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J Appl Physiol 99: 1668-1675, 2005. First published July 7, 2005; doi:10.1152/japplphysiol.01200.2004
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NaHCO3-induced alkalosis reduces the phosphocreatine slow component during heavy-intensity forearm exercise

S. C. Forbes,1,3 G. H. Raymer,2,3 J. M. Kowalchuk,1,3 and G. D. Marsh1,2,3

1School of Kinesiology, 2Department of Medical Biophysics, and 3Lawson Health Research Institute, The University of Western Ontario, London, Ontario, Canada

Submitted 25 October 2004 ; accepted in final form 4 July 2005


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 ACKNOWLEDGMENTS
 REFERENCES
 
During heavy-intensity exercise, the mechanisms responsible for the continued slow decline in phosphocreatine concentration ([PCr]) (PCr slow component) have not been established. In this study, we tested the hypothesis that a reduced intracellular acidosis would result in a greater oxidative flux and, consequently, a reduced magnitude of the PCr slow component. Subjects (n = 10) performed isotonic wrist flexion in a control trial and in an induced alkalosis (Alk) trial (0.3g/kg oral dose of NaHCO3, 90 min before testing). Wrist flexion, at a contraction rate of 0.5 Hz, was performed for 9 min at moderate- (75% of onset of acidosis; intracellular pH threshold) and heavy-intensity (125% intracellular pH threshold) exercise. 31P-magnetic resonance spectroscopy was used to measure intracellular [H+], [PCr], [Pi], and [ATP]. The initial recovery data were used to estimate the rate of ATP synthesis and oxidative flux at the end of heavy-intensity exercise. In repeated trials, venous blood sampling was used to measure plasma [H+], [HCO3–], and [Lac–]. Throughout rest and exercise, plasma [H+] was lower (P < 0.05) and [HCO3–] was elevated (P < 0.05) in Alk compared with control. During the final 3 min of heavy-intensity exercise, Alk caused a lower (P < 0.05) intracellular [H+] [246 (SD 117) vs. 291 nmol/l (SD 129)], a greater (P < 0.05) [PCr] [12.7 (SD 7.0) vs. 9.9 mmol/l (SD 6.0)], and a reduced accumulation of [ADP] [0.065 (SD 0.031) vs. 0.098 mmol/l (SD 0.059)]. Oxidative flux was similar (P > 0.05) in the conditions at the end of heavy-intensity exercise. In conclusion, our results are consistent with a reduced intracellular acidosis, causing a decrease in the magnitude of the PCr slow component. The decreased PCr slow component in Alk did not appear to be due to an elevated oxidative flux.

sodium bicarbonate; phosphorus-31 magnetic resonance spectroscopy; acid-base status; muscle energetics

AT THE ONSET OF EXERCISE, phosphocreatine (PCr) hydrolysis via the creatine kinase reaction provides an immediate means of ATP resynthesis. In addition, the tight coupling between the kinetics of PCr degradation and that of the pulmonary O2 uptake (O2) (33, 39–41) has provided support for the notion that the creatine kinase reaction plays a key role in the regulation of oxidative phosphorylation. Specifically, PCr hydrolysis is associated with an increase in [ADP] (where brackets denote concentration), and [ADP], either alone (5) or in combination with other metabolites (e.g., the phosphorylation potential) (34), has been generally accepted to play an important role in regulating the rate of oxidative phosphorylation.

Following a step increase in work rate to moderate-intensity exercise, [PCr] decreases in a monoexponential pattern (primary component), with the rate of oxidative phosphorylation increasing in a mirror-type fashion. When ATP requirements are matched by ATP supply from oxidative phosphorylation, [PCr] reaches steady state. During heavy-intensity exercise, steady state is not obtained. PCr degradation is characterized by a primary component similar to that obtained in moderate-intensity exercise and an additional slower decline in [PCr] (slow component) (16, 39). The PCr slow component typically becomes discernible 2–4 min after the onset of heavy-intensity, constant-load exercise (34).

