From the Cover: Shaking up glycolysis: Sustained, high lactate flux during aerobic rattling

Abstract

Substantial ATP supply by glycolysis is thought to reflect cellular anoxia in vertebrate muscle. An alternative hypothesis is that the lactate generated during contraction reflects sustained glycolytic ATP supply under well-oxygenated conditions. We distinguished these hypotheses by comparing intracellular glycolysis during anoxia to lactate efflux from muscle during sustained, aerobic contractions. We examined the tailshaker muscle of the rattlesnake because of its uniform cell properties, exclusive blood circulation, and ability to sustain rattling for prolonged periods. Here we show that glycolysis is independent of the O 2 level and supplies one-third of the high ATP demands of sustained tailshaking. Fatigue is avoided by rapid H ؉ and lactate efflux resulting from blood flow rates that are among the highest reported for vertebrate muscle. These results reject the hypothesis that glycolysis necessarily reflects cellular anoxia. Instead, they demonstrate that glycolysis can provide a high and sustainable supply of ATP along with oxidative phosphorylation without muscle fatigue.31 P magnetic resonance spectroscopy ͉ high-energy phosphates ͉ rattlesnake L actate generation by tissues is often taken as a sign of tissue hypoxia (1). This thinking has led to the anaerobic threshold hypothesis, which links lactate generation in exercising muscle to an intracellular O 2 limitation to respiration of pyruvate (2). However, many tissues seem to generate lactate under aerobic conditions (so-called aerobic glycolysis) in a process linking glycolytic ATP supply to ion transport (3-5). For example, lactate generation by muscles exercising at rates well below their aerobic maximum (6) supports this alternative explanation. Muscle PO 2 seems to be well above limits to mitochondrial respiration in these muscles as indicated by indirect measures of tissue oxygenation involving myoglobin saturation (6, 7). This finding questions an O 2 limit to respiration as the basis for lactate generation in exercising muscle. However, these indirect measures do not have the spatial resolution to determine intracellular PO 2 to rule out local tissue anoxia (8).A definitive test of whether an O 2 limitation is responsible for lactate generation during sustained contractions is possible by using a direct comparison of glycolysis during anoxic and sustained aerobic contractions. New magnetic resonance (MR) techniques for partitioning intracellular ATP supply in vivo permitted this comparison in human muscles and were used to show that glycolytic flux is independent of O 2 state (9). The similarities of flux under anoxia and aerobic conditions in that study indicated that glycolysis is not mutually exclusive with oxidative phosphorylation. This glycolytic flux should generate a high lactate efflux into the blood during aerobic contractions and serves as a test for MR results. The rattlesnake tailshaker muscles are well suited for a direct comparison of intracellular glycolysis to lactate efflux. First, the uniform muscle fiber properties (10, 11...

Evaluation of in vivo mitochondrial bioenergetics in skeletal muscle using NMR and optical methods

“…Therefore, this approach quantifies not only mitochondrial ATP production, but also resting ATP demand of the muscle, because under steady-state conditions ATP supply must equal demand. Comparison of non-invasive 31 P spectroscopy with traditional arterial-venous oxygen differences across the muscle using Fick’s Law supports the use of this approach to measure ATP demand in skeletal muscle[19, 24]. …”

Limits of aerobic metabolism in cancer cells

“…The postulation of a maximum oxygen consumption rate results in model predictions mimicking aerobic glycolysis in yeast 17 and the Warburg effect (unpublished data). However, under aerobic conditions, further increase of the oxygenation level does not abrogate aerobic glycolysis 16 , 18 . Simulations of genome scale models of mammalian cell metabolism also predict the utilization of glutamine at high proliferation rates 19 , 20 .…”

“…However, cells from healthy mammalian tissues also manifest aerobic glycolysis and glutaminolysis during fast growth 12 – 15 . Furthermore, aerobic glycolysis is observed with concomitant high rates of respiratory metabolism in cancer 15 and muscle 16 cells.…”

Mathematical models of cancer metabolism

“…The observed saturation in the oxygen consumption rate could be due to a limitation in the oxygen supply or a limitation in the oxidative phosphorylation capacity. Work in the context of muscle cell metabolism has shown that a further increase in the oxygen supply during aerobic conditions does not alter the respiration and aerobic glycolysis rates [ 31 , 32 ], ruling out a limitation in the oxygen supply. Regarding the other alternative, a limitation in the oxidative phosphorylation capacity, cells could in principle increase their mitochondrial content to satisfy their higher energy demands.…”