Abstract
Blood levels of hypoxanthine (HX) have been suggested as potential biomarkers associated with intramuscular metabolic dynamics in response to exercise. This pilot randomized crossover trial (UMIN000036520) aimed to investigate the changes in plasma HX after whole-body vibration exercise (WBVE) and their relationships with body composition and muscle-related parameters, enrolling eighteen healthy male volunteers. In the WBVE-alone intervention, the study subjects performed 20-min of WBVE. In the OGTT → WBVE intervention, a 75-g oral glucose load (OGL) was administered 30 min prior to the start of the WBVE intervention. Blood samples were collected before the start and 10 min after the end of WBVE in both interventions. WBVE resulted in a significant increase in plasma HX levels, which was accompanied by increased blood ammonia, pyruvic acid, and lactic acid levels. The HX increase following WBVE was suppressed by prior OGL. In the WBVE-alone intervention, there were no significant correlations between the post-WBVE changes in plasma HX (ΔHX) levels and any of the clinical parameters. On the other hand, in the OGTT → WBVE intervention, ΔHX showed significant negative correlations with muscle mass (ρ = –0.62, p = 0.01), strength (ρ = –0.71, p = 0.005), and muscle quality (ρ = –0.81, p = 0.0007) in the legs. In conclusion, these findings suggest possible associations between post-WBVE increases in plasma HX levels and muscle status, particularly under the glucose-supplemented condition. The measurement of plasma HX concentrations following WBVE may have clinical applications in the identification of high-risk populations for sarcopenia.
Introduction
In an aging society such as Japan, sarcopenia poses a significant challenge in extending healthy life expectancy. Sarcopenia, characterized by the loss of muscle mass and strength with aging, not only affects physical function but also increases the risk of falls, fractures, and other adverse health outcomes, including cognitive decline, cardiovascular disease, and mortality [1, 2]. Sarcopenia often involves a decrease in the efficiency of muscle energy metabolism, including mitochondrial dysfunction, which can contribute to muscle weakness and loss of mass [3]. Therefore, in addition to traditional approaches such as aerobic exercise, resistance training, and adequate protein intake to preserve skeletal muscle function [4], it is important to understand the metabolic dynamics associated with muscle status in the early or presymptomatic stage of the disease.
During exercise, muscles rely on several pathways to generate adenosine triphosphate (ATP) to meet increased energy demands [5]. One of the main pathways for ATP production during short bursts of high-intensity exercise is the breakdown of creatine phosphate stored in muscle cells. Another important pathway is glycolysis, which involves the breakdown of glucose into pyruvate, resulting in the production of ATP, and pyruvate can then be further metabolized to produce additional ATP through either aerobic or anaerobic metabolism. Additionally, in the adenylate kinase reaction, two molecules of adenosine diphosphate (ADP) are converted into one molecule of ATP and one molecule of adenosine monophosphate (AMP). As a metabolite that can be associated with ATP metabolism in skeletal muscle, previous reports have highlighted the phenomenon of elevated blood hypoxanthine (HX) levels following exercise [6]. HX is a purine metabolite produced by the degradation of inosine monophosphate (IMP) through inosine. In addition to de novo purine synthesis through the pentose phosphate pathway, IMP is generated with ammonia by AMP deaminase (AMPD)-mediated AMP degradation. Several previous reports demonstrated that the post-exercise increase in blood HX levels dynamically changed during an annual training cycle; thus, HX was indicated to be a useful biomarker during the training process in highly trained athletes [7-10]. However, the relationships between exercise-induced changes in HX and muscle status remain unclear, particularly in nonathlete, healthy subjects.
Whole-body vibration exercise (WBVE) has emerged as a convenient, safe, and effective intervention for enhancing muscle activity and function [11, 12]. WBVE involves standing, sitting, or lying on a platform that generates mechanical vibrations, leading to involuntary muscle contractions and stimulation of neuromuscular pathways. This modality of exercise has been shown to improve muscle strength, power, and flexibility, making it attractive even for the elderly or individuals with limited mobility [13, 14].
