Scott R. Murgatroyd

Human Exercise-Induced Circulating Progenitor Cell Mobilization Is Nitric Oxide-Dependent and Is Blunted in South Asian Men

Objective—Circulating progenitor cells (CPC) have emerged as potential mediators of vascular repair. In experimental models, CPC mobilization is critically dependent on nitric oxide (NO). South Asian ethnicity is associated with reduced CPC. We assessed CPC mobilization in response to exercise in Asian men and examined the role of NO in CPC mobilization per se. Methods and Results—In 15 healthy, white European men and 15 matched South Asian men, CPC mobilization was assessed during moderate-intensity exercise. Brachial artery flow-mediated vasodilatation was used to assess NO bioavailability. To determine the role of NO in CPC mobilization, identical exercise studies were performed during intravenous separate infusions of saline, the NO synthase inhibitor l-NMMA, and norepinephrine. Flow-mediated vasodilatation (5.8%±0.4% vs 7.9%±0.5%; P=0.002) and CPC mobilization (CD34+/KDR+ 53.2% vs 85.4%; P=0.001; CD133+/CD34+/KDR+ 48.4% vs 73.9%; P=0.05; and CD34+/CD45− 49.3% vs 78.4; P=0.006) was blunted in the South Asian group. CPC mobilization correlated with flow-mediated vasodilatation and l-NMMA significantly reduced exercise-induced CPC mobilization (CD34+/KDR+ −3.3% vs 68.4%; CD133+/CD34+/KDR+ 0.7% vs 71.4%; and CD34+/CD45− −30.5% vs 77.8%; all P<0.001). Conclusion—In humans, NO is critical for CPC mobilization in response to exercise. Reduced NO bioavailability may contribute to imbalance between vascular damage and repair mechanisms in South Asian men.

A raised metabolic rate slows pulmonary O2 uptake kinetics on transition to moderate-intensity exercise in humans independently of work rate

During exercise below the lactate threshold (LT), the rate of adjustment (τ) of pulmonary O2 uptake ( ) is slowed when initiated from a raised work rate. Whether this is consequent to the intrinsic properties of newly recruited muscle fibres, slowed circulatory dynamics or the effects of a raised metabolism is not clear. We aimed to determine the influence of these factors on using combined in vivo and in silico approaches. Fifteen healthy men performed repeated 6 min bouts on a cycle ergometer with work rates residing between 20 W and 90% LT, consisting of the following: (1) two step increments in work rate (S1 and S2), one followed immediately by the other, equally bisecting 20 W to 90% LT; (2) two 20 W to 90% LT bouts separated by 30 s at 20 W to raise muscle oxygenation and pretransition metabolism (R1 and R2); and (3) two 20 W to 90% LT bouts separated by 12 min at 20 W allowing full recovery (F1 and F2). Pulmonary O2 uptake was measured breath by breath by mass spectrometry and turbinometry, and quadriceps oxygenation using near‐infrared spectroscopy. The influence of circulatory dynamics on the coupling of muscle and lung was assessed by computer simulations. The in R2 (32 ± 9 s) was not different (P > 0.05) from S2 (30 ± 10 s), but both were greater (P < 0.05) than S1 (20 ± 10 s) and the F control bouts (26 ± 10 s). The slowed kinetics in R2 occurred despite muscle oxygenation being raised throughout, and could not be explained by slowed circulatory dynamics ( predicted by simulations: S1 = R2 < S2). These data therefore suggest that the dynamics of muscle O2 consumption are slowed when exercise is initiated from a less favourable energetic state.

The intramuscular contribution to the slow oxygen uptake kinetics during exercise in chronic heart failure is related to the severity of the condition

The mechanism for slow pulmonary O(2) uptake (Vo(2)) kinetics in patients with chronic heart failure (CHF) is unclear but may be due to limitations in the intramuscular control of O(2) utilization or O(2) delivery. Recent evidence of a transient overshoot in microvascular deoxygenation supports the latter. Prior (or warm-up) exercise can increase O(2) delivery in healthy individuals. We therefore aimed to determine whether prior exercise could increase muscle oxygenation and speed Vo(2) kinetics during exercise in CHF. Fifteen men with CHF (New York Heart Association I-III) due to left ventricular systolic dysfunction performed two 6-min moderate-intensity exercise transitions (bouts 1 and 2, separated by 6 min of rest) from rest to 90% of lactate threshold on a cycle ergometer. Vo(2) was measured using a turbine and a mass spectrometer, and muscle tissue oxygenation index (TOI) was determined by near-infrared spectroscopy. Prior exercise increased resting TOI by 5.3 ± 2.4% (P = 0.001), attenuated the deoxygenation overshoot (-3.9 ± 3.6 vs. -2.0 ± 1.4%, P = 0.011), and speeded the Vo(2) time constant (τVo(2); 49 ± 19 vs. 41 ± 16 s, P = 0.003). Resting TOI was correlated to τVo(2) before (R(2) = 0.51, P = 0.014) and after (R(2) = 0.36, P = 0.051) warm-up exercise. However, the mean response time of TOI was speeded between bouts in half of the patients (26 ± 8 vs. 20 ± 8 s) and slowed in the remainder (32 ± 11 vs. 44 ± 16 s), the latter group having worse New York Heart Association scores (P = 0.042) and slower Vo(2) kinetics (P = 0.001). These data indicate that prior moderate-intensity exercise improves muscle oxygenation and speeds Vo(2) kinetics in CHF. The most severely limited patients, however, appear to have an intramuscular pathology that limits Vo(2) kinetics during moderate exercise.

