Effects of hyperventilation on phosphocreatine kinetics and muscle deoxygenation during moderate-intensity plantar flexion exercise.

The effects of controlled voluntary hyperventilation (Hyp) on phosphocreatine (PCr) kinetics and muscle deoxygenation were examined during moderate-intensity plantar flexion exercise. Male subjects (n = 7) performed trials consisting of 20-min rest, 6-min exercise, and 10-min recovery in control [Con; end-tidal Pco(2) (Pet(CO(2))) approximately 33 mmHg] and Hyp (Pet(CO(2)) approximately 17 mmHg) conditions. Phosphorus-31 magnetic resonance and near-infrared spectroscopy were used simultaneously to monitor intramuscular acid-base status, high-energy phosphates, and muscle oxygenation. Resting intracellular hydrogen ion concentration ([H(+)](i)) was lower (P < 0.05) in Hyp [90 nM (SD 3)] than Con [96 nM (SD 4)]; however, at end exercise, [H(+)](i) was greater (P < 0.05) in Hyp [128 nM (SD 19)] than Con [120 nM (SD 17)]. At rest, [PCr] was not different between Con [36 mM (SD 2)] and Hyp [36 mM (SD 1)]. The time constant (tau) of PCr breakdown during transition from rest to exercise was greater (P < 0.05) in Hyp [39 s (SD 22)] than Con [32 s (SD 22)], and the PCr amplitude was greater (P < 0.05) in Hyp [26% (SD 4)] than Con [22% (SD 6)]. The deoxyhemoglobin and/or deoxymyoglobin (HHb) tau was similar between Hyp [13 s (SD 8)] and Con [10 s (SD 3)]; however, the amplitude was increased (P < 0.05) in Hyp [40 arbitrary units (au) (SD 23)] compared with Con [26 au (SD 17)]. In conclusion, our results indicate that Hyp-induced hypocapnia enhanced substrate-level phosphorylation during moderate-intensity exercise. In addition, the increased amplitude of the HHb response suggests a reduced local muscle perfusion in Hyp compared with Con.     

NaHCO3-induced alkalosis reduces the phosphocreatine slow component during heavy-intensity forearm exercise.

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.     

ATP synthesis and proton handling in muscle during short periods of exercise and subsequent recovery.

We used (31)P-magnetic resonance spectroscopy to study proton buffering in finger flexor muscles of eight healthy men (25-45 yr), during brief (18-s) voluntary finger flexion exercise (0.67-Hz contraction at 10% maximum voluntary contraction; 50/50 duty cycle) and 180-s recovery. Phosphocreatine (PCr) concentration fell 19 +/- 2% during exercise and then recovered with half time = 0.24 +/- 0.01 min. Cell pH rose by 0.058 +/- 0.003 units during exercise as a result of H(+) consumption by PCr splitting, which (assuming no lactate production or H(+) efflux) implies a plausible non-P(i) buffer capacity of 20 +/- 3 mmol. l intracellular water(-1). pH unit(-1). There was thus no evidence of significant glycogenolysis to lactate during exercise. Analysis of PCr kinetics as a classic linear response suggests that oxidative ATP synthesis reached 48 +/- 2% of ATP demand by the end of exercise; the rest was met by PCr splitting. Postexercise pH recovery was faster than predicted, suggesting "excess proton" production, with a peak value of 0.6 +/- 0.2 mmol/l intracellular water at 0.45 min of recovery, which might be due to, e.g., proton influx driven by cellular alkalinization, or a small glycolytic contribution to PCr resynthesis in recovery.     