The purpose of this pilot study was to investigate the change in plasma HX concentration following WBVE and its associations with body composition and muscle-related parameters in healthy male subjects.
Materials and Methods
Study design
The present study is a post hoc analysis of a previously reported study, originally aiming to determine the acute effects of WBVE on post-load glucose metabolism [15]. In the study, a 2-h 75-g oral glucose tolerance test (OGTT) was performed to examine the acute effects of WBVE on glucose metabolism in healthy men. The detailed study design has been described previously [15]. The study was an open-label, randomized, crossover trial conducted at Osaka University from July 2019 to July 2020 involving 18 healthy male volunteers. Each study participant received all of the three following interventions in random order: (1) receiving a 75-g oral glucose load (OGL) without any exercise including WBVE (OGL-alone), (2) 20 min of WBVE before receiving an OGL (WBVE-alone), and (3) 20 min of WBVE after receiving an OGL (OGL → WBVE). The present study aimed to examine blood metabolites altered by WBVE; therefore, the blood samples obtained from the (2) WBVE-alone and (3) OGL → WBVE interventions were reanalyzed. OGLs were administered at approximately 10:00 AM after an overnight fast. In the OGL → WBVE intervention, WBVE was started 30 min after glucose ingestion.
This study was conducted in accordance with the Declaration of Helsinki and approved by the Institutional Review Board of Osaka University Hospital (approval numbers 18452-3, 21387-4, and 24081). Prior to participation, all participants provided written informed consent. The trial was registered at the UMIN Clinical Trials Registry under the identifier UMIN000036520.
Study participants and allocation
As previously reported [15], 18 healthy male volunteers aged 20–59 years were enrolled in the trial. The exclusion criteria included a history of diabetes or glucose intolerance, diseases potentially related to abnormal glucose homeostasis, symptoms that could be worsened by vibration (such as dizziness), and a risk of falling. Allocation concealment was ensured using sequentially numbered opaque sealed envelopes. The sequences and interventions were not blinded and were opened after the assignment.
WBVE
WBVE was performed using a WBV platform (Personal Power Plate®, Performance Health Systems, LLC, Northbrook, IL), which mechanically generated a three-dimensional WBV load in the vertical, horizontal, and sagittal planes. The duration and type of WBVE were determined based on previous report [16]. In brief, the subjects performed a 60-s static squat on the platform 10 times with a 60-s rest between each squat. During the static squat, the subjects stood on the platform with their knees slightly bent (knee angle of 90°–110° flexion) and their feet shoulder-width apart. The WBV platform had a preset frequency of 35 Hz and a vertical amplitude of 2–4 mm, while the horizontal and sagittal axes had an amplitude of approximately one-seventh of the vertical amplitude.
Blood measurements
Blood samples were obtained through an indwelling venous catheter and immediately centrifuged at 1,000 rpm for 10 min at 4°C. The resulting plasma and serum were stored at –20°C until analysis. Plasma HX, xanthine (Xan), and uric acid (UA) concentrations were measured as previously described [17]. In brief, the plasma samples obtained using blood collection tubes coated with ethylenediaminetetraacetic acid (EDTA)-2K were added to methanol containing [13C3, 15N]-HX, [13C2, 15N2]-Xan, and [13C2, 15N2]-UA as internal standards and then centrifuged at 3,000 × g at 4°C for 15 min. The supernatant (40 μL) was diluted with distilled water (160 μL), and the concentrations of HX, Xan, and UA were determined by LC/TQMS (Nexera, Shimadzu/QTRAP 4500, SCIEX). Measurements of the ammonia, pyruvic acid, lactic acid, creatine kinase (CK), and lactate dehydrogenase (LDH) levels were performed by SRL, Inc. (Tokyo, Japan).