Skeletal muscle ATP turnover by 31P magnetic resonance spectroscopy during moderate and heavy bilateral knee extension

Heavy‐intensity exercise causes a progressive increase in energy demand that contributes to exercise limitation. This inefficiency arises within the locomotor muscles and is thought to be due to an increase in the ATP cost of power production; however, the responsible mechanism is unresolved. We measured whole‐body O2 uptake and skeletal muscle ATP turnover by combined pulmonary gas exchange and magnetic resonance spectroscopy during moderate and heavy exercise in humans. Muscle ATP synthesis rate increased throughout constant‐power heavy exercise, but this increase was unrelated to the progression of whole‐body inefficiency. Our data indicate that the increased ATP requirement is not the sole cause of inefficiency during heavy exercise, and other mechanisms, such as increased O2 cost of ATP resynthesis, may contribute.

Data collection, handling, and fitting strategies to optimize accuracy and precision of oxygen uptake kinetics estimation from breath-by-breath measurements

Phase 2 pulmonary oxygen uptake kinetics (ϕ2 τV̇o2P) reflect muscle oxygen consumption dynamics and are sensitive to changes in state of training or health. This study identified an unbiased method for data collection, handling, and fitting to optimize V̇o2P kinetics estimation. A validated computational model of V̇o2P kinetics and a Monte Carlo approach simulated 2 × 105 moderate-intensity transitions using a distribution of metabolic and circulatory parameters spanning normal health. Effects of averaging (interpolation, binning, stacking, or separate fitting of up to 10 transitions) and fitting procedures (biexponential fitting, or ϕ2 isolation by time removal, statistical, or derivative methods followed by monoexponential fitting) on accuracy and precision of V̇o2P kinetics estimation were assessed. The optimal strategy to maximize accuracy and precision of τV̇o2P estimation was 1-s interpolation of 4 bouts, ensemble averaged, with the first 20 s of exercise data removed. Contradictory to previous advice, we found optimal fitting procedures removed no more than 20 s of ϕ1 data. Averaging method was less critical: interpolation, binning, and stacking gave similar results, each with greater accuracy compared with analyzing repeated bouts separately. The optimal procedure resulted in ϕ2 τV̇o2P estimates for transitions from an unloaded or loaded baseline that averaged 1.97 ± 2.08 and 1.04 ± 2.30 s from true, but were within 2 s of true in only 47-62% of simulations. Optimized 95% confidence intervals for τV̇o2P ranged from 4.08 to 4.51 s, suggesting a minimally important difference of ~5 s to determine significant changes in τV̇o2P during interventional and comparative studies.NEW & NOTEWORTHY We identified an unbiased method to maximize accuracy and precision of oxygen uptake kinetics (τV̇o2P) estimation. The optimum number of bouts to average was four; interpolation, bin, and stacking averaging methods gave similar results. Contradictory to previous advice, we found that optimal fitting procedures removed no more than 20 s of phase 1 data. Our data suggest a minimally important difference of ~5 s to determine significant changes in τV̇o2P during interventional and comparative studies.

The power–duration relationship of high‐intensity exercise: from mathematical parameters to physiological mechanisms

