2-agonists">Beta2-agonists signal through the beta2-adrenoceptor and are frequently used worldwide in the treatment of respiratory disorders such as asthma and chronic obstructive pulmonary disease. In addition to their bronchodilatory effects, 2-agonists">beta2-agonists have also been shown to be ergogenic (8) and may therefore be misused by athletes with intent to enhance performance. Several studies have found improved sprint performance following acute administration of 2-agonists">beta2-agonists (10,23,28,32), but the underlying mechanisms are not fully understood. Glycolysis is a major provider of adenosine triphosphate (ATP) during supramaximal exercise (4,5) and hence plays an important role in fatigue development (2) and performance (4,5). Therefore, based on observations of elevated venous lactate following maximal sprinting (24,28,32) and increased accumulation of muscle lactate and breakdown of glycogen during submaximal exercise (25,29), beta2-adrenergic improvements in sprint performance have been suggested to involve an increased provision of ATP from glycolysis (9,28). Obviously, venous lactate is only an indirect marker of muscle glycolysis, and measurements of muscle ATP, phosphocreatine (PCr), glycogenolysis, and glycolysis are warranted to completely understand the importance of glycogenolysis and glycolysis in beta2-adrenergic improvements in sprint performance.
Different fiber types have different contractile and metabolic properties (12). In animal models, beta2-adrenergic stimulation has been shown to increase peak twitch force and tetanic force in fast-twitch muscles (1,6,7,22), whereas peak twitch force and tetanic force are either increased (7,20) or decreased (1,6,22) in slow-twitch muscles. The effects of beta2-adrenergic stimulation on glycogen metabolism have also been shown to vary between fiber types in rats (18,36). Although several studies have investigated the effects of beta2-adrenergic stimulation on muscles with different fiber types in animal models, there is limited knowledge of beta2-adrenergic effects on different fiber types in humans. Further knowledge will be useful in understanding whether beta2-adrenergic improvements in supramaximal performance are related to fiber type–specific effects. During maximal exercise lasting <10 s, performance depends on the provision of energy from intramuscular PCr stores (5,16). Therefore, the effects of beta2-adrenergic stimulation on ATP and PCr utilization in different fiber types during supramaximal exercise may provide new information about the mechanisms behind the performance-enhancing effects of 2-agonists">beta2-agonists during maximal sprinting.
Thus, the aim of the present study was to investigate the effects of the beta2-agonist terbutaline (TER) on muscle metabolism and power output during a 10-s cycle sprint. Another aim was to examine the effects of TER on ATP and PCr utilization in different fiber types during the cycle sprint. We hypothesized that TER increases power output and that this is associated with greater muscle anaerobic ATP utilization due to increased rates of glycogenolysis and glycolysis, along with increased depletion of PCr and ATP stores.
Nine moderately trained male cyclists took part in the study (Table 1). Subjects were informed about the risks and discomforts related to the different tests and procedures in the study. Written informed consent was collected from all subjects before the study. The study was approved by the local scientific ethics committee of Copenhagen (H-4-2012-110) and was performed in accordance with the Helsinki II Declaration.
The study was performed as a randomized double-blind cross-over study consisting of one screening visit followed by two identical study visits with inhalation of either TER or placebo (PLA). At the screening visit, subjects completed an incremental test on a cycle ergometer (Monark 839E; Monark, Vansbro, Sweden) to determine maximal oxygen uptake (V˙O2max), followed by three 10-s sprints that served to familiarize subjects with the equipment and the sprint protocol used in the study. Three days following the screening visit, subjects reported to the laboratory 3 h after consuming a light meal. Under local anesthesia (1 mL of 20 mg·mL−1 lidocaine without epinephrine), two incisions (spaced at least 5 cm apart) were made through the skin and fascia of the right vastus lateralis muscle to allow quick sampling of muscle biopsy. Hereafter, subjects inhaled 15 mg of TER (30 doses of 0.5 mg) or PLA and warmed up on a cycle ergometer at 150 W for 15 min. After 3 min of rest, a biopsy sample was collected using a 5-mm Bergstroem needle with suction. A 10-s sprint was performed 15 s later, and a biopsy sample was collected immediately after the sprint. The biopsy sample collected before the sprint was taken as close to the time of the start of the sprint as possible to get the most accurate measurement of the muscular milieu right before the sprint. The 10-s sprint test was performed 25 min after inhalation of the first dose of 0.5 mg of TER, taking into account the time required to inhale 15 mg of TER (7 min). At this time point, serum concentrations of TER are peaking (13), with minimal receptor desensitization (34). Thus, the sprint test was carried out at a time point where drug effects were expected to be maximal. After 16.8 ± 2.3 d, subjects reported to the laboratory and underwent the exact same procedures, but with the opposite treatment. Subjects were told to maintain their regular exercise regimens throughout the study and to abstain from caffeine, alcohol, and strenuous exercise 48 h before each visit at the laboratory. Food intake and fluid intake were registered 48 h before the first study visit and were duplicated before the second visit. Test procedures were performed at the same time of day to standardize hormone concentrations and body temperature.