Phosphocreatine recovery kinetics following low- and high-intensity exercise in human triceps surae and rat posterior hindlimb muscles

Previous studies have suggested the recovery of phosphocreatine (PCr) after exercise is at least second-order in some conditions. Possible explanations for higher-order PCr recovery kinetics include heterogeneity of oxidative capacity among skeletal muscle fibers and ATP production via glycolysis contributing to PCr resynthesis. Ten human subjects (28 +/- 3 yr; mean +/- SE) performed gated plantar flexion exercise bouts consisting of one contraction every 3 s for 90 s (low-intensity) and three contractions every 3 s for 30 s (high-intensity). In a parallel gated study, the sciatic nerve of 15 adult male Sprague-Dawley rats was electrically stimulated at 0.75 Hz for 5.7 min (low intensity) or 5 Hz for 2.1 min (high intensity) to produce isometric contractions of the posterior hindlimb muscles. [(31)P]-MRS was used to measure relative [PCr] changes, and nonnegative least-squares analysis was utilized to resolve the number and magnitude of exponential components of PCr recovery. Following low-intensity exercise, PCr recovered in a monoexponential pattern in humans, but a higher-order pattern was typically observed in rats. Following high-intensity exercise, higher-order PCr recovery kinetics were observed in both humans and rats with an initial fast component (tau < 15 s) resolved in the majority of humans (6/10) and rats (5/8). These findings suggest that heterogeneity of oxidative capacity among skeletal muscle fibers contributes to a higher-order pattern of PCr recovery in rat hindlimb muscles but not in human triceps surae muscles. In addition, the observation of a fast component following high-intensity exercise is consistent with the notion that glycolytic ATP production contributes to PCr resynthesis during the initial stage of recovery.     

Plasma acid-base regulation above and below ventilatory threshold in late gestation.

Stewart's physicochemical approach was used to study the effects of pregnancy on acid-base regulation in arterialized blood. Responses of 15 healthy pregnant women (PG; gestational age, 37.1 +/- 0.2 wk) were compared with those of 15 nonpregnant controls (CG) at rest and during cycling at 70 and 110% of the ventilatory threshold (T(vent)). Hydrogen ion concentration ([H(+)]) was lower in the PG vs. CG at rest and during exercise (P < 0.05 at rest and 70% T(vent)). Exercise-induced changes in [H(+)] were similar between groups. Lower resting [H(+)] values in the PG vs. CG resulted from lower values for arterialized PCO(2) (Pa(CO(2))) and total weak acid ([A](tot)), which were partly offset by a lower strong-ion difference ([SID]). Reductions in [A](tot) and [SID] at rest were primarily the result of reductions in albumin [Alb] and sodium [Na(+)], respectively. In the transition from rest to 70% T(vent), small increases in Pa(CO(2)) and [A](tot) contributed to moderate increases in [H(+)] in both groups, however [SID] increased in the PG and decreased in the CG (P < 0.05 between groups). In the transition from rest to 110% T(vent), decreases in [SID] made a significantly greater contribution to changes in [H(+)] in the CG vs. PG. Exercise-induced increases in [H(+)] are similar in the pregnant vs. nonpregnant state, but there is a reduced contribution of [SID] both above and below T(vent) during pregnancy.     

Metabolic effects of induced alkalosis during progressive forearm exercise to fatigue.

