INTRODUCTION

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INCREASED WHOLE BODY OXYGEN FLUX during exhaustive aerobic exercise may elicit potentially toxic pertubations in cellular homeostasis via increased free radical production. The measurement of free radicals in biological systems is difficult because of their high reactivity and low steady-state concentration. Electron spin resonance (ESR) spectroscopy is arguably the most sensitive, specific, and direct method of measuring free radical species and is currently underutilized in the clinical and physiological environment. Until recently, ESR spectroscopy had only been used in the animal model to demonstrate increases in free radical concentration after exercise (4, 18). Using ESR spectroscopy, we recently reported that maximal aerobic exercise resulted in significant increases in the concentration of the -phenyl-tert-butylnitrone (PBN) adduct and also lipid hydroperoxides (LH) and malondialdehyde (MDA) in the venous circulation of healthy human volunteers (2). LH are considered to be the major initial reaction products of free radical attack on the cell membrane, whereas MDA is formed as a decomposition product of LH, thus justifying their use as indirect determinants of free radical-mediated oxidative damage. Free radicals, defined as any species containing an unpaired electron that is capable of independent existence, are, by definition, highly reactive and cause damage to DNA, cell membranes, and proteins (8). Oxygen radicals such as superoxide anion are continually generated in vivo by a number of pathways including mitochondrial electron-transport chain, xanthine oxidase, and activated phagocytes (17). Additionally, superoxide may combine with nitric oxide (NO ·) to form the damaging peroxynitrite (ONOO), as shown in Eq. 1.

(1)

Therefore, if superoxide is produced at an increased rate during exercise because of highly respiring mitochondria, one potential consequence of this may be increased endothelial damage via increased peroxynitrite formation. Alternatively, superoxide may inhibit the vascular relaxant effects of nitric oxide, leading to altered endothelial function.

Ascorbic acid is a water-soluble antioxidant, able to scavenge aqueous superoxide, peroxyl, and alkoxyl radicals and inhibit lipid peroxidation (3). Importantly, decreased levels of plasma ascorbic acid have been reported in physically active men in the United Kingdom (9). Recently, however, it has been suggested that ascorbic acid exhibits both antioxidant and prooxidant properties in vivo (15). Thus the aim of the present study was to determine the effect of ascorbic acid supplementation on the ESR signal intensity of the PBN adduct in the venous circulation of healthy human volunteers after maximal aerobic exercise. In addition to this, the effect of ascorbic acid supplementation on supporting assays of exercise-induced lipid peroxidation is also reported.

	METHODS

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Subject characteristics. Ten subjects volunteered for this study and were required to perform an incremental exercise test to exhaustion (control phase). The same subjects were then required 8 wk later to repeat the exercise test after supplementation with ascorbic acid (supplementation phase). Subjects were healthy male students (aged 18-30 yr; height 1.77 ± 1.6 m; body mass 78.6 ± 3.3 kg). All were nonsmokers and free of any physician-diagnosed disease. Subjects taking vitamin supplements were excluded. Written informed consent was obtained before participation, and ethical approval was obtained from a Local Research Ethics Committee.

Ascorbic acid supplementation. An acute oral dose of 1,000 mg of L-ascorbic acid (Hoffman-LaRoche, UK) was given in two 500-mg tablets 2 h before the subjects performed the maximal oxygen uptake (O_{2 max}) test, allowing plasma levels to increase and resulting in saturation of the plasma with ascorbic acid.

Blood sampling. Blood was collected from an antecubital forearm vein by using a vacutainer system (Becton-Dickinson, Oxford, UK). The resting (preexercise) blood sample was taken with the subject seated in a chair and resting for 5 min, whereas the postexercise samples were taken immediately on cessation of exercise. After immediate centrifugation at 3,500 rpm for 12 min, the samples were frozen within 30 min to 80°C and stored for a maximum of 8 wk before analysis. However, without exception, all ESR samples underwent same-day analysis. Additional blood samples were taken and used to detect exercise-induced hemoconcentration via changes in hematocrit level.