The origins of intolerance during exercise have perplexed scientists for well over a century. A better understanding of the processes that contribute to limiting high-intensity exercise have far-reaching implications, not only for elite exercise performance, but also for the wide spectrum of health, quality of life and mortality. It is intriguing, therefore, that there has existed for about 50 years a mathematical model that has the capacity to predict intolerance during high-intensity constant-work rate exercise. This hyperbolic power–duration (P–d) relationship was first described by Monod and Scherrer for a single muscle group in 1965, and has since been extended to whole-body exercise (e.g. Poole et al. 1988). This model characterises a ‘critical’ power output (CP) or speed (CS) that, once exceeded, will lead to exhaustion in a duration predicted by the completion of a constant amount of work (W ′) (Morton, 2006). CP, which lies between the lactate threshold and maximum oxygen uptake (V̇O2 max), reflects an intrinsic threshold of aerobic energy provision – defining the highest constant-work rate for which steady states in ventilation, gas exchange (e.g. V̇O2 ) and metabolic (e.g. muscle and blood acid–base status) variables can be achieved (Poole et al. 1988). That CP reflects a parameter of aerobic function is supported by the fact that it is sensitive to interventions affecting oxygen transport and utilisation, such as breathing hypoxic gas mixtures or endurance exercise training. While W ′ has been likened to a (predominantly) anaerobic energy source (Monod & Scherrer, 1965; potentially comprising stores of intramuscular glycogen, high-energy phosphates and oxygen), relatively less is known about the physiological underpinnings of this parameter. Importantly, the robust nature of the P–d relationship is demonstrated in its ability to characterise exercise tolerance for a wide range of exercise modalities (including cycling, running, swimming, kayaking, rowing and knee-extension over durations of ∼2–30 min; Morton, 2006), for different subject populations (ranging from adolescents to the elderly, and from elite athletes to patients with chronic heart or lung diseases), and for different species (humans, lungless salamanders, ghost crabs, mice, horses and now also rats; Copp et al. 2010). It is our belief, therefore, that a better understanding of the basis of the P–d relationship will elucidate the factor (or factors) that contribute to limiting exercise performance – the secrets of which are defined within the parameters that shape its curve. Surprisingly, however, there is relatively little evidence supporting the physiological origins of the P–d relationship. As such, the underlying physiological equivalents of the defining mathematical P–d parameters, CP and W ′, remain conjectural.

Selecting Constant Work Rates for Endurance Testing in COPD: The Role of the Power-Duration Relationship

Abstract Constant work rate (CWR) exercise testing is highly responsive to therapeutic interventions and reveals physiological and functional benefits. No consensus exists, however, regarding optimal methods for selecting the pre-intervention work rate. We postulate that a CWR whose tolerated duration (tlim) is 6 minutes (WR6) may provide a useful interventional study baseline. WR6 can be extracted from the power-duration relationship, but requires 4 CWR tests. We sought to develop prediction algorithms for easier WR6 identification using backward stepwise linear regression, one in 69 COPD patients (FEV1 45 ± 15% pred) and another in 30 healthy subjects (HLTH), in whom cycle ergometer ramp incremental (RI) and CWR tests with tlim of ∼6 minutes had been performed. Demographics, pulmonary function, and RI responses were used as predictors. We validated these algorithms against power-duration measurements in 27 COPD and 30 HLTH (critical power 43 ± 18W and 231 ± 43W; curvature constant 5.1 ± 2.7 kJ and 18.5 ± 3.1 kJ, respectively). This analysis revealed that, on average, only corrected peak work rate ( = WRpeak–1 min × WRslope) in RI was required to predict WR6 (COPD SEE = 5.0W; HLTH SEE = 5.6W; R2 > 0.96; p < 0.001). In the validation set, predicted and actual WR6 were strongly correlated (COPD R2 = 0.937; HLTH 0.978; p < 0.001). However, in COPD, unlike in HLTH, there was a wide range of tlim values at predicted WR6: COPD 8.3 ± 4.1 min (range 3.6 to 22.2 min), and HLTH 5.5 ± 0.7 min (range 3.9 to 7.0 min). This analysis indicates that corrected WRpeak in an incremental test can yield an acceptable basis for calculating endurance testing work rate in HLTH, but not in COPD patients.

Reply to Francescato et al.: Interpreting the averaging methods to estimate oxygen uptake kinetics parameters

to the editor: We thank Dr. Francescato and colleagues for their interest in our study ([1][1]) and for their letter ([3][2]). The authors requested clarification of why we concluded that 1-s interpolation of breath-by-breath pulmonary oxygen uptake (Vo2P) data resulted in the most accurate and

97 Increasing skeletal muscle oxygenation by prior moderate-intensity exercise increases aerobic energy provision in chronic heart failure

