorporated DHA even tually would be secreted within lipoproteins, calculated liver DHA secretion rates equaled 2.19 and 0.82 mmol/day in the n-3 PUFA-adequate and -deprived rats, respectively. These rates exceed the published rates of brain DHA consumption by 6-and 10-fold, respectively, and should be sufficient to maintain normal and reduced brain DHA concentrations, respectively, in the two dietary conditions.-