Determination of V˙O2max
V˙O2max was determined in an incremental cycle test with breath-by-breath measurements. Pulmonary oxygen uptake was measured using a gas-analyzing system (JAEGER MasterScreen CPX; Viasys Healthcare GmbH, Hoechberg, Germany). Calibration was performed with a 3-L syringe and with gasses of known O2 and CO2 concentrations. Subjects were instructed to maintain a cadence between 80 and 90 rpm. The test started at 150 W, and the load was increased by 30 W every minute until exhaustion. Exhaustion was reached when pedaling frequency fell below 70 rpm for more than 5 s. V˙O2max was defined as the highest value averaged in any 30-s period. A respiratory exchange ratio above 1.15 was used as criterion for achievement of V˙O2max.
The protocol for the 10-s sprint was designed with the Monark 839E analysis software. Subjects were told to pedal for 15 s with a cadence of 70–80 rpm (against a resistance of 6 N), after which resistive load was increased to 0.9 N·kg body mass−1, and subjects were told to pedal as fast as possible for 10 s. Subjects were instructed to remain seated during the test. Flywheel velocity (rpm) and acceleration (rpm·s−1) were recorded at 1-s intervals on a computer. The instantaneous power generated during the sprints was corrected for changes in kinetic energy and averaged over 1-s intervals, taking into account the work performed in accelerating the flywheel. Flywheel velocity was multiplied by effective load (resistive load plus acceleration balancing load), as previously described (31). Although the resistive load was known (0.9 N·kg body mass−1), the acceleration balancing load was calculated from the creation of several deceleration curves, using a series of different resistive loads (31). These curves were obtained by setting the flywheel in motion at 100 rpm and by plotting deceleration from the cessation of pedaling. Curves were created for resistive loads of 10, 20, 30, 40, 50, 60, and 70 N. By plotting deceleration against load, we created a linear regression equation where flywheel acceleration was inserted to calculate the acceleration balancing load:
Peak power (highest power output recorded at any second during the test), mean power (average power output during the test), peak pedal speed (highest pedal speed recorded at any second during the test), mean pedal speed (average pedal speed during the test), time to peak power, and time to peak pedal speed were registered. Coefficients of variation (standard deviation/mean × 100) for peak power and mean power were calculated based on the results of the screening visit and the PLA trial. The coefficient of variation was 1.5% for peak power and 2.4% for mean power.
Muscle biopsy samples from only seven subjects were analyzed because it was not possible to obtain biopsy samples from the two other subjects after the sprint on the second study visit. These two subjects contracted the vastus lateralis muscle during the invasive procedure, which unfortunately made it impossible to collect biopsy samples. Biopsy samples were frozen immediately in liquid nitrogen and stored at −80°C for later analyses. The time delay from the cessation of sprint to the freezing of biopsy samples in liquid nitrogen was not different between the TER trial and the PLA trial (9.6 ± 0.9 and 9.9 ± 1.0 s, respectively). Biopsy samples were freeze-dried and dissected free of blood, connective tissue, and fat at 18°C and <30% humidity using a stereomicroscope.