Metabolic alkalosis induced by sodium bicarbonate (NaHCO(3)) ingestion has been shown to enhance performance during brief high-intensity exercise. The mechanisms associated with this increase in performance may include increased muscle phosphocreatine (PCr) breakdown, muscle glycogen utilization, and plasma lactate (Lac(-)(pl)) accumulation. Together, these changes would imply a shift toward a greater contribution of anaerobic energy production, but this statement has been subject to debate. In the present study, subjects (n = 6) performed a progressive wrist flexion exercise to volitional fatigue (0.5 Hz, 14-21 min) in a control condition (Con) and after an oral dose of NaHCO(3) (Alk: 0.3 g/kg; 1.5 h before testing) to evaluate muscle metabolism over a complete range of exercise intensities. Phosphorus-31 magnetic resonance spectroscopy was used to continuously monitor intracellular pH, [PCr], [P(i)], and [ATP] (brackets denote concentration). Blood samples drawn from a deep arm vein were analyzed with a blood gas-electrolyte analyzer to measure plasma pH, Pco(2), and [Lac(-)](pl), and plasma [HCO(3)(-)] was calculated from pH and Pco(2). NaHCO(3) ingestion resulted in an increased (P < 0.05) plasma pH and [HCO(3)(-)] throughout rest and exercise. Time to fatigue and peak power output were increased (P < 0.05) by approximately 12% in Alk. During exercise, a delayed (P < 0.05) onset of intracellular acidosis (1.17 +/- 0.26 vs. 1.28 +/- 0.22 W, Con vs. Alk) and a delayed (P < 0.05) onset of rapid increases in the [P(i)]-to-[PCr] ratio (1.21 +/- 0.30 vs. 1.30 +/- 0.30 W) were observed in Alk. No differences in total [H(+)], [P(i)], or [Lac(-)](pl) accumulation were detected. In conclusion, NaHCO(3) ingestion was shown to increase plasma pH at rest, which resulted in a delayed onset of intracellular acidification during incremental exercise. Conversely, NaHCO(3) was not associated with increased [Lac(-)](pl) accumulation or PCr breakdown.     

Acid-base balance during repeated cycling sprints in boys and men.

The aim of this study was to investigate the acid-base balance during repeated cycling sprints in children and adults. Eleven boys (9.6 +/- 0.7 yr) and ten men (20.4 +/- 0.8 yr) performed ten 10-s sprints on a cycle ergometer separated by 30-s passive recovery intervals. To measure the time course of lactate ([La]), hydrogen ions ([H(+)]), bicarbonate ions ([HCO(3)(-)]), and base excess concentrations and the arterial partial pressure of CO(2), capillary blood samples were collected at rest and after each sprint. Ventilation and CO(2) output were continuously measured. After the 10th sprint, concentrations of boys vs. men were as follows: [La], 8.5 +/- 2.1 vs. 15.4 +/- 2.0 mmol/l; [H(+)], 43.8 +/- 1.3 vs. 66.9 +/- 9.9 nmol/l (P < 0.001). Significant correlations showed that, for a given [La], [H(+)] was lower in the boys compared with the men (P < 0.001). Significant relationships also indicated that, for a given [La], [HCO(3)(-)] and base excess concentration were similar in the boys compared with the men. Moreover, significant relationships revealed that, for a given [H(+)] or [HCO(3)(-)], arterial partial pressure of CO(2) was lower in the boys compared with the men (P < 0.001). The ventilation-to-CO(2) output ratio was higher in the boys during the first five rest intervals and was then higher in the men during the last five sprints. To conclude, during repeated sprints, the ventilatory regulation related to the change in acid-base balance induced by lactic acidosis was more important during the first rest intervals in the boys compared with the men.     

Muscle oxidative metabolism accelerates with mild acidosis during incremental intermittent isometric plantar flexion exercise

BACKGROUND: It has been thought that intramuscular ADP and phosphocreatine (PCr) concentrations are important regulators of mitochondorial respiration. There is a threshold work rate or metabolic rate for cellular acidosis, and the decrease in muscle PCr is accelerated with drop in pH during incremental exercise. We tested the hypothesis that increase in muscle oxygen consumption (o2mus) is accelerated with rapid decrease in PCr (concomitant increase in ADP) in muscles with drop in pH occurs during incremental plantar flexion exercise. METHODS: Five male subjects performed a repetitive intermittent isometric plantar flexion exercise (6-s contraction/4-s relaxation). Exercise intensity was raised every 1 min by 10% maximal voluntary contraction (MVC), starting at 10% MVC until exhaustion. The measurement site was at the medial head of the gastrocnemius muscle. Changes in muscle PCr, inorganic phosphate (Pi), ADP, and pH were measured by 31P-magnetic resonance spectroscopy. o2mus was determined from the rate of decrease in oxygenated hemoglobin and/or myoglobin using near-infrared continuous wave spectroscopy under transient arterial occlusion. Electromyogram (EMG) was also recorded. Pulmonary oxygen uptake (o2pul ) was measured by the breath-by-breath gas analysis. RESULTS: EMG amplitude increased as exercise intensity progressed. In contrast, muscle PCr, ADP, o2mus, and o2pul did not change appreciably below 40% MVC, whereas above 40% MVC muscle PCr decreased, and ADP, o2mus, and o2pul increased as exercise intensity progressed, and above 70% MVC, changes in muscle PCr, ADP, o2mus, and o2pul accelerated with the decrease in muscle pH (~6.78). The kinetics of muscle PCr, ADP, o2mus, and o2pul were similar, and there was a close correlation between each pair of parameters (r = 0.969~0.983, p < 0.001). CONCLUSION: With decrease in pH muscle oxidative metabolism accelerated and changes in intramuscular PCr and ADP accelerated during incremental intermittent isometric plantar flexion exercise. These results suggest that rapid changes in muscle PCr and/or ADP with mild acidosis stimulate accelerative muscle oxidative metabolism.     

