Human skeletal muscle energy metabolism: when a physiological model promotes the search for new technologies

Modeling oxygenation in venous blood and skeletal muscle in response to exercise using near-infrared spectroscopy

Noninvasive, continuous measurements in vivo are commonly used to make inferences about mechanisms controlling internal and external respiration during exercise. In particular, the dynamic response of muscle oxygenation (Sm(O(2))) measured by near-infrared spectroscopy (NIRS) is assumed to be correlated to that of venous oxygen saturation (Sv(O(2))) measured invasively. However, there are situations where the dynamics of Sm(O(2)) and Sv(O(2)) do not follow the same pattern. A quantitative analysis of venous and muscle oxygenation dynamics during exercise is necessary to explain the links between different patterns observed experimentally. For this purpose, a mathematical model of oxygen transport and utilization that accounts for the relative contribution of hemoglobin (Hb) and myoglobin (Mb) to the NIRS signal was developed. This model includes changes in microvascular composition within skeletal muscle during exercise and integrates experimental data in a consistent and mechanistic manner. Three subjects (age 25.6 +/- 0.6 yr) performed square-wave moderate exercise on a cycle ergometer under normoxic and hypoxic conditions while muscle oxygenation (C(oxy)) and deoxygenation (C(deoxy)) were measured by NIRS. Under normoxia, the oxygenated Hb/Mb concentration (C(oxy)) drops rapidly at the onset of exercise and then increases monotonically. Under hypoxia, C(oxy) decreases exponentially to a steady state within approximately 2 min. In contrast, model simulations of venous oxygen concentration show an exponential decrease under both conditions due to the imbalance between oxygen delivery and consumption at the onset of exercise. Also, model simulations that distinguish the dynamic responses of oxy-and deoxygenated Hb (HbO(2), HHb) and Mb (MbO(2), HMb) concentrations (C(oxy) = HbO(2) + MbO(2); C(deoxy) = HHb + HMb) show that Hb and Mb contributions to the NIRS signal are comparable. Analysis of NIRS signal components during exercise with a mechanistic model of oxygen transport and metabolism indicates that changes in oxygenated Hb and Mb are responsible for different patterns of Sm(O(2)) and Sv(O(2)) dynamics observed under normoxia and hypoxia.

High-energy phosphate metabolism in the calf muscle of healthy humans during incremental calf exercise with and without moderate cuff stenosis

It is known that the relevance of a peripheral stenosis for muscle function increases with exercise. Our intention was to investigate the impact of a moderate cuff stenosis (CS) at 120 mmHg of the superficial femoral artery on high-energy phosphate (HEP) metabolism during isotonic, incremental calf exercise. Serial phosphorus 31 magnetic resonance spectroscopy (31P MRS) and velocity-encoded phase-contrast MR imaging (VEPC MRI) were carried out in each leg of ten healthy male volunteers. Each leg underwent four increments of calf exercise (2, 3, 4 and 5 W) followed by recovery during separate exercise sessions with and without a CS at 120 mmHg. The serial 31P MRS measurements had a time resolution of 10 s. VEPC MRI was performed at the end of each increment during separate sessions. During all increments, we detected significant differences (P < 0.05) in the phosphocreatine (PCr) time constants and the amount of PCr hydrolysis between the sessions without and with CS. Regarding the time courses of the PCr, inorganic phosphate (Pi) and pH level, we observed significant differences (P < 0.002) during exercise and recovery. During both conditions, the end-increment PCr levels as well as blood flow correlated significantly with the mechanical power. The PCr time constants during exercise significantly correlated with the intramuscular pH, but not with blood flow or mechanical power. However, the PCr recovery time constants correlated significantly with blood flow and end-exercise pH. Our study shows that reduction of blood flow due to a peripheral stenosis results in a prolongation of PCr time constants, decreased PCr and pH level as well as increased Pi level during exercise. We believe that 31P MRS during incremental exercise might provide additional information for assessing the relevance of a peripheral stenosis and its impact on muscle function.

