Excess purine degradation in exercising muscles of patients with glycogen storage disease types V and VII

Preclinical Research in McArdle Disease: A Review of Research Models and Therapeutic Strategies

McArdle disease is an autosomal recessive disorder of muscle glycogen metabolism caused by pathogenic mutations in the PYGM gene, which encodes the skeletal muscle-specific isoform of glycogen phosphorylase. Clinical symptoms are mainly characterized by transient acute “crises” of early fatigue, myalgia and contractures, which can be accompanied by rhabdomyolysis. Owing to the difficulty of performing mechanistic studies in patients that often rely on invasive techniques, preclinical models have been used for decades, thereby contributing to gain insight into the pathophysiology and pathobiology of human diseases. In the present work, we describe the existing in vitro and in vivo preclinical models for McArdle disease and review the insights these models have provided. In addition, despite presenting some differences with the typical patient’s phenotype, these models allow for a deep study of the different features of the disease while representing a necessary preclinical step to assess the efficacy and safety of possible treatments before they are tested in patients.

A sex-related mediating effect of uric acid in the association between body composition and blood pressure in children and adolescents

The increase in blood pressure (BP) during somatic growth might have direct determinants but also mediating factors. We investigated whether uric acid (UA) and other metabolic factors would mediate the association between body composition components and BP. A cross-sectional study was conducted in 928 children and adolescents (aged 6-18 years), in which body composition and blood biochemistry were evaluated. Structural equation modeling was performed to test the direct and indirect pathways between systolic blood pressure (SBP) and body composition parameters. Muscle mass (MM) showed a strong direct effect on BP, regardless of sex. In girls, a mediating pathway through UA was not significant, but the association between fat mass (FM) and MM with SBP was mediated by the cluster of metabolic factors. In boys, both MM and FM were associated with SBP through a mediating pathway via UA, but not via the cluster of metabolic factors. The association between body composition and BP in children and adolescents has a complex design and also has a sex-specific mediating component. The increase in the UA levels may affect BP levels early in boys. Also, metabolic changes elicited by FM contribute to the increase in BP at an early age in girls. Novelty: MM showed a strong direct effect on BP, regardless of sex. In girls, the association between FM and MM with SBP was mediated by the cluster of metabolic factors. In boys, both MM and FM were associated with SBP through a mediating pathway via UA.

McArdle Disease: Clinical, Biochemical, Histological and Molecular Genetic Analysis of 60 Patients

A clinical, biochemical, histological and molecular genetic analysis of 60 McArdle patients (33 males and 27 females; mean age at diagnosis: 37 years) was performed. The objective of this study was to identify a possible genotype–phenotype correlation in McArdle disease. All patients complained of exercise-induced myalgia and fatigue; permanent weakness was present in 47% of the patients. Five percent of patients conveyed of masticatory muscle weakness. Age of onset was <15 years in 92% patients. Serum creatine kinase was elevated 5 to13-fold. Forearm ischemic test showed decreased lactate production but excessively increased ammonia upon exercise (n = 16). Muscle biopsies revealed highly reduced or missing myophosphorylase activity (n = 20) (mean: 0.17 ± 0.35 U/g tissue; normal: 12–61) and histologically, sub-sarcolemmal glycogen accumulation (n = 9). Molecular genetic analysis revealed the common p.Arg50Ter mutation in 68% of the patients. Other rather frequent mutations were p.Arg270Ter (allele frequency: 5%) followed by c.2262delA and p.Met1Val (allele frequencies: 3%). Twenty-four other rare mutations were also identified. No genotype–phenotype correlation was observed. The analysis highlights that testing of the p.Arg50Ter mutation could be performed first in molecular genetic testing of patients with exercise intolerance possibly due to McArdle disease. However, there is enormous mutation heterogeneity in McArdle disease thus sequencing of the myophosphorylase gene is needed in patients highly suspicious of McArdle disease.

Influence of muscle mass on the serum uric acid levels in children and adolescents

BACKGROUND AND AIM

Uric acid (UA) is an end-product of purine catabolism and its increase in blood is a risk factor for several diseases. UA levels in men are usually higher than in women. This difference is partially due to sex hormones. We sought to investigate the onset of sexual difference in UA levels during pubertal development and the determinants of UA levels in children and adolescents.