Sample extraction procedure and ESR analysis. The sample extraction procedure using HPLC-grade toluene that was previously scanned by ESR for the presence of artifactual radicals, combined with vacuum degassing employed in the present study, is identical to that previously reported (2). Room-temperature ESR analysis was carried out on a JEOL RE2X series X-band spectrometer with 100-KHz field modulation by using the following operating conditions: microwave frequency 9.436 GHz; incident microwave power 10 MW; scan width ± 4.000 mT; modulation amplitude 0.1000 mT; magnetic field center 334.6 mT; scan time 4.0 min; time constant 0.10 or 0.30 s. ESR conditions were identical before and after exercise and between studies, with the exception of spectrometer gain. Additionally, samples from the ascorbic acid-supplemented subjects were analyzed by using increased spectrometer gain to attempt to detect the presence of any small ESR signal.

Measurement of plasma lipid peroxidation and ascorbic acid concentration. Lipid peroxidation was assessed by using two established assays. MDA was measured by HPLC with fluorometric detection in EDTA plasma (23). This method overcomes the lack of specificity generally associated with the measurement of MDA. LH concentration was measured by using the ferrous iron-xylenol orange assay in a clotted serum sample (13). This method measures the susceptibility to iron-induced LH formation in serum. The presence of iron ions in the assay procedure may, therefore, yield slightly higher LH values compared with other methods. Plasma ascorbic acid was measured by using a fluorometric technique (20). The technique is based on the kinetics of fluorescence development by condensation of dehydroascorbic acid with 1,2-phenylenediamine. The enzymatic oxidation of ascorbic acid with ascorbate oxidase confers specificity to the assay without the need for chromatographic separation. After centrifugation, the blood plasma was immediately stabilized and deproteinated by the addition of 900 µl of 5% metaphosphoric acid to 100 µl EDTA plasma. Plasma total antioxidant capacity was measured by using enhanced chemiluminescence and is expressed as Trolox equivalents (22).

Exercise protocol. The exercise test employed in this study is identical to that previously reported (2). Briefly, the subjects were required to cycle to volitional exhaustion on a calibrated cycle ergometer (Monark 824, Monark, Stockholm, Sweden). The test is incremental and progressive and designed to elicit O_{2 max}. Breath-by-breath oxygen uptake was continually recorded during the test by using a computerized on-line gas-analysis system (Medgraphics, Manchester, UK). Heart rate was also continually recorded by using a portable electrocardiograph-calibrated heart rate telemetry system (Polar Sport Tester, Kenilworth, UK). Subjects were instructed to refrain from exercise and alcohol for 24 h before the test. Criteria for objective determination of O_{2 max} were as follows: respiratory exchange ratio >1.15 at termination of test; plateauing of oxygen uptake curve where observed; heart rate approximating 220 beats/min  age; and failure of subjects to cycle at 60 rpm despite verbal encouragement. The tests were carried out in the morning after an overnight fast.

Statistical analysis. Statistical analysis was carried out by using a statistical package for social sciences (SPSS, Surrey, UK). Results are expressed as means ± SE, and P < 0.05 was considered statistically significant. Identification of significant differences was carried out via the Wilcoxon signed-rank matched-pairs test, whereas the Spearman correlation coefficient was used to determine the strength of relationship between two dependent variables.

