Skeletal muscle metabolism in heart failure: a 31P nuclear magnetic resonance spectroscopy study of leg muscle

Assessment of the Physiological Adaptations to Chronic Hypoxemia in Eisenmenger Syndrome

OBJECTIVE

Eisenmenger syndrome is characterized by severe and lifelong hypoxemia and pulmonary hypertension. Despite this, patients do surprisingly well and report a reasonable quality of life. The aim of this study was to investigate whether these patients undergo adaptation of their skeletal and cardiac muscle energy metabolism which would help explain this paradox.

DESIGN AND SETTING

Ten patients with Eisenmenger syndrome and eight age- and sex-matched healthy volunteers underwent symptom-limited treadmill cardiopulmonary exercise testing, transthoracic echocardiography and (31) P magnetic resonance spectroscopy of cardiac and skeletal muscle. Five subjects from each group also underwent near infrared spectroscopy to assess muscle oxygenation.

RESULTS

Despite having a significantly lower peak VO2 , patients with Eisenmenger syndrome have a similar skeletal muscle phosphocreatine (PCr) recovery, a measure of oxidative capacity, when compared to healthy controls (34.9 s ± 2.9 s vs. 35.2 s ± 1.7 s, P = .9). Furthermore their intracellular pH falls to similar levels during exercise suggesting they are not reliant on early anaerobic metabolism (0.3 ± 0.06 vs. 0.28 ± 0.04, P = .7). While their right ventricular systolic function remained good, the Eisenmenger group had a lower cardiac PCr/ATP ratio compared to the control group (1.55 ± 0.10 vs. 2.17 ± 0.15, P < .05).

CONCLUSIONS

These results show that adult patients with Eisenmenger syndrome have undergone beneficial physiological adaptations of both skeletal and cardiac muscle. This may, in part, explain their surprisingly good survival despite a lifetime of severe hypoxemia and adverse cardiopulmonary hemodynamics.

Central haemodynamics and peripheral muscle function during exercise in patients with chronic heart failure

In this narrative review of the current literature, we examine the central and peripheral mechanisms responsible for the exercise intolerance of chronic heart failure and highlight briefly the benefits of exercise training in the treatment of this debilitating disorder. Specifically, we identify the common finding of reduced cardiac output reserve during exercise conditions leading to decreased exercise tolerance. We also reveal that the stroke volume response to exercise varies depending on the individual patient, the presence of mitral regurgitation, and the aetiology of heart failure. Chronic heart failure patients with left ventricular systolic dysfunction appear able to use the Frank-Starling mechanism to compensate (in part) for their decreased contractile reserve. Patients with left ventricular diastolic dysfunction have normal contractile function; however, they are unable to make use of the Frank-Starling mechanism during exercise conditions. We also reveal that pericardial constraint may limit diastolic filling and exercise capacity in patients with chronic heart failure. It appears that interventions that reduce pericardial constraint and mitral regurgitation enhance diastolic filling and increase exercise tolerance. A series of peripheral muscle changes also occur, including changes in muscle mass, cellular structure, energy metabolism, and blood flow. Each of these factors is associated with decreased exercise capacity and the symptoms of chronic heart failure. Exercise training has been shown to improve both central haemodynamics and peripheral muscle function leading to improvements in exercise capacity, functional status, and overall quality of life in patients with chronic heart failure.

Why does chronic heart failure cause breathlessness and fatigue?

Traditional explanations for the symptoms of fatigue and breathlessness experienced by patients with chronic heart failure (CHF) focus on how reduced cardiac output on exercise leads to impaired skeletal muscle blood supply, thus causing fatigue, and on how the requirement for a raised left ventricular filling pressure to maintain cardiac output results in reduced pulmonary diffusion owing to interstitial edema, thus causing breathlessness. However, indices of left ventricular function relate poorly to exercise capacity and symptoms, suggesting that the origin of symptoms may lie elsewhere. There is a specific heart failure myopathy that is present early in the condition which may contribute largely to the sensation of fatigue. Receptors present in skeletal muscle sensitive to work (ergoreceptors) are overactive in patients with CHF, presumably as a consequence of the myopathy, and their activity is related both to the ventilatory response to exercise and breathlessness, and to the sympathetic overactivity of CHF. In the present paper, we review the systemic consequences of left ventricular dysfunction to understand how they relate to the symptoms of heart failure.