Post‐Exercise Blood Pressure Reduction Is Greater Following Intermittent Than Continuous Exercise and Is Influenced Less by Diurnal Variation

Recently, we reported that circadian variation exists in the response of blood pressure (BP) following a bout of uninterrupted exercise. The usual phenomenon of post‐exercise hypotension was absent or reversed when such exercise was performed between 04:00–08:00 h. Nevertheless, research examining BP changes following bouts of intermittent exercise at different times of the day is scarce, even though this type of activity is probably more popular. Therefore, we aimed to compare post‐exercise BP reductions of continuous (CONT) and intermittent (INT) exercise protocols performed at 08:00 h and 16:00 h. At both of these times of day, eight normotensive males completed 30 min of continuous cycling in the CONT and three 10 min bouts of cycling separated by 10 min of rest in the INT protocol. The exercise intensity was set at 70% V˙O_2peak during both protocols. Heart rate, systolic (S) and diastolic (D) BP, and mean arterial pressure (MAP) were measured 5 min before and 20 min after exercise. Changes from pre‐exercise baseline were analyzed using linear mixed modeling. MAP was 8±1 mm Hg lower following INT compared with CONT exercise (p<0.05). SBP and DBP were also significantly lower following INT compared with CONT exercise (p<0.05). Diurnal variation in MAP was evident, with attenuated hypotension being observed after morning exercise (p<0.05), although this diurnal variation was less marked following INT compared with CONT exercise (p<0.05). We conclude that intermittent exercise mediates greater post‐exercise hypotension compared with a single continuous bout of equivalent work and that this protocol‐dependent difference is greatest in the afternoon. Therefore, a bout of afternoon exercise that is occasionally interrupted with short rest periods is recommended for lowering BP acutely.     

The effect of higher ATP cost of contraction on the metabolic response to graded exercise in patients with chronic obstructive pulmonary disease

To better understand the metabolic implications of a higher ATP cost of contraction in chronic obstructive pulmonary disease (COPD), we used (31)P-magnetic resonance spectroscopy ((31)P-MRS) to examine muscle energetics and pH in response to graded exercise. Specifically, in six patients and six well-matched healthy controls, we determined the intracellular threshold for pH (T(pH)) and inorganic phosphate-to-phosphocreatine ratio (T(Pi/PCr)) during progressive dynamic plantar flexion exercise with work rate expressed as both absolute and relative intensity. Patients with COPD displayed a lower peak power output (WRmax) compared with controls (controls 25 ± 4 W, COPD 15 ± 5 W, P = 0.01) while end-exercise pH (controls 6.79 ± 0.15, COPD 6.76 ± 0.21, P = 0.87) and PCr consumption (controls 82 ± 10%, COPD 70 ± 18%, P = 0.26) were similar between groups. Both T(pH) and T(Pi/PCr) occurred at a significantly lower absolute work rate in patients with COPD compared with controls (controls: 14.7 ± 2.4 W for T(pH) and 15.3 ± 2.4 W for T(Pi/PCr); COPD: 9.7 ± 4.5 W for T(pH) and 10.0 ± 4.6 W for T(Pi/PCr), P < 0.05), but these thresholds occurred at the same percentage of WRmax (controls: 63 ± 11% WRmax for T(pH) and 67 ± 18% WRmax for T(Pi/PCr); COPD: 59 ± 9% WRmax for T(pH) and 61 ± 12% WRmax for T(Pi/PCr), P > 0.05). Indexes of mitochondrial function, the PCr recovery time constant (controls 42 ± 7 s, COPD 45 ± 11 s, P = 0.66) and the PCr resynthesis rate (controls 105 ± 21%/min, COPD 91 ± 31%/min, P = 0.43) were similar between groups. In combination, these results reveal that when energy demand is normalized to WRmax, as a consequence of higher ATP cost of contraction, patients with COPD display the same metabolic pattern as healthy subjects, suggesting that skeletal muscle energy production is well preserved in these patients.     

