Transport and phosphorylation of 2-deoxy-D-glucose by amphibian retina. Effects of light and darkness.

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Abstract
We studied the uptake of 2-deoxy-D-glucose (2DG) and the synthesis of its phosphorylated product 2DG-6-phosphate (2DG-6P) by the retinas of the clawed frog (Xenopus laevis) and the bullfrog (Rana catesbeiana). Autoradiographs showed that most of the retinal 2DG uptake is by the photoreceptor layer. The 2DG accumulation by isolated Xenopus retinas was time and concentration dependent. The Kt for transport was 5.05 mM; Vmax was 6.99 X 10(-10) mol . mg-1 tissue wet weight min-1. The Km for 2DG-6P formation was estimated to be 2-3 mM and Vmax to be approximately 4 x 10(-9) mol . mg-1 min-1. 2DG uptake was inhibited competitively by glucose with a Ki of 2.29 mM. Exposure to light reduced 2DG uptake by no more than 10% as compared with dark uptake. Low sodium or ouabain (10(-4)-10(-7) M) treatment did not significantly alter 2DG uptake as compared with control retinas. In experiments upon intact, anesthetized bullfrogs, light reduced both the total amount of radioactivity acquired by the retina and the fraction of 2DG-6P present. The results are discussed in terms of the fraction of energy consumed by the retina required to maintain the photoreceptor dark current.