Mechanisms of exercise training in patients with heart failure

BACKGROUND

The reduction of exercise capacity because of fatigue and dyspnea in patients with heart failure can be improved with exercise training. We sought to examine the mechanisms of exercise training as an adjunctive treatment strategy for patients with heart failure.

METHODS

We reviewed the published data on the possible mechanisms of effect of exercise training in heart failure.

RESULTS

Symptoms of heart failure may be explained on the basis of abnormal skeletal muscle perfusion and structure and endothelial function. Exercise training has been shown to engender changes in muscle structure and biochemistry and vascular function, although effects on cardiac function have not been detected uniformly and may require longer training periods.

CONCLUSIONS

A suitable, long-term program of exercise training may reverse unfavorable interactions among the heart, vessels, and skeletal muscles. These improvements may be preserved with an ongoing maintenance program.

Heart failure accompanied by sick euthyroid syndrome and exercise training

Sick euthyroid syndrome is defined as the decrease of serum free triiodothyronine with normal free L-thyroxin and thyrotropin. Its appearance in patients with chronic heart failure is an indicator of severity. Exercise training through a wide variety of mechanisms reverses sick euthyroid syndrome (normalization of free triiodothyronine levels) and improves the ability to exercise. There is a connection during exercise among dyspnea, hyperventilation, fatigue, catecholamines, a decrease in the number and function of beta-blocker receptors, and elevation of serum free triiodothyronine. It is not known whether sick euthyroid syndrome contributes to the development of heart failure or is only an attendant syndrome.

Apoptosis in the skeletal muscle of patients with heart failure: investigation of clinical and biochemical changes

OBJECTIVE

To investigate the contribution of apoptosis in the development of the skeletal myopathy in chronic heart failure.

DESIGN

The electrophoretic pattern of myosin heavy chains (MHC), fibre cross sectional area, number of in situ nick end labelling (TUNEL) positive apoptotic myocyte nuclei, and the tissue levels of caspase-3, Bcl-2, and ubiquitin were determined in biopsies taken from the vastus lateralis muscle. The study involved nine patients with severe chronic heart failure caused by ischaemic heart disease and hibernating myocardium and five controls.

RESULTS

In chronic heart failure patients the vastus lateralis showed a significant increase of MHC(2a) and MHC(2b) and a greater degree of fibre atrophy, as demonstrated by the decreased cross sectional area. There was also an increased number of TUNEL positive apoptotic myocyte nuclei. Tissue concentrations of Bcl-2 were decreased, while those of caspase-3 and ubiquitin were increased. Peak oxygen consumption (VO(2)) was negatively correlated with the number of TUNEL positive nuclei and the fibre cross sectional area. There was a correlation between the number of apoptotic nuclei and the fibre cross sectional area, but no correlation between myosin heavy chains and number of apoptotic nuclei.

CONCLUSIONS

Myocyte apoptosis occurs in the skeletal muscle of patients with chronic heart failure, and its magnitude is associated with the severity of exercise capacity limitation and the degree of muscle atrophy. Muscle atrophy contributes to the limitation of exercise capacity, together with the increased synthesis of fast, more fatiguable myosin heavy chains.

Dissociation between muscle metabolism and oxygen kinetics during recovery from exercise in patients with chronic heart failure

OBJECTIVE

To estimate muscle metabolism and oxygen delivery to skeletal muscle in patients with chronic heart failure.

METHODS

13 patients with chronic heart failure and 15 controls performed calf plantar flexion for six minutes at a constant workload of 50% of one repetition maximum. During recovery from exercise, skeletal muscle content of oxygenated haemoglobin (oxy-Hb) and the level of phosphocreatine (PCr) were measured by near-infrared spectroscopy and (31)P-magnetic resonance spectroscopy, respectively.

RESULTS

The mean (SD) time constants of PCr and oxy-Hb during recovery from exercise were significantly greater in patients with chronic heart failure than in normal subjects (tau PCr: 76.3 (30.2) s v 36.5 (5.8) s; tau oxy-Hb: 48.3 (7.3) s v 30.1 (7.7) s; p < 0.01). Both time constants were similar in normal subjects, while the tau PCr was significantly greater than the tau oxy-Hb in patients with chronic heart failure.

CONCLUSIONS

The slower recovery of PCr compared with oxy-Hb in patients with chronic heart failure indicates that haemoglobin resaturation is not a major rate limiting factor of PCr resynthesis. It is suggested that muscle metabolic recovery may depend more on oxygen utilisation than on haemoglobin resaturation or oxygen delivery in patients with chronic heart failure.

