31P MRS measurement of mitochondrial function in skeletal muscle: reliability, force-level sensitivity and relation to whole body maximal oxygen uptake

Cardiovascular factors explain genetic background differences in VO2max

The purpose of this study was to further explore factors that may be related to ethnic differences in the maximum rate at which an individual can consume oxygen (VO2max) between 20 African American (AA) and 30 European American (EA) sedentary women who were matched for body weight (kg) and fat-free mass (FFM). VO2max (l/min) was determined during a graded treadmill exercise test. Submaximal steady-state heart rate and submaximal VO2 were determined at a treadmill speed of 1.3 m/sec and a 2.5% grade. Hemoglobin (Hb) was determined by the cyanide method, muscle oxidative capacity by 31P magnetic resonance spectroscopy (ADP time constant), and FFM (kg) by dual-energy x-ray absorptiometry. Genetic classification was self-reported, and in a subset of the sample (N = 32), the determinants of ethnicity were measured by African genetic admixture. AA women had significantly reduced VO2max, Hb levels, and muscle oxidative capacity (longer ADP time constants, P < or = 0.05) than EA women. Submaximal oxygen pulse (O2Psubmax), ADP time constant, Hb, and ethnic background were all significantly related to VO2max (ml/kg/min and ml/kg FFM/min, all P < or = 0.01). By multiple regression modeling, Hb, O2Psubmax, muscle oxidative capacity, and ethnicity were found to explain 61% and 57% of the variance of VO2max in ml/kg/min and ml/kg FFM/min, respectively. Muscle oxidative capacity and O2Psubmax were both significantly and independently related to VO2max in all three models (P < or = 0.05), whereas Hb and ethnicity were not. These results suggest that mitochondrial muscle oxidative capacity and oxygen delivery capabilities, as determined by O2Psubmax, account for most if not all of the ethnic differences in VO2max.

Dynamic MRS and MRI of skeletal muscle function and biomechanics

MR is a powerful technique for studying the biomechanical and functional properties of skeletal muscle in vivo in health and disease. This review focuses on 31P, 1H and 13C MR spectroscopy for assessment of the dynamics of muscle metabolism and on dynamic 1H MRI methods for non-invasive measurement of the biomechanical and functional properties of skeletal muscle. The information thus obtained ranges from the microscopic level of the metabolism of the myocyte to the macroscopic level of the contractile function of muscle complexes. The MR technology presented plays a vital role in achieving a better understanding of many basic aspects of muscle function, including the regulation of mitochondrial activity and the intricate interplay between muscle fiber organization and contractile function. In addition, these tools are increasingly being employed to establish novel diagnostic procedures as well as to monitor the effects of therapeutic and lifestyle interventions for muscle disorders that have an increasing impact in modern society.

Muscle metabolic function and free-living physical activity

We have previously shown that muscle metabolic function measured during exercise is related to exercise performance and subsequent 1-yr weight gain. Because it is well established that physical activity is important in weight maintenance, we examined muscle function relationships with free-living energy expenditure and physical activity. Subjects were 71 premenopausal black and white women. Muscle metabolism was evaluated by 31P magnetic resonance spectroscopy during 90-s isometric plantar flexion contractions (45% maximum). Free-living energy expenditure (TEE) was measured using doubly labeled water, activity-related energy expenditure (AEE) was calculated as 0.9 × TEE − sleeping energy expenditure from room calorimetry, and free-living physical activity (ARTE) was calculated by dividing AEE by energy cost of standard physical activities. At the end of exercise, anaerobic glycolytic rate (ANGLY) and muscle concentration of phosphomonoesters (PME) were negatively related to TEE, AEE, and ARTE ( P < 0.05). Multiple regression analysis showed that both PME (partial r = −0.29, <0.02) and ANGLY (partial r = −0.24, P < 0.04) were independently related to ARTE. PME, primarily glucose-6-phosphate and fructose-6-phosphate, was significantly related to ratings of perceived exertion ( r = 0.21, P ≤ 0.05) during a maximal treadmill test. PME was not related to ARTE after inclusion of RPE in the multiple regression model, suggesting that PME may be obtaining its relationship with ARTE through an increased perception of effort during physical activity. In conclusion, physically inactive individuals tend to be more dependent on anaerobic glycolysis during exercise while relying on a glycolytic pathway that may not be functioning optimally.