The underlying mechanisms of the PCr slow component are presently unknown. Many investigators have suggested that an increased ATP demand occurs during heavy-intensity exercise due to a progressive recruitment of less efficient fibers (1, 37, 41). Isolated mouse (9) and cat (13) skeletal muscle composed of primarily type II fibers has been shown to have an approximately threefold greater ATP cost of force production than muscle composed of primarily type I fibers. However, studies utilizing integrated electromyography (iEMG) and power density frequency measurements have failed to indicate an altered recruitment pattern within 2–4 min of the onset of heavy-intensity exercise (44, 45, 49).

Alternatively, an important factor contributing to the PCr slow component may involve intracellular acidosis (a characteristic of heavy-intensity exercise), causing a slowing of the rate of oxidative phosphorylation, which would be expected to result in greater PCr hydrolysis to maintain ATP supply. This explanation is consistent with previous studies that have shown that acidosis reduces the rate of oxidative phosphorylation (14, 21, 50). Possible mechanisms for acidosis slowing oxidative flux include a decreased signal for oxidative phosphorylation (e.g., [ADP]) due to a shift in the creatine kinase equilibrium (7) and a reduction in the mitochondria sensitivity of the signal that drives oxidative phosphorylation (21).

A recent study from our laboratory (38) examined the effects of an induced alkalosis (Alk) by sodium bicarbonate (NaHCO3) ingestion using phosphorus magnetic resonance spectroscopy (31P-MRS). This study found that Alk resulted in a reduced intracellular acidosis at intensities above the intracellular and lactate threshold during forearm exercise (38). Therefore, using a similar intervention, the present study utilized NaHCO3 ingestion to examine the effect of a reduced intracellular acidosis on the magnitude of the PCr slow component. We tested the hypothesis that the magnitude of the PCr slow component would be reduced following Alk, and that the reduced PCr slow component would be the result of an attenuated acidosis, causing a greater oxidative flux.

A secondary purpose of this study was to evaluate the effects of NaHCO3 ingestion at rest and during moderate-intensity exercise. We hypothesized that, at rest and during moderate-intensity exercise, Alk would have no effect on intracellular acid-base status, [PCr], or plasma lactate concentration ([Lac–]pl), because the muscle membrane is relatively impermeable to HCO3– (4). However, during heavy-intensity exercise, we hypothesized that a steeper [H+] gradient between muscle and plasma in Alk would cause a greater Lac– (and H+) efflux, resulting in a greater [Lac–]pl and a reduced intracellular acidosis.


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 ACKNOWLEDGMENTS
 REFERENCES
 
Subjects.   Healthy active men [n = 10; mean age 26 yr (SD 2)] participated in the study. The experimental protocol and all associated risks were explained, and an informed, written consent was obtained from each subject. The study was approved by The University of Western Ontario Review Board for Health Sciences Research Involving Human Subjects.

Protocol.   An incremental ramp wrist-flexion protocol was performed initially to determine work rates that corresponded to the moderate- and heavy-intensity exercise domains. Each subject was studied during an Alk and control (Con) isotonic wrist-flexion exercise protocol. Two constant-load exercise tests were performed in each condition: 1) while obtaining data from 31P-MRS, and 2) while collecting venous blood samples.

The exercise was performed in a supine position with the arm placed in full extension, abducted at 90°, and positioned in a custom-built, wrist-flexion ergometer. During the 31P-MRS trials, the ergometer was located in the bore of the magnet, whereas, during the blood sampling trials, the ergometer was secured to a table. The wrist ergometer used a cable-and-pulley system that raised and lowered a suspended reservoir. Resistance was increased by adding a known quantity of water to the reservoir. The wrist-flexion exercise consisted of repeatedly depressing a lever with the hand in a pronated position. Exercise was done at a frequency of 0.50 Hz (1-s contraction, 1-s relaxation) through a 70° range of motion (ROM). Full ROM and appropriate pacing of the contractions (set by a metronome) was ensured by an investigator monitoring the exercise.

