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J Appl Physiol 85: 2140-2145, 1998;
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Vol. 85, Issue 6, 2140-2145, December 1998

Recovery of free ADP, Pi, and free energy of ATP hydrolysis in human skeletal muscle

Henning Wackerhage1, Uwe Hoffmann1, Dieter Essfeld1, Dieter Leyk1, Klaus Mueller2, and Jochen Zange2

1 Department of Physiology, German Sports University, D-50933 Cologne; and 2 Institute of Aerospace Medicine, Deutsches Zentrum fuer Luft und Raumfahrt, D-51170 Cologne, Germany

    ABSTRACT
Top
Abstract
Introduction
Methods
Results
Discussion
References

We measured significant undershoots of the concentrations of free ADP ([ADP]) and Pi ([Pi]) and the free energy of ATP hydrolysis (Delta GATP) below initial resting levels during recovery from severe ischemic exercise with 31P-nuclear magnetic resonance spectroscopy in 11 healthy sports students. Undershoots of the rate of oxidative phosphorylation would be predicted if the rate of oxidative phosphorylation would depend solely on free [ADP], [Pi], or Delta GATP. However, undershoots of the rate of oxidative phosphorylation have not been reported in the literature. Furthermore, undershoots of the rate of oxidative phosphorylation are unlikely because there is evidence that a balance between ATP production and consumption cannot be achieved if an undershoot of the rate of oxidative phosphorylation actually occurs. Therefore, oxidative phosphorylation seems to depend not only on free [ADP], [Pi], or Delta GATP. An explanation is that acidosis-related or other factors control oxidative phosphorylation additionally, at least under some conditions.

phosphorus nuclear magnetic resonance spectroscopy; exercise; recovery; free energy

    INTRODUCTION
Top
Abstract
Introduction
Methods
Results
Discussion
References

UNDERSHOOTS OF THE CONCENTRATIONS of free ADP ([ADP]) (1, 2) and Pi ([Pi]) (4) below initial resting levels have been measured during recovery of human skeletal muscle from exercise. An undershoot of free ADP and the free energy of ATP hydrolysis (Delta GATP) was observed during hypercapnic acidosis in resting cat skeletal muscle (13).

The undershoot of Pi was measured with a 4.7-T 31P-nuclear magnetic imaging (NMR) magnet (4). The fact that Pi and phosphomonoester (PME) peaks are difficult to resolve with a conventional 31P-NMR magnet may be an explanation for the difficulty in detecting Pi undershoots in studies where 31P-NMR magnets with lower field strength are used.

The undershoots of free ADP and Pi below initial resting levels occurred after similar types of exercise. For this reason, undershoots of free ADP and Pi at the same time may be measurable with a high-field-strength 31P-NMR magnet under certain conditions. If so, and if the ATP changes are small, a pronounced undershoot of Delta GATP will result.

Undershoots of free ADP, Pi, and Delta GATP below initial resting levels pose several questions regarding reactions that precede and follow the ATP synthesis step as well as the control of energy metabolism.

An undershoot of Delta GATP would require a more negative minimum free energy (Delta G) of reactions that precede the ATP synthesis step and would allow a more negative Delta G of reactions that depend on Delta GATP.

Free ADP, Pi at low concentrations, and Delta GATP are putative control factors of mitochondrial oxidative phosphorylation. In the most widely cited oxidative phosphorylation control models, it is assumed that in skeletal muscle 1) oxidative phosphorylation is controlled by free ADP (and Pi at low concentrations); Michaelis-Menten or kinetics of a higher order are assumed (8, 16, 21); and 2) the rate of oxidative phosphorylation depends linearly (9, 23) or quasi-linearly on Delta GATP (15).

According to the above-mentioned models, undershoots of free ADP, Pi, and Delta GATP would result in a decrease in oxidative phosphorylation, which must be compensated for by either decreased ATP consumption or increased nonoxidative ATP production to maintain the balance between ATP production and consumption.

The purpose of this study was to investigate the time courses of free [ADP], [Pi], and Delta GATP in human calf muscles by using a 4.7-T 31P-NMR magnet during and after moderate and severe calf muscle exercise. We want to discuss the findings especially with respect to the control of oxidative phosphorylation.

    METHODS
Top
Abstract
Introduction
Methods
Results
Discussion
References

Subjects. One woman and ten men who were healthy and endurance-trained sports students (runners and triathletes), participated in this study. Their age, height, and weight were 28 ± 3 (SD) yr, 180 ± 4 cm, and 76 ± 8 kg, respectively. The subjects were informed about the possible risks involved in the tests before their voluntary consent was obtained.