Relationship between insulin sensitivity and in vivo mitochondrial function in skeletal muscle

Recent data have shown that individuals with low insulin sensitivity (S(I)) also have reduced whole body maximal oxygen uptake. The objectives of this study were to determine 1) whether muscle mitochondrial function was independently related to S(I) after being adjusted for known determinants of S(I) and 2) whether lower S(I) among African-American (AA) vs. Caucasian-American (CA) women was due to lower muscle mitochondrial function among AA women. Subjects were 37 CA and 22 AA premenopausal women (age: 33.6 +/- 6.3 yr). Mitochondrial function [time constant of ADP (ADP(tc))] was assessed during a 90-s unilateral isometric contraction using (31)P magnetic resonance spectroscopy, S(I) with an intravenous glucose tolerance test, body composition by dual-energy X-ray absorptiometry, and visceral adipose tissue (VAT) with computed tomography. ANOVA was used to compare AA and CA groups, and multiple linear regression modeling was used to identify independent predictors of S(I). Between-race comparisons indicated that muscle oxidative capacity was lower among AAs vs. CAs (ADP(tc): 25.6 +/- 9.8 vs. 21.4 +/- 9.9 s). Multiple linear regression models for the dependent variable S(I) contained 1) VAT and race and 2) VAT, race, and ADP(tc). Significant independent effects for all predictor variables were observed in both the first (r(2) = 0.345) and second (r(2) = 0.410) models. The partial correlation for race was lower in the second model (-0.404 vs. -0.300), suggesting that muscle mitochondrial function contributed to the racial difference in S(I). Lower muscle mitochondrial function among AAs may in part explain lower S(I) among them.

Reliability of 31P-magnetic resonance spectroscopy during an exhaustive incremental exercise test in children

This study examined the reliability of (31)P-magnetic resonance spectroscopy (MRS) to measure parameters of muscle metabolic function in children. On separate days, 14 children (7 boys and 7 girls) completed three knee-extensor incremental tests to exhaustion inside a whole-body scanner (1.5 T, Phillips). The dynamic changes in the ratio of inorganic phosphate to phosphocreatine (Pi/PCr) and intracellular muscle pH were resolved every 30 s. Using plots of Pi/PCr and pH against power output (W), intracellular thresholds (ITs) for each variable were determined using both subjective and objective procedures. The IT(Pi/PCr) and IT(pH) were observed subjectively in 93 and 81% of their respective plots, whereas the objective method identified the IT(Pi/PCr) in 88% of the plots. The IT(pH) was undetectable using the objective method. End exercise (END) END(Pi/PCr), END(pH), IT(Pi/PCr) and IT(pH) were examined using typical error statistics expressed as a % coefficient of variation (CV) across all three exercise tests. The CVs for the power output at the subjectively determined IT(Pi/PCr) and IT(pH) were 10.6 and 10.3%, respectively. Objective identification of the IT(Pi/PCr) had a CV of 16.3%. CVs for END(pH) and END(Pi/PCr) were 0.9 and 50.0%, respectively. MRS provides a valuable window into metabolic changes during exercise in children. During knee-extensor exercise to exhaustion, END(pH) and the subjectively determined IT(Pi/PCr) and IT(pH) demonstrate good reliability and thus stable measures for the future study of developmental metabolism. However, the objectively determined IT(Pi/PCr) and END(Pi/PCr) displayed poor reliability.