Animal models of heart failure

Animal models of heart failure present homogenous groups of animals all with heart failure produced by a well defined lesion at a particular stage of evolution, in contrast to humans, who present with heart failure of uncertain duration from a wide variety of causes and with marked variation in age and pre-morbid health and fitness. Animal models of heart failure provide diseased groups of animals in which experimental procedures, not possible in humans, can be evaluated and in which new treatments can be tested before their safety is established in humans. An ideal model should have a common human counterpart and should closely mimic heart failure in humans. Thus the haemodynamic changes should include increased cardiac filling pressures and low cardiac output. There should be evidence of activation of the sympathetic nervous system and increased secretion of hormones such as renin, angiotensin, aldosterone, vasopressin, atrial natriuretic factor and endothelin. The clinical features of the human syndrome such as cardiomegaly, lung and peripheral oedema and decreased exercise tolerance should be present. Lastly, the model should be inexpensive and technically simple to produce and study. This paper reviews some commonly used models of heart failure in relation to the criteria listed above. There is no perfect animal model of heart failure and in practice one should match the model to the purpose of the study.

Lactate kinetics at rest and during exercise in lambs with aortopulmonary shunts

In a previous study [G. C. M. Beaufort-Krol, J. Takens, M. C. Molenkamp, G. B. Smid, J. J. Meuzelaar, W. G. Zijlstra, and J. R. G. Kuipers. Am. J. Physiol. 275 (Heart Circ. Physiol. 44): H1503-H1512, 1998], a lower systemic O2 supply was found in lambs with aortopulmonary left-to-right shunts. To determine whether the lower systemic O2 supply results in increased anaerobic metabolism, we used [1-13C]lactate to investigate lactate kinetics in eight 7-wk-old lambs with shunts and eight control lambs, at rest and during moderate exercise [treadmill; 50% of peak O2 consumption (VO2)]. The mean left-to-right shunt fraction in the shunt lambs was 55 +/- 3% of pulmonary blood flow. Arterial lactate concentrations and the rate of appearance (Ra) and disappearance (Rd) of lactate were similar in shunt and control lambs, both at rest (lactate: 1, 201 +/- 76 vs. 1,214 +/- 151 micromol/l; Ra = Rd: 12.97 +/- 1.71 vs. 12.55 +/- 1.25 micromol. min-1. kg-1) and during a similar relative workload. We found a positive correlation between Ra and systemic blood flow, O2 supply, and VO2 in both groups of lambs. In conclusion, shunt lambs have similar lactate kinetics as do control lambs, both at rest and during moderate exercise at a similar fraction of their peak VO2, despite a lower systemic O2 supply.

Improved exercise tolerance after losartan and enalapril in heart failure: correlation with changes in skeletal muscle myosin heavy chain composition

BACKGROUND

In congestive heart failure, fatigue-resistant, oxidative, slow type I fibers are decreased in leg skeletal muscle, contributing to exercise capacity (EC) limitation. The mechanisms by which ACE inhibitors and AII antagonists improve EC is still unclear. We tested the hypothesis that improvement in EC is related to changes in skeletal muscle composition toward type I fibers.

METHODS AND RESULTS

Eight patients with congestive heart failure, NYHA classes I through IV, were treated for 6 months with enalapril (E) 20 mg/d, and another 8 with losartan (L) 50 mg/d. EC was assessed with maximal cardiopulmonary exercise testing at baseline and after treatment. Myosin heavy chain (MHC) composition of the gastrocnemius was studied after electrophoretic separation of slow MHC1, fast oxidative MHC2a, and fast glycolytic MHC2b isoforms from needle microbiopsies obtained at baseline and after 6 months. EC improved in both groups. Peak V(O2) increased from 21.0+/-4.7 to 27.6+/-4.3 mL . kg-1 . min -1 (P=0.011) in the L group and from 17.5+/-5.0 to 25.0+/-5.5 mL . kg-1 . min -1 (P=0.014) in the E group. Similarly, ventilatory threshold changed from 15.0+/-4.0 to 19.9+/-4.9 mL (P=0. 049) with L and from 12.0+/-1.9 to 15.4+/-3.5 mL (P=0.039) with E. MCH1 increased from 61.2+/-11.2% to 75.4+/-7.6% with L (P=0.012) and from 60.6+/-13.1% to 80.1+/-10.9% (P=0.006) with E. Similarly, MHC2a decreased from 21.20+/-9.5% to 12.9+/-4.4% (P=0.05) with L and from 19.9+/-7.8% to 11.8+/-7.9% (P=0.06) with E. MHC2b changed from 17. 5+/-6.5% to 11.7+/-5.2% (P=0.07) with L and from 19.5+/-6.4% to 8. 1+/-4.6% (P=0.0015) with E. There was a significant correlation between net changes in MHC1 and absolute changes in peak V(O2) (r2=0.29, P=0.029) and a trend to significance for MHC2a and 2b.