Experimental protocol. Subjects lay supine with the upper body elevated by 25°. An air cuff was placed around the lower part of the right thigh, and the right foot was fixed on the foot support of a calf muscle ergometer. A 70° angle between the pedal and the horizontal plane was adjusted.

Force was measured with a strain-gauge force transducer (model U2A, Hottinger-Baldwin Messtechnik, Darmstadt, Germany) and was displayed to the subjects.

Three tests were performed. In the test force (TF) test, maximum voluntary force was measured during three bouts lasting 5 s, with 60-s recovery between each bout. The highest individual force averaged over 1 s measured at the force transducer was used as an estimate of the subject's TF. In the 20% TF and 50% TF tests (Fig. 1), after 4 min of rest the thigh cuff was rapidly inflated to between 280 and 300 mmHg (37-40 kPa). One minute later, the subjects performed an isometric plantar flexion at either 20 or 50% of their TF for 4 min or until exhaustion. After another 90 s, the thigh cuff was deflated and the subjects remained in their position for 20 min.


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  		Fig. 1.   Experimental protocol for 20% test force (TF) and 50% TF test. Plantar flexion, volunteers performed an isometric foot plantar flexion with 20 or 50% of their individual TF. Ischemia, arterial occlusion at lower thigh level. In Figs. 2-8, time 0 indicates opening of occlusion cuff (end of occlusion).

The 20% TF and the 50% TF test were performed in random order with normally 1 day and, in some cases, several hours between tests. The TF test was performed at least 20 min before the first of the following two tests.

31P-NMR. 31P-NMR measurements were performed on a Bruker Biospec 47/40 (4.7-T, 40-cm bore; Bruker, Karlsruhe, Germany) by using a 5-cm-diameter surface coil. Vector size was 4 k, and spectral width was 10,000 Hz. The repetition time was 0.3 s, and the flip angle was 60° in the center of the coil. For one spectrum, 64 free induction decays were acquired in 20 s. The spectra were analyzed by using the software package UXNMR (Bruker). The integral borders were 7.4-5.9 parts/million (ppm) for the PME peak, 5.8-3.3 ppm for the Pi peak, 1.1 to -1.1 ppm for the phosphocreatine (PCr) peak, and -14.9 to -17.4 ppm for the beta -ATP peak. To correct for partial saturation, correction factors were determined by comparing 50-100 spectra, measured with the parameters given above, with 5-10 fully relaxed spectra in 5 resting subjects. The average PCr integral ratio of the fully relaxed to the partially relaxed spectra was set to one. The correction factors for the other signals were calculated relative to this value. They were 1.94 ± 0.36 for PME, 1.23 ± 0.13 for Pi, and 0.80 ± 0.02 for ATP. It was difficult to determine the correction factor for the PME signal accurately because this signal is small at rest. The integrals of the PME, Pi, PCr, and beta -ATP signals were corrected with the average correction factors and their relative proportions from total phosphate ([PME] + [Pi] + [PCr] + [ATP]) were determined. Intramuscular pH was calculated from the distance between the maximums of the Pi and PCr peaks (delta ) as
pH = 6.75 + log [(&dgr; − 3.27) /(5.69 − &dgr;)] (1)
where delta  is given in ppm (e.g., see Ref. 2).

Estimation of absolute concentrations and calculation of free [ADP] and Delta GATP. The absolute [PME], [Pi], [PCr], and [ATP] were estimated by multiplying the peak areas of these signals by the factor that was obtained by converting the resting average beta -ATP peak area into an assumed [ATP] of 8.2 mmol/l in cell water (2). Total creatine concentrations ([Cr]tot; [PCr] + [Cr]) were assumed to be constant throughout a test. [Cr]tot was estimated by assuming a [PCr]-to-[Cr]tot ratio of 0.85 at initial rest for each individual (15). [Cr] was calculated as [Cr]tot - [PCr].