	RESULTS

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There was no significant difference in any of the physiological parameters between the control and supplementation phases. Mean whole body O_{2 max} was measured as 49.85 ± 2.12 and 47.43 ± 1.95 ml · kg¹ · min¹, whereas mean postexercise respiratory exchange ratio was measured as 1.22 ± 0.01 and 1.21 ± 0.02 for the control and supplementation groups, respectively. Maximal postexercise heart rates were 192 ± 4 and 187 ± 3 beats/min for the control and supplementation groups, respectively, whereas time to exhaustion was 15.30 ± 0.30 and 16.29 ± 1.03 min, again, for control and supplementation groups, respectively. There was no significant change in hematocrit values after exercise for either the control group (43.2 ± 1.1% packed cell volume preexercise vs. 44.8 ± 1.9% packed cell volume postexercise) or supplementation group (42.8 ± 1.5% packed cell volume preexercise vs. 43.9 ± 2.4% packed cell volume postexercise). Supplementation with 1,000 mg of ascorbic acid resulted in significant increases in plasma ascorbic acid concentration, from 26.28 ± 5.77 µmol/l presupplementation to 117.54 ± 8.96 µmol/l postsupplementation, P = 0.005; and plasma total antioxidant capacity increased from 510 ± 45.1 µmol/l presupplementation to 1,680 ± 36.1 µmol/l postsupplementation (Trolox equivalents), P = 0.002. Results for the unsupplemented group demonstrate a parallel stimulation by exercise in all oxidative stress-related assays, whereas after supplementation with ascorbic acid, strenuous aerobic exercise resulted in no significant increase in free radical production in vivo (see Table 1).

	DISCUSSION

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The purpose of the present study was to examine the effect of antioxidant intervention on exercise-induced increases in free radical production as measured by ESR and indexes of free radical-mediated lipid peroxidation. Figure 1 demonstrates clear postexercise increases in the intensity of the PBN adduct, indicating increased free radical production, since the intensity of the signal is proportional to the concentration of radicals in the sample. The administration of an acute dose of 1,000 mg of L-ascorbic acid prevented significant increases by exercise in all of the free radical-related parameters measured, which suggests that ascorbic acid is an effective antioxidant in the prevention of exercise-induced oxidative stress. Importantly, the postexercise ESR intensity seen in the supplemented subjects (Fig. 2) was similar to the resting ESR signal in the controls. Thus the dramatic increase in postexercise PBN adduct formation is not seen after ascorbic acid supplementation.

View larger version (22K): [in this window] [in a new window]   Fig. 1.   Pre- (A) and postexercise (B) electron spin resonance (ESR) spectra of -phenyl-tert-butylnitrone (PBN) adduct in human plasma after maximal aerobic exercise (control phase) without ascorbic acid supplementation.

View larger version (11K): [in this window] [in a new window]   Fig. 2.   Pre- (A) and postexercise (B) ESR spectra of PBN adduct in human plasma after supplementation with ascorbic acid.

The hyperfine coupling constants recorded from the ESR spectra of the PBN adduct were, for nitrogen and hydrogen, respectively, a_N = 1.37 mT, a_H = 0.19 mT for control group and a_N = 1.37 mT, a_H = 0.16 mT for the supplemented group, suggesting that the species detected in the present study are secondary alkoxyl radicals formed as a consequence of primary oxygen-centered radical attack on membrane phospholipids. The coupling constants seen in the present study compare favorably to those previously reported (a_N = 1.36 mT; a_H = 0.15 mT) in reperfused rat heart (7). Garlick et al. (7) suggested that the species found in the reperfused rat heart were either carbon-centered or alkoxyl radicals formed via reaction of primary oxygen-centered radicals with membrane lipids, which supports the conclusion drawn in the present study. Additionally, Tortolani et al. (19) have reported similar values of coupling constants (a_N = 1.36 mT and a_H = 0.19 mT), which they attributed to secondary carbon-centered or alkoxyl radical formation in the blood of patients undergoing elective cardioplegia, again supporting the interpretation of the ESR data seen in the present study. However, whereas the hyperfine coupling constants are similar among the various reported studies, the observed differences may reflect differences in experimental design, e.g., choice of solvent, which may influence the hyperfine coupling constants.