The incremental ramp wrist-flexion protocol (250 ml water/min) to volitional fatigue has been described previously (25, 31). Briefly, power output was calculated using the product of mass (mass of reservoir plus water), the contraction frequency, and the vertical displacement traveled by the reservoir per contraction (0.10 m). With this protocol, exercise commenced at 0.47 W and increased by ~0.10 W/min until fatigue. For each subject, intracellular pH was plotted against power output, and piecewise linear regression was used to determine the onset of intracellular acidosis or intracellular pH threshold (TpHi). Based on TpHi, power outputs corresponding to moderate (75% TpHi) and heavy (125% TpHi) exercise were chosen (Table 1) and were confirmed during the Con trial by ensuring that moderate-intensity exercise demonstrated a plateau in intracellular [PCr], [Pi], and [H+] ([H+]i), and that, for heavy-intensity exercise, a plateau in these variables was either not present or delayed.


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  		Table 1. Power output at intracellular pH threshold during a ramped-function wrist-flexion exercise protocol, and mean moderate- and heavy-intensity workloads used during experimental trials

 
Before the experimental trials, the subjects reported to the laboratory at least 2 h after abstaining from food consumption and at least 4 h after a heavy meal. Caffeine and exercise were not permitted 24 h before testing. Fluid consumption was allowed ad libitum; however, during the subsequent trial, the same total volume of fluid was consumed before each trial. Subjects ingested gel capsules containing NaHCO3 (total dose, 0.3 g/kg body mass) over a 15-min period, beginning 90 min before the start of the exercise test. A placebo was not administered before the Con trial because, in our experience, the side effects of NaHCO3 ingestion (minor bloating, discomfort, gastrointestinal distress) are recognizable by the subjects. Each subject performed the protocols at approximately the same time of day, and 1–2 wk were allowed between each exercise session.

During the constant-load trials, each subject used the same loads for both Con and Alk conditions. Each trial consisted of collecting data during rest (3 min), warm-up (3 min, lifting an empty reservoir; mass = 1.075 kg), moderate- (9 min) and heavy-intensity (9 min) exercise, and recovery (15 min). The increases in exercise intensity (from warm-up to moderate and moderate-to-heavy exercise) occurred over a 5-s period and without warning to the subject by an investigator pouring a predetermined volume of water into the reservoir while the subject continued to exercise.

31P-MRS.   31P-MRS data were collected by using a 30-cm bore, 1.89-T superconducting magnet interfaced with a SMIS console (Surrey Medical Imaging Systems, Guilford, UK). A 4-cm dual-tuned 1H/31P surface coil was placed under the belly of the forearm, ~7–9 cm distal to the medial epicondyle of the humerus. From this position, the data collected were primarily from the flexor digitorum superficialis muscle (unpublished observations of MRI measurements). The 1H signal was used to shim the magnet homogeneity and as a result improve spectral resolution. The magnetic field homogeneity was adjusted until the proton signal from water produced a peak with a full width at one-half maximum of 50–70 Hz and was approximately Lorentzian in shape.

Spectra were collected continuously throughout the entire protocol. Each spectrum consisted of an average of six scans obtained over 36 s. All spectra were acquired by using a 3-ms adiabatic 90° radio frequency pulse, 12-µs delay time, 3.33-kHz receiver bandwidth, and 2,048 complex data points. The first 1.5 ms of data were not used to eliminate the very broad phosphorus components originating from large inhomogeneities or bone.

Data analysis.   Quantification of metabolite data was performed in the time domain by fitting the free induction decay data to a sum of damped sinusoids, which could be varied in amplitude, phase, delay time, damping constant, and frequency. This method used a prior knowledge and a nonlinear least squares algorithm to iteratively reduce the difference between the data and the experimental model (2). The concentrations of the phosphate metabolites Pi, PCr, and {alpha}-, {gamma}-, and {beta}-ATP were determined from the amplitude of the exponential model function at time = 0. ATP peak area was determined from the average of the {alpha}-, {gamma}-, and {beta}-ATP peaks. PCr, Pi, and ATP peaks were corrected for partial saturation (47), and absolute concentrations were calculated by using PCr-to-ATP and Pi-to-ATP ratios and by assuming a resting ATP concentration of 8.2 mmol/l (15). Free [ADP] was calculated by assuming a total creatine concentration of 42 mmol/l (15) and that the creatine kinase reaction was at equilibrium. The equilibrium constant was adjusted for [H+]i (12). The pHi was determined from the chemical shift of Pi relative to PCr (47). The peak areas and positions determined for each spectrum were assumed to represent metabolite and pH data at the midpoint of the acquisition time for each spectrum. It previously has been shown that, with this assumption, any "error" that may result is negligible (30). [H+]i was calculated from pH.