Rapid adaptation of pulmonary oxygen uptake (VO2) at exercise onset reduces the reliance on limited anaerobic energy stores and is associated with increased exercise tolerance. These VO2 kinetics, however, are slow in patients with chronic heart failure (CHF). This could be due to limitations in the control of muscle O2 consumption and/or O2 delivery. Recent evidence in CHF of a transient overshoot in microvascular deoxygenation at exercise onset supports the latter. As prior exercise is known to increase muscle blood flow in healthy individuals, we examined whether it could attenuate the fall in microvascular deoxygenation and speed VO2 kinetics on transition to moderate exercise in CHF patients. Thirteen CHF patients (NYHA class I: n=3, II: n=9, and III: n=1) performed a ramp test on a cycle ergometer for estimation of lactate threshold (LT) and VO2max. Patients subsequently repeated two 6-min moderate-intensity exercise transitions (bout 1, bout 2) from rest to 90%LT, separated by 6-min of rest. Measurements included breath-by-breath VO2 using a turbine and mass spectrometer (MSX, NSpire, UK), and tissue oxygenation index (TOI) of the vastus lateralis by spatially resolved near-infrared spectroscopy (NIRO200, Hamamatsu, Japan). The exponential time-constant (τ) for TOI and phase II VO2 were estimated using non-linear least-squares regression. The τVO2/τTOI, or “kinetic index”, was taken to reflect the relative matching of muscle oxygenation to its instantaneous requirement. LT and VO2max were 9.9±1.7 (mean±SD) and 15.0±3.2 ml/kg/min, respectively. Prior exercise increased resting TOI by 10±3% (p<0.05), attenuated the transient overshoot in muscle deoxygenation by ∼50% (p<0.05) and slowed the rate of deoxygenation in the transient (τTOI: 10±1 vs 21±13 s; p<0.05). Both τVO2 (46±20 vs 39±18 s; p<0.05) and the kinetic index (4.5±1.8 vs 2.2±0.9; p<0.05) were reduced following prior exercise. τVO2 was well correlated to the kinetic index (R2=0.92) in bout 1. However, although a lower τVO2 was typically reflected in a reduced kinetic index in bout 2, VO2 kinetics remained slowed in 4 patients. These patients had a higher NYHA class (2.3±0.5 vs 1.6±0.5; p=0.06) and greater initial τVO2 (62±17 vs 33±9 s; p<0.05) than the others. In CHF prior moderate-intensity exercise improved the dynamic matching of muscle oxygenation to its instantaneous requirement and speeded VO2 kinetics in all patients. This suggests that slow VO2 kinetics in CHF are due, at least in part, to a dynamic limitation in O2 delivery. However, this approach revealed an apparent limitation in the control of muscle O2 consumption in the most severe patients, which was only partly ameliorated by improving O2 delivery. Nevertheless, these findings suggest that an acute intervention to improve muscle oxygenation can increase aerobic energy provision on transition to exercise in CHF patients.

020 Human exercise induced circulating progenitor cell mobilisation is nitric oxide dependent and is blunted in South Asian men

Background Circulating progenitor cells (CPCs) have emerged as potential mediators of vascular repair. In experimental models CPC mobilisation is critically dependent on nitric oxide (NO). South Asian ethnicity is associated with reduced CPCs; we assessed CPC mobilisation in response to exercise in Asian men and examined the role of NO in CPC mobilisation per se. Methods In 15 healthy white European men and 15 matched South Asian men, CPC mobilisation (relative % increase from pre- to post-exercise) was assessed during moderate intensity exercise using flow cytometry. Three subsets of CPCs were studied (CD34+/KDR+, CD133+/CD34+/KDR+ or CD34+/CD45−) and were expressed as a % of circulating lymphocytes). Brachial artery flow mediated vasodilatation (FMD) was used to assess NO bioavailability. To determine the role of NO in CPC mobilisation, identical exercise studies were performed during intravenous separate infusions of saline, the NO synthase inhibitor L-NMMA, and norepinephrine. Results Groups were well matched for age, blood pressure, indices of obesity and circulating lipids; however the South Asian group exhibited insulin resistance as measured by the HOMA-IR score (1.3±0.2 vs 0.8±0.1; p=0.047). FMD (5.8±0.4% vs 7.9±0.5%; p=0.002) and basal CPC numbers (CD34+/KDR+: 0.030% vs 0.046%, p=0.05; CD133+/CD34+/KDR+: 0.008% vs 0.014%, p=0.026; CD34+/CD45−: 0.011% vs 0.018%, p=0.044) were lower in the South Asian group. CPC mobilisation (CD34+/KDR+ 53.2% vs 85.4%, p=0.001; CD133+/CD34+/KDR+ 48.4% vs 73.9%, p=0.05; CD34+/CD45− 49.3% vs 78.4, p=0.006) was blunted in the South Asian group. CPC mobilisation correlated with FMD and L-NMMA significantly reduced exercise induced CPC mobilisation (CD34+/KDR+ −3.3% vs 68.4%; CD133+/CD34+/KDR+ 0.7% vs 71.4%; CD34+/CD45− −30.5% vs 77.8%; all p<0.001). Conclusions In humans NO is critical for CPC mobilisation in response to exercise. Reduced NO bioavailability may contribute to imbalance between vascular damage and repair mechanisms in South Asian men. This project was supported by the British Heart Foundation.