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R. E. Flynn, III, R. A. Linsenmeier; Modeling ATP and Oxygen Consumption in the Photoreceptors. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1274.
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To create a mathematical model relating photoreceptor ATP and oxygen consumption to the characteristics of the underlying cellular processes that use energy.
ATP consumption (QATP) in the photoreceptors was simulated using a mathematical model derived from known cellular mechanisms. Each mechanism was broken into components to determine the total energy requirement of the process. For example, at steady state the amount of Na+ pumping by a photoreceptor must balance the Na+ current through the outer segment cation channels and the Na+ entering via Na+/Ca2+ and Na+/H- exchange. The equations for these processes under dark and light adapted conditions can be written in terms of parameters such as dark current, Ca2+ concentration, and flash intensity. Values of the parameters can be obtained from in vitro and in vivo measurements in the retina of toads, rats, monkeys, rabbits, and other species. The model calculates the total number of ATP used, and then determines oxygen consumption (QO2) through the stoichiometry of oxidative metabolism.
Na+ pumping and cyclic nucleotide turnover are to be the largest predicted consumers of ATP in the cell. The model predicts that Na+ pumping should use 50 to 60% of the total energy, with 73% of this being due to Na+ entry via light dependent channels. Most of the remaining energy is required for Na+/Ca2+ exchange. This is in agreement with experimental data from rabbit retina obtained by Ames et al. 1992, which indicates that Na pumping uses 50% of the energy in dark adaptation.
Because so much is known about photoreceptor function, it is possible to relate it to the metabolic demand of the photoreceptor in a model that can now be used to predict ATP utilization and oxygen consumption under a variety of different conditions. We have accurately modeled the Na+ pumping mechanism of the photoreceptor and determined the amount of QO2 required by this process.
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