CONCLUSIONS

Six months' treatment with L and with E produces an improvement in EC of similar magnitude. These changes are accompanied by a reshift of MHCs of leg skeletal muscle toward the slow, more fatigue-resistant isoforms. Magnitude of MHC1 changes correlates with the net peak V(O2) gain, which suggests that improved EC may be caused by favorable biochemical changes occurring in the skeletal muscle.

Skeletal muscle myosin heavy chains in heart failure: correlation between magnitude of the isozyme shift, exercise capacity, and gas exchange measurements

BACKGROUND

Patients with congestive heart failure (CHF) have a reduced exercise capacity because of the early appearance of fatigue and dyspnea. Qualitative changes in the skeletal muscle composition and metabolism can be responsible for the origin of symptoms

METHODS

We correlated the myosin heavy chain (MHC) composition of the gastrocnemius in 20 patients with different degrees of CHF to NYHA class, diuretic consumption, echocardiographic parameters, and expiratory gases measured during cardiopulmonary exercise testing. MHC composition was determined electrophoretically in skeletal muscle needle microbiopsies and the percent distribution was calculated by densitometry. Maximal cardiopulmonary exercise testing was performed on a treadmill with a modified Naughton protocol. A capnograph was used.

RESULTS

There was no correlation between ejection fraction, left ventricular end systolic diameter, left ventricular end diastolic diameter, and MHC composition. We found a significant positive correlation between the percentage of MHC 1 (slow aerobic isoform) and NYHA class (r2 = 0.62, p < 0.0001), peak VO2 (r2 = 0.5, p < 0.0004), ventilatory threshold (VT) (r2 = 0.33, p = 0.008) and O2 pulse (peak VO2/HR) (r2 = 0.40, p = 0.003). There was a negative correlation between both MHC2a (fast oxidative) and MHC2b (fast glycolytic) with peak VO2 (r2 = 0.38, p = 0.004 and r2 = 0.37, p = 0.004, respectively), VT (r2 = 0.2, p = 0.046 and r2 = 0.34, p = 0.007, respectively), and O2 pulse (peak VO2/HR) (r2 = 0.39, p = 0.003 and r2 = 0.23, p = 0.03). NYHA class was also correlated positively with MHC2a and MHC2b (r2 = 0.46, p = 0.001 and r2 = 0.41, p < 0.006, respectively) and negatively with the same clinical and functional parameters.

CONCLUSIONS

The correlation between the magnitude of the MHC shift from the slow aerobic to the fast glycolytic and fast oxidative with both functional and objective measurements of exercise capacity (peak VO2, VT, O2 pulse) seem to suggest that changes in skeletal muscle composition may play a determining role in exercise tolerance in patients with CHF.

Exercise limitation in chronic heart failure: central role of the periphery

The symptoms of chronic heart failure (CHF) are predominantly shortness of breath and fatigue during exercise and reduced exercise capacity. Disturbances of central hemodynamic function are no longer considered to be the major determinants of exercise capacity. The two symptoms of fatigue and breathlessness are often considered in isolation. A pulmonary abnormality is usually considered to be the cause of abnormal ventilation, and increased dead space ventilation has come to be accepted as a major cause of the increased ventilation relative to carbon dioxide production seen in CHF. Rather than decreased skeletal muscle perfusion, an intrinsic muscle abnormality is considered to be responsible for fatigue. Another abnormality seen in CHF is persistent sympathetic nervous system activation, which is difficult to explain on the basis of baroreflex activation. There is increasing evidence for the importance of skeletal muscle ergoreceptors or metaboreceptors in CHF. These receptors are sensitive to work performed, and activation results in increased ventilation and sympathetic activation. The ergoreflex appears to be greatly enhanced in CHF. We put forward the "muscle hypothesis" as an explanation for many of the pathophysiologic events in CHF. Impaired skeletal muscle function results in ergoreflex activation. In turn, this causes increased ventilation, thus linking the symptoms of breathlessness and fatigue. Furthermore, ergoreflex stimulation may be responsible for persistent sympathetic activation.