The oxidative ATP production rate at the end of heavy-intensity exercise was estimated by the rate of PCr resynthesis during the initial 18 s of recovery, as previously described (6, 10, 21). Briefly, oxidative flux during the initial recovery period was equal to the change in [PCr] due to oxidative phosphorylation divided by 18 s. PCr resynthesis via oxidative phosphorylation ({Delta}[PCr]ox phos) was determined by subtracting PCr resynthesis due to glycolysis ({Delta}[PCr]glycol) from the total PCr resynthesis ({Delta}[PCr]total) (Eq. 1).

(1)
{Delta}[PCr]glycol was calculated by using the stoichiometry of ATP production per [H+] generation of 1.5. The [H+] buffered in the cell and the [H+] consumed in the breakdown of PCr were included in the calculation of [H+] generation by glycolysis ({Delta}[H+]glycol) (Eq. 2):

(2)
where {Delta}pH is the change in muscle pH, {beta}tot is the total muscle buffer capacity (sum of intracellular and Pi buffers), {gamma} is the proton stoichiometric coefficient of PCr hydrolysis (27), and {Delta}[PCr] is the change in [PCr].

Kinetic analysis.   [PCr] kinetic responses were fit with a monoexponential model of the formula described by Eq. 3 using nonlinear least squares regression techniques (SigmaPlot 2000). Responses from each work transition were analyzed separately.

(3)
where [PCr]0 is the steady state [PCr] before the work transition, {Delta}[PCr]ss is the amplitude change from baseline ([PCr]0) to the [PCr] plateau in which the simple exponential phase projects, and {tau} is the time constant of the response. The data were fit starting at the time corresponding to the transition in work rate and proceeded to the end of transition (moderate intensity) or to the onset of the PCr slow component (heavy intensity).

For the analysis of the PCr kinetics during heavy-intensity exercise, the methods were similar to that used by Rossiter et al. (42). The fitting window was lengthened iteratively (beginning at onset to, initially, 2.1 min) until the exponential model fit demonstrated a discernible deviation from the measured response. The goodness of fit was determined by 1) maintenance of a flat residual profile, and 2) the highest correlation (R value) obtained between the measured data and the simple exponential modeled fit. The magnitude of the slow component for [PCr] was then estimated from the difference between the projected [PCr] from the simple exponential fit and the measured [PCr] at 9 min of heavy-intensity exercise.

Blood sampling.   Venous blood draining the active forearm muscle was drawn from the antecubital vein at the elbow joint during rest, at end of warm-up, and at minutes 3, 6, and 9 of moderate- and heavy-intensity exercise. Blood was drawn into syringes containing lithium heparin, then mixed, placed in ice water, and analyzed within 60 min. Whole blood samples (200 µl) were analyzed for plasma pH, PCO2, and [Lac–] (Statprofile 9 Plus blood gas-electrolyte analyzer, Nova Biomedical Canada, Mississauga, ON). Electrodes were calibrated before each test and at regular intervals during analysis. [HCO3–] was calculated from measured pH and PCO2 by using the Henderson-Hasselbach equation. Plasma [H+] ([H+]pl) was calculated from measured pH.

Statistical analysis.   Statistical analyses were performed by using SPSS version 10.0 statistical program for the personal computer (SPSS, Chicago, IL). Before analyses, the intracellular metabolic data (H+, PCr, Pi, and ATP) were each binned and averaged over 3-min periods: rest, warm-up, and the initial (Early), middle (Mid), and final (Late) 3 min of moderate- and heavy-intensity exercise. Intracellular and plasma measurements were analyzed for condition, time, and interaction effects by two-way repeated-measures ANOVA. Also, PCr kinetic parameter estimates were analyzed for condition, intensity, and interaction effects. When a significant interaction effect was found, a post hoc analysis was performed by using Student-Newman-Keuls post hoc test at specific time points. Significance was set at the P < 0.05 level for all comparisons. Data presented in the text and Tables 1 and 2 are reported as means (SD), and Figs. 1–6 are represented by means ± SE.


