Limits to sustainable muscle performance: interaction between glycolysis and oxidative phosphorylation
1 Department of Radiology,
2 Department of Physiology and Biophysics and
3 Department of Bioengineering, University of Washington Medical Center, Seattle, WA 98195-7115, USA
*Address for correspondence: Department of Radiology, Box 357115, University of Washington Medical Center, Seattle, WA 98195-7115, USA (e-mail: kconley{at}u.washington.edu)
Accepted July 2, 2001
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Summary |
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Key words: 31P magnetic resonance spectroscopy, muscle, energetics, human muscle, rattlesnake.
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Sustainable versus maximal performance |
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Sustained power output beyond a few minutes is characterized by a constant muscle force production and a continuous ATP supply, which both reflect the effects of mitochondrial oxidative phosphorylation. Recycling of the products of PCr breakdown and oxidation of the glycolytic products by the mitochondria ensures that the build-up of metabolites stabilizes at a level that permits sustained muscle force production. Mitochondrial oxidative phosphorylation also provides the longer-term supply of ATP needed to sustain muscle force production, as shown in the upper right diagram in Fig.1. This dual role of the mitochondria shown in Fig.1 illustrates our first and second contentions that sustainable power output is determined at the muscle level and that this power output is determined by ATP supply.
The simple scenario depicted in Fig.1 (upper right) needs to be modified by two experimental findings. The first finding is that mitochondrial function alone does not determine the level of sustained performance, since sustained respiration is typically well below the maximal rate of oxygen uptake (Hammond and Diamond, 1997; Peterson et al., 1990). Sustainable muscle performance is also lower than that at the aerobic maximum. For example, human cyclists can maintain for 30min approximately 80% of the power output achieved in an aerobic capacity test (so-called V·O2max test). This finding suggests that muscles have a higher capacity than the oxidative flux maintained during steady-state exercise (Hoppeler et al., 1985). Rodent and human exercise studies have shown a region of muscle work where there is no sign of fatigue, but where the muscle is not in metabolic steady state, as indicated by falling [PCr] and pH (McCully et al., 1993). Eventually, muscle force production drops as Pi and H+ accumulate, but it is clear that the failure to achieve a steady-state [PCr] and pH precedes the decline in muscle force. This range of muscle work rates has been termed the transitional phase between steady-state and fatiguing contractions (Meyer and Foley, 1996). The paradox is that the muscle is not working at its oxidative capacity, yet the falling [PCr] indicates that ATP supply does not meet demand (hence the need to break down [PCr] to meet contractile needs).
The second finding modifying the upper right diagram in Fig.1 may explain this paradox. Glycolytic pyruvate supply often exceeds oxidative needs, resulting in accumulation of H+ and lactate in fully aerobic tissue (Connett and Sahlin, 1996). The consequence of a higher than expected glycolytic rate is substantial non-oxidative ATP supply (so-called substrate-level phosphorylation) and the accumulation of inhibitory by-products during sustained contractions. We show below that this glycolytic flux contributes significantly to sustained ATP supply and generates H+. We argue that this high glycolytic flux has the important consequence of restricting oxidative ATP supply to below the oxidative capacity during sustained contractions.
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Partitioning of ATP supply |
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![]() | (1) |
where [ADP] is the adenosine diphosphate concentration and [Cr] is the free creatine concentration. We use the initial rate of PCr breakdown during exercise to quantify the muscle ATPase activity (Blei et al., 1993). Glycolysis is apparent as H+ generation, from which we determine glycolytic ATP synthesis (Conley et al., 1997; Conley et al., 1998). After exercise has halted, [PCr] recovers to the resting level, and this recovery rate reflects mitochondrial ATP synthesis.
We partitioned ATP supply and demand using an extreme example of sustained performance: tailshaking by the rattlesnake, in which 50Hz contractions can be maintained for hours (Martin and Bagby, 1973). The tailshaker muscle has many advantages over human muscle for the study of intracellular energetics. This muscle is uniform in its cell properties (Clark and Schultz, 1980; Schultz et al., 1980) and all its fibers appear to be recruited during rattling (Schaeffer et al., 1996), allowing unambiguous quantification of metabolic flux. To our surprise, glycolysis supplies more than one-third of the sustained ATP production during rattling (Kemper et al., 2001). The higher than expected fraction of ATP supplied by glycolytic flux during aerobic contractions is not restricted to the rattlesnake. We have reported that the glycolytic ATP contribution estimated for three human muscles studied in our laboratory is higher than the 8% contribution to ATP supply expected if (i) glycogenolysis is the sole source of pyruvate and (ii) pyruvate is the sole source of substrate for oxidative phosphorylation (Kemper et al., 2001). All the human muscles have contributions of 1012% due to glycolytic flux, which generates H+ and lactate. Including the 8% of ATP produced by glycolysis on the way to oxidation of pyruvate (with the above assumptions), this contribution of glycolysis to ATP supply can reach a total of up to 20%. It has long been recognized that glycolytic flux occurs during muscle exercise below the aerobic capacity (Connett and Sahlin, 1996), but these direct muscle measurements are the first to document the substantial fraction contributed by glycolysis to the total ATP flux.