Clinical examination and definitions
The fat mass and muscle masses of the arms, legs, and trunk were measured using an InBody 770 system (InBody Japan Inc.), which applies a bioelectrical impedance analysis (BIA) method. The muscle mass measured by the BIA method also correlates with that measured by dual-energy X-ray absorptiometry [18]. Grip strength (kg) was measured in the standing position using an isokinetic dynamometer (Smedley’s hand dynamometer) on the right and left hands, and then the average grip strength was calculated. Leg strength (kg) was measured using a pull-type handheld dynamometer (Mobie MT-100, Sakai Medical), in which the isometric knee flexion strength was isometrically measured twice, alternating between the left and right sides [19]. The maximum values from both sides were averaged.
The skeletal muscle index (SMI) was defined as the height-adjusted appendicular skeletal muscle mass—the muscle mass of the arms and legs/height2 (kg/m2)—based on the definition recommended by the Asian Working Group for Sarcopenia (AWGS). Arm muscle quality was defined as the ratio of the grip strength to the entire arm muscle mass [grip strength (kg)/arm muscle mass (kg)] [20]. Similarly, leg muscle quality was defined as the ratio of leg strength to the entire leg muscle mass [leg strength (kg)/leg muscle mass (kg)]. These values were measured on both sides and then averaged.
Statistical analysis
All values are presented as the number of subjects and the median (interquartile range [IQR]) for continuous variables. The Wilcoxon rank sum test was used to determine the differences between the WBVE-alone and OGL → WBVE interventions in terms of the plasma HX, Xan, and UA levels at each time point (pre- and post-WBVE) and the post-WBVE changes in the plasma HX (ΔHX) level. The time-course changes in the plasma HX, Xan, and UA levels after WBVE were analyzed using the Wilcoxon signed rank test for each intervention. The correlations between ΔHX and changes in the Xan (ΔXan) level after WBVE and each clinical parameter were analyzed using Spearman’s correlation coefficient. In all cases, a two-sided test was used and p values <0.05 were considered statistically significant. All analyses were conducted using JMP Statistical Discovery Software 15.0 (SAS Institute, Cary, NC, USA).
Results
Changes in blood levels of HX and other exercise-related metabolites after WBVE
First, to assess post-WBVE changes in the blood levels of exercise-related metabolites, including HX, blood samples were collected from one healthy male volunteer before WBVE and 20, 25, and 30 min after the start of the 20-min WBVE (Fig. 1A). As shown in Fig. 1B, the plasma HX concentration was initially low at baseline, but it gradually increased after WBVE, reaching a peak at 30 min (10 min after the end of WBVE). WBVE caused an increase in the level of ammonia, a metabolite produced when AMP is metabolized to IMP by AMPD, which peaked earlier than that of HX (Fig. 1C). Blood concentrations of pyruvic acid (Fig. 1D) and lactic acid (Fig. 1E) also increased during WBVE. On the other hand, WBVE did not affect serum CK (Fig. 1F) or LDH (Fig. 1G) levels. These results suggest that WBVE can induce an immediate increase in the plasma HX level, which is associated with AMP degradation and glycolysis but not muscle tissue damage.
Pre-exercise glucose supplementation can affect skeletal muscle metabolism, including ATP production through glycolysis [21-23]. We next investigated the impact of an OGL on the post-WBVE changes in plasma HX levels in the 18 healthy male volunteers. Table 1 shows the clinical characteristics of the participants, who had a median age of 33 years (IQR 31–36) and a median body mass index (BMI) of 21.3 kg/m2 (IQR 19.4–22.8). For 15 subjects who underwent body composition measurements, the percent body fat mass was 18.7% (IQR 15.1–24.0), and the skeletal muscle mass was 30.5 kg (IQR 27.7–33.0) (SMI; 7.7 kg/m2 (IQR 7.2–8.3)). Grip and leg strength were measured in 15 and 14 subjects and were 44.0 kg (IQR 42.0–48.5) and 40.8 kg (IQR 38.0–51.3), respectively. According to the AWGS criteria, 4 subjects had low skeletal muscle mass (SMI <7.0 kg/m2), but none had weak grip strength (<28 kg); thus, no subjects met the diagnosis of sarcopenia. Arm muscle quality, defined as the ratio of grip strength (kg) to arm muscle mass (kg), was 17.5 (IQR 15.3–19.1) (n = 15), and leg muscle quality, defined as the ratio of leg strength (kg) to leg muscle mass (kg), was 4.5 (IQR 4.1–5.4) (n = 13).
