New isolation method of retina and interphotoreceptor matrix

The hepatic retinyl ester hydrolase activity is depressed at the onset of diabetes in BB rats

Dietary vitamin A as retinyl ester is hydrolysed and re-esterified with long-chain fatty acids in the small intestine. The esterified vitamin A is subsequently stored in the liver, where it is hydrolysed to free retinol to be transported by carrier proteins to the target tissue. A decreased availability of retinol carrier proteins has been suggested to be responsible for affecting metabolic availability of vitamin A in type 1 diabetes. Using BB Wistar rats, the present study was undertaken to examine whether the presence of a hyperglycaemic state modifies retinyl ester hydrolase (REH) activity in the intestine and the liver. At the onset of diabetes, hepatic REH enzymatic activity was significantly decreased. However, REH activity remained unaffected in the small intestine, including both ileum and jejunum. Diabetes also resulted in decreased plasma and liver concentrations of retinol. An in vitro study was conducted to examine the effect of diabetes on the intestinal uptake of retinyl palmitate. Jejunum and ileum from diabetic and non-diabetic BB rats were incubated with labelled retinyl palmitate at different concentrations ranging from 32 to 256 nmol/l. The uptake of retinyl palmitate was increased in both diabetic and non-diabetic rats together with the increase of substrate concentration. However, no significant difference was observed in the uptake of retinyl palmitate between diabetic and non-diabetic rats. These present results suggest that the depressed hepatic REH activities may contribute to the diabetes-associated metabolic derangement of vitamin A.

Dietary supplementation of N-3 fatty acids and hydroperoxide levels in rat retinas

Docosahexaenoic acid (DHA) plays an important role in visual and neural development in mammals. In the present study, effect of dietary supplementation with n-3 fatty acids, primarily docosahexaenoic acid (DHA) with high purity, on the fatty acid composition of photoreceptor cells of young rats (fed from 4 weeks) was investigated. DHA in rod outer segment (ROS) membranes was significantly increased in the group of high DHA feeding (9.69% total energy). Other n-3 fatty acids (alpha-linolenic acid (ALA) and eicosapentaenoic acid (EPA)) included in the diets with DHA (0.95%-5.63% total energy) also significantly increased the proportion of DHA compared with the linoleic acid diet groups. However, the proportions of arachidonic acid (ARA) and other long chain n-6 fatty acids (22:4n6 and 22:5n6) were suppressed in these n-3 fatty acids-fed groups. Phospholipid hydroperoxides in ROS membranes were determined using a highly sensitive analytical technique, chemiluminescence-high performance liquid chromatography (CL-HPLC). There was no increasing tendency in the hydroperoxide levels of ROS membranes containing high content of DHA, and phosphatidylethanolamine hydroperoxide (PEOOH) was much lower than phosphatidylcholine hydroperoxide (PCOOH) under normal light conditions, which implies that DHA supplementation does not much affect the peroxidizability of ROS membranes in vivo. But UV irradiation on separated ROS membranes accelerated the formation of phospholipid hydroperoxides in high DHA feeding rats, and PEOOH was produced more efficiently than PCOOH in vitro.

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 membrane long-chain and very-long-chain fatty acid composition, rhodopsin content, and rhodopsin kinetics in developing rat photoreceptor cells. The dietary fats were formulated based on the fat composition of a conventional infant formula providing an 18:2n-6/18:3n-3 ratio of 7:1 (SMA, Wyeth Nutritionals), which served as the control fat blend. This dietary fat blend was modified 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). Dams 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 from pups at 2, 3, and 6 wk of age for fatty acid analysis of individual phospholipids, rhodopsin content, and rhodopsin disappearance kinetics after light exposure. Feeding AA + DHA in the diet increased 22:6n-3 levels in phosphatidylcholine 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 rats fed diets containing AA and/or DHA. The kinetics of rhodopsin disappearance after light exposure was highest in rats fed DHA 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 membrane lipid and visual pigment content and kinetics in the developing photoreceptor cell.