Age and Activity Status Affect Muscle Reoxygenation Time after Maximal Cycling Exercise

PURPOSE

The purpose of this study was to determine the interaction of age and habitual physical activity on recovery time of muscle oxygenation following maximal cycling exercise (CycEXmax).

METHODS

Twelve sedentary middle-aged (50+/-6), 13 sedentary elderly (66+/-3), 13 active middle-aged (53+/-5), and 20 active elderly (67+/-5) women participated in this study. We evaluated the peak pulmonary oxygen uptake (VO2peak) during CycEXmax and the half-recovery time of muscle oxygenation (T1/2reoxy time) using near-infrared spectroscopy at the vastus lateralis (VL) during the recovery phase after CycEXmax.

RESULTS

T1/2reoxy time was significantly greater in the elderly subjects than in the middle-aged subjects in both sedentary (P<0.05) and active groups (P<0.01). T1/2reoxy time of the active group was lower (P<0.01) than that of the sedentary group regardless of age. Age was significantly correlated to T1/2reoxy time in both sedentary and active groups (in both sedentary and active groups: P<0.01). The slope of T1/2reoxy time against age in the sedentary group was significantly greater (VL: P<0.05) than that of the active group. VO2peak showed significant inverse correlation with T1/2reoxy time at the VL in both sedentary and active groups. The slope of VO2peak against T1/2reoxy time showed no significant differences between middle-aged and elderly subjects.

CONCLUSION

The results of this study suggest that T1/2reoxy time was prolonged with aging, regardless of habitual physical activity levels. However, habitual physical activity may prevent the age-related prolongation in T1/2reoxy time after CycEXmax. VO2peak appears to be one of the major factors determining T1/2reoxy time, not age.

Computing the B1 field of the toroidal MRI coil

We present an analytic solution for the B1 field produced in a gapped toroidal cavity resonator designed as a probe for high field MRI. This resonator supports standing TEM waves, so its electric and magnetic fields are identical to those produced by a stationary planar current source with the same (constant) cross-section multiplied by a complex exponential propagation factor. An explicit expression for the field may therefore be found by solving Laplace's equation for the static potential, which is accomplished with a two-dimensional logarithmic conformal transformation algorithm. The equipotential curves are also the contours of the field strength B, and the B (vector) field at any point is directed along the contour passing through that point. With this information, we construct the solution by computing the angle made by the equipotential curve with the horizontal axis at each point, using this angle to analyze the B field into its x and y components, and adding the contributions from the current sources to obtain the magnitude and direction of B at each point in the region of interest. Some proposed extensions of this algorithm are also discussed.

Exercise over-stress and maximal muscle oxidative metabolism: a 31P magnetic resonance spectroscopy case report

OBJECTIVE

31P magnetic resonance spectroscopy (MRS) was used to document long lasting losses in muscle oxidative capacity after bouts of intense endurance exercise.

METHODS

The subject was a 34 year old highly fit female cyclist (VO2MAX = 53.3 ml/kg/min). Over a five month period, she participated in three separate intense bouts of acute unaccustomed exercise. 31P MRS measurements were performed seven weeks after the first bout and every two weeks for 14 more weeks. In all cases, 31P MRS measurements followed three days after each bout.

RESULTS

The subject showed a decreased ability to generate ATP from oxidative phosphorylation and an increased reliance on anaerobic ATP production during the 70% and 100% maximal voluntary contractions after the exercise bouts. Increased rates of fatigue and increased indicators of exercise difficulty also accompanied these reductions in muscle oxidative capacity. Increased oxidative and anaerobic ATP production were needed to maintain the work level during a submaximal 45% maximal voluntary contraction exercise.

CONCLUSIONS

Acute increases in intensity accompanied by a change in exercise mode can influence the ability of muscle to generate ATP. The muscles were less economical and required more ATP to generate force during the submaximal exercises. During the maximal exercises, the muscle's mitochondria showed a reduced oxidative capacity. However, these reductions in oxidative capacity at the muscle level were not associated with changes in whole body maximal oxygen uptake. Finally, these reductions in muscular oxidative capacity were accompanied by increased rates of anaerobic ATP production, fatigue, and indicators of exercise difficulty.