Approximately 2 mg of dry weight (dw) muscle tissue was extracted in 1 N HCl and hydrolyzed at 100°C for 3 h. Glycogen was determined by the hexokinase method, as previously described (33). Approximately 2 mg of dw muscle tissue was extracted in a solution of 1.5 M perchloric acid and 1 mM ethylenediaminetetraacetic acid, neutralized to pH 7.0 with 2.2 M KHCO3, and stored at −80°C until analyzed enzymatically for muscle metabolites, as previously described (31). For determination of ATP and PCr, a reagent solution (pH 8.1) consisting of 95 mM Tris, 4.8 mM glucose, 2.4 mM AMP, 4.8 mM MgCl2, 2.4 mM dithiothreitol (DTT), 0.6 mM adenosine diphosphate, 0.02 mM P1, P5-di(adenosine-5′)pentaphosphate, 0.3 mM nicotinamide adenine dinucleotide phosphate (NADP), and 0.7 U·mL−1 glucose-6-phosphate (G-6-P) dehydrogenase (10127671001; Roche, Copenhagen, Denmark) was created. Each extracted biopsy sample was added to three wells of a 96-well microplate (10 μL in each well), and 200 μL of reagent solution was added to each well. The samples were mixed, and fluorescence intensity was read in a Flouroscan Ascent™ FL microplate fluorometer (Thermo Scientific, Copenhagen, Denmark), whereafter hexokinase (11426362001; Roche) was added to the samples, giving a final activity of 0.7 U·mL−1. Thirty minutes later, the plate was read again, and creatine kinase (10736988001; Roche) was added to the samples, giving a final activity of 0.26 U·mL−1. The plate was read 30 min later. A stock solution of 2 mM PCr disodium salt hydrate (P6502; Sigma-Aldrich, Brondby, Denmark) and H2O was used to create a standard curve. The difference in fluorescence intensity between the first two measurements was used to determine ATP, whereas the difference between the last two measurements was used to determine PCr. For determination of creatine, a reagent solution (pH 7.5) consisting of 41 mM Hepes, 143 mM KCl, 24 mM MgCl, 0.02 mM P1,P5-di(adenosine-5′) pentaphosphate, 0.1 mM nicotinamide adenine dinucleotide, 0.14 mM phospoenolpyruvate, 1 mM ATP, 5.3 U·mL−1 lactate dehydrogenase (LDH) (10127876001; Roche), and 6.7 U·mL−1 pyruvate kinase (10109045001; Roche) was created. A standard curve was created from a stock solution of 2 mM creatine monohydrate (C3630; Sigma-Aldrich, Copenhagen, Denmark). The same procedures as for ATP and PCr determination were carried out, except that the plate was read 20 min after addition of reagent solution and 40 min after addition of creatine kinase (10736988001; Roche), giving a final activity of 0.26 U·mL−1. For lactate, a reagent solution consisting of 1.5 M glycylglycine, 1.5 mM NAD+, and 10.2 mM glutamic acid was created. A standard curve was made from a stock solution of 4.44 mM lactate (826-10; Trinity Biotech, Bray, Ireland). The microplate was read 60 min after glutamic–pyruvic transaminase (10105589001; Roche) and LDH (10127876001; Roche) was added to the samples, giving final activities of 3.1 and 4.8 U·mL−1, respectively. For G-6-P, a reagent solution (pH 8.1) of 95 mM Tris, 30 mM MgCl2, 2.4 mM DTT, 2.5 mM ATP, and 0.5 mM nicotinamide adenine dinucleotide phosphate was used, and a standard curve was made from a stock solution of 0.01 mM G-6-P (G7879; Sigma-Aldrich). The microplate was read 30 min after G-6-P dehydrogenase (10127671001; Roche) was added to the samples, giving a final activity of 0.7 U·mL−1. For pyruvate, a reagent solution (pH 7.0) consisting of 0.25 M kalium dihydrogen phosphate, 2.8 mM ethylenediaminetetraacetic acid, and 30 μM nicotinamide adenine dinucleotide was used, and a standard curve was made from a stock solution of 0.2 mM pyruvate (S8636; Sigma-Aldrich). The microplate was read 30 min after LDH (10127876001; Roche) was added to the samples, giving a final activity of 4.8 U·mL−1. In contrast to 10 μL of sample added to each well to determine ATP, PCr, creatine, and lactate, 25 and 50 μL of samples were added to determine G-6-P and pyruvate, respectively. All measurements were carried out at room temperature. For all measurements, the excitation wavelength was 355 nm, and the emission wavelength was 460 nm.
In the present article, the difference in muscle metabolite concentration between the biopsy sample obtained before the sprint and the biopsy sample obtained immediately after the sprint is denoted Δ. For lactate, pyruvate, and G-6-P, Δ represents net accumulation during the sprint. For ATP, PCr, and glycogen, Δ represents net reduction during the sprint.
In mixed-muscle fibers, anaerobic ATP utilization during the sprint was estimated from differences (Δ) in muscle ATP, PCr, lactate, and pyruvate, as previously described (5):
The mean rate of glycolysis and glycogenolysis during the sprint was estimated from Δlactate, Δpyruvate, ΔG-6-P, and sprint duration (10 s):
where 0.33ΔG-6-P was used as estimate of glucose-1-phosphate and fructose-6-phosphate accumulation, as previously described (26).