We used the rattlesnake tailshaker muscle to validate our in vivo determinations using direct measurement of lactate generation and oxygen uptake (Kemper et al., 2001). This muscle has an exclusive blood circulation, allowing us to measure lactate generation by the tailshaker muscle alone during aerobic contractions. We found close agreement between the glycolytic ATP supply determined by in vivo magnetic resonance spectroscopy (MRS) and that determined from direct measurement of lactate generation during rattling (Kemper et al., 2001). These findings confirm the results from 31P studies in human muscle (Conley et al., 1997; Conley et al., 1998) that glycolysis is a high-flux pathway that makes a significant contribution to ATP generation during muscle work.
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Limits to oxidative phosphorylation |
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The basis of this limitation to sustained ATP supply probably lies in the effects of both muscle contraction and glycolysis on the signal for oxidative phosphorylation [ADP]. At the onset of exercise, an imbalance between ATP supply and demand causes [PCr] to decrease (Fig.3) and [ADP] to increase according to the creatine kinase (CK) reaction, as shown in Fig.4:
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![]() | (2) |
where Keq is the CK equilibrium constant. The key signaling molecule ADP is at too low a concentration (micromolar levels) to be routinely measured by 31P MRS. However, the value calculated from the CK reaction has been confirmed in intact muscle in which [ADP] was raised to high levels for a long enough period to be directly measured by 31P MRS (Fisher and Dillon, 1988).
The elevation in [ADP] activates oxidative phosphorylation in isolated mammalian heart mitochondria (Fig.4B), and Fig.6B shows that the same relationship holds for human muscle in vivo. The drop in [PCr] continues with exercise until [ADP] increases enough to activate sufficient mitochondrial oxidative phosphorylation to balance ATP supply to ATP demand. Once an ATP balance has been attained, [PCr] breakdown is no longer needed to meet ATP demands and [PCr] reaches a steady-state level.
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What limits [ADP]? |
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The evidence for an effect of pH on oxidative phosphorylation comes from recent MRS studies in animals and humans (McCully et al., 1993). First, steady-state exercise studies have shown that steady-state [PCr] is depleted to only approximately half the resting level at the highest steady-state exercise level. Higher exercise levels cannot be maintained because pH drops throughout the course of the exercise (McCully et al., 1993). In addition, rodent and human studies have shown that the rate of [PCr] recovery slows at lower pH (Paganini et al., 1997; Walter et al., 1997). Some evidence exists for a direct effect of pH on oxidative phosphorylation (Harkema et al., 1997b), but we suggest that the effect of pH is on the signal activating oxidative phosphorylation [ADP]. Thus, the elevation of [H+] during sustained exercise does not directly inhibit force production, as occurs at higher [H+], but rather affects sustained ATP production by limiting the rise in [ADP]. How can [ADP] ever rise high enough to elicit the mitochondrial oxidative capacity?
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Keep [PCr] low and/or pH high |
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The tailshaker muscle is able to elicit a high [ADP] and maintain it during sustained rattling by using the second strategy: keep pH high (Kemper et al., 1999). Two properties of the tailshaker muscle achieve this goal by ensuring a rapid H+ and lactate efflux rate: small fiber size and extremely high muscle blood flow rates. The tailshaker muscle fiber diameter is approximately half (33µm)(Clark and Schultz, 1980) the diameter of human quadriceps fibers (67µm)(Hoppeler et al., 1985), resulting in a small diffusion distance to the fiber periphery. Removal of protons from the fiber is facilitated by muscle blood flow rates (468ml100g-1min-1)(Kemper et al., 2001) exceeding those found in human athletes (350ml100g-1min-1)(Richardson et al., 1995) or racehorses (<300ml100g-1min-1)(Armstrong et al., 1992). The end result is an intracellular pH in tailshaker muscle of 7.2 (Kemper et al., 1999) compared with 6.5 in the human quadriceps at V·O2max (Richardson et al., 1995). This high pH combined with a low sustained [PCr] (10mmoll-1) in tailshaker muscle result in an [ADP] of 0.15mmoll-1. This [ADP] is well above any other reported sustained level and should elicit more than 80% of the oxidative capacity of mitochondria according to Fig.6 (Kemper et al., 1999). The high aerobic flux and accompanying blood flow permit a high H+ and lactate efflux and keep pH high. These results suggest that muscles can generate an [ADP] that comes close to the level that elicits the oxidative capacity in mitochondria. Thus, the limit to sustained oxidative ATP synthesis reflects the ability to activate mitochondria fully, which depends upon the interactions between glycolytic flux, H+ efflux and oxidative phosphorylation (Fig.2).
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Concluding remarks |
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Acknowledgments |
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References |
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