Rod outer segment lipids in vitamin A-adequate and -deficient rats

Weanling albino rats were fed a vitamin-A-adequate diet or vitamin-A-deficient diet and maintained in a cyclic light or dark environment for up to 14 weeks. One half of the rats were supplemented with additional dietary linolenic acid in the form of linseed oil. The lipid composition and rhodopsin-opsin contents of isolated rod outer segments were determined after 6-7 weeks or 12-14 weeks on diet. This study shows that feeding rats a standard vitamin A-adequate or -deficient diet results in an age-dependent loss of omega three docosahexaenoic acid and a concomitant increase in omega six docosapentanoic acid in the rod outer segments. The loss of docosahexaenoate appears to be caused by insufficient dietary omega three fatty acids. The increase in omega six docosapentanoic acid appears to arise from the high concentration of linoleic acid in standard diets containing either cottonseed, or peanut oil or supplemental corn oil. Feeding rats diets supplemented with linseed oil, however, results in a rod outer-segment lipid profile which is the same as for chow-fed animals. The same effects were seen in the fatty-acid profile of lipids from liver, although the content of polyunsaturates was much lower than in rod outer segments. Vitamin A deficiency, itself, does not lead to changes in the fatty-acid composition of either the rod outer segments or liver. After 6-7 weeks on A+ or A- diet, rhodopsin levels were, as expected, higher in dark-reared rats than in cyclic-light animals. Although the rhodopsin levels in dark-reared vitamin A-adequate rats were significantly higher than in vitamin A-deficient animals, measurements of the lipid to opsin ratio of rod outer segments indicate that the rods of vitamin A-deficient rats are not markedly different than those of vitamin A-adequate rats. It is concluded that these diets may be useful in providing a means for evaluating the role of docosahexaenoic acid in visual cell death from damaging light.