Relating pulmonary oxygen uptake to muscle oxygen consumption at exercise onset: in vivo and in silico studies

Assessment of the rate of muscle oxygen consumption, UO(2m), in vivo during exercise involving a large muscle mass is critical for investigating mechanisms regulating energy metabolism at exercise onset. While UO(2m) is technically difficult to obtain under these circumstances, pulmonary oxygen uptake, VO(2p), can be readily measured and used as a proxy to UO(2m). However, the quantitative relationship between VO(2p) and UO(2m) during the nonsteady phase of exercise in humans, needs to be established. A computational model of oxygen transport and utilization--based on dynamic mass balances in blood and tissue cells--was applied to quantify the dynamic relationship between model-simulated UO(2m) and measured VO(2p) during moderate (M), heavy (H), and very heavy (V) intensity exercise. In seven human subjects, VO(2p) and muscle oxygen saturation, StO(2m), were measured with indirect calorimetry and near infrared spectroscopy (NIRS), respectively. The dynamic responses of VO(2p) and StO(2m) at each intensity were in agreement with previously published data. The response time of muscle oxygen consumption, tauUO(2m) estimated by direct comparison between model results and measurements of StO(2m) was significantly faster (P < 0.001) than that of pulmonary oxygen uptake, tauVO(2p) (M: 13 +/- 4 vs. 65 +/- 7 s; H: 13 +/- 4 vs. 100 +/- 24 s; V: 15 +/- 5 vs. 82 +/- 31 s). Thus, by taking into account the dynamics of oxygen stores in blood and tissue and determining muscle oxygen consumption from muscle oxygenation measurements, this study demonstrates a significant temporal dissociation between UO(2m) and VO(2p) at exercise onset.

High-energy phosphate metabolism in the calf muscle during moderate isotonic exercise under different degrees of cuff compression: A phosphorus 31 magnetic resonance spectroscopy study

BACKGROUND

The purpose of this study was to investigate phosphocreatine (PCr) and inorganic phosphate levels as well as pH changes in exercising muscle at a workload of 4.5 W under progressive cuff stenoses, whereby the flow reduction due to cuff compression was quantified by flow-sensitive magnetic resonance imaging.

METHODS

By using a whole-body 1.5-T magnetic resonance scanner and an exercise bench, serial phosphorus 31 (31P) magnetic resonance spectroscopy with a time resolution of 30 seconds was performed in 10 healthy men. Percentage changes in PCr, inorganic phosphate (Pi), and pH were statistically evaluated in comparison with baseline. The exercise protocol was characterized by a constant workload level of 4.5 W. Ischemic conditions were achieved by a cuff that was placed at the upper leg. Consecutively, increments of 0, 60, 90, 120, and 150 mm Hg were applied. Each increment lasted for 3 minutes. The following rest period was 10 minutes.

RESULTS

Blood flow increased significantly immediately after the onset of muscle exercise. No significant changes in blood flow were detected as long as the air pressure of the pneumatic cuff was 60 to 90 mm Hg. Significant reductions in blood flow were observed immediately after inflation of the cuff to 120 and 150 mm Hg. PCr passed into a steady state during the first increment with 0 mm Hg and showed no substantial changes during the increment with 60, 90, and 120 mm Hg. PCr hydrolysis seemed progressive during the 150-mm Hg increment. Pi passed into a plateau level at the onset of exercise and increased significantly at the increment of 150 mm Hg. The pH turned into a steady state with no significant changes during the increments up to 120 mm Hg. At 150 mm Hg, pH decreased progressively. PCr levels at the end of the 150-mm Hg increment correlated significantly and moderately with the reduction in blood flow.

CONCLUSIONS

Our study shows that the ischemic condition during constant muscle exercise is clearly characterized by PCr and Pi kinetics, as well as by pH changes. The correlation between the degree of blood flow reduction and PCr levels in the exercising muscle groups, which are supplied by the stenosed arteries, is the first essential of using 31P magnetic resonance spectroscopy in the assessment of the effect of arterial stenoses on muscle function in claudicants.

Saturation of the Lactate Clearance Mechanisms Different from the “Lactate Shuttle” Determines the Anaerobic Threshold: Prediction from the Bioenergetic Model

It is demonstrated, that the bioenergetic model combined with the mathematical constraints determined by the experimental knowledge of the aerobic metabolism and the Lohmann reaction dictates the exact lactate (La)-time relationship during exercise. The theory predicts that La is necessarily produced (above the resting baseline), even during extremely low work loads, where the metabolism was usually considered in the past to be "pure" aerobic. The La rate of production increases linearly as a function of the work load. The anaerobic threshold is strictly determined by the saturation of the La clearance mechanisms of the body different from the "La shuttle" and not by the involvement of a sudden increased La production at the cellular level. These results imply that the half time of the PCr breakdown kinetics at the onset of a constant load exercise can be expressed as a function of the onset speed of the aerobic and of the anaerobic metabolism, even in the case of a very low mechanical power. The PCr half-time does not depend on the workload and represents a physiological invariant. The bioenergetic model was created during a long historical period, when it was believed that the La production was not present at all for very low exercise levels but, actually, the bioenergetic model predicts exactly the opposite result!