Inverse relationship between exercise economy and oxidative capacity in muscle

An inverse relationship has been shown between running and cycling exercise economy and maximum oxygen uptake (VO2max). The purposes were: 1) determine the relationship between walking economy and VO2max; and 2) determine the relationship between muscle metabolic economy and muscle oxidative capacity and fiber type. Subjects were 77 premenopausal normal weight women. Walking economy (1/VO2max) was measured at 3 mph and VO2max during graded treadmill test. Muscle oxidative phosphorylation rate (OxPhos), and muscle metabolic economy (force/ATP) were measured in calf muscle using 31P MRS during isometric plantar flexion at 70 and 100% of maximum force, (HI) and (MI) respectively. Muscle fiber type and citrate synthase activity were determined in the lateral gastrocnemius. Significant inverse relationships (r from -0.28 to -0.74) were observed between oxidative metabolism measures and exercise economy (walking and muscle). Type IIa fiber distribution was inversely related to all measures of exercise economy (r from -0.51 to -0.64) and citrate synthase activity was inversely related to muscle metabolic economy at MI (r = -0.56). In addition, Type IIa fiber distribution and citrate synthase activity were positively related to VO2max and muscle OxPhos at HI and MI (r from 0.49 to 0.70). Type I fiber distribution was not related to any measure of exercise economy or oxidative capacity. Our results support the concept that exercise economy and oxidative capacity are inversely related. We have demonstrated this inverse relationship in women both by indirect calorimetry during walking and in muscle tissue by 31P MRS.

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.

Muscle metabolic function, exercise performance, and weight gain

PURPOSE

The study purpose was to determine the relationship: 1) of muscle metabolism to exercise performance and 2) of exercise performance to rate of weight gain.

METHODS

Eighty-three black and white premenopausal women were evaluated for maximum oxygen uptake (VO2max ), isometric quadriceps and triceps surae strength, and 31P magnetic resonance spectroscopy of calf muscle metabolic capacity. Rate of weight gain was determined 1 yr later. Multiple regression was used to model dependent variables.

RESULTS

Muscle aerobic capacity and strength of the quadriceps muscle independently contributed to endurance time on the treadmill (ET) in one model (overall R = 0.47, P < 0.01), and VO2max and strength of the quadriceps muscle independently contributed to ET in another model (R = 0.85, P < 0.001). In models of muscle strength, maximum creatine kinase activity and maximum anaerobic glycolytic rate independently contributed to triceps surae strength, after adjusting for triceps surae cross-section area (R = 0.63, P < 0.001). In another model, maximum creatine kinase activity was related to quadriceps strength independent of leg lean tissue (R = 0.31, P < 0.05). Rate of weight gain was related to muscle metabolic economy (r = -0.25, = 0.04), quadriceps strength (r = -0.34, P < 0.01), VO2max (r = -0.22, = 0.04), and ET (r = -0.21, = 0.04). Rate of weight gain was modeled by muscle metabolic economy, VO2max, and quadriceps strength (R = 0.48, P < 0.01).

CONCLUSIONS

Implications of findings are 1) greater strength and aerobic fitness-at the muscle and whole-body levels-improve endurance; 2) greater muscle anaerobic metabolism is associated with greater muscle strength, independent of muscle size; and 3) greater exercise endurance reduces weight gain.