Abnormalities in exercising skeletal muscle in congestive heart failure can be explained in terms of decreased mitochondrial ATP synthesis, reduced metabolic efficiency, and increased glycogenolysis

OBJECTIVE

To distinguish between the effects of reduced oxidative capacity and reduced metabolic efficiency on skeletal muscle bioenergetics during exercise in patients with congestive heart failure.

DESIGN AND PATIENTS

Patients were studied by 31P magnetic resonance spectroscopy during aerobic exercise and recovery, and results compared with controls.

RESULTS

In flexor digitorum superficialis muscle (26 patients) there was a 30% decrease in oxidative capacity compared with control (mean (SE) 36 (2) v 51 (4) mM/min) and also a 40% decrease in "effective muscle mass" (5 (1) v 9 (1) arbitrary units), probably at least partly the result of reduced metabolic efficiency. Both contribute to increased phosphocreatine depletion and intracellular acidosis during exercise. However, an increased concentration of ADP (an important mitochondrial regulator) during exercise permitted near-normal rates of oxidative ATP synthesis. Results were similar in gastrocnemius muscle (20 patients), with a 30% decrease in maximum oxidative capacity (29 (4) v 39 (3) mM/min) and a 65% decrease in effective muscle mass (5 (1) v 13 (2) arbitrary units). Exercise training improved maximum oxidative capacity in both muscles, and in gastrocnemius effective muscle mass also.

CONCLUSIONS

Skeletal muscle exercise abnormalities in patients with congestive heart failure results more from decreased metabolic efficiency than from the abnormalities in mitochondrial oxidation. Both decreased efficiency and defective mitochondrial oxidation result in an increased activation of glycogen phosphorylase, and may be improved by exercise training.

Magnetic resonance spectroscopy in congenital heart disease

OBJECTIVE

To determine the feasibility of studying myocardial and skeletal muscle bioenergetics using 31P magnetic resonance spectroscopy (MRS) in babies and young children with congenital heart disease.

SUBJECTS

16 control subjects aged 5 months to 24 years and 18 patients with CHD, aged 7 months to 23 years, of whom 11 had cyanotic CHD, five had cardiac failure, and two had had a Senning procedure.

DESIGN

31P MRS was carried out using a 1.9 Tesla horizontal 65 cm bore whole body magnet to study the myocardium in 10 patients and skeletal muscle (gastrocnemius) in 14 patients, eight of whom were exercised, together with appropriate controls.

RESULTS

In hypoxaemic patients, in skeletal muscle at rest intracellular pH (pHi) was abnormally high [7.06 (SEM 0.04) v 7.04 (0.05), P < 0.01] and showed a positive correlation with haemoglobin (P < 0.03). On exercise, hypoxaemic patients fatigued more quickly but end-exercise pHi and phosphocreatine recovery were normal, implying that an equivalent but smaller amount of work had been performed. End-exercise ADP concentration was lower. On recovery, the initial rate of phosphocreatine resynthesis was low. Skeletal muscle bioenergetics were within normal limits in those in heart failure. In the myocardium, the phosphocreatine/ATP ratio was similar in controls and hypoxaemic subjects, but low in those in heart failure.

CONCLUSIONS

In heart failure, the myocardial phosphocreatine/ATP ratio was reduced, as in adults, while resting skeletal muscle studies were normal. By contrast, hypoxaemic children had normal myocardial bioenergetics, but showed skeletal muscle alkalinity, and energy reserves were more readily depleted on exercise. On recovery, the initially slow phosphocreatine resynthesis rate reflects a low rate of mitochondrial ATP synthesis, probably due to an inadequate oxygen supply. 31P MRS offers a safe, non-invasive method of studying myocardial and skeletal muscle bioenergetics in children as young as 5 months.

Rat skeletal muscle metabolism in experimental heart failure: effects of physical training

Skeletal muscle metabolic abnormalities exist in chronic heart failure. The influence of physical training on muscle metabolism after myocardial infarction was studied in a rat model. 31P magnetic resonance spectroscopy and enzyme assays were performed in Wistar rats 12 weeks after coronary artery ligation. Infarcted rats were allocated randomly to either 6 weeks of training or non-training. Spectra were collected from the calf muscles during sciatic nerve stimulation at 2 Hz. Fibre typing and enzymatic assays were performed on the muscles of the contralateral non stimulated leg. Post-mortem rats were also divided into severe and moderate heart failure according to the lung weight per body weight. At 200 g twitch tension, phosphocreatine and pH were found to be significantly lower in the non-trained severe heart failure group compared with the other groups. Phosphocreatine recovery half-time was significantly longer in the non-trained group with severe heart failure and correlated with the citrate synthase activity in the muscle. The training did not induce a change in the enzyme activities in the infarcted animals with moderate heart failure but did correct the lower citrate synthase activity in the non-trained severe heart failure animals. This normalization of muscle metabolism was achieved by training without any change in calf muscle mass, making atrophy unlikely to be the sole cause of the metabolic changes in heart failure. Training in rats with severe heart failure can reverse the abnormalities of skeletal muscle metabolism, implicating decreased physical activity in the aetiology of these changes.