The most common aqueous radical is the hydroperoxyl radical, which is generated in equal amounts with superoxide and is scavenged by ascorbic acid, whereas ascorbic acid may make a relatively greater contribution than vitamin E to the plasma antioxidant defense mechanism (21). If this is the case, then ascorbic acid will scavenge peroxyl and alkoxyl radicals, preventing any increase in the postexercise ESR signal intensity. This is further supported by a lack of increase in the supporting assays of free radical-mediated lipid peroxidation, which is a radical chain reaction. Supplementation with ascorbic acid resulted in a 50% decrease in the baseline ESR signal compared with controls, suggesting a suppression of resting free radical production, whereas increases in the plasma total antioxidant capacity imply improved antioxidant status. The mechanism of action of the antioxidant properties of ascorbic acid involves direct interaction and scavenging of aqueous lipid-derived peroxyl radicals and chain breaking in lipid peroxidation (3). Additionally, the indirect antioxidant properties of ascorbic acid include regeneration of vitamin E from the tocopheroxyl radical at the aqueous-lipid interface (12), whereas ascorbic acid is itself regenerated by glutathione (10). The two-step reversible oxidation of ascorbic acid yields dehydroascorbic acid with the formation of the intermediate ascorbate free radical. The delocalized nature of the unpaired electron on the ascorbyl radical makes it comparatively unreactive, although it is able to donate an electron to other free radicals, thereby stabilizing the radical and preventing propagation of radicals leading to lipid peroxidation. The identification of the species as secondary alkoxyl radicals, probably derived from membrane phospholipids, and their location in the aqueous phase of the blood, provide a plausible biochemical explanation for the effectiveness of ascorbic acid in the present study. However, what is not clear from the present study is precisely where ascorbic acid acted to scavenge the radicals produced by exercise. It is possible that ascorbic acid acted intracellularly, since cells are saturated at doses of 200 mg. We suggest that the most likely site of ascorbic acid scavenging is at the aqueous-lipid interface of the cell, which would allow ascorbic acid to not only scavenge intracellular aqueous radicals but also regenerate vitamin E in the phospholipid bilayer. However, it is equally plausible that the effect of ascorbic acid was only a blood phenomenon, since plasma is saturated at doses of 1,000 mg, and thus ascorbic acid could scavenge any blood-borne radicals. Further work is undoubtedly required to answer this question.

Mean baseline levels of lipid peroxidation also appeared to be decreased after ascorbic acid supplementation compared with the control phase, although this difference was not statistically significant. Ascorbic acid supplementation did, however, prevent a significant increase in LH and MDA after maximal aerobic exercise. This may be an unusual observation in that ascorbic acid is a water-soluble antioxidant and perhaps as such would not be expected to inhibit lipid peroxidation or scavenge what may be lipid-derived radicals in origin. However, it has been recently reported (1) that exercise-induced lipid peroxidation was highest in healthy, physically active male subjects when they were not supplemented with ascorbic acid; thus the lipid peroxidation data in the present study are in agreement with reports in the literature. Also, in a randomized, placebo-controlled trial, supplementation with 1,000 mg of ascorbic acid was reported to enhance the total radical-trapping ability of the plasma (11). One possible mechanism by which the effect of ascorbic acid may be explained is that, since it is an effective reducing agent, donation of an electron by ascorbic acid to a peroxyl radical would have the effect of stabilizing the radical, thus preventing propagation of lipid peroxidation; it may also act further up the chain by scavenging superoxide.

Increased oxygen flux through intermediate metabolism during exercise increases the rate of oxygen free radical production and alters cellular antioxidant status (14). Furthermore, exercise participation can itself modulate interactions between nutritional status and immune function, especially via increased intake of antioxidants to protect the physically active person against an augmented production of free radicals due to increased tissue metabolism and minor muscle injuries (16). Ascorbic acid has been described as an outstanding antioxidant in human blood plasma (5), and Frei et al. (6) have further suggested that a simple controlled regimen of ascorbic acid supplementation may prove helpful in preventing the formation of LH, which might not otherwise be detoxified by the endogenous plasma antioxidants, thus causing damage to critical targets (6). The present study demonstrates a decrease in all parameters associated with oxidative damage and an enhancement of the antioxidant defenses in healthy human subjects before and after maximal aerobic exercise. It demonstrates an attenuation by ascorbic acid of the ESR signal and free radical-mediated lipid peroxidation products in human blood pre- and postexercise. It is concluded from these results that an acute ascorbic acid supplementation prevents the significant increase in the concentration of the PBN adduct and lipid peroxidation and may be considered to be an effective antioxidant in the prevention of exercise-induced oxidative stress.