Artificial gravity with ergometric exercise preserves the cardiac, but not cerebrovascular, functions during 4 days of head-down bed rest

Cardiovascular and musculoskeletal deconditioning occurring in long-term spaceflight requires new strategies to counteract these adverse effects. We previously reported that a short-arm centrifuge produced artificial gravity (AG), together with ergometer, has an approving effect on promoting cardiovascular function. The current study sought to investigate whether the cardiac and cerebrovascular functions were maintained and improved using a strategy of AG combined with exercise training on cardiovascular function during 4-day head-down bed rest (HDBR). Twelve healthy male subjects were assigned to a control group (CONT, n=6) and an AG combined with ergometric exercise training group (CM, n=6). Simultaneously, cardiac pumping and systolic functions, cerebral blood flow were measured before, during, and after HDBR. The results showed that AG combined with ergometric exercise caused an increase trend of number of tolerance, however, there was no significant difference between the two groups. After 4-day HDBR in the CONT group, heart rate increased significantly (59±6 vs 66±7 beats/min), while stroke volume (98±12 vs 68±13 mL) and cardiac output (6±1 vs 4±1 L/min) decreased significantly (p<0.05). All subjects had similar drops on cerebral vascular function. Volume regulating hormone aldosterone increased in both groups (by 119.9% in CONT group and 112.8% in the CM group), but only in the CONT group there were a significant changes (p<0.05). Angiotensin II was significantly increased by 140.5% after 4-day HDBR in the CONT group (p<0.05), while no significant changes were observed in the CM group. These results indicated that artificial gravity with ergometric exercise successfully eliminated changes induced by simulated weightlessness in heart rate, volume regulating hormones, and cardiac pumping function and partially maintained cardiac systolic function. Hence, a daily 1h alternating +1.0 and +2.0 Gz with 40 W exercise training appear to be an effective countermeasure against cardiac deconditioning.     

Muscle metabolism in older subjects using 31P magnetic resonance spectroscopy.

We used phosphorus magnetic resonance spectroscopy to study the calf muscles of elderly normal (mean +/- SD) (80.0 +/- 5.12 years), elderly impaired (80.7 +/- 0.58 years), old normal (66.8 +/- 1.92 years), and young normal people (24.6 +/- 4.72 years). Relative levels of inorganic phosphate (Pi), phosphocreatine (PCr), and adenosine triphosphate were measured with a 1.9-tesla, 30-cm bore magnet at rest and following plantra flexon exercise. No differences were found at rest or during recovery from exercise in the elderly normal subjects with respect to gender or the presence of stable medical problems treated with medication. At rest there was an age-related decrease in the ratio of PCr/Pi. After exercise, the time constant of PCr recovery increased with age. A mild 7-week exercise regimen consisting of plantar flexion had no effect on time constant of PCr recovery in the elderly subjects. Four elderly impaired subjects had lower PCr/Pi ratios at rest and slower time constant of PCr recovery after exercise than normal elderly subjects. We conclude that gender and the presence of stable medical problems had no effect on muscle metabolism in the elderly and that the elderly recovered slower than young controls. This slower recovery was not corrected with a mild exercise program.     

