Effects of a vitamin-A-free diet on tissue vitamin A concentration and dark adaptation of aging rats.

Changes in serum, hepatic, and ocular vitamin A levels were followed in 9-month-old Long Evans rats fed either a vitamin-A-free or vitamin-A-sufficient diet for up to 18 months. After 18 months, hepatic total vitamin A fell exponentially from 2542 +/- 433 to 48 +/- 12 micrograms/g wet weight, but these stores maintained normal serum retinol and ocular total vitamin A concentrations in vitamin-A-deprived rats. However, retinal dark-adaptation time, a functional indicator of photoreceptor integrity, was prolonged in elderly vitamin-A-deprived rats as compared to vitamin-A-sufficient controls (p less than 0.05). Aging appeared to be the most important factor that depressed the rate of dark adaptation, with vitamin A status having a secondary effect. The proportion of total ocular retinaldehyde decreased (p less than 0.05) and the concentration of retinyl ester increased (p less than 0.05) in the eyes of 27-month-old vitamin-A-deprived rats compared to age-matched vitamin-A-sufficient animals. Possible mechanisms of visual dysfunction include abnormal retinal rhodopsin recycling caused by a dependence on newly absorbed dietary vitamin A (e.g., chylomicron). Alternatively, in the vitamin-A-deprived rat, decreased hepatic oxidation of pentobarbital (used for electroretinographic anesthesia) could prolong retinal anesthetic exposure and either directly depress neural transmission or indirectly alter transmission by affecting rhodopsin recycling.