Abstract
Purpose: :
A novel cone cycle has recently been located in the chicken retina. In this retinoid cycle, light induced an increase in 11–cis retinyl ester in the retina, the level of which was rapidly reduced to baseline level after subsequent dark adaptation. The purpose of this study is to show how light intensity and the duration of light exposure affect the accumulation of 11–cis retinyl ester in the chicken retina.
Methods: :
Chickens were dark–adapted overnight before exposure to incandescent light at different intensities: 1,000, 2,000 and 3,000 Lux for 20 min. Additional groups were exposed to 2,000 Lux for 10, 20 and 40 min. before being dark adapted for 20 min. Animals were sampled at regular time intervals and retinal and retinyl esters in the retina were analyzed by HPLC.
Results: :
Dark adapted retinas contain 0.23 nmol of cis retinal/mg retinal proteins (n=4) and 0.24 nmol of cis retinyl esters/mg (n=4). Exposure to 1,000 Lux for 20 min reduced the level of cis retinal to 0.13 nmol/mg (n=4) and increased cis retinyl ester to 0.37 nmol/mg (n=4). Exposure to 2,000 Lux resulted in the reduction of cis retinal to 0.08 nmol/mg (n=4) and an increase in the amount of cis retinyl ester (to 0.75 nmol/mg, n=4). Further increase in light intensity (to 3,000 Lux) did not result in significant reduction of retinal or increase in retinyl ester. After exposure to light (at 2,000 Lux) for 10 min., chicken retina contained 0.35 nmol/mg cis retinyl esters (n=4). In comparison, 20 min. light exposure induced ∼2 times the amount of cis retinyl esters (to 0.75 nmol/mg, n=4). However, light exposure for 40 min. resulted in a much smaller increase in retinyl esters (to 0.85 nmol/mg, n=4). Dark adaptations subsequent to light exposures (for 10, 20 or 40 min.) rapidly returned cis retinyl esters to baseline level (0.2 nmol/mg, n=6).
Conclusions: :
Accumulation and mobilization of cis retinyl esters in the chicken retina are dependent on both light intensity and duration of light exposure. Increase in light intensity bleaches more cone pigments leading to significant increase in the amount of cis retinyl esters in the retina. At one light intensity (2,000 Lux), the increase in the duration of light exposure from 10 to 20 min. resulted in a proportional increase in the amount of retinyl esters. However, an additional increase of only 0.1 nmol/mg was observed when chicken was exposed for an additional 20 min. This suggests that there may be a limit on the size of this retinoid pool in the retina (∼0.9 nmol/mg). The maximum rates of accumulation and mobilization were ∼0.05 nmol/mg/min and ∼0.10 mol/mg/min, respectively.
Keywords: photoreceptors • retina • retinoids/retinoid binding proteins