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B.S. Winkler, C.A. Starnes, Z. Firouzgan; Energy Requirements, Ion Transport and Metabolic Overload in Retinal Cells . Invest. Ophthalmol. Vis. Sci. 2004;45(13):3279.
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Purpose:To examine the balance between energy supply and demand in relation to activation of ion transport in normal and mitochondrial–inhibited retinal cells. Methods: Cultured retinal cells, including transformed Müller cells (rMC–1), photoreceptor cells (661W) and ganglion cells (RGC–5) and passaged human RPE cells were incubated in oxygenated culture media in the presence and absence of Antimycin A, an inhibitor of mitochondrial electron transport, and in the presence and absence of 0.01 mM monensin, an antibiotic that enhances sodium entry resulting in stimulation of the activity of the ATP–dependent sodium–potassium pump. Measurements were made of the rate of lactic acid production, ATP content and morphology. Results: Under the control incubation condition, each retinal cell type produced lactate aerobically, maintained ATP content and had well preserved structure over many hrs. When the cells were incubated in media containing monensin, aerobic lactic acid production increased 2–3 fold (depending on cell type) and ATP content and structure were similar to the control condition. When cells were incubated with Antimycin A, lactic acid production increased 2–3 fold (Pasteur Effect) and ATP content and morphology were well preserved. Interestingly, addition of monensin to Antimycin A–containing media did not increase the rate of lactic acid production in the cells beyond that found with Antimycin A alone. However, there was a substantial decline in ATP content and signs of cell death began to appear over time, as demand for ATP exceeded its supply from glycolysis. Conclusions:These results have significant implications for understanding metabolic compensatory responses in retinal cells exposed to stress. Under normal conditions (mitochondria functional), a monensin–induced increase in sodium entry and pump activity signals an increased need for energy production which is met by a substantial stimulation in the rate of aerobic glycolysis (and likely respiration, too). This homeostatic mechanism preserves ATP content and morphology. However, when mitochondria are inhibited with Antimycin A an increase in sodium entry does not result in a further increase in energy production via glycolysis, since this pathway is already at its maximum. Thus, the extent to which retinal cells tolerate an impairment in mitochondrial function depends on the status of sodium transport and by extension to all signals which influence the rate of entry of this ion. Metabolic overload is relevant to models of ischemia and retinal degeneration, e.g., rd mice, where mitochondrial swelling and enhanced sodium influx are early signs of damaging reactions.
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