The calculation of glycogenolytic rate was not overestimated due to phosphorylation of muscle glucose by hexokinase II, leading to accumulation of G-6-P, lactate, and pyruvate. Formation of G-6-P from breakdown of muscle glycogen occurs extremely fast following initiation of maximal sprinting; because hexokinase II is strongly inhibited by its product (G-6-P), the formation of G-6-P from phosphorylation of glucose by hexokinase II is thought to be very small (35). The pyruvate oxidized and the lactate released to the circulation were not included in the calculations; however, due to the short duration of the sprint, the underestimation of anaerobic ATP turnover, glycolytic rate, and glycogenolytic rate was probably less than 10% (8).
Single-muscle fiber analysis
Fragments of single-muscle fibers were dissected manually from seven subjects using a stereomicroscope. Approximately 20 fibers were dissected from each biopsy sample, and one third of each fiber fragment was cut off and placed in microcentrifuge tubes for fiber type determination by Western blot analysis, as previously described (37). Briefly, 12 μL of 6× Laemmli buffer (0.7 mL of 0.5 M Tris base, 3 mL of glycerol, 0.93 g of DTT, 1 g of sodium dodecyl sulfate, and 1.2 mg of bromophenol blue) diluted (1:3) in double-distilled H2O was added to each fiber. Single-fiber fragments were loaded on 26-well 4%–15% Tris–HCl Criterion gels (Bio-Rad Laboratories, Hercules, CA), and proteins were separated by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (60 mA per gel and a maximum of 150 V) for 2 h before being semidry-transferred to polyvinylidene difluoride membranes (Millipore A/S, Copenhagen, Denmark) for 2 h at 70 mA per gel and a maximum of 25 V. Membranes were blocked for 45 min in Tris-buffered saline with 0.1% Tween-20 (TBST) with 2% skimmed milk and incubated with primary antibodies for 2 h. After blocking, membranes were washed three times in TBST and incubated for 1 h with horseradish peroxidase–conjugated secondary antibody (DAKO, Copenhagen, Denmark) diluted 1:5000 in TBST with 2% skimmed milk. The membrane was washed three times (15 min each) in TBST, after which bands were visualized using chemiluminescent detection (Luminata Forte Western HRP substrate; Millipore A/S), and images were collected from a ChemiDoc MP Imaging System (Bio-Rad Laboratories, Berkeley, CA). Single-muscle fibers were characterized as either Type I or Type II by expression of major histocompatibility complex (MHC) I and Type II–specific SERCA I protein (41). The following antibodies were used: MHC Type I, 0.5 μg·mL−1; mouse monoclonal IgM A4.840 (developed by Dr. Blau, Developmental Studies Hybridoma Bank, University of Iowa); SERCA I protein, 0.1 μg·mL−1; and mouse monoclonal MA3-912 (Thermo Scientific). Fibers were excluded if a clear band was identified for both MHC I and SERCA I protein, indicating a hybrid fiber type or that several fibers had been loaded in one well. In total, 133 Type I and 135 Type II fiber fragments were identified. In the TER trial, 4.29 ± 0.52 Type I fibers and 5.14 ± 0.63 Type II fibers were identified in each biopsy sample collected before the sprint, whereas 5.43 ± 0.57 Type I fibers and 4.57 ± 0.61 Type II fibers were identified in each biopsy sample collected after the sprint. In the PLA trial, 4.14 ± 0.34 Type I fibers and 4.86 ± 0.26 Type II fibers were identified in each biopsy sample collected before the sprint, whereas 5.14 ± 0.46 Type I fibers and 4.71 ± 0.36 Type II fibers were identified in each biopsy sample collected after the sprint.
The remaining fragment of the fibers classified as either Type I or Type II fibers was used for single-fiber ATP and PCr determination via a luminometric method, as previously described (44). Briefly, the fragment was weighed on a quartz-fiber fishpole balance. The balance was calibrated by measuring the quartz-fiber deformation of several p-nitrophenol crystals, with known weights applied to the tip of the quartz fiber. The weight of single fibers ranged from ∼1 to 8 μg and was calculated from the deformation of the quartz fiber. After the weight had been determined, each fragment was extracted in 200 μL of trichloroacetic acid (2.5%) and neutralized in 20 μL of 2.2 M KHCO3. Then, 50 μL of extract was added to a sucrose buffer containing d-luciferin (BioThema, Handen, Sweden). The assay was carried out at 25°C in a luminometer (1251; Bio Orbit Oy, Turku, Finland) fitted with a temperature-controlled 25-position sample carousel, three automatic dispensing units, and a computer.