Differential contribution of central command to the cardiovascular responses during static exercise of ankle dorsal and plantar flexion in humans

To examine whether central command contributes differently to the cardiovascular responses during voluntary static exercise engaged by different muscle groups, we encouraged healthy subjects to perform voluntary and electrically evoked involuntary static exercise of ankle dorsal and plantar flexion. Each exercise was conducted with 25% of the maximum voluntary force of the right ankle dorsal and plantar flexion, respectively, for 2 min. Heart rate (HR) and mean arterial blood pressure (MAP) were recorded, and stroke volume, cardiac output (CO), and total peripheral resistance were calculated. With voluntary exercise, HR, MAP, and CO significantly increased during dorsal flexion (the maximum increase, HR: 12 ± 2.3 beats/min; MAP: 14 ± 2.0 mmHg; CO: 1 ± 0.2 l/min), whereas only MAP increased during plantar flexion (the maximum increase, 6 ± 2.0 mmHg). Stroke volume and total peripheral resistance were unchanged throughout the two kinds of voluntary static exercise. With involuntary exercise, there were no significant changes in all cardiovascular variables, irrespective of dorsal or plantar flexion. Furthermore, before the force onset of voluntary static exercise, HR and MAP started to increase without muscle contraction, whereas they had no significant changes with involuntary exercise at the moment. The present findings indicate that differential contribution of central command is responsible for the different cardiovascular responses to static exercise, depending on the strength of central control of the contracting muscle.     

The Recovery of Repeated-Sprint Exercise Is Associated with PCr Resynthesis,while Muscle pH and EMG Amplitude Remain Depressed

The physiological equivalents of power output maintenance and recovery during repeated-sprint exercise (RSE) remain to be fully elucidated. In an attempt to improve our understanding of the determinants of RSE performance we therefore aimed to determine its recovery following exhaustive exercise (which affected intramuscular and neural factors) concomitantly with those of intramuscular concentrations of adenosine triphosphate [ATP], phosphocreatine [PCr] and pH values and electromyography (EMG) activity (a proxy for net motor unit activity) changes. Eight young men performed 10, 6-s all-out sprints on a cycle ergometer, interspersed with 30 s of recovery, followed, after 6 min of passive recovery, by five 6-s sprints, again interspersed by 30 s of passive recovery. Biopsies of the vastus lateralis were obtained at rest, immediately after the first 10 sprints and after 6 min of recovery. EMG activity of the vastus lateralis was obtained from surface electrodes throughout exercise. Total work (TW), [ATP], [PCr], pH and EMG amplitude decreased significantly throughout the first ten sprints (P<0.05). After 6 min of recovery, TW during sprint 11 recovered to 86.3±7.7% of sprint 1. ATP and PCr were resynthesized to 92.6±6.0% and 85.3±10.3% of the resting value, respectively, but muscle pH and EMG amplitude remained depressed. PCr resynthesis was correlated with TW done in sprint 11 (r = 0.79, P<0.05) and TW done during sprints 11 to 15 (r = 0.67, P<0.05). There was a ∼2-fold greater decrease in the TW/EMG ratio in the last five sprints (sprint 11 to 15) than in the first five sprints (sprint 1 to 5) resulting in a disproportionate decrease in mechanical power (i.e., TW) in relation to EMG. Thus, we conclude that the inability to produce power output during repeated sprints is mostly mediated by intramuscular fatigue signals probably related with the control of PCr metabolism.     