Table 1 Clinical characteristics of the study subjects
| Clinical parameters |
| Age (years, n = 18) |
33.0 [31.0–35.8] (28–50) |
| BW (kg, n = 18) |
65.5 [60.0–71.9] (50–107) |
| BMI (kg/m2, n = 18) |
21.3 [19.4–22.5] (17.5–36.6) |
| Fat mass (kg, n = 15) |
11.9 [10.2–16.6] (5.8–29.6) |
| Percent body fat (%, n = 15) |
18.7 [15.1–24.0] (9.6–33.1) |
| Skeletal muscle mass (kg, n = 15) |
30.5 [27.7–33.0] (23.9–36.3) |
| Arm muscle mass (kg, n = 15) |
5.4 [4.9–6.2] (4.0–7.0) |
| Trunk muscle mass (kg, n = 15) |
22.6 [21.8–25.1] (18.6–27.3) |
| Leg muscle mass (kg, n = 15) |
18.5 [17.3–20.0] (13.3–21.2) |
| SMI (kg/m2, n = 15) |
7.7 [7.2–8.3] (6.4–9.6) |
| Grip strength (kg, n = 15) |
44.0 [42.0–48.5] (39.0–60.0) |
| Leg strength (kg, n = 14) |
40.8 [38.0–51.3] (22.6–57.5) |
| Arm muscle quality (n = 15) |
17.5 [15.3–19.1] (11.9–21.1) |
| Leg muscle quality (n = 13) |
4.5 [4.1–5.4] (3.0–5.7) |
The data are presented as the number of subjects or median [interquartile range (IQR)] (minimum-maximum). BW, body weight; BMI, body mass index; SMI, skeletal muscle index. SMI = muscle mass of the arms and legs (kg)/height (m)2. Arm muscle quality = grip strength (kg)/arm muscle mass (kg). Leg muscle quality = leg strength (kg)/leg muscle mass (kg).
As shown in Fig. 2A, the study participants underwent WBVE with or without an OGL in a crossover design (OGL → WBVE intervention or WBVE-alone intervention). For the OGL → WBVE intervention, a 75-g OGL was administered 30 min prior to the start of WBVE. Blood samples were collected before the start (Pre) and 10 min after the end (Post) of the 20-min WBVE in both interventions. Consistent with the results obtained from one subject (Fig. 1B), the WBVE-alone intervention resulted in significantly increased plasma HX levels (Fig. 2B, OGL(–)/Pre-WBVE vs. OGL(–)/Post-WBVE). In the OGL → WBVE intervention, the baseline plasma HX concentrations were slightly but significantly lower than those in the WBVE-alone intervention (Fig. 2B, OGL(–)/Pre-WBVE vs. OGL(+)/Pre-WBVE). Furthermore, the post-WBVE increase in HX concentrations was significantly suppressed by prior OGL (Fig. 2B, OGL(–)/Post-WBVE vs. OGL(+)/Post). Overall, changes in plasma HX levels after WBVE (ΔHX) were significantly decreased in the OGL → WBVE intervention compared to that in the WBVE-alone intervention (Fig. 2C). However, interestingly, we also found that, in certain subjects with elevated HX levels in the WBVE-alone intervention, ΔHX was not suppressed or only slightly suppressed with prior OGL (as indicated by the arrows in Fig. 2C). Changes in the plasma levels of Xan and UA, metabolites of HX produced by xanthine oxidoreductase (XOR), were also examined. Similar to HX, plasma Xan levels increased significantly after WBVE, and this increase was suppressed by OGL (Fig. 2D). However, the changes in plasma Xan levels were much smaller than those of plasma HX. On the other hand, either with or without OGL, WBVE did not cause any changes in the plasma UA levels (Fig. 2E). These findings indicate that there are significant individual differences in the WBVE-induced increases in plasma HX levels and whether they are suppressed by prior OGL.