TER (Bricanyl; 0.5 mg per dose) and PLA were provided by AstraZeneca (London, United Kingdom) and administered by inhalation from identically looking turbohalers. TER and PLA were administered by a person not involved in other parts of the study. Before the study, all subjects were given oral and written information about potential side effects of TER. Seven of nine subjects reported tachycardia and tremors after inhalation of 15 mg of TER. Four subjects received TER at the first visit, and five subjects received PLA at the first visit.
SPSS 18.0 (IBM, Armonk, NY) was used for statistical analysis. Sample size for a cross-over design was determined (with “mean power” as primary outcome) as previously described (12). Power was set to 0.8, significance level was set to 0.05, and effect size was based on previous observations of increased mean power following administration of 2-agonists">beta2-agonists (10). Sample size calculations revealed that eight subjects were necessary to obtain a power of 0.8. With the risk of one dropout, nine subjects were included. Normality was tested with Shapiro–Wilk test, and all data were normally distributed. Comparisons of mean power, peak power, anaerobic ATP utilization, glycogen utilization, glycolytic rate, and changes (Δ) in muscle metabolites were analyzed with paired t-test. Two-way repeated-measures ANOVA was used to compare power output, muscle metabolites from homogenates (treatment × sampling point), and utilization of ATP and PCr in single fibers (treatment × fiber type). Three-way repeated-measures ANOVA was used to compare ATP and PCr in single fibers (treatment × sampling point × fiber type). In case of a significant ANOVA, Student–Newman–Keuls post hoc test was applied. Pearson correlation analyses were used to examine relationships between mean power output and anaerobic ATP utilization, glycogenolytic rate, and glycolytic rate. Significance was set to 0.05.
Power output during the 10-s sprint
At every second of the sprint, power output was higher (P < 0.05) with TER than with PLA (Fig. 1A). The mean power and peak power output during the sprint were 8.3% ± 1.1% and 7.8% ± 2.5% higher (P < 0.05), respectively, with TER than with PLA (Fig. 1B). For all subjects, the mean power and peak power were higher with TER than with PLA (Fig. 1B). Peak pedal speed (167.1 ± 4.2 vs 159.1 ± 4.6 rpm) and mean pedal speed (148.8 ± 3.2 vs 140.6 ± 3.3 rpm) were higher (P < 0.05) with TER than with PLA. In both trials, peak power was reached (P < 0.05) earlier than peak pedal speed (TER: 2.67 ± 0.24 vs 3.78 ± 0.15 s; PLA: 2.78 ± 0.22 vs 3.67 ± 0.17 s), whereas no difference was observed between treatments.
Metabolites in mixed-muscle fibers
With TER, muscle ATP after the sprint was not different from that before the sprint. With PLA, muscle ATP was lower (P < 0.05) after the sprint than before the sprint. The decrease in ATP with PLA tended to be higher (P = 0.07) than the decrease in ATP with TER (8.3 ± 1.9 vs 3.9 ± 2.9 mmol·kg dw−1). Muscle PCr and net breakdown of PCr during the sprint were similar between treatments (Table 2).
Muscle glycogen before and after the sprint was not different between TER and PLA, but the net rate of glycogen breakdown was greater (P < 0.05) with TER than with PLA (6.0 ± 0.6 vs 3.9 ± 0.6 mmol glucosyl units·kg dw−1·s−1) (Table 2). Before and after the sprint, G-6-P and lactate were higher (P < 0.05) with TER than with PLA. Net accumulation of both G-6-P and lactate was higher (P < 0.05) with TER than with PLA (Table 2). Pyruvate before and after the sprint was the same with TER and PLA; however, with PLA, muscle pyruvate after the sprint was higher (P < 0.05) than muscle pyruvate before the sprint. There was no difference in net muscle pyruvate accumulation during the sprint between TER and PLA. The net rates of glycogenolysis (TER: 6.5 ± 0.8 mmol glucosyl units·kg dw−1·s−1; PLA: 3.1 ± 0.7 mmol glucosyl units·kg dw−1·s−1) and glycolysis (TER: 2.4 ± 0.2 mmol glucosyl units·kg dw−1·s−1; PLA: 1.6 ± 0.2 mmol glucosyl units·kg dw−1·s−1) during the sprint were higher (P < 0.05) with TER than with PLA.