Harkema and Meyer (13) point out that errors may arise at low pH if free [ADP] and Delta GATP are calculated by using the formulas given in Lawson and Veech (20) and/or Kushmerick et al. (19). To avoid such errors, free [ADP] and Delta GATP were calculated by using the formulas and constants of Harkema and Meyer. At first, the observed equilibrium constant (Kobs) for the creatine kinase (CK) reaction was calculated as
<IT>K</IT><SUB>obs</SUB> = <IT>K</IT><SUB>CK</SUB> ⋅ (f<SUB>ADP</SUB> ⋅ f<SUB>PCr</SUB> ⋅ [H<SUP>+</SUP>])/f<SUB>ATP</SUB> (2)
where the equilibrium constant for the CK reaction, KCK, is 1.75 · 108 mol-1 · l-1, and fractions of total reactants, fADP, fPCr, and fATP, are [ADP3-]/[Sigma ADP], [PCr2-]/[Sigma PCr], and [ATP4-]/[Sigma ATP], respectively. The latter terms were calculated as in Harkema and Meyer (13), using the same formulas and constants. Free [ADP] was then calculated as
Free [ADP] = ([ATP] ⋅ [Cr])/(<IT>K</IT><SUB>obs</SUB> ⋅ [PCr]) (3)
Delta GATP was calculated by using Eq. 12 of Harkema and Meyer (13). Because the CK reaction is assumed to be in equilibrium (10, 22), Delta GATP is assumed to match the free energy of PCr hydrolysis. Delta GATP was calculated as
&Dgr;<IT>G</IT><SUB>ATP</SUB> = &Dgr;<IT>G</IT><SUB>PCr</SUB> = −<IT>R</IT>T ⋅ (<IT>K</IT><SUB>ATP</SUB> ⋅ <IT>K</IT><SUB>CK</SUB>) −<IT>R</IT>T ⋅ ln (f<SUB>PCr</SUB>/f<SUB>P<SUB>i</SUB></SUB>) 
+ <IT>R</IT>T ⋅ ln ([P<SUB>i</SUB>] ⋅ [Cr]/[PCr]) (4)
where R is the gas constant and T is the absolute temperature, fPi is [HPO2-4]/[Sigma Pi], and KATP, the equilibrium constant for the ATP hydrolysis reaction, is assumed to be 0.129 (13).

Statistics. All statistical tests were computed with PlotIt for Windows 3.1 (Scientific Programming Enterprises) or Statistica for Windows (Statsoft, Tulsa, OK).

The time constant tau  for [PCr] recovery was calculated by using the following equation
[PCr] (<IT>t</IT>) = [PCr]<SUB>20 s</SUB> + &Dgr;[PCr] (1 − <IT>e</IT><SUP>−<IT>t</IT>/&tgr;</SUP>) (5)
In this equation, [PCr]20 s represents [PCr] 20 s after ischemia, and Delta [PCr] is the difference between [PCr]20 s and the steady-state [PCr] at the end of recovery.

To estimate the ATP that was consumed by [PCr] recovery, the first derivation of Eq. 5 was obtained
<FR><NU>d[PCr]</NU><DE>d<IT>t</IT></DE></FR> = <FR><NU>&Dgr;[PCr]</NU><DE>&tgr;</DE></FR> e<SUP>−<IT>t</IT>/&tgr;</SUP> (6)
The rate of [PCr] increase was estimated by applying Eq. 6 to the results of each experiment. The average rate of [PCr] recovery for 2 min after ischemia was calculated to obtain a minimum estimate for ATP consumption at that point.

Deviations from means are given as ±SD in the text and displayed as SE bars in Figs. 2-8.

Two-factorial Huyn-Feld adjusted ANOVAs were performed as a priori tests to test the hypothesis that the means were the same in the 20% TF and 50% TF test, and at different periods. The parameters tested were free [ADP], [Pi], and Delta GATP. The factors were "test" (level 1: 20% TF test; level 2: 50% TF test) and "time" (level 1: resting average; level 2: mean from 2 to 5 min after ischemia; level 3: mean from 17 until 20 min after ischemia). In case the two-factorial ANOVA yielded a significant result, a Newman-Keuls test was performed as an a posteriori test.

A least-squares linear regression was calculated to analyze the linear relationship between [PME] and [Pi] in the 50% TF test after ischemia.

P < 0.05 was regarded as significant for all tests.

    RESULTS
Top
Abstract
Introduction
Methods
Results
Discussion
References

All subjects performed the ischemic foot plantar flexion for 4 min at 20% of their TF. In the 50% TF test, eight subjects performed the ischemic foot plantar flexion for 4 min. Three subjects, however, stopped the foot plantar flexion after 129, 181, and 201 s. The resting and recovery data of these subjects were used in the study.

During ischemic exercise, [PCr] decreased significantly to values of 24.7 ± 3.0 and 8.0 ± 5.7 mmol/l in the 20% TF and 50% TF tests, respectively. After the occlusion cuff was opened, [PCr] recovered significantly faster in the 20% TF test, with a tau  of 28 ± 15 s (range: 10-53 s), than in the 50% TF test, with a tau  of 43 ± 14 s (range: 26-63 s) (Fig. 2).