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  		Table 2. Phosphocreatine kinetics during transition from warm-up to moderate-intensity exercise (transition 1) and during transition from moderate- to heavy-intensity exercise (transition 2)

 


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  		Fig. 1. Intracellular H+ concentration ([H+]), venous plasma [H+], and [H+] gradient (difference between intracellular [H+] and extracellular [H+]) during rest, warm-up, and moderate- and heavy-intensity exercise in control (Con) and alkalosis (Alk) trials. At all time points, plasma H+ was greater (P < 0.05) in Alk than Con. Intracellular H+ values were different (P < 0.05) between conditions during the final 3 min of heavy-intensity exercise. No significant differences in the [H+] gradient between conditions were present. Values are means ± SE. *Difference between conditions (P < 0.05); {dagger}different from time 0 min (P < 0.05).

 


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  		Fig. 6. Plasma lactate concentration ([Lac–]) during rest, warm-up, and moderate- and heavy-intensity exercise in Con and Alk trials. Values are means ± SE. No significant differences between conditions were present. {dagger}Different from time 0 min (P < 0.05).

 

   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 ACKNOWLEDGMENTS
 REFERENCES
 
Acid-base status and intramuscular metabolites.   During preexercise resting conditions (postingestion), [H+]i was not different between Alk [pH 7.09 (SD 0.02)] and Con [pH 7.06 (SD 0.03)]. No differences in [H+]i were found between Alk and Con during moderate-intensity exercise; however, during Late-heavy, [H+]i was lower (P < 0.05) in Alk [pH 6.61 (SD 0.17)] than Con [pH 6.54 (SD 0.18)] (Fig. 1).

[PCr] was not different between Con and Alk at rest and during warm-up and moderate-intensity exercise. During heavy-intensity exercise, [PCr] was not different between conditions during the early stages of the exercise transition, but, during Late-heavy, the [PCr] in Con was lower (P < 0.05) than in Alk (Fig. 2). To provide further insight into the PCr response, the time course of adaptation of [PCr] was analyzed during the transition from warm-up to moderate- and to heavy-intensity exercise. Although our study was limited to one trial per condition and the time resolution was 36 s, the adequacy of our PCr kinetic analysis was shown by the time constants being in the expected range (~20–65 s) during the Con trial (52), the high correlations (R = 0.91–0.99) between the data and the model best fit line, and visual inspection of the residuals, which fell squarely on either side of the "zero-difference" line. An example of a representative subject is shown in Fig. 3. The time constant and amplitude of the primary component of PCr degradation during the transition to moderate- and to heavy-intensity exercise were not different between conditions (Table 2). [PCr] was greater in Alk compared with Con during Late-heavy and was a consequence of a reduced or abolished PCr slow component (Table 2).



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  		Fig. 2. Phosphocreatine concentration ([PCr]) and calculated cytosolic adenosine triphosphate concentration ([ADP]) during Alk and Con trials at rest and during warm-up and moderate- and heavy-intensity exercise. Values are means ± SE. *Difference between conditions (P < 0.05); {dagger}different from time 0 min (P < 0.05).

 


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  		Fig. 3. PCr kinetic responses during transition from warm-up to moderate- and to heavy-intensity exercise in a representative subject. Residuals are shown for each predicted fit.

 
The [Pi] presented as a mirror-type image of PCr degradation, with [Pi] increasing in direct proportion to [PCr] decreasing. [Pi] was not different between Con [3.7 mmol/l (SD 0.8)] and Alk [4.1 mmol/l (SD 0.5)] at rest and during warm-up or throughout moderate-intensity exercise. During heavy-intensity exercise, [Pi] was not different between conditions during the initial ~6 min of exercise, but was greater (P < 0.05) in Con than Alk at end exercise [Con, 28.6 mmol/l (SD 7.0) vs. Alk, 25.8 mmol/l (SD 7.7)]. [ATP] was not different between conditions during rest and it was not affected by warm-up or moderate- or heavy-intensity exercise.