Anaerobic ATP utilization in mixed-muscle fibers
Anaerobic ATP utilization during the sprint was 9.2% ± 4.0% higher (P < 0.05) with TER than with PLA (109 ± 8 vs 100 ± 6 mmol·kg dw−1) (Fig. 2). Estimated ATP production from glycolysis was 55% ± 18% higher (P < 0.05) with TER than with PLA, whereas ATP production from PCr was similar between treatments.
Relationship between mean power and anaerobic ATP utilization, glycogenolytic rate, and glycolytic rate
Mean power was correlated with anaerobic ATP utilization (r = 0.81, P < 0.01), glycogenolytic rate (r = 0.78, P < 0.01), and glycolytic rate (r = 0.70, P < 0.01). The difference in mean power between TER and PLA was correlated with the differences in anaerobic ATP utilization (r = 0.77, P < 0.05), glycogenolytic rate (r = 0.80, P < 0.05), and glycolytic rate (r = 0.78, P < 0.05) (Fig. 3).
ATP and PCr in single-muscle fibers
Before the sprint, there was no difference in ATP between Type I and Type II fibers with either TER or PLA (Fig. 4A). In Type I fibers, ATP before the sprint was not different from ATP after the sprint with either TER or PLA. With PLA, ATP content in Type II fibers was 6.9 ± 2.0 mmol·kg dw−1 lower (P < 0.05) after the sprint, whereas ATP was not lowered with TER (Fig. 4A). The decrease in ATP content in either Type I or Type II fibers was not different between treatments (Fig. 4B).
Before the sprint, PCr was 8.3 ± 3.0 mmol·kg dw−1 higher (P < 0.05) in Type II fibers (vs Type I fibers) with PLA, whereas no difference was observed between fiber types with TER (Fig. 4C). PCr in Type II fibers was 15.9 ± 4.0 mmol·kg dw−1 lower (P < 0.05) before the sprint with TER than with PLA, with no difference between TER and PLA in Type I fibers. After the sprint, there was no difference in PCr content in Type I and Type II fibers between treatments. With PLA, the net breakdown of PCr was 11.8 ± 3.8 mmol·kg dw−1 higher (P < 0.05) in Type II fibers (vs Type I fibers), whereas no difference was observed between fiber types with TER (Fig. 4D). There was no between-treatment difference in the breakdown of PCr in either Type I or Type II fibers (Fig. 4D).
In agreement with our hypothesis, TER increased power output and anaerobic ATP utilization during 10 s of maximal sprinting, which were associated with higher rates of glycogenolysis and glycolysis. In disagreement with our hypothesis, in mixed-muscle fibers, there was no difference in PCr breakdown between treatments, and ATP content was only reduced in the PLA trial. Moreover, in contrast to PLA, TER did not lead to a reduction in ATP in Type II fibers or a greater breakdown of PCr in Type II fibers (vs Type I fibers) during the sprint. These observations are somewhat surprising because accumulation of muscle lactate is usually related to depletion of ATP and PCr (27). Thus, despite greater power output and anaerobic ATP utilization, the greater rates of glycogenolysis and glycolysis in the TER trial counteracted depletion of ATP.
Although previous studies have reported improved peak power and mean power during 30 s of maximal sprinting following administration of 2-agonists">beta2-agonists (10,24,32), little information about power output at various time points during these tests has been reported. This information is useful in understanding at what time points 2-agonists">beta2-agonists exert the greatest effects and, hence, whether beta2-adrenergic improvements in sprint ability should be attributed to improvements during a specific time period. This was elucidated in the present study by reporting of power output at every second of the sprint. Data revealed that power output was higher at all time points, indicating that the effects of TER on power output are immediate and persistent during 10 s of maximal sprinting.