We next examined the relationships between ΔHX and body composition and muscle-related parameters using Spearman correlation analysis. In the WBVE-alone intervention, ΔHX showed no significant correlations with any of the examined parameters (Table 2, OGL (–)). On the other hand, in the OGL → WBVE intervention, negative correlations were observed between the ΔHX and skeletal muscle mass (ρ = –0.49, p = 0.06), especially leg muscle mass (ρ = –0.62, p = 0.01) (Table 2, OGL (+)). Additionally, ΔHX under the OGL condition showed significant negative correlations with leg strength (ρ = –0.71, p = 0.005) and leg muscle quality (ρ = –0.81, p = 0.0007), but no correlation was observed for grip strength or arm muscle quality (Table 2, OGL (+)). Similar to ΔHX, changes in plasma Xan levels after WBVE (ΔXan) also showed a significant negative correlation with leg muscle quality in the OGL → WBVE intervention (ρ = –0.65, p = 0.02) (Supplementary Table 1, OGL (+)). However, this correlation was relatively weak compared to that for ΔHX. These results suggest that, particularly under glucose-supplying conditions, a post-WBVE increase in plasma HX levels is associated with decreased muscle mass and muscle quality in the legs.
Table 2 Correlation analyses of the factors associated with changes in plasma HX concentrations after WBVE with or without OGL
|
OGL (–) |
OGL (+) |
| ρ |
p value |
ρ |
p value |
| BMI (n = 18) |
0.28 |
0.26 |
–0.21 |
0.39 |
| Fat mass (n = 15) |
0.45 |
0.09 |
0.11 |
0.70 |
| Percent body fat (n = 15) |
0.49 |
0.07 |
0.27 |
0.33 |
| Skeletal muscle mass (n =
15) |
–0.11 |
0.71 |
–0.49 |
0.06 |
| Arm muscle mass (n = 15) |
–0.05 |
0.86 |
–0.47 |
0.08 |
| Trunk muscle mass (n = 15) |
–0.08 |
0.77 |
–0.48 |
0.07 |
| Leg muscle mass (n = 15) |
–0.11 |
0.69 |
–0.62 |
0.01 |
| SMI (n = 15) |
–0.13 |
0.64 |
–0.51 |
0.05 |
| Grip strength (n = 15) |
0.02 |
0.95 |
–0.36 |
0.19 |
| Leg strength (n = 14) |
–0.40 |
0.15 |
–0.71 |
0.005 |
| Arm muscle quality (n =
15) |
–0.05 |
0.86 |
0.33 |
0.23 |
| Leg muscle quality (n =
13) |
–0.52 |
0.07 |
–0.81 |
0.0007 |
HX, hypoxanthine; WBVE, whole-body vibration exercise; OGL, oral glucose load; BMI, body mass index; SMI, skeletal muscle index.
Discussion
The main findings of the present study are as follows. Overall, WBVE significantly increased plasma HX concentrations in healthy male subjects, and the increase was suppressed by a pre-exercise OGL. In addition, the changes in HX levels under the OGL condition, but not the WBVE-alone condition, showed a significant negative correlation with leg muscle quality.
