Energy Metabolism in the Cone Dominant Ground Squirrel Retina

Abstract

Purpose: : There is essentially no information on the pathways of energy metabolism in cone–dominant retinas. Our purpose was to provide quantitative information on the rates of glycolysis and mitochondrial glucose oxidation in dark– and light–adapted cone–dominant ground squirrel retinas.

Methods: : Ground squirrel eyecups (retina/choroid/sclera) were incubated in bicarbonate–buffered, oxygenated media of normal ionic composition in the presence of ¹⁴C–glucose (10 mM) and the production of ¹⁴CO₂ was measured. Measurements were also made of lactic acid production in the presence and absence of oxygen. Effects of light and dark, inhibiting the Na–K ATPase with ouabain, and blocking glutamatergic synaptic transmission (CNQX, MK–801, APB) on metabolic activities were examined. NMR was used to track metabolites generated from ¹³C–glucose.

Results: : Dark–adapted ground squirrel eyecups produced lactate in the presence of oxygen (aerobic glycolysis) at an averaged rate of 4.5 µmoles/hr (n=6). Light–adapted eyecups produced lactate at an averaged rate of 3.7 µmoles/hr (n=8), or about 20% less than the rate in darkness. In the presence of nitrogen, the rate of lactate production increased by 50% in the dark and light (n=6). The rate of aerobic glycolysis in darkness was decreased 25% by ouabain (n=4) but was unchanged following blockade of glutamate receptors (n=3). The rate of mitochondrial glucose oxidation in dark–adapted squirrel eyecups amounted to 0.10 µmoles/hr (n=5), while in the light this rate was 0.13 µmoles/hr (n=5). Ouabain caused a 70% decrease in mitochondrial glucose oxidation in the light (n=4) and dark (n=4), while blockade of glutamate receptors in dark–adapted squirrel eyecups had little effect on glucose oxidation. The major product formed from ¹³C–glucose in ground squirrel retinas was lactate. Other products, at much smaller amounts, included glutamate, aspartate, glutamine, alanine and GABA.

Conclusions: : Lactate, not CO₂, is quantitatively the major product of the in vitro aerobic metabolism of glucose in the cone–dominant ground squirrel retina, accounting for approx. 85% of glucose utilization. There are only small differences in the rates of glycolysis and mitochondrial glucose oxidation in dark– and light–adapted squirrel retinas. Activity of the sodium pump is linked more closely to ATP generated by mitochondria than to ATP generated by glycolysis in the cone–dominant ground squirrel retina.

Keywords: metabolism • retina • mitochondria