Physicochemical analysis of phasic menstrual cycle effects on acid-base balance

In accordance with Stewart's physicochemical approach, the three independent determinants of plasma hydrogen ion concentration ([H(+)]) were measured at rest and during exercise in the follicular (FP) and luteal phase (LP) of the human menstrual cycle. Healthy, physically active women with similar physical characteristics were tested during either the FP (n = 14) or LP (n = 14). Arterialized blood samples were obtained at rest and after 5 min of upright cycling at both 70 and 110% of the ventilatory threshold (T(Vent)). Measurements included plasma [H(+)], arterial carbon dioxide tension (Pa(CO(2))), total weak acid ([A(Tot)]) as reflected by total protein, and the strong-ion difference ([SID]). The transition from rest to exercise in both groups resulted in a significant increase in [H(+)] at 70% T(Vent) versus rest and at 110% T(Vent) versus both rest and 70% T(Vent). No significant between-group differences were observed for [H(+)] at rest or in response to exercise. At rest in the LP, [A(Tot)] and Pa(CO(2)) were significantly lower (acts to decrease [H(+)]) compared with the FP. This effect was offset by a reduction in [SID] (acts to increase [H(+)]). After the transition from rest to exercise, significantly lower [A(Tot)] during the LP was again observed. Although the [SID] and Pa(CO(2)) were not significantly different between groups, trends for changes in these two variables were similar to changes in the resting state. In conclusion, mechanisms regulating [H(+)] exhibit phase-related differences to ensure [H(+)] is relatively constant regardless of progesterone-mediated ventilatory changes during the LP.     

Creatine reduces human muscle PCr and pH decrements and P(i) accumulation during low-intensity exercise.

The purpose of this study was to examine with (31)P-magnetic resonance spectroscopy energy metabolism during repeated plantar flexion isometric exercise (Ex-1-Ex-4) at 32 +/- 1 and 79 +/- 4% of maximal voluntary contraction (MVC) before and during a creatine (Cr) feeding period of 5 g/day for 11 days. Eight trained male subjects participated in the study. ATP was unchanged with Cr supplementation at rest and during exercise at both intensities. Resting muscle phosphocreatine (PCr) increased (P < 0.05) from 18.3 +/- 0.9 (before) to 19.6 +/- 1.0 mmol/kg wet wt after 9 days. At 79% MVC, PCr used, P(i) accumulated, and pH at the end of Ex-1-Ex-4 were similar after 4 and 11 days of Cr supplementation. In contrast, PCr utilization and P(i) accumulation were lower and pH was higher for exercise at 32% MVC with Cr supplementation, suggesting aerobic resynthesis of PCr was more rapid during exercise. These results suggest that elevating muscle Cr enhances oxidative phosphorylation during mild isometric exercise, where it is expected that oxygen delivery matches demands and predominantly slow-twitch motor units are recruited.     

Human muscle performance and PCr hydrolysis with varied inspired oxygen fractions: a 31P-MRS study.

The purpose of this study was to use 31P-magnetic resonance spectroscopy to examine the relationships among muscle PCr hydrolysis, intracellular H+ concentration accumulation, and muscle performance during incremental exercise during the inspiration of gas mixtures containing different fractions of inspired O2 (FIO2). We hypothesized that lower FIO2 would result in a greater disruption of intracellular homeostasis at submaximal workloads and thereby initiate an earlier onset of fatigue. Six subjects performed plantar flexion exercise on three separate occasions with the only variable altered for each exercise bout being the FIO2 (either 0.1, 0.21, or 1.00 O2 in balance N2). Work rate was increased (1-W increments starting at 0 W) every 2 min until exhaustion. Time to exhaustion (and thereby workload achieved) was significantly (P < 0.05) greater as FIO2 was increased. Muscle phosphocreatine (PCr) concentration, Pi concentration, and pH at exhaustion were not significantly different among the three FIO2 conditions. However, muscle PCr concentration and pH were significantly reduced at identical submaximal workloads (and thereby equivalent rates of respiration) above 4-5 W during the lowest FIO2 condition compared with the other two FIO2 conditions. These results demonstrate that exhaustion during all FIO2 occurred when a particular intracellular environment was achieved and suggest that during the lowest FIO2 condition, the greater PCr hydrolysis and intracellular acidosis at submaximal workloads may have contributed to the significantly earlier time to exhaustion.     