Streptozotocin-induced diabetes in rats is associated with impaired metabolic availability of vitamin A (retinol)

Using streptozotocin-induced diabetic Wistar rats, studies were carried out to examine the metabolic availability of vitamin A in the plasma, liver and the retina of the eye. Control and diabetic rats were fed ad lib. on a semi-purified diet either with or without (basal) vitamin A supplementation, or pair-fed on the basal diet for 4 weeks. Despite the fact that diabetic rats consumed 48% more feed, they had lower plasma concentrations of retinol (P < 0.003). The decrease in plasma retinol concentration was a response to diabetes (or diabetes-induced trauma), since neither pair-feeding (P < 0.01) nor vitamin A supplementation altered this effect (P < 0.05). Furthermore, the hepatic concentrations of the vitamin in these animals remained elevated and this increase was greater in the supplemented diabetic group (P < 0.001). Decreases in 11-cis retinal (a component of rhodopsin) concentrations in the retina were also observed in diabetic animals. The increased hepatic and the decreased plasma and retina vitamin A levels suggest a defect in the transport of the vitamin from the liver.

Relationship between dietary supply of long-chain fatty acids and membrane composition of long- and very long chain essential fatty acids in developing rat photoreceptors

The present study was designed to determine if dietary supply of long-chain fatty acid (LCFA, C20:4n-6, and/or C22:6n-3), reflecting levels that might be incorporated into infant formulas, influences the fatty acid composition of the visual cell membrane. The rod outer segment (ROS) of the retina was analyzed from rats fed diets varying in the ratio of 18:2n-6 to 18:3n-3 with or without 20:4n-6 [arachidonic acid (AA)] and 22:6n-3 (docosahexaenoic acid) from birth to six weeks of age. The level of very long chain fatty acids (VLCFA, C24-C36) was identified using gas chromatography and gas chromatography-mass spectrometry. In the ROS, the highest relative percent of AA was attained in phosphatidylcholine (PC) and phosphatidylethanolamine (PE) of animals fed 1% AA diet, whereas feeding 0.7% docosahexaenoic acid (DHA) diet significantly increased the DHA level in PC, phosphatidylserine, and phosphatidylinositol compared to feeding diets containing AA. VLCFA of n-6 and n-3 up to C36 were found in PC, with the most abundant fatty acids being C32 and C34. In PC, phosphatidylserine and PE, the n-6 tetraenoic VLCFA level was highly increased in animals fed 1% AA compared to other dietary groups. This study suggests that dietary fat containing small amounts of AA or DHA is an important factor influencing membrane fatty acid composition of the visual cell during development.

Dietary fat alters membrane composition in rod outer segments in normal and diabetic rats: impact on content of very-long-chain (C > or = 24) polyenoic fatty acids

The effect of high n - 3 (5.8%, w/w) vs. a low n - 3 (1.2%, w/w) fatty acids in a diet with a low ratio of polyunsaturated/saturated fatty acids (P/S = 0.27) content was investigated to determine the effect of diet on the level of long- and very-long-chain fatty acids (VLCFA C > or = 24) in phospholipids of rod outer segments (ROS) of normal and diabetic rats. After 6 weeks of feeding, diets high in n - 3 fatty acids increased the levels of 22:5(n - 3) and 22:6(n - 3), while decreasing the 22:5(n - 6) level in all major phospholipid classes. n - 6 and n - 3 VLCFA of C24 to C34 with 4, 5 and 6 double bonds were found only in phosphatidylcholine (PC) while other phospholipid classes contained only C24 fatty acids as minor components. The content of VLCFA in PC was approx. 6.7% (w/w) of total fatty acids in the ROS. Feeding a high n - 3 fatty acid diet significantly reduced n - 6 tetraenoic VLCFA in all phospholipids. In the diabetic state, the levels of n - 6 tetraenes and pentaenes in individual phospholipids were different from control animals. This study demonstrates that the VLCFA content of photoreceptor cells reflects the dietary level of n - 3 fatty acids fed. The unique polyenoic n - 6 and n - 3 VLCFA appear to be synthesized from shorter chain precursors which respond to altering the ratio of n - 6/n - 3 fatty acids fed.