Calculated free cytosolic [ADP] was not different between conditions during rest, warm-up, and throughout moderate-intensity exercise. During heavy-intensity exercise, [ADP] was greater (P < 0.05) in Con than Alk during Mid- and Late-heavy (Fig. 2). The phosphorylation potential (calculated as ln[ATP]/[ADP][Pi]) was not different between conditions during rest and moderate-intensity exercise. However, during Late-heavy, the phosphorylation potential was greater in Alk [8.46 l/mmol (SD 0.89)] than Con [8.03 l/mmol (SD 0.95)].

The PCr resynthesis rate during the initial 18 s of recovery following heavy-intensity exercise provides an estimate of the total ATP synthesis rate from both oxidative and glycolytic flux (see Eq. 1). This initial recovery period was assumed to represent oxidative and glycolytic ATP production rates at the end of heavy-intensity exercise. These findings suggest that the ATP synthesis rate was similar in Con [0.48 mmol·l–1·s–1 (SD 0.06)] and Alk [0.44 mmol·l–1·s–1 (SD 0.04)]. Also, the glycolytic flux was calculated (Eq. 2) and then subtracted from the total ATP synthesis rate to provide an estimate of the oxidative flux (Eq. 1). The oxidative flux was similar in the conditions (Fig. 4).



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  		Fig. 4. Oxidative flux at the end of heavy-intensity exercise in Con and Alk trials. Values are means ± SE. No significant differences between conditions were present.

 
Plasma metabolites.   Venous plasma [HCO3–] during preexercise rest was greater (P < 0.05) in Alk than Con, and the elevation was maintained throughout exercise (Fig. 5). Similarly, venous [H+]pl was lower (P < 0.05) at rest in Alk, and differences were maintained throughout exercise (Fig. 1). At rest and during exercise, venous [Lac–]pl was not different between Alk and Con (Fig. 6).



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  		Fig. 5. Plasma bicarbonate concentration ([HCO3–]) during rest, warm-up, and moderate- and heavy-intensity exercise in Con and Alk. Values are means ± SE. *Difference between conditions (P < 0.05); {dagger}different from time 0 min (P < 0.05).

 
H+ gradient.   The muscle-to-plasma H+ gradient increased (P < 0.05) with exercise intensity, with no differences at any time between Alk and Con (Fig. 1).


   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
ACKNOWLEDGMENTS
 REFERENCES
 
In this study, we examined the metabolic responses to constant-load moderate- and heavy-intensity forearm exercise following NaHCO3 ingestion. The main finding of this study was that the magnitude of the PCr slow component was reduced following NaHCO3 ingestion, consistent with our hypothesis that an attenuated intracellular acidosis reduces the PCr slow component. However, in contrast to our hypothesis, oxidative flux was similar in the conditions at the end of heavy-intensity exercise. Our findings suggest that, in the less acidic condition, a reduced ADP (or associated) signal for oxidative phosphorylation was required to maintain the same rate of oxidative flux during heavy-intensity exercise. An additional important finding was that the ATP demand appeared to be reduced in Alk compared with Con at the end of heavy-intensity exercise.

Rest.   In Alk, the increase in resting plasma [HCO3–] (Fig. 5) and decrease in [H+]pl (Fig. 1) following NaHCO3 were similar to previous studies that had used 0.3 g/kg dose (32). Similar to the present study, several other studies have demonstrated no differences in resting [H+]i between conditions (8, 19, 35, 38). The lack of any effect of NaHCO3 ingestion on [H+]i at rest is consistent with the hypothesis that muscle cells are relatively impermeable to HCO3– (4, 29).

Moderate-intensity exercise.   Because PCr kinetics have been shown to provide a reasonable estimate of the adaptation of muscle O2 utilization during the transition to moderate-intensity exercise (33, 39), the similar time constants and amplitude of PCr change in Alk and Con (Table 2) suggest that the rate of increase in oxidative phosphorylation also was similar in the conditions. Other studies (36, 55) examining the pulmonary O2 kinetics during transition to moderate-intensity exercise following NaHCO3 ingestion found no differences between Alk and Con in the phase II O2 time constant, reflecting a similar response of muscle O2 consumption. Thus, despite significant elevation in extracellular [HCO3–] and decreases in extracellular [H+] in Alk, intracellular metabolic events appear not to be affected during moderate-intensity exercise.