Exercise intolerance in chronic heart failure is not associated with impaired recovery of muscle function or submaximal exercise performance

OBJECTIVES

This study investigated whether recovery of skeletal muscle function is impaired in patients with heart failure and whether impaired recovery is associated with abnormal submaximal systemic exercise tolerance during repeated testing.

BACKGROUND

Patients with heart failure experience fatigue during daily activities. Because abnormalities of skeletal muscle play a role in their exercise intolerance, these symptoms may reflect a delay in muscle recovery and a resulting limitation in submaximal exercise tolerance.

METHODS

Two protocols were used. In protocol 1, knee extensor strength and endurance, and their recovery after fatiguing exercise, were evaluated in 11 patients (mean [+/- SEM] age 62 +/- 5 years, New York Heart Association functional class 2.3 +/- 0.2, ejection fraction 24 +/- 5%) and in 10 age-matched sedentary control subjects. Protocol 2 examined the recovery of knee extensor endurance and submaximal exercise tolerance, as quantified on a self-powered treadmill, over 24 h in 18 patients (mean age 65 +/- 3 years, functional class 2.4 +/- 0.2, ejection fraction 23 +/- 3%) and in 10 control subjects.

RESULTS

Peak oxygen consumption was reduced in both heart failure groups (15.4 +/- 1.4 and 15.6 +/- 1.0 ml/kg per min) compared with that in the respective control groups (23.1 +/- 2.9 and 25.6 +/- 1.0 ml/kg per min, both p < 0.05), as was muscle endurance but not muscle strength. In protocol 1, knee extensor endurance recovered more slowly in the patients than in control subjects (to 62 +/- 4% and 87 +/- 7% of the baseline value after 5 min, respectively, p < 0.05). In protocol 2, submaximal exercise tolerance was lower in the patients with heart failure than in control subjects (1,075 +/- 116 vs. 1,390 +/- 110 m), but knee extensor endurance and walking distance recovered fully by 10 and 30 min, respectively.

CONCLUSIONS

Although these findings confirm earlier studies that demonstrated impaired muscle endurance in patients with heart failure, the results provide no evidence that recovery of either muscle function or submaximal exercise tolerance is delayed beyond the initial 5 to 10 min after exercise.

Effects of exercise training in patients with congestive heart failure: a critical review

Congestive heart failure is a potentially debilitating disorder that affects a significant number of patients. The age-adjusted death rate has doubled over the past decade. Patients live an average of 4 to 5 years, and nearly all suffer from fatigue and breathlessness, which limits exercise capacity and produces a poor quality of life. Patients have usually been advised to avoid exercise because of concerns that they would experience a further decline in cardiac function. However, it has been demonstrated that exercise capacity is not related to the degree of left ventricular systolic dysfunction. This has led to the suggestion that peripheral changes in skeletal muscle and blood supply may play a major role in determining the exercise capacity of patients with congestive heart failure. Studies have demonstrated abnormalities of skeletal muscle blood flow, metabolism and structure, all of which are consistent with the impaired performance observed in these patients. Although the effects of exercise training have been examined in only a relatively few number of patients, the results have been promising. Exercise training has been found to improve exercise capacity and reduce symptoms. However, to our knowledge no data exist as to the impact of exercise training on left ventricular function, hospital stay or mortality in this population. Even though the early results are promising, they require confirmation of feasibility, clinical benefit and safety in larger, long-term randomized trials. It should be determined whether training has a long-term beneficial impact on measures more closely related to daily activities and quality of life. Ultimately, it would be important to determine whether training has an impact on mortality and morbidity.

