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A. O. Chertov, J. B. Hurley; Energy Production in Photoreceptors, Is It a Question of Supply or Demand?. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1659.
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The architecture of photoreceptors makes it so that the site of energy production and the site of its consumption are separated. Such a cellular layout requires a mechanism to relate the needs of the consumers with the output levels of the producers. The distances between sites of ATP production and expenditure also makes it unlikely that it is simply the matter of elevated ADP levels during periods of high ATP consumption leading to faster rates of oxidative phosphorylation. It is our goal to shed light on this dilemma and discover the mechanism that relates energy production with utilization in photoreceptors. In order to accomplish this goal we plan to evaluate energy production and consumption independently under conditions of high and low energy use and try to find a signal that ties them together.
Mice were dark adapted over night and their eyes removed in the dark utilizing night vision goggles. The retinas were isolated from the enucleated eyes and submerged in oxygenated physiologic solution containing glucose. The retinas were then exposed to conditions meant to change the energy consumption rate and then homogenized in perchloric acid. Energy production in the retina was inhibited by the addition of iodoacetic acid, sodium cyanide and oligomycin. Ion exchange HPLC was used to evaluate energetic nucleotide levels along with creatine phosphate in the retinal extracts as a means to evaluate cellular energy levels.
Preliminary data indicate that changes in energy production can be enacted independently of changes in energy consumption. The major environmental change that photoreceptors experience is the appearance of light, so we looked for secondary messengers that undergo changes in concentration in response to light. Calcium's known effects on phototransduction, glycolysis and oxidative phosphorylation made it a great candidate. By removing the extra-cellular Ca2+ we induced a decrease in the ATP/ADP without a corresponding increase in the ATP consumption rate. A similar observation was made when dark adapted retinas were exposed to light. The ATP/ADP ratio decreased with the appearance of light instead of increasing initially and then decreasing with time as the mitochondria begin to sense the lack of ADP.
These studies indicate that photoreceptors can change their rate of energy production to anticipate changes in energy demand. Our work so far suggests that Ca2+ might be play an important role in this process.
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