Skeletal muscle phosphocreatine recovery in exercise-trained humans is dependent on O2 availability.

In skeletal muscle, phosphocreatine (PCr) recovery from submaximal exercise has become a reliable and accepted measure of muscle oxidative capacity. During exercise, O2 availability plays a role in determining maximal oxidative metabolism, but the relationship between O2 availability and oxidative metabolism measured by 31P-magnetic resonance spectroscopy (MRS) during recovery from exercise has never been studied. We used 31P-MRS to study exercising human gastrocnemius muscle under conditions of varied fractions of inspired O2 (FIO2) to test the hypothesis that varied O2 availability modulates PCr recovery from submaximal exercise. Six male subjects performed three bouts of 5-min steady-state submaximal plantar flexion exercise followed by 5 min of recovery in a 1.5-T magnet while breathing three different FIO2 concentrations (0.10, 0. 21, and 1.00). Under each FIO2 treatment, the PCr recovery time constants were significantly different, being longer in hypoxia [33. 5 +/- 4.1 s (SE)] and shorter in hyperoxia (20.0 +/- 1.8 s) than in normoxia (25.0 +/- 2.7 s) (P 相似文献   

Effect of weight reduction, obesity predisposition, and aerobic fitness on skeletal muscle mitochondrial function

We used (31)P magnetic resonance spectroscopy to measure maximal mitochondrial function in 12 obesity-prone women before and after diet-induced weight reduction and in 12 matched, never-obese, and 7 endurance-trained controls. Mitochondrial function was modeled after maximum-effort plantar flexion from the phosphocreatine recovery time constant (TC(PCr)), the ADP recovery time constant (TC(ADP)), and the rate of change in PCr during the first 14 s of recovery (OxPhos). Weight reduction was not associated with a significant change in mitochondrial function by TC(PCr), TC(ADP), or OxPhos. Mitochondrial function was not different between postobese and never-obese controls by TC(PCr) [35.1 +/- 2.5 (SE) vs. 34.6 +/- 2.5 s], TC(ADP) (22.9 +/- 1.8 vs. 21.2 +/- 1.8 s), or OxPhos (0.26 +/- 0. 03 vs. 0.25 +/- 0.03 mM ATP/s), postobese vs. never-obese, respectively. However, TC(ADP) was significantly faster (14.5 +/- 2. 3 s), and OxPhos was significantly higher (0.38 +/- 0.04 mM ATP/s) in the endurance-trained group. These results suggest that maximal mitochondrial function is not impaired in normal-weight obesity-prone women relative to their never-obese counterparts but is increased in endurance-trained women.     

Energy metabolism during repeated sets of leg press exercise leading to failure or not

This investigation examined the influence of the number of repetitions per set on power output and muscle metabolism during leg press exercise. Six trained men (age 34 ± 6 yr) randomly performed either 5 sets of 10 repetitions (10REP), or 10 sets of 5 repetitions (5REP) of bilateral leg press exercise, with the same initial load and rest intervals between sets. Muscle biopsies (vastus lateralis) were taken before the first set, and after the first and the final sets. Compared with 5REP, 10REP resulted in a markedly greater decrease (P<0.05) of the power output, muscle PCr and ATP content, and markedly higher (P<0.05) levels of muscle lactate and IMP. Significant correlations (P<0.01) were observed between changes in muscle PCr and muscle lactate (R(2) = 0.46), between changes in muscle PCr and IMP (R(2) = 0.44) as well as between changes in power output and changes in muscle ATP (R(2) = 0.59) and lactate (R(2) = 0.64) levels. Reducing the number of repetitions per set by 50% causes a lower disruption to the energy balance in the muscle. The correlations suggest that the changes in PCr and muscle lactate mainly occur simultaneously during exercise, whereas IMP only accumulates when PCr levels are low. The decrease in ATP stores may contribute to fatigue.