Contribution of specific skeletal muscle metabolic abnormalities to limitation of exercise capacity in patients with chronic heart failure: a phosphorus 31 nuclear magnetic resonance study

Several studies of phosphorus 31 (31P) magnetic resonance spectroscopy (MRS) have demonstrated the presence of skeletal muscle metabolic abnormalities during exercise in patients with chronic heart failure (CHF). We studied the contribution of these abnormalities to the limitation of exercise capacity in CHF. In 25 patients (age 57 +/- 2 years, left ventricular ejection fraction [LVEF] 28% +/- 1.6%, peak oxygen consumption (VO2) 16 +/- 1.2 ml/kg/mm) (mean +/- SEM), we studied the calf muscle at rest and during plantar flexion with 31P MRS. The phosphocreatine (PCr) depletion rate was significantly negatively correlated to peak VO2 (r = -0.62, p = 0.001) but not to LVEF. Muscle pH was correlated with the inorganic phosphorus (Pi)/PCr ratio (r = -0.69, p = 0.0001) and with the PCr/adenosine triphosphate beta (ATP beta) ratio (which negatively relates to adenosine diphosphate [ADP] concentration) (r = 0.65, p = 0.00001). Although muscle ATP (ATP/sum of phosphorus [sigma P] remained stable, in 8 patients ATP/sigma P decreased significantly (-15% +/- 4%, p = 0.0002). In this ATP-depleted group, peak VO2 was significantly lower than that of the nondepleted group and PCr depletion more rapid, whereas LVEF did not differ. Skeletal muscle metabolic abnormalities in CHF contribute markedly to the alteration of exercise capacity. Rapid PCr depletion and muscle acidosis are the most relevant abnormalities. ATP depletion and excessive increase in ADP during exercise may contribute further to exercise limitation specifically in patients with more marked CHF.

Postexercise phosphocreatine resynthesis is slowed in multiple sclerosis

To determine whether skeletal muscle oxidative metabolism is impaired in multiple sclerosis (MS), phosphorus magnetic resonance spectroscopy was used to measure the rate of intramuscular phosphocreatine (PCr) resynthesis following exercise in MS and controls. Thirteen MS patients underwent intermittent isometric tetanic contractions of the dorsiflexor muscles elicited by stimulation of the peroneal nerve. Eight healthy control subjects performed voluntary isometric exercise of the same muscles. During exercise, there were no differences between groups in the fall of either PCr or pH. However, the half-time (T1/2) of PCr recovery following exercise was significantly longer in MS (2.3 +/- 0.3 min) compared to controls (1.2 +/- 0.1 min, P < 0.02). These data provide evidence of slowed PCr resynthesis following exercise in MS, which indicates impaired oxidative capacity in the skeletal muscle of this group. This finding suggests that intramuscular changes consistent with deconditioning may be important in the altered muscle function of persons with MS.

Evaluation of myocardial energy status in vivo by NMR spectroscopy

NMR spectroscopy is a powerful and non-invasive technique with which to study cardiac energy metabolism in vivo. This method makes use of the "spin" properties of certain atomic nuclei. The naturally occurring phosphorus nucleus (P-31) is visible by NMR and phosphorus-31 NMR spectra contain signals from the major components of energy metabolism. In vivo, the phosphocreatine to ATP ratio (PCr/ATP) is used as an index of the energy status and viability of the myocardium. However, it is the response of this metabolic index to differing physiological and pharmacological stresses that has helped to elucidate the mechanisms that regulate cellular respiration and to highlight abnormalities in heart failure. As there are many technical difficulties involved with cardiac NMR, 31-phosphorus studies of skeletal muscle have provided an indirect way of studying abnormalities in myocardial metabolism in vivo. One of the unique features of NMR is that it permits in vivo measurements of fluxes through key enzymes in energy metabolism using magnetization transfer. Determination of the rates of energy transfer through the creatine kinase reaction and energy turnover in vivo will provide new insights into the control of energy metabolism in health and disease. Alternatively, carbon-13 NMR can be used to measure fluxes through the different metabolic pathways of synthesis and catabolism following administration of selectively labelled carbon-13 substrates. In conclusion, the non-invasive and versatile nature of NMR spectroscopy makes it an ideal method to assess and evaluate energy metabolism in vivo.

Physical training improves skeletal muscle metabolism in patients with chronic heart failure

OBJECTIVES

This study investigated the effects of physical training on skeletal muscle metabolism in patients with chronic heart failure.

BACKGROUND

Skeletal muscle metabolic abnormalities in patients with chronic heart failure have been associated with exercise intolerance. Muscle deconditioning is a possible mechanism for the intrinsic skeletal muscle metabolic changes seen in chronic heart failure.

