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Nuclear Magnetic Resonance and Biochemical Measurements of Glucose Utilization in the Cone-Dominant Ground Squirrel Retina

¹From the Eye Research Institute and the ²Department of Chemistry, Oakland University, Rochester, Michigan.

PURPOSE. To provide quantitative information on glucose utilization in cone-dominant ground squirrel retinas.

METHODS. Ground squirrel eyecups were incubated in medium containing ¹⁴C-glucose, and the production of ¹⁴CO₂ was measured. Measurements were also made of lactic acid production (glycolysis). Nuclear magnetic resonance (NMR) was used to track metabolites generated from ¹³C-1 glucose.

RESULTS. Ground squirrel eyecups produced lactate at a high rate and exhibited normal histology. Light-adaptation reduced glycolysis by 20%. Ouabain decreased glycolysis by 25% and decreased ¹⁴CO₂ production by 60%. Blockade of glutamate receptors had little effect on the glycolysis and ¹⁴CO₂ produced. When metabolic responses were restricted to photoreceptors, light caused a 33% decrease in ¹⁴CO₂ production. The rate of ¹⁴CO₂ production was less than 10% of lactate production. Lactate was the major product formed from ¹³C-glucose. Other ¹³C-labeled compounds included glutamate, aspartate, glutamine, alanine, taurine, and GABA. Lactate was the only product detected in the medium bathing the ground squirrel retinas. The rod-dominant rat retina exhibited a similar pattern of metabolites formed from glucose.

CONCLUSIONS. Lactate, not CO₂, is the major product of glucose metabolism in both ground squirrel and rat retinas. Active Na⁺ transport, however, depends more on ATP produced by mitochondria than by glycolysis. A relatively high fraction of ATP production from glycolysis and glucose oxidation continues in the absence of active Na⁺ pumping and glutamatergic transmission. Major neurotransmitters are synthesized from the aerobic metabolism of glucose; anoxia-induced impairment in retinal synaptic transmission may be due to depletion of neurotransmitters.

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