Several clinical studies to date have shown that blood HX levels increase after various types of exercise such as treadmill running [10] and prolonged cycling [24]. We observed for the first time that plasma HX concentrations can be significantly elevated after WBVE. Similar to previous results obtained after other types of exercise [10, 24], a post-WBVE increase in plasma HX levels was accompanied by elevated blood ammonia levels, indicating activation of the adenylate kinase reaction pathway (2ADP↔ATP + AMP) in response to the increased ATP demand and subsequent AMP degradation during WBVE. The changes in plasma HX concentration after WBVE in healthy men observed in the present study (4.46 ± 4.87 μM) were smaller than those observed after an incremental treadmill exercise test (>30 μM) reported in a previous study in amateur runners [10], which may reflect the lower intensity of WBVE exercise compared to treadmill exercise. HX is known to be transported into and out of cells, by equilibrative nucleoside transporters, namely ENT1 [25] and ENT2 [26], which are members of the solute carrier (SLC) family. WBVE did not alter serum CK or LDH levels, which are indicative of muscle tissue damage. Consequently, the increase in plasma HX levels following WBVE should be attributed to increased HX production and intracellular concentration in myocytes, resulting in its release into the circulation by these SLC transporters according to a concentration gradient. Regarding the other purine metabolites, WBVE did not alter plasma UA levels, whereas plasma Xan concentrations were significantly increased, similar to HX. However, the change in Xan concentrations was smaller than that in HX. This finding is consistent with our previous report showing that XOR, the rate-limiting enzyme involved in the metabolism of HX to Xan and Xan to UA, is primarily expressed in the liver and small intestine, with limited expression in skeletal muscle in humans [27]. Thus, the majority of HX produced in skeletal muscle is thought to be secreted into the circulation without being metabolized to Xan and UA. Taken together, among these purine metabolites, the plasma levels of HX were the most sensitive to changes during WBVE, owing to alterations in ATP metabolism within skeletal muscle.
Notably, glucose loading prior to WBVE affected the WBVE-induced changes in plasma HX levels. The crossover design employed in this study demonstrated that prior OGL administration significantly suppressed the post-WBVE increase in plasma HX levels. Although muscle glycogen is considered the primary source of carbohydrate fuel for muscle contraction by exercise, blood glucose also plays an important role in the overall glycolytic pathway, particularly during prolonged intense exercise. Muscle contraction associated with exercise stimulates the translocation of glucose transporter 4 (GLUT4) from intracellular vesicles to the muscle cell surface through multiple signaling pathways that are independent of insulin, including AMP-activated protein kinase, Ca2+-calmodulin-dependent protein kinase, nitric oxide synthase, and GTPase Rac1 activation [28]. Circulating glucose is then taken up by muscle cells through facilitated diffusion from GLUT4 translocated to the plasma membrane, and subsequently metabolized as an energy source during exercise. Pre-exercise carbohydrate intake has been reported to increase muscle glycogen availability [29] and improve performance during exercise such as endurance running [30] and cycling [31]. Increased levels of pyruvic acid and lactic acid were followed by a post-WBVE increase in plasma HX levels (Fig. 1D, E), indicating that the glycolytic pathway was activated when HX was produced in skeletal muscle. Therefore, in the OGL → WBVE intervention, increased ATP production through glycolysis and oxidative phosphorylation using the glucose supplied prior to WBVE may have compensated for the ATP demand during exercise, resulting in reduced HX production and its release into the circulation. Accordingly, our results suggest that the OGL-mediated suppression of the post-WBVE increase in HX levels may reflect the ability of skeletal muscle to generate ATP by utilizing blood glucose during exercise.