METHODS

We used phosphorus-31 nuclear magnetic resonance spectroscopy to study muscle metabolism during exercise in 12 patients with stable ischemic chronic heart failure undergoing 8 weeks of home-based bicycle exercise training in a randomized crossover controlled trial. Changes in muscle pH and concentrations of phosphocreatine and adenosine diphosphate (ADP) were measured in phosphorus-31 spectra of calf muscle obtained at rest, throughout incremental work load plantar flexion until exhaustion and during recovery from exercise. Results were compared with those in 15 age-matched control subjects who performed a single study only.

RESULTS

Before training, phosphocreatine depletion, muscle acidification and the increase in ADP during the 1st 4 min of plantar flexion exercise were all increased (p < 0.04) compared with values in control subjects. Training produced an increase (p < 0.002) in incremental plantar flexion exercise tolerance. After training, phosphocreatine depletion and the increase in ADP during exercise were reduced significantly (p < 0.003) at all matched submaximal work loads and at peak exercise, although there was no significant change in the response of muscle pH to exercise. After training, changes in ADP were not significantly different from those in control subjects, although phosphocreatine depletion was still greater (p < 0.05) in trained patients than in control subjects. The phosphocreatine recovery half-time was significantly (p < 0.05) shorter after training, although there was no significant change in the half-time of adenosine diphosphate recovery. In untrained subjects, the initial rate of phosphocreatine resynthesis after exercise (a measure of the rate of oxidative adenosine triphosphate [ATP] synthesis) and the inferred maximal rate of mitochondrial ATP synthesis were reduced compared with rates in control subjects (p < 0.003) and both were significantly increased (p < 0.05) by training, so that they were not significantly different from values in control subjects.

CONCLUSIONS

The reduction in phosphocreatine depletion and in the increase in ADP during exercise, and the enhanced rate of phosphocreatine resynthesis in recovery (which is independent of muscle mass) indicate that a substantial correction of the impaired oxidative capacity of skeletal muscle in chronic heart failure can be achieved by exercise training.

Human skeletal muscle digitalis glycoside receptors (Na,K-ATPase)--importance during digitalization

The aims of the present study were to evaluate in humans the putative importance of skeletal muscle digitalis glycoside receptors (Na,K-ATPase) in the volume of distribution of digoxin and to assess whether therapeutic digoxin exposure might cause digitalis receptor upregulation in skeletal muscle. Samples of the vastus lateralis were obtained postmortem from 11 long-term (9 months to 9 years) digitalized (125-187.5 micrograms daily) and eight undigitalized subjects. In intact samples from digitalized patients, vanadate-facilitated 3H-ouabain binding increased 15% (p < 0.02) from 150 +/- 18 to 173 +/- 13 pmol/g wet wt. (mean +/- SEM) after clearing receptors of bound digoxin by washing samples in excess specific digoxin antibody fragments. 3H-ouabain binding in the untreated group was 257 +/- 28 and 274 +/- 26 pmol/g wet wt. (7%, p > 0.30) before and after washing in specific digoxin antibody fragments, respectively. Thus, the present study indicates a approximately 13% occupancy of skeletal muscle digitalis glycoside receptors with digoxin during digitalization. In light of the large skeletal muscle contribution to body mass, this indicates that the skeletal muscle Na,K-ATPase pool constitutes a major volume of distribution for digoxin during digitalization. The results gave no indication of skeletal muscle digitalis glycoside receptor upregulation in response to digoxin treatment. On the contrary, there was evidence of significantly lower (37%, p < 0.005) digitalis glycoside receptor concentration in the vastus lateralis of the digitalized patients, which may be of importance for skeletal muscle incapacity in heart failure.

Skeletal muscle function, morphology, and metabolism in patients with congestive heart failure

Work by a number of laboratories over the last decade has revealed that skeletal muscle in patients with congestive heart failure (CHF) exhibits altered metabolism, biochemistry, and histology. These alterations appear to be at least in part independent of systemic hemodynamic abnormalities and they lead to abnormal muscle function. Furthermore, muscle dysfunction may play a role in limiting exercise capacity. While blood flow to exercising muscle may be impaired in CHF, both the functional and metabolic changes are, to some degree, independent of the changes in blood flow. Deconditioning and muscle atrophy may be responsible for some of these functional metabolic alterations in muscle, but other factors appear to be operating as well. Finally, the finding that many of the changes in skeletal muscle associated with CHF can be reversed by exercise training suggests that activity should be encouraged in patients with CHF.