METHODS AND RESULTS

The muscle mass and fat mass were measured by multi-frequency bioelectrical impedance in a cross-sectional study involving 823 children and adolescents (both sexes; 6-18 years). Serum UA was determined using a commercially available kit. UA levels started to become higher in boys (5.0 ± 1.0 mg/dL) than in girls (4.1 ± 0.9 mg/dL) around 13 years. Boys in the highest quartile of muscle mass presented higher UA levels (5.2 ± 0.7 mg/dL) when compared with the third (4.2 ± 0.7 mg/dL), second (3.7 ± 0.9 mg/dL) and first (3.4 ± 0.9 mg/dL). Similarly, girls in the highest quartile of muscle mass presented higher UA levels (4.2 ± 0.7 mg/dL) when compared with the second (3.8 ± 0.9 mg/dL) and first (3.3 ± 0.9 mg/dL). Muscle mass explained 43.0% and 7.7% of the variability of UA in boys and girls, respectively.

CONCLUSION

Sexual differences in serum UA levels begin at puberty and partially result from a direct influence of muscle mass.

Beneficial Effects of Ketogenic Diet on Phosphofructokinase Deficiency (Glycogen Storage Disease Type VII)

Background: A deficiency of muscle phosphofructokinase (PFKM) causes a rare metabolic muscle disease, the Tarui disease (Glycogen storage disease type VII, GSD VII) characterized by exercise intolerance with myalgia due to an inability to use glucose as an energy resource. No medical treatment for GSD VII currently exists. The aim of this study was to determine whether a dietary intervention with excessive fat intake would benefit GSD VII.

Patient and Methods: A ketogenic diet (KD) intervention implemented as a modified Atkins diet was established for one patient with PFKM deficiency, with a low late lactate response and very high ammonia levels associated with exercise. We recorded the KD intervention for a total of 5 years with clinical and physiotherapeutic evaluations and regular laboratory parameters. Cardiopulmonary exercise testing, including breath gas analysis and venous lactate and ammonia measurements, was performed before KD and at 3, 8 months and 5 years after initiation of KD.

Results: During the 5 years on KD, the patient's muscle symptoms had alleviated and exercise tolerance had improved. In exercise testing, venous ammonia had normalized, the lactate profile remained similar, but oxygen uptake and mechanical efficiency had increased and parameters showing ventilation had improved.

Conclusions: This study is the first to show a long-term effect of KD in GSD VII with an alleviation of muscle symptoms, beneficial effects on breathing, and improvement in exercise performance and oxygen uptake. Based on these findings, KD can be recommended under medical and nutritional supervision for selected patients with GSD VII, although further research of this rare disease is warranted.

Higher oxidative stress in skeletal muscle of McArdle disease patients

McArdle disease (MCD) is an autosomal recessive condition resulting from skeletal muscle glycogen phosphorylase deficiency. The resultant block in glycogenolysis leads to an increased flux through the xanthine oxidase pathway (myogenic hyperuricemia) and could lead to an increase in oxidative stress. We examined markers of oxidative stress (8-isoprostane and protein carbonyls), NAD(P)H-oxidase, xanthine oxidase and antioxidant enzyme (superoxide dismutase, catalase and glutathione peroxidase) activity in skeletal muscle of MCD patients (N = 12) and controls (N = 12). Eight-isoprostanes and protein carbonyls were higher in MCD patients as compared to controls (p < 0.05). There was a compensatory up-regulation of catalase protein content and activity (p < 0.05), mitochondrial superoxide dismutase (MnSOD) protein content (p < 0.01) and activity (p < 0.05) in MCD patients, yet this increase was not sufficient to protect the muscle against elevated oxidative damage. These results suggest that oxidative stress in McArdle patients occurs and future studies should evaluate a potential role for oxidative stress contributing to acute pathology (rhabdomyolysis) and possibly later onset fixed myopathy.

Normal activities of AMP-deaminase and adenylate kinase in patients with McArdle disease

During physical activity in McArdle patients, little or no lactate is released in the skeletal muscle. However, excessive ammonia production has frequently been reported in these patients. Production of ammonia is catalysed by AMP deaminase (AMPD) and adenylate kinase (AK). The activities of AMPD and AK along with housekeeping enzyme phosphoglucoisomerase (PGI) were measured in 11 genetically confirmed McArdle patients and compared with 27 healthy controls. The AMPD and AK activities were not significantly different in patients and controls. The activity of PGI was significantly higher in patients than in controls suggesting compensation of the impaired glycogenolysis in McArdle. The ratios of activities of AMPD and AK over PGI were significantly lower in patients than in controls. High ammonia production in McArdle patients is not based on enzyme induction of AMPD and AK but possibly due to kinetic activation of the enzyme AMPD by increased concentration of the substrate AMP.

Contribution of branched-chain amino acids to purine nucleotide cycle: a pilot study

BACKGROUND/OBJECTIVES

Branched-chain amino acids (BCAAs) and purine nucleotide cycle (PNC) are both associated with energy metabolism. The purpose of this study was to explore the influences of BCAA supplementation on the PNC activity of male athletes in response to a bout of endurance running exercise.