It is noteworthy that individual differences were observed not only in WBVE-induced changes in plasma HX levels but also in the OGL-mediated suppressive effect on the post-WBVE increase in HX levels (Fig. 2C). In some participants whose plasma HX levels were increased after WBVE, the increase was strongly suppressed by the pre-exercise glucose load, while in others, the response was minimal (as indicated by the arrows in Fig. 2C). Several studies have reported associations between post-exercise increases in HX levels and physical activity or exercise capacity. In healthy older adults, serum HX concentrations after a 5-min walk test (approximately 70% of maximal heart rate) were significantly greater in the sedentary group than in the active group, based on daily physical activity [32]. Compared to nonobese subjects, obese subjects with lower exercise capacity reserves and anaerobic threshold values exhibited significant elevations in serum HX levels in a treadmill exercise test [33]. In highly trained athletes, blood HX concentrations after treadmill exercise exhibited greater changes than lactic acid and ammonia concentrations during a one-year training cycle, presumably reflecting training status. However, no such annual change was observed in amateurs [10]. In the present study, ΔHX in the WBVE-alone intervention did not correlate with any of the clinical parameters. However, under the glucose-supplying condition, we observed significant correlations between ΔHX and muscle-related parameters. When participants received OGL prior to WBVE, ΔHX was negatively correlated with lower muscle mass, strength, and quality. Such correlations were found to be more significant in the legs than in the arms. This may be because WBVE was performed in a static squat position, which places a greater load on the lower limbs. Additionally, ΔXan was much smaller than ΔHX, although it showed a significant correlation with leg muscle quality under the OGL condition. Moreover, among the muscle-related parameters evaluated in this study, ΔHX under the OGL condition showed the strongest correlation with the leg muscle quality, which represents the maximum power output per unit of muscle mass. In disease states such as aging and sarcopenia, changes in mitochondrial function are recognized as contributing factors in muscle performance and quality. Previous studies have shown a decline in oxidative capacity and mitochondrial content in aging muscle [34, 35]. Furthermore, reduced maximal mitochondrial ATP production and mitochondrial respiration rates were reported to be associated with fatigability and walking speed in the elderly [36-38]. Exercise training improved glucose availability owing to the enhanced capacity for glucose uptake by increasing muscle GLUT4 protein expression [39]. Thus, even in healthy subjects, those with lower muscle quality may have a potential reduction in the ability to generate ATP in skeletal muscle through glycolysis and oxidative phosphorylation, possibly due to decreased glucose availability or impaired mitochondrial function, which might be manifested by the WBVE-induced increase in plasma HX levels, despite pre-exercise glucose supplementation.
Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) is considered a potential enzyme that contributes to individual differences in WBVE-induced increases in plasma HX levels. HGPRT is a key enzyme in purine recycling that catalyzes the conversion of HX to IMP through the salvage pathway. IMP generated during exercise is thought to be partially resynthesized into AMP in the purine nucleotide cycle to restore the adenine nucleotide pool [40]. Although a significant portion of IMP is dephosphorylated to inosine and further degraded to HX, IMP can be resynthesized from HX by HGPRT and recovered as intramuscular stores [6]. Zielinski et al. reported that high-intensity training increased erythrocyte HGPRT activity, enhanced HX to IMP resynthesis, and reduced post-exercise HX levels [7-9, 41]. According to these findings, decreased HGPRT activity may have contributed to the increase in plasma HX levels after WBVE. However, in this study, the associations between ΔHX and muscle-related parameters were only evident under the OGL condition; therefore, it remains unclear whether altered activities of HGPRT were involved in the reduced muscle quality observed in our nonathlete, healthy subjects.
In addition to the small number of participants, a major limitation of this study is that some participants showed little or no increase in HX levels under the setting and duration of WBVE examined. Therefore, different results may be obtained even in the WBVE-alone intervention by providing a more rigorous load that would be expected to augment plasma HX levels to a greater extent. Moreover, further longitudinal studies are necessary to determine whether the effect of OGL on the post-WBVE increase in HX levels is affected by interventions that enhance muscle quality, such as aerobic and resistance training, and whether this increase is associated with a risk of developing musculoskeletal disorders.
