Glycolytic flux in muscle declines rapidly after exercise stops, indicating that muscle activation is a key controller of glycolysis. The mechanism underlying this control could be 1) a Ca(2+)-mediated modulation of glycogenolysis, which supplies substrate (hexose phosphates, HP) to the glycolytic pathway, or 2) a direct effect on glycolytic enzymes. To distinguish between these possibilities, HP levels were raised by voluntary 1-Hz exercise, and glycolytic flux was measured after the exercise ceased. Glycolytic H(+) and ATP production were quantified from changes in muscle pH, phosphocreatine concentration, and P(i) concentration as measured by 31P magnetic resonance spectroscopy. Substrate (HP) and metabolite (P(i), ADP, and AMP) levels remained high when exercise stopped because of the occlusion of blood flow with a pressure cuff. Glycolytic flux declined to basal levels within approximately 20 s of the end of exercise despite elevated levels of HP and metabolites. Therefore, this flux does not subside because of insufficient HP substrate; rather, glycolysis is controlled independently of glycogenolytic HP production. We conclude that the inactivation of glycolysis after exercise reflects the cessation of contractile activity and is mediated within the glycolytic pathway rather than via the control of glycogen breakdown.

Why does the onset of glycolytic flux in muscle lag the start of exercise? We tested the hypothesis that both elevated metabolite levels and muscle activity are required for flux to begin. Glycolytic flux was determined from changes in muscle pH, phosphocreatine concentration, and P(i) concentration ([P(i)]) as measured by 31P magnetic resonance spectroscopy. Eight subjects performed rapid ankle dorsiflexions to approximately 45% of maximal voluntary contraction force under ischemia at a rate of 1 contraction/s. Subjects completed two bouts of exercise separated by 1 min of ischemic rest. Glycolytic flux was activated by 27 s in the first bout, ceased during the ischemic rest period, and was activated more quickly in the second bout. Because the onset in both bouts occurred at approximately the same [P(i)], ADP concentration, and AMP concentration, the activation of glycolysis appears to be related to the elevation of these metabolite concentrations. However, because no glycolytic flux occurred at rest, even when metabolite levels were high, both muscle activity and elevated metabolites are needed to turn on this pathway. We conclude that the delayed onset of glycolytic flux during exercise reflects the time needed to raise metabolites to flux-activating levels.

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...