Relations Between Intracellular Ions and Energy Metabolism Under Acidotic Conditions: A Study with Nigericin in Synaptosomes, Neurons, and C6 Glioma Cells

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Abstract
Effects of nigericin were investigated in rat brain synaptosomes, cultured neurons, and C6 glioma cells to characterize the relations among ATP synthesis, [Na+]i., [K+]i, and [Ca2+]i, and pH under conditions when [H+]i is substantially increased and transmembrane electrical potential is decreased. Intracellular acidification and loss of K+ were accompanied by enhanced oxygen consumption and lactate production and a decrease in cellular energy level. Changes in the last three parameters were attenuated by addition of 1 mM ouabain. In synaptosomes treated with nigericin, neither respiration nor glycolysis was affected by 0.3 μM tetrodotoxin, whereas 1 mM amiloride reduced lactate production by 20% but did not influence respiration. In C6 cells, amiloride decreased the nigericin‐stimulated rate of lactate generation by about 50%. The enhancement by nigericin of synaptosomal oxygen uptake and glycolytic rate decreased with time. However, there was only a small reduction in respiration and none in glycolysis in C6 cells. Measurements with ion‐selective microelectrodes in neurons and C6 cells showed that nigericin also caused a rise in [Ca2+], and [Na+]., The increase in [Na+], in C6 cells was partially reversed by 1 mM amiloride. It is concluded that nigericin‐induced loss of K+ and subsequent depolarization lead to an increase in Na+ influx and stimulation of the Na+/K+ pump with a consequent rise in energy utilization; that acidosis inhibits mitochondrial ATP production; that a rise in [H+] does not decrease glycolytic rate when the energy state (a fall in [ATP] and rises in [ADP] and [AMP]) is simultaneously reduced; that a fall in [K+], depresses both oxidative phosphorylation and glycolysis; and that the nigericin‐induced alterations in ion levels and activities of energy‐producing pathways can explain some of the deleterious effects of ischemia and hypoxia.