Nutritional deprivation of alpha-linolenic acid decreases but does not abolish turnover and availability of unacylated docosahexaenoic acid and docosahexaenoyl-CoA in rat brain

We applied our in vivo fatty acid method to examine concentrations, incorpo ration, and turnover rates of docosahexaenoic acid (22:6 n-3) in brains of rats subject to a dietary deficiency of a-linolenic acid (18:3 n-3) for thr ee generations. Adult deficient and adequate rats of the F3 generation were infused intravenously with [4,5-H-3]docosahexaenoic acid over 5 min, after which brain uptake and distribution of tracer were measured. Before infusi on, the plasma 22:6 n-3 level was 0.2 nmol ml(-1) in 18:3 n3-deficient comp ared with 10.6 nmol ml(-1) in control rats. Brain unesterified 22:6 n-3 was not detectable, whereas docosahexaenoyl-CoA content was reduced by 95%, an d 22:6 n-3 content in different phospholipid classes was reduced by 83-88% in deficient rats. Neither plasma or brain arachidonic acid (20:4 n-6) leve l was significantly changed with diet. Docosapentaenoic acid (22:5 n-6) rec iprocally replaced 22:6 n-3 in brain phospholipids. Calculations using oper ational equations from our model indicated that 22:6 n-3 incorporation from plasma into brain was reduced 40-fold by 18:3 n-3 deficiency. Recycling of 22:6 n-3 due to deacylation-reacylation within phospholipids was reduced b y 30-70% with the deficient diet, but animals nevertheless continued to pro duce 22:6 n-3 and docosahexaenoyl-CoA for brain function. We propose that f unctional brain effects of n-3 deficiency reflect altered ratios of n-6 to n-3 fatty acids.