Dietary 20 : 4n-6 and 22 : 6n-3 modulates the profile of long- and very-long-chain fatty acids, rhodopsin content, and kinetics in developing photoreceptor cells

The objective of this study was to determine whether addition of dietary 20 :4n-6 and 22:6n-3 to a conventional infant formula fat blend influences mem brane long-chain and very-long-chain fatty acid composition, rhodopsin cont ent, and rhodopsin kinetics in developing rat photoreceptor cells. The diet ary fats were formulated based on the fat composition of a conventional inf ant formula providing an 18:2n-6/18:3n-3 ratio of 7:1 (SMA, Wyeth Nutrition als), which served as the control fat blend. This dietary fat blend was mod ified to contain 20:4n-6 [arachidonic acid (AA)], 22:6n-3 [docosahexaenoic acid (DHA)], AA + DHA, or an 18:2n-6/18:3n-3 ratio of 4:1 (alpha-linolenic acid). Darns were fed diets from birth, and rat pups were fed the same diet after weaning. Retinas and rod outer segments were prepared in the dark fr om pups at 2, 3, and 6 wk of age for fatty acid analysis of individual phos pholipids, rhodopsin content, and rhodopsin disappearance kinetics after li ght exposure. Feeding AA + DHA in the diet increased 22:6n-3 levels in phos phatidylcholine and phosphatidylethanolamine. In phosphatidylcholine, total n-6 tetraenoic very-long-chain fatty acids and total n-3 pentaenoic and n- 3 hexaenoic very-long-chain fatty acids increased after feeding AA and DHA, respectively. Developmental changes were characterized by a decrease in 20 :4n-6 in the major phospholipids, whereas 22:6n-3 increased with age in rod outer segments. The highest rhodopsin content occurred in the retina of ra ts fed diets containing AA and/or DHA, The kinetics of rhodopsin disappeara nce after light exposure was highest in rats fed DIIA at 6 wk of age. This study demonstrates that small manipulations of the dietary level of 20:4n-6 and 22:6n-3 are important determinants of fatty acid composition of membra ne lipid and visual pigment content and kinetics in the developing photorec eptor cell.