SUBJECTS/METHODS

Twelve male athletes (20.3±1.4 years) participated in the study. Each of the athletes received 12 g of a BCAA supplement (leucine 54%, isoleucine 19% and valine 27%) per day during the study. They performed two identical 60-min running exercises (65-70% maximum heart rate reserved) before and after receiving the BCAA supplements for 15 days. In addition to body composition measurement, plasma and urinary samples were also collected. Plasma samples were examined for the concentrations of glucose, lactate, BCAAs, alanine, glutamine, aspartate, hypoxanthine and uric acid. Urinary samples were examined for the concentrations of urea nitrogen, hydroxyproline, 3-methylhistidine and creatinine.

RESULTS

Body composition and the concentrations of urinary metabolites were not affected by BCAA supplementation, whereas clearance of plasma lactate after recovery from exercise was enhanced by BCAA supplementation (P<0.05). Plasma aspartate concentration was increased (P<0.05), whereas plasma glutamine, hypoxanthine and uric acid concentrations were decreased (P<0.05) by BCAA supplementation.

CONCLUSIONS

The findings suggest that BCAA supplements not only provided additional substrate to meet the energy demands of the athletes during endurance exercise but also reduced their PNC activity, and subsequently decreased uric acid production and reduced the incidence of gout in a person engaging in endurance exercise.

Unique Exercise Lactate Profile in Muscle Phosphofructokinase Deficiency (Tarui Disease); Difference Compared with McArdle Disease

INTRODUCTION

Glycogen storage disease V (GSDV, McArdle disease) and GSDVII (Tarui disease) are the most common of the rare disorders of glycogen metabolism. Both are associated with low lactate levels on exercise. Our aim was to find out whether lactate response associated with exercise testing could distinguish between these disorders.

METHODS

Two siblings with Tarui disease, two patients with McArdle disease and eight healthy controls were tested on spiroergometric exercise tests with follow-up of venous lactate and ammonia.

RESULTS

A late increase of lactate about three times the basal level was seen 10-30 min after exercise in patients with Tarui disease being higher than in McArdle disease and lower than in the controls. Ammonia was increased in Tarui disease.

DISCUSSION

Our results suggest that follow-up of lactate associated with exercise testing can be utilized in diagnostics to distinguish between different GSD diseases.

McArdle disease: a unique study model in sports medicine

McArdle disease is arguably the paradigm of exercise intolerance in humans. This disorder is caused by inherited deficiency of myophosphorylase, the enzyme isoform that initiates glycogen breakdown in skeletal muscles. Because patients are unable to obtain energy from their muscle glycogen stores, this disease provides an interesting model of study for exercise physiologists, allowing insight to be gained into the understanding of glycogen-dependent muscle functions. Of special interest in the field of muscle physiology and sports medicine are also some specific (if not unique) characteristics of this disorder, such as the so-called 'second wind' phenomenon, the frequent exercise-induced rhabdomyolysis and myoglobinuria episodes suffered by patients (with muscle damage also occurring under basal conditions), or the early appearance of fatigue and contractures, among others. In this article we review the main pathophysiological features of this disorder leading to exercise intolerance as well as the currently available therapeutic possibilities. Patients have been traditionally advised by clinicians to refrain from exercise, yet sports medicine and careful exercise prescription are their best allies at present because no effective enzyme replacement therapy is expected to be available in the near future. As of today, although unable to restore myophosphorylase deficiency, the 'simple' use of exercise as therapy seems probably more promising and practical for patients than more 'complex' medical approaches.

Phenotype consequences of myophosphorylase dysfunction: insights from the McArdle mouse model

KEY POINTS

This is the first study to analyse the effect of muscle glycogen phosphorylase depletion in metabolically different muscle types. In McArdle mice, muscle glycogen phosphorylase is absent in both oxidative and glycolytic muscles. In McArdle mice, the glycogen debranching enzyme (catabolic) is increased in oxidative muscles, whereas the glycogen branching enzyme (anabolic) is increased in glycolytic muscles. In McArdle mice, total glycogen synthase is decreased in both oxidative and glycolytic muscles, whereas the phosphorylated inactive form of the enzyme is increased in both oxidative and glycolytic enzymes. In McArdle mice, glycogen content is higher in glycolytic muscles than in oxidative muscles. Additionally, in all muscles analysed, the glycogen content is higher in males than in females. The maximal endurance capacity of the McArdle mice is significantly lower compared to heterozygous and wild-type mice.

ABSTRACT

McArdle disease, caused by inherited deficiency of the enzyme muscle glycogen phosphorylase (GP-MM), is arguably the paradigm of exercise intolerance. The recent knock-in (p