In summary, our study highlights the significant correlation between post-WBVE increase in plasma HX levels and muscle quality, particularly under the glucose-supplemented condition (Graphical Abstract). These findings imply the importance of the metabolic link between skeletal muscle status and muscular glucose availability during exercise. Given that WBVE is a relatively simple and safe method of exercise loading, even for elderly individuals, the measurement of plasma HX levels following WBVE may have the potential for clinical applications in identifying high-risk populations for sarcopenia, including diabetic patients, and promoting early therapeutic intervention.
Graphical Abstract
Whole-body vibration exercise (WBVE) for healthy male subjects resulted in significant increases in plasma hypoxanthine (HX) concentrations, possibly reflecting increased ATP demand and subsequent AMP degradation during exercise. Although there were individual differences, receiving a pre-exercise oral glucose load (OGL) significantly suppressed the increases in plasma HX levels after WBVE. In addition, the changes in HX levels under the OGL condition showed a significant negative correlation with leg muscle quality.
Author Contributions
T.H. analyzed the data and wrote the manuscript. Y.F. and H.N. conceived the study and wrote the manuscript. Y.K. analyzed the data and reviewed the manuscript. T.N., S.A., and Y.O. measured the plasma HX, Xan, and UA concentrations by the LC/TQMS method, and reviewed the manuscript. H.W. designed the study and contributed to the blood sample collection. T.S., Y.O., H.N., and S.F. were involved in the data analysis and interpretation. M.T. designed the study and reviewed the manuscript. T.K., N.K., and I.S. reviewed the manuscript. All the authors have read and approved the final manuscript.
Acknowledgment
We thank all members of the Adiposcience Laboratory at the Department of Metabolic Medicine, Graduate School of Medicine, Osaka University, for their helpful discussions and suggestions.
Funding Information
This work was supported, in part, by research grant from the Gout and Uric Acid Foundation (to Y.F. and H.N.) and Grants-in-Aid for Scientific Research (C) no. 23K08006 (to H.N.). The funding agencies had no role in the study design, data collection and analysis, decision to publish, or manuscript preparation.
Conflicts of Interest Statement
S.A., T.N., and Y.O. are full-time employees of Sanwa Kagaku Kenkyusho Co., Ltd. All other authors declare no conflict of interest.
Data Availability Statement
The data presented in this study are available upon request from the corresponding author.
Supplementary Table 1 Correlation analyses of the factors associated with changes in plasma Xan concentrations after WBVE with or without OGL
|
OGL (–) |
OGL (+) |
| ρ |
p value |
ρ |
p value |
| BMI (n = 18) |
0.34 |
0.17 |
0.19 |
0.45 |
| Fat mass (n = 15) |
0.48 |
0.07 |
0.38 |
0.16 |
| Percent body fat (n = 15) |
0.48 |
0.07 |
0.45 |
0.09 |
| Skeletal muscle mass (n = 15) |
0.03 |
0.92 |
–0.05 |
0.86 |
| Arm muscle mass (n = 15) |
0.13 |
0.65 |
–0.05 |
0.87 |
| Trunk muscle mass (n = 15) |
0.09 |
0.75 |
–0.04 |
0.90 |
| Leg muscle mass (n = 15) |
–0.05 |
0.86 |
–0.13 |
0.63 |
| SMI (n = 15) |
–0.02 |
0.95 |
–0.08 |
0.79 |
| Grip strength (n = 15) |
0.04 |
0.90 |
0.10 |
0.73 |
| Leg strength (n = 14) |
–0.26 |
0.37 |
–0.37 |
0.19 |
| Arm muscle quality (n = 15) |
–0.20 |
0.48 |
0.15 |
0.60 |
| Leg muscle quality (n = 13) |
–0.32 |
0.28 |
–0.65 |
0.02 |
Xan, xanthine; WBVE, whole-body vibration exercise; OGL, oral glucose load; BMI, body mass index; SMI, skeletal muscle index.
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