To our knowledge, the present study is the first to show that higher power output with 2-agonists">beta2-agonists is associated with higher anaerobic energy production related to elevated rates of glycogenolysis and glycolysis. In order to evaluate the importance of anaerobic ATP utilization, glycogenolysis, and glycolysis for beta2-adrenergic enhancements in power output, we performed several correlation analyses. In the present study, correlations between mean power and anaerobic ATP utilization, glycogenolytic rate, and glycolytic rate suggest that these factors—all related to anaerobic metabolism— are important for performance during a 10-s sprint. Therefore, the fact that the TER-induced increase in mean power was correlated with the increases in anaerobic ATP utilization, glycogenolytic rate, and glycolytic rate suggests that increased rates of glycogenolysis and glycolysis may be important for a TER-induced increase in mean power during 10 s of maximal sprinting. In support of this, the increased rate of glycogenolysis in the TER trial greatly increased the formation of G-6-P, which presumably reduced the accumulation of inorganic phosphate and thereby perhaps counteracted development of fatigue (2,43). The reason that formation of G-6-P reduces the accumulation of Pi is that Pi is used in the formation of G-6-P from the breakdown of glycogen. Moreover, as there was no between-treatment difference in PCr breakdown during the sprint and ATP was only reduced in the PLA trial, it is evident that the increased rate of glycolysis in the TER trial counteracted a reduction in ATP, thereby perhaps postponing fatigue development by reducing the accumulation of Pi, free Mg2+, and AMP (2,43). Single-fiber analysis revealed that ATP in Type II fibers was only lowered with PLA, indicating that the TER-induced counteraction of ATP depletion measured in mixed-muscle fibers should be attributed to effects on Type II fibers. As breakdown of PCr was not greater in Type II fibers with TER, this counteraction in Type II fibers may be explained by either increased production of ATP from glycolysis or reduced ATP utilization during the sprint. It is not likely for ATP utilization in Type II fibers to be lower in the TER trial because animal studies have shown increased peak twitch force and tetanic force in fast-twitch muscles upon beta2-adrenergic stimulation (1,6,7,22), suggesting a greater utilization of ATP in Type II fibers during maximal force production. Therefore, it is more plausible that an increased rate of glycolysis in the TER trial counteracted the reduction of ATP in Type II fibers. In support of this, epinephrine has been shown to increase glycogenolysis and release of lactate during electrical subtetanic stimulations (30–60 contractions per minute) in fast-twitch muscles in rats (36).
The mechanisms behind the higher rates of glycogenolysis and glycolysis observed in the TER trial may involve activation of the key enzymes glycogen phosphorylase (GP) and phosphofructokinase (PFK) by factors related to cellular energy state (such as Pi and AMP) or by a protein kinase A (PKA)–dependent covalent modulation (30,39). However, based on the findings of a lower decline in ATP, unchanged breakdown of PCr, and greater accumulation of G-6-P in the TER trial, it is not likely for the increased rates of glycogenolysis and glycolysis during the sprint to be attributed primarily to the activation of GP and PFK by AMP or Pi, as accumulation of AMP and Pi is closely related to breakdown of ATP and PCr (19,40). Therefore, it is more likely for the increased rates of glycogenolysis and glycolysis to be attributed primarily to a PKA-dependent covalent modulation of GP and PFK. PKA-dependent phosphorylation of phosphorylase kinase transforms GP from the less active form to the more active form. In rats unable to transform GP from the inactive form to the active form, glycogen breakdown is markedly lower during the first 10 s of electrical stimulation (11), supporting the notion that an increased fraction of GP in the active form may have increased the glycogenolytic rate during the sprint. Furthermore, phosphorylation of PFK by PKA increases the activity of PFK (39). In support of a PKA-dependent activation of GP and PFK, muscle G-6-P and lactate were elevated before the sprint with no difference in ATP or PCr, indicating that the differences in G-6-P and lactate were not a result of the accumulation of the activator AMP or Pi (30,39).
Despite the fact that there was no difference in PCr or in the breakdown of PCr in mixed-muscle fibers, PCr in Type II fibers was lower before the sprint in the TER trial. The lower PCr in Type II fibers before the sprint in the TER trial may reflect a greater breakdown of PCr during warm-up or a limited rate of resynthesis after warm-up until the sampling of muscle biopsy. This greater breakdown of PCr may be due to an increased utilization of ATP by the sarcoplasmic reticulum (SR) Ca2+ ATPase and Na+–K+ ATPase, as both proteins are stimulated by 2-agonists">beta2-agonists in fast-twitch muscles (15,37). PCr in Type I fibers was not reduced before the sprint in the TER trial probably because the potentially greater ATP requirement by Ca2+ ATPase and Na+–K+ ATPase was covered by oxidative metabolism and hence did not lead to a reduction in PCr. Thus, the different observations in Type I and Type II fibers may be explained by a greater reliance on oxidative metabolism in Type I fibers and on anaerobic metabolism in Type II fibers (14) to cover the greater ATP requirement induced by TER. The lower PCr in Type II fibers before the sprint in the TER trial led to a reduced availability of PCr during the sprint. This reduced availability plausibly explains why the greater breakdown of PCr in Type II fibers (vs Type I fibers) observed in the PLA trial and in other studies (19) was abolished with TER.