Age is independently related to muscle metabolic capacity in premenopausal women

The purpose of this study was to determine whether muscle metabolic capacity was inversely related to age after adjusting for physical activity in sedentary premenopausal women. Eighty-three women (ages 23-47 yr) had their free-living, activity-related energy expenditure evaluated with doubly labeled water procedures, and room calorimeter determined sleeping energy expenditure. Maximum O(2) uptake and strength were evaluated in all subjects, whereas 31P-magnetic resonance spectroscopy determined metabolic economy during maximal exercise, and muscle biopsy maximal enzyme activity was evaluated in subsets of the sample (48 and 18 subjects, respectively). Age was significantly related to whole body treadmill endurance time (r = -0.32), plantar flexion strength (r = -0.29), maximum O(2) uptake (r = -0.27), (31)P-magnetic resonance spectroscopy ADP recovery rate (r = -0.44), and anaerobic glycolytic capacity (r = -0.37), and muscle biopsy citrate synthase activity (r = -0.48), glyceraldehyde-3-phosphate dehydrogenase (r = -0.54), phosphofructokinase (r = -0.62), and phosphorylase (r = -0.58) activity even after adjusting for activity-related energy expenditure. These data suggest that, in sedentary premenopausal women, both oxidative and glycolytic muscle capacity decrease with age even when physical activity is taken into account.

Relation between in vivo and in vitro measurements of skeletal muscle oxidative metabolism

The relationships between in vivo (31)P magnetic resonance spectroscopy (MRS) and in vitro markers of oxidative capacity (mitochondrial function) were determined in 27 women with varying levels of physical fitness. Following 90-s isometric plantar flexion exercises, calf muscle mitochondrial function was determined from the phosphocreatine (PCr) recovery time constant, the adenosine diphosphate (ADP) recovery time constant, the rate of change of PCr during the initial 14 s of recovery, and the apparent maximum rate of oxidative adenosine triphosphate (ATP) synthesis (Q(max)). Muscle fiber type distribution (I, IIa, IIx), citrate synthase (CS) activity, and cytochrome c oxidase (COX) activity were determined from a biopsy sample of lateral gastrocnemius. MRS markers of mitochondrial function correlated moderately (P < 0.05) with the percentage of type IIa oxidative fibers (r = 0.41 to 0.66) and CS activity (r = 0.48 to 0.64), but only weakly with COX activity (r = 0.03 to 0.26, P > 0.05). These results support the use of MRS to determine mitochondrial function in vivo.

Hemoglobin, muscle oxidative capacity, and VO2max in African-American and Caucasian women

PURPOSE

The purpose of this paper was to determine whether differences in hemoglobin (Hb) and muscle aerobic capacity exist between African-American (AA) and Caucasian (CA) premenopausal women and to determine whether Hb and aerobic capacity of the muscle are associated with the racial differences in maximum oxygen uptake (VO2max).

METHODS

43 AA and 46 CA sedentary premenopausal women were subjects. Percent body fat was determined by four-compartment model, leg lean tissue by dual energy x-ray absorptiometry, VO2max during a graded exercise test, aerobic capacity of the calf muscle by 31P magnetic resonance spectroscopy, and serum Hb by the cyanide method.

RESULTS

AA women had reduced VO2max (AA 29.3 +/- 3.0 vs CA 33.6 +/- 5.6 mL.kg(-1) bdw(-1).min, P < 0.01), reduced muscle aerobic capacity (AA 24.3 +/- 5.8 vs CA 21.3 +/- 4.8 s, P = 0.01, where lower values indicate higher aerobic capacity), and reduced Hb (AA 11.8 +/- 1.3 vs CA 12.9 +/- 0.8 g.dL(-1), P < 0.01). The racial difference in VO2max persisted whether the values were unadjusted or adjusted for fat-free mass or leg lean tissue. Multiple regression analysis revealed that both Hb and muscle aerobic capacity were related to VO2max after adjusting for each other, race, and either fat-free mass or leg lean tissue. Being AA was associated with reduced VO2max in mL O2.kg leg lean tissue(-1).min(-1) (zero-order simple Pearson-product correlation -0.60, P < 0.01). When multiple regression was used, the correlation between race and VO2max decreased but persisted (-0.40, <0.01) after adjusting for Hb and muscle aerobic capacity.