Attenuated intracellular acidosis.   Like the present study, a lower [H+]i during heavy-intensity exercise in Alk compared with Con was found in repeated 1-min cycling sprints (8), a 1-h cycling time trial (46), and 5 min of dynamic high-intensity handgrip exercise (35). In contrast, others have not shown a reduced [H+]i following Alk (3, 18, 19). Differences between studies may be explained by methodical differences. Hollidge-Horvat et al. (18) estimated [H+]i based on muscle [Lac–], subjects in Bishop et al. (3) performed more work in Alk than Con, and in the study by Hood et al. (19) the exercise involved squeezing a ball and, therefore, the work was not able to be accurately quantified.

A common explanation provided for a reduced intracellular acidosis in Alk is that a steeper H+ gradient exists between muscle and plasma, causing a greater efflux of H+ and [Lac–] through the Lac–-H+ cotransporter (32). In the present study, a greater H+ gradient was not evident in Alk. Furthermore, if the reduced intramuscular acidosis in Alk was due to a greater efflux of H+ and Lac–, a greater [Lac–]pl would be expected but was not found. An alternative explanation of the reduced intracellular acidosis in Alk during the later stages of heavy-intensity exercise in the present study may involve an elevated intracellular strong ion difference due to an increased intracellular [Na+] through enhanced activity of the H+/Na+ exchangers (22). Although we did not measure intramuscular [Na+], NaHCO3 ingestion was associated with an elevated muscle [Na+] during exercise in the perfused rat hindlimb model (28). An increase in intramuscular [Na+] would cause an increase in the intracellular strong ion difference ([sum of strong cations] – [sum of stong anions]) and, at constant muscle PCO2 and weak acid concentration, would contribute to a decrease in [H+]i through physicochemical changes (24).

Effect of acidosis on oxidative flux.   In the present study, the finding that the estimated oxidative flux was similar in Alk and Con at the end of heavy-intensity exercise is consistent with previous studies that have shown that NaHCO3 ingestion does not alter pulmonary O2 at the end of 6 min (23, 43, 54) and 30 min (17) of heavy-intensity cycling. Our results indicated a similar oxidative flux in conditions, despite Alk demonstrating a reduced signal (i.e., ADP or phosphorylation potential) that drives oxidative phosphorylation. This finding suggests that acidosis impairs the effectiveness of the signal on mitochondrial oxidative phosphorylation. The stronger potentiation of the signal in Alk may have resulted in a decrease in the quantity of the signal required, and this was reflected by a reduced magnitude of the PCr slow component.

Consistent with our findings, Jubrais et al. (21) demonstrated that oxidative flux was not elevated, despite a greater [ADP] in acidic (i.e., pH < 6.88) hand and leg exercise compared with sustainable exercise, where pH remained close to resting levels. In this study, the highest oxidative flux occurred in bouts of exercise that did not involve acidosis. An effect of acidosis on mitochondrial function has been previously shown in isolated cat muscle (14). In that study, an induced acidosis by hypercapnia resulted in an approximately threefold decrease in oxidative capacity. Walsh et al. (50) demonstrated that at a pH of 6.6, skinned fibers had a lower oxidative flux compared with a pH of 7.0 at constant, submaximal [ADP]. However, at an [ADP] of 0.4 mmol, which elicits maximal rates of oxidative phosphorylation, oxidative flux was not altered in the different pH conditions. These results support that the inhibiting effects of acidosis on oxidative phosphorylation can be compensated by a greater quantity of the signal, in congruence with the finding of the present study that [ADP] was elevated to a greater extent in the more acidic condition (Con) to maintain a similar level of oxidative flux. Therefore, since a signal for oxidative phosphorylation is linked to the creatine kinase reaction, to elevate the quantity of the signal, PCr must be degraded to a greater extent, thus consistent with the greater magnitude of the PCr slow component in the more acidic condition (Con).