The higher power output during the sprint in the TER trial may have been caused by increased rates of Ca2+ release and reuptake from the SR in skeletal muscles, as TER has been shown to modulate force by altering the amplitude and decay rate of Ca2+ transients (7,20,37). It is also possible that a TER-induced increase in Na+–K+ ATPase activity (15) may have contributed to the higher power output by counteracting loss of muscle membrane excitability through increased reuptake of K+ from the interstitium (38). Increased Ca2+ release and reuptake from the SR, through PKA-dependent phosphorylation of the ryanodine receptor and phospholamban (3,37), is thought to have contributed to the greater power output observed at all seconds of the sprint test (3,7,20), whereas increased reuptake of K+ from the interstitium by Na+–K+ ATPase is thought to have little importance in the initial phase of the sprint and the greatest importance in the last phase of the sprint. This is because accumulation of K+ increases gradually throughout the sprint, leading to a greater possibility of loss of membrane excitability in the last phase of the sprint (38), which may be counteracted by beta2-adrenergic reuptake of K+ from the interstitium by Na+–K+ ATPase.
We cannot exclude the possibility that increased ATP utilization from aerobic pathways contributed to the higher power output observed with TER, but this is unlikely because a recent study found no effects of TER on oxygen uptake during a 30-s sprint (24). Furthermore, even though oxygen stored as myoglobin may produce up to 12 mmol ATP·kg dw−1 during a 10-s sprint (21), the contribution of aerobic metabolism to total ATP utilization is likely to be less than 20% (17) and hence of minor importance to performance.
A limitation of the present study was that seven of nine subjects reported side effects, such as tachycardia and tremor, after drug administration, questioning the double-blind design of the study. The sensation of side effects may have motivated some of the subjects to perform better during the 10-s sprint. On the other hand, it is also possible that, in some of the subjects, the unpleasant sensation of side effects may have affected performance negatively (42). Nevertheless, subjects who experienced side effects had a TER-induced increase in mean power and peak power of 10.9% and 7.6%, respectively, compared with 7.7% and 7.8% in the two subjects who did not experience side effects. Based on these observations, it is evident that the sensation of side effects did not influence the effects of TER on performance. Another limitation of the present study was that the rates of glycogenolysis and glycolysis in single fibers were not determined. These measurements would have provided essential information about anaerobic ATP production in Type I and Type II fibers, which is important in understanding whether beta2-adrenergic improvements in sprint performance should be attributed to effects on a specific fiber type.
In summary, TER increased power output during 10 s of maximal sprinting, which was associated with higher anaerobic ATP utilization as a result of higher rates of glycogenolysis and glycolysis, with no difference in breakdown of PCr. The present findings also suggest that the effects of TER on ATP and PCr during maximal sprinting vary between fiber types. Thus, in contrast to the PLA trial, there was no difference in PCr breakdown during the sprint between Type I fibers and Type II fibers in the TER trial, which may be attributed to the lower PCr content in Type II fibers before the sprint in the TER trial (vs the PLA trial). Despite a lower PCr content in Type II fibers before the sprint in the TER trial, ATP in Type II fibers was not reduced after the sprint, suggesting that a TER-induced increase in glycogenolysis and glycolysis may postpone the development of fatigue in Type II fibers during maximal sprinting by counteracting the accumulation of Pi, AMP, and free Mg2+ (2). Therefore, the performance-enhancing effects of 2-agonists">beta2-agonists during maximal sprinting may perhaps involve improved fatigue resistance in Type II fibers due to increased rates of glycogenolysis and glycolysis. Future studies should investigate the effects of 2-agonists">beta2-agonists on glycogenolysis and glycolysis in different fiber types (including hybrid fibers) and try to uncover the mechanisms by which 2-agonists">beta2-agonists stimulate glycogenolytic and glycolytic rates during supramaximal exercise in humans.
We would like to thank Jens Jung Nielsen and Martin Thomassen (Department of Nutrition, Exercise, and Sports) for great technical assistance. Furthermore, we would like to thank Michael Kreiberg, Christoffer Haase, and Victoria Becker (Respiratory Research Unit, Bispebjerg University Hospital) for their help with the dissection of muscle biopsy samples.
This study was supported by the World Anti-Doping Agency.
None of the authors have any conflicts of interest.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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