CONCLUSIONS

These data suggest that differences in Hb and aerobic capacity of muscle are related to reduced VO2max in AA women. However, Hb and aerobic capacity of the muscle can only partially explain the racial differences in VO2max.

Skeletal muscle metabolism in overweight and post-overweight women: an isometric exercise study using (31)P magnetic resonance spectroscopy

OBJECTIVE

To investigate whether skeletal muscle anaerobic metabolism, oxidative metabolism or metabolic economy during controlled sub-maximal and near-maximal exercises is altered in overweight women after diet-induced weight reduction, and whether these parameters are different between normal-weight, obesity-prone and normal-weight obesity-resistant women with similar physical fitness levels.

DESIGN

A prospective weight loss study of overweight women and their comparison with never overweight controls.

SUBJECTS

Thirty overweight, nondiabetic, premenopausal women and 28 never overweight controls were included in this analysis. All were participating in a longitudinal investigation of the role of energy metabolism in the etiology of obesity. The overweight women were recruited specifically to have a positive family history of obesity and have a body mass index (BMI) between 27 and 30 kg/m(2) and were studied in the overweight state and after reduction to a normal weight. The never-overweight controls were recruited specifically to have no personal and family history of obesity and were group matched with the weight-reduced post-overweight subjects in terms of premenopausal status, age, BMI, race and sedentary lifestyle.

MEASUREMENTS

All testing was performed following one month of weight maintenance and during the follicular phase of the menstrual cycle. Hydrostatic weighing was performed to measure body composition and a whole-body maximal oxygen uptake (VO(2max)) test was done to measure aerobic fitness. (31)P MRS was used to determine ATP production from oxidative phosphorylation (OxPhos), 'anaerobic' glycolysis (AnGly), and creatine kinase (CK), as well as muscle metabolic economy. The time constant of ADP (TC(ADP)), V(PCr) (ie the initial rate of PCr resynthesis following exercise), and Q(max) (ie the apparent maximal oxidative ATP production rate) were also calculated as additional markers of mitochondrial function.

RESULTS

Diet-induced weight loss did not have any effects on the anaerobic metabolism markers (AnGly and CK). The aerobic metabolism markers calculated from the initial recovery data (OxPhos and V(PCr)) were unaffected by diet-induced weight loss. However, diet-induced weight loss resulted in improvements in the TC(ADP) and Q(max) in the post-overweight state as compared to their overweight state. There were no differences in any of the anaerobic (AnGly and CK) or oxidative metabolism markers (OxPhos, V(PCr), Q(max) and TC(ADP)) between the post-overweight and control groups.

CONCLUSIONS

Once the overweight women were reduced to a normal-weight state, their skeletal muscle energy metabolism and economy was similar to the never overweight control women. In overweight women, oxidative metabolism or mitochondrial function may be limited by blood flow to the muscle following the cessation of exercise.

Muscle metabolic economy is inversely related to exercise intensity and type II myofiber distribution

It is not known what causes the well-established inverse relationship between whole-body exercise economy and exercise intensity. The purpose of this study was to: (1) evaluate muscle exercise economy at 45%, 70%, and maximum isometric strength using 31P magnetic resonance spectroscopy (31P-MRS); and (2) determine the relationship between percent type II muscle fiber cross-section, whole-body exercise economy, and muscle exercise economy. Subjects included 32 premenopausal women. Muscle exercise economy was significantly different across the three exercise intensities (28.1 +/- 10.4, 24.8 +/- 8.2, and 20.2 +/- 7.5 N/cm2. mmol/L adenosine triphosphate [ATP] for the 45%, 70%, and maximum intensities, respectively). Percent type II muscle area was significantly related to whole-body metabolic economy during activities of daily living (r = -0.68) and 31P-MRS muscle metabolic economy during isometric plantar flexion (r = -0.53). These data suggest that skeletal muscle becomes less economical as force production increases, and that these decreases in metabolic economy may be related to increased dependence on inefficient type II muscle.

Current awareness

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