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A. Tsin, E.T. Villazana–Espinoza, A. Muniz, D.M. Allen; Retinoid Cycle in the Isolated Chicken Retina . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1061.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:Retinas in cone–dominated species (chicken/ground squirrel), in comparison to retinas in rod–dominated species (bovine/human), store a large amount of 11–cis retinyl esters (11–cis RE; 10 molar equivalents) but the functional role of this RE pool is not known. Our ARVO 2003 report showed that an active retinoid cycle exists in intact retina (i.e. retina including the underlying retinal pigment epithelium [RPE], choroid and sclera) of chickens subjected to light/dark exposure. In the present study, we conducted additional experiments to show that similar retinoid cycle also exists in isolated chicken retina (i.e. retinas separated from underlying RPE, choroid and sclera) subjected to light/dark exposure. Methods:Retinas were dissected from chicken eyes after 2 hr dark adaptation. They were placed in aerated avian buffer in test tubes and subjected for 1 hr (at 4 oC) to illumination for an exhaustive bleach before 1 hr of dark adaptation. At 30 min. intervals, tissues were homogenized, extracted, saponified, and analyzed for retinol by HPLC. Retinoid levels were determined by previously established calibration standards. Previous studies established that chicken retinas have minimal amounts of free retinols under all light/dark conditions. Results:Upon illumination, the level of 11–cis RE in the isolated chicken retina increased from 0.44 to 1.44 nmol/retina within 1.5 hr from the onset of light adaptation. Upon dark adaptation, levels of 11–cis RE in the isolated retina returned to 0.54 nmol/retina. Additionally, all–trans RE (AT RE) in the isolated retina also increased from 0.20 to 0.77 nmol/retina within 1 hr of light exposure. Upon dark adaptation, the level of AT RE returned to 0.4 nmol/retina. In comparison, 11–cis RE in the intact retinas of chicken subjected to similar light/dark adaptations had an increase of 11–cis RE from 0.57 nmol/retina to 5.30 nmol/retina. Upon dark adaptation for 1 hr, the level of 11–cis RE was returned to baseline, 0.6 nmol/retina. The level of AT RE increased from 0.32 to 0.93 nmol/retina upon light adaptation and returned to 0.32 nmol/retina upon dark adaptation. Conclusions:Isolated chicken retinas responded to light illumination by rapid accumulation of 11–cis RE, followed by a depletion of 11–cis RE upon dark adaptation. Significantly, the retinoid cycle in the isolated retina is similar to that observed in the intact retina. The lower level of 11–cis RE accumulation in the isolated retina can be attributed to impaired visual cycle enzyme activities (dehydrogenase, isomerase and esterifying enzyme) due to ambient conditions such as buffer and low temperature (4 oC).
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