There are several possible explanations of why acidosis could decrease the effectiveness of the signals driving oxidative phosphorylation. Acidosis has been suggested to have a direct effect on the mitochondrial membrane by causing a dissociation of creatine kinase enzyme from the inner mitochondrial membrane, resulting in a reduced [ADP] in proximity to the mitochondria (50). Alternatively, a more optimum [H+]i during heavy-intensity exercise may increase substrate provision to the electron transport chain by providing a more favorable acid-base status for certain oxidative flux enzymes. Pyruvate dehydrogenase (PDH) phosphatase has been shown to have an optimum pH of 6.7–7.1 (20), and an enhanced PDH activity by dichloroacetate administration (an inhibitor of PDH kinase) has been previously demonstrated to reduce the PCr slow component (41). An enhanced PDH activity and a subsequent greater NADH accumulation would result in a greater redox drive (48). Consistent with Wilson's model of metabolic control (53), if the redox drive was greater in Alk compared with Con, a reduced drive from the phosporylation potential would be expected (and, therefore, less PCr degradation) for the same rate of oxidative phosphorylation.

ATP cost of contractions.   The finding that both Alk and Con had similar ATP synthesis rates from the oxidative and glycolytic pathways, in combination with a reduced amplitude of the PCr slow component in Alk, suggests a lower ATP demand in Alk at the end of heavy-intensity exercise. Because PCr degradation has been thought to indicate an imbalance between ATP supply from oxidative phosphorylation and ATP demand (7), the reduced degradation of PCr in Alk indicates that ATP supply from oxidative flux better matched ATP demand. A reduced acidosis in Alk may have delayed fatigue in fibers by reducing inhibition on the myofibrillar ATPase, Ca2+ binding to troponin C, and the sarcoplasmic ATPase (11). Therefore, Alk would have resulted in a lessened requirement to recruit additional, presumably less efficient, type II fibers. The ATP cost of force production has been shown to be greater in type II fibers than type I fibers in vitro (9) and in primarily type II (biceps) than type I (soleus) cat muscle (13). However, because the present study did not measure recruitment patterns and because the effects of acidosis on fatigue have been recently questioned (51), this hypothesis clearly requires further investigation. Nonetheless, the possibility that an increased ATP demand may be an important contributing factor to the PCr slow component should not be ruled out. Support for the PCr slow component being caused by a progressive increase in ATP demand during heavy-intensity exercise comes from the finding that the PCr slow component has been shown to coincide temporally with the pulmonary O2 slow component (42). This temporal relationship may imply that the underlying mechanism for the O2 and PCr slow component is the same. Recently, using muscle biopsy techniques, Krustrup et al. (26) demonstrated that the onset of the O2 slow component coincided with the recruitment of type II fibers during cycling at 80% of maximum O2. In constrast, the lack of association between the onset of the slow component and altered recruitment patterns in previous studies utilizing iEMG (44, 45, 49) is difficult to reconcile. However, the differences in findings may be due to limitations of iEMG to detect changes in muscle recruitment during heavy-intensity dynamic exercise.

In summary, our findings suggest that a reduced intracellular acidosis decreased the magnitude of the PCr slow component. Our results are consistent with a reduced intracellular acidosis being associated with a lower ATP demand. Furthermore, the finding of a lower [ADP] in Alk, despite a similar rate of oxidative flux at the end of heavy-intensity exercise, is consistent with the hypothesis that, at submaximal exercise intensities, the inhibition of acidosis on the rate of oxidative phosphorylation can be overcome by a greater [ADP].


   ACKNOWLEDGMENTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 ACKNOWLEDGMENTS
REFERENCES
 
We thank Dr. R. T. Thompson in the Imaging Division of the Lawson Health Research Institute for technical assistance and for providing research time on the MRS unit.


   FOOTNOTES
 

Address for reprint requests and other correspondence: G. D. Marsh, School of Kinesiology, The Univ. of Western Ontario, London, Ontario, Canada N6A-3K7 (e-mail: gdmarsh{at}uwo.ca)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 ACKNOWLEDGMENTS
 REFERENCES
 

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