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E. Rungger-Brandle, P.S. Senanayake, A.A. Dosso, J.G. Hollyfield; Glycogen in RPE and Choroid of the Diabetic Rat . Invest. Ophthalmol. Vis. Sci. 2003;44(13):395.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To evaluate steady state levels and storage of glycogen in the RPE-choroid complex after streptozotocin injection. Methods: The RPE-choroid complex was isolated from pigmented Long Evans rats at one, 4, and 12 weeks after induction of diabetes, digested with proteinase K, and twice ethanol precipitated. Supernatant and precipitate fractions were derivatized with the fluorescent tag 2-aminoacridone (AMAC), either directly to identify endogenous saccharides such as free glucose with free aldehyde or, after digestion with glucoamylase to determine total glycogen. After separation by electrophoresis, the bands were digitized and their intensities quantified. For EM histochemical demonstration of particulate glycogen (glycosomes), thin sections were stained by the periodic acid-thiocarbohydrazide-silver proteinate method. Results: The levels of glucose, total glycogen and the ratio of glycogen to glucose in the fasting state were similar in the two groups, 4 weeks after the induction of diabetes. In the fed state, glucose and total glycogen were increased in diabetic rats but the ratio of glycogen to glucose was similar to the control. At 12 weeks postinjection, levels of glucose and total glycogen in the fed state were higher in the diabetic rats but the ratio of glycogen to glucose was lower than in the controls. Glycosomes were present in choroidal fibroblasts and smooth muscle cells in the controls and significantly enriched in the diabetic tissue as soon as one week postinjection. By contrast, virtually no glycosomes were detectable in the RPE and the endothelium of the choriocapillaris. Conclusions: Our data document the progression from normal to altered glucose and total glycogen levels during the development of diabetes. The decreased ratio of glycogen to glucose at later time points of diabetes suggests that the glycogen stock is not replenished. The fact that almost no glycosomes can be detected in the RPE of hyperglycemic rats may be due to rapid glycogen turnover and minimal storage. Moreover, our observations hint to cell type-specific stability of glycogen, storage being stable in choroidal fibroblasts and smooth muscle cells and highly labile in RPE and endothelium.
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