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  		Fig. 2.   Phosphocreatine concentration ([PCr]) in right calf before and after isometric ischemic foot plantar flexion with 20 (open circle ; n = 11) and 50% (bullet ; n = 11) of subjects' TF. Values are means ± SE. Time constant tau  is measurement of velocity of [PCr] recovery obtained by fitting a monoexponential equation to [PCr] recovery. t, Time.

After the opening of the occlusion cuff, the pH recovered more slowly than did [PCr] in both tests (cf. Figs. 2 and 3).


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  		Fig. 3.   pH in right calf before and after isometric ischemic foot plantar flexion with 20 (open circle ; n = 11) and 50% (bullet ; n = 11) of subjects' TF. Rest arrow, resting level of open circle  and bullet . Values are means ± SE.

The Pi and PME peaks could be resolved (Fig. 4). After ischemia, [Pi] dropped significantly below its initial resting concentration in both tests, whereas the undershoot in the 50% TF test was more pronounced than in the 20% TF test (Fig. 5, Table 1).


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  		Fig. 4.   31P-nuclear magnetic resonance spectroscopy (31P-NMR) spectra acquired during a 50% TF test. a, Rest; b, end of exercise; c, recovery (2 min 40 s after ischemia), in which PCr is recovered, Pi is lower than at rest, and phosphomonoester (PME) peak is visible (arrow); d, late recovery (11 min 40 s after ischemia), in which Pi is largely recovered and PME peak is hardly discernible. ppm, Parts/million.


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  		Fig. 5.   Pi concentration ([Pi]) in right calf before and after isometric ischemic foot plantar flexion with 20 (open circle ; n = 11) and 50% (bullet ; n = 11) of subjects' TF. Values are means ± SE.

                              
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  		Table 1.   Parameters related to the control of oxidative phosphorylation at rest and during recovery

At the beginning of postexercise ischemia in the 50% TF test, [PME] was 5.9 ± 2.0 mmol/l and significantly higher than at initial rest. After ischemia, [PME] slowly recovered to its initial resting value.

[PME] and [Pi] correlated negatively after complete [PCr] recovery in the 50% TF test (Fig. 6).


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  		Fig. 6.   Significant negative correlation between PME concentration ([PME]) and [Pi] in right calf after isometric ischemic exercise with 50% of subjects' TF and complete [PCr] recovery. Values are means ± SE. Calculation was performed for period lasting from 4 to 20 min after ischemia.

Calculated free [ADP] decreased significantly below its initial resting concentration in the 50% TF test but not in the 20% TF test (Fig. 7, Table 1).


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  		Fig. 7.   Calculated free ADP concentration ([ADP]) in right calf before and after isometric ischemic foot plantar flexion with 20 (open circle ; n = 11) and 50% (bullet ; n = 11) of subjects' TF. Values are means ± SE.

The undershoots of both [Pi] and free [ADP] resulted in an undershoot of Delta GATP (Fig. 8, Table 1).


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  		Fig. 8.   Free energy of ATP hydrolysis (Delta GATP) in right calf before and after isometric ischemic foot plantar flexion with 20 (open circle ; n = 11) and 50% (bullet ; n = 11) of subjects' TF. Values are means ± SE.

    DISCUSSION
Top
Abstract
Introduction
Methods
Results
Discussion
References

First, the measurements and calculations and their limitations are discussed. Following that, we discuss the implication of the undershoots of free ADP, Pi, and Delta GATP with respect to reactions that precede and follow the ATP synthesis step and with respect to the control of oxidative phosphorylation.

Undershoots of free [ADP], [Pi], and Delta GATP below their initial resting concentrations. Undershoots of [Pi] after exercise (Fig. 5, Table 1) were also measured in other 31P-NMR studies (e.g., see Ref. 4). The most likely explanation is that Pi was temporarily bound in glycolytic sugar phosphates, which appear as a separate PME peak in a high-field-strength 31P-NMR spectrum during and after intensive exercise (4). Results of muscle biopsy studies are in line with this hypothesis because they show millimolar increases in glycolytic intermediates during intensive exercise (17). The negative correlation between [PME] and [Pi] after complete [PCr] recovery (Fig. 6) was probably mostly due to Pi liberation in the course of glycogen resynthesis out of glycolytic intermediates. An uptake of millimolar [Pi] into mitochondria, as suggested by Iotti et al. (14), is an unlikely cause for the [Pi] undershoot because mitochondrial volume is far too small for this.

Undershoots of free [ADP] below the resting concentration (Fig. 7, Table 1) after ischemic and nonischemic heavy exercise have also been calculated previously by Argov et al. (1) and Arnold et al. (2), respectively. Mathematically, undershoots of free [ADP] may be caused either by a slower proton (pH) recovery, compared with the recovery of the other metabolites of the CK reaction, or by a [PCr] overshoot (19) or by both. A free [ADP] undershoot has also been calculated for a noncontracting skeletal muscle in which acidosis has been induced by hypercapnia (13). However, because free [ADP] is calculated, its correctness depends on the correctness of the assumptions made (3). The most important prerequisite is that the CK reaction was in, or very near to, chemical equilibrium during the period of the undershoot. The chemical equilibrium of this reaction has been demonstrated by means of 31P-NMR (10, 22). Because ATPase flux was low during recovery, the CK reaction should have been in chemical equilibrium at the point of the undershoot if this reaction can generally equilibrate in vivo. With the approach of Harkema and Meyer (13), an underestimation of free [ADP] at low pH was probably avoided; however, this depends on the correctness of the assumed Mg2+ and K+ concentrations, the metabolite dissociation constants, and the correct mathematical formulation (13).

The undershoot of Delta GATP in the 50% TF test was caused both by an undershoot of free [ADP] and of [Pi], whereas [ATP] did not change much throughout the experiments.

Undershoots of free ADP, Pi, and Delta GATP: implications. The most striking finding with respect to the Delta GATP undershoot in the 50% TF test is its magnitude of 5 kJ/mol. Because of this, higher electrochemical potential gradients could theoretically have been produced, e.g., by ATP-dependent pumps like the Ca2+ pump of the sarcoplasmic reticulum and by Na+-K+-ATPase. The real electrochemical potential gradients produced by these pumps, however, depended also on Delta G losses, e.g., because of leaking, nonideal coupling, and on the pumping stoichiometry. Furthermore, the Delta GATP undershoot requires that the minimum Delta G of preceding steps, e.g., the protonmotive force, must have been 5 kJ/mol more negative or that the H+/ATP stoichiometry must have changed to yield a net flux toward ATP synthesis.

According to all models that use solely free [ADP], [Pi], or Delta GATP (3, 7, 9, 15, 16, 21, 23) as control factors of oxidative phosphorylation, the rate of oxidative phosphorylation would have dropped considerably below its initial resting level during recovery in the 50% TF test. This would also be true if higher-order kinetics with respect to free ADP would be assumed (16, 21). There are two arguments against an actual undershoot of oxidative phosphorylation below resting level after exercise. To our knowledge, no undershoots of whole body or isolated muscle oxygen uptake under initial resting levels after intensive exercise have been reported in the literature. In particular, there is no evidence that pronounced oxidative phosphorylation undershoots occur as early as 2 min after exercise, as would be expected from the free [ADP], [Pi], or Delta GATP time courses (Figs. 5, 7, 8). The second argument is that it is unlikely that ATP resynthesis could have matched ATP consumption if an undershoot of oxidative phosphorylation had actually occurred. If an undershoot of oxidative phosphorylation had actually occurred, either glycolytic ATP resynthesis would have increased or ATP consumption would have decreased compared with initial rest, so as to maintain the balance between ATP production and consumption. There is no evidence for this.

[PCr] and pH were constant during postexercise ischemia in both experiments. This confirms the previous finding that glycolytic activity is negligible during postexercise ischemia (24). It seems unlikely that glycolytic activity increased after ischemia. This is supported by indirect evidence: during recovery, the known glycogenolytic and glycolytic control factors (free ADP, AMP, Pi, pH, and Ca2+) were the same as at rest or activating glycogenolysis and glycolysis less than at rest. This indicates that glycolytic ATP production was not increased over initial resting level at the time when the undershoots of [ADP], [Pi], and Delta GATP occurred in the 50% TF test.

There is also no evidence for decreased ATP consumption at the time when the undershoots of [ADP], [Pi], and Delta GATP occurred in the 50% TF test. The opposite is the case at the beginning of the undershoots: 2 min after ischemia, free [ADP], [Pi], and Delta GATP were already lower than at initial rest. At this point the mean [PCr] increase was 1.65 ± 1.31 mmol · l-1 · min-1. Therefore, by itself the ATP need for PCr resynthesis at this point was 340% of the basal metabolic rate of 0.48 ± 0.12 mmol · l-1 · min-1 that has been measured with 31P-NMR in human skeletal muscle (5).

Thus it is highly unlikely that a balance between ATP production and ATP consumption can be achieved if it is assumed that free [ADP], [Pi], and Delta GATP are the only control factors of oxidative phosphorylation. This implies that control factors other than free [ADP], [Pi], or Delta GATP (3, 7, 9, 15, 16, 21, 23) contribute to the control of oxidative phosphorylation, at least at the point where the undershoots of [ADP], [Pi], and Delta GATP occurred in the 50% TF test.

Possible explanations. Undershoots in free [ADP] and Delta GATP below resting levels at constant oxygen consumption can also be induced by hypercapnic acidosis in a resting muscle (13). This indicates that the additional control is not necessarily related to muscular contraction. The fact that the free [ADP] undershoot in Harkema and Meyer's study (13) and our study was due to acidosis suggests that acidosis has an effect on oxidative phosphorylation. In vitro studies in renal cortex mitochondria suggest that changes in the transmembrane pH gradient can occur if the pH of the surrounding medium is altered (11). If the cytosolic pH decreases, the inner membrane pH gradient and the protonmotive force may increase, although proton leaking will diminish this increase (6). If the protonmotive force increases, the rate of oxidative phosphorylation will also rise (25), and this may compensate for the lower activation by free ADP, Pi, or Delta GATP at the point of their undershoots.

Acidosis also affects the free energy available from NADH oxidation. However, because the pH changes in our studies were smaller and because the Delta GATP changes were larger in our study than in others (13), this effect was negligible in our study.

31P-NMR yields cytosolic estimates of free ADP, Pi, or Delta GATP. Oxidative phosphorylation, however, takes part in the matrix of the mitochondrion because the catalyzing F1 portion of the F0F1-ATPase is orientated toward the matrix. If the transport of ADP, Pi, and ATP would be affected by acidosis, ischemia, or contraction, the relationship between extra- and intramitochondrial free [ADP], [Pi], and Delta GATP would also change. Pi is transported together with a proton inside the mitochondrion (26). Thus a change in the transmembrane pH gradient will alter the intramitochondrial [Pi] at a given cytosolic [Pi]. ATP and ADP are exchanged via an electrogenic carrier (18). Changes in the transmembrane voltage, e.g., because of changes in the ionic composition of the cytosol, would alter the intramitochondrial [ATP]-to-[ADP] ratio at a given cytosolic [ATP]-to-[ADP] ratio.

Some contraction-related effects may have played a role in our study. If the cytosolic Ca2+ concentration increases during muscular contraction, more Ca2+ is transported into the mitochondria via a Ca2+ carrier. Intramitochondrial Ca2+ activates dehydrogenases of the Krebs cycle (12), which may affect the [NADH]-to-[NAD+] ratio and, via this, the protonmotive force. Ca2+ might also release a Ca2+-sensitive inhibitor protein from the H+-transporting ATP synthase in skeletal muscle (27).

A recent study suggests that aldosterone may have a rapid nongenomic effect on [PCr] resynthesis after exercise (28). Finally, all the control mechanisms discussed above might control oxidative phosphorylation differently in different muscle fiber types (19).

In conclusion, oxidative phosphorylation seems to depend not only on free [ADP], [Pi], or Delta GATP. Several acidosis-related or other factors may control oxidative phosphorylation additionally, at least under some conditions.

    FOOTNOTES

Address for reprint requests: H. Wackerhage, Dept. of Applied Biology, Univ. of Central Lancashire, Preston PR1 2HE, UK (E-mail: h.wackerhage{at}uclan.ac.uk).

Received 3 December 1997; accepted in final form 4 August 1998.

    REFERENCES
Top
Abstract
Introduction
Methods
Results
Discussion
References

1.   Argov, Z., N. Di Stefano, and D. L. Arnold. ADP recovery after a brief ischemic exercise in normal and diseased human muscle---a 31P MRS study. NMR Biomed. 9: 165-172, 1996[Medline].

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11.   Hager, S. R., and D. P. Simpson. Influence of bicarbonate buffer system on pH gradient in rabbit renal cortex mitochondria. Mol. Physiol. 2: 203-208, 1982.

12.   Hansford, R. G. Role of calcium in respiratory control. Med. Sci. Sports Exerc. 26: 44-51, 1994[Medline].

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J APPL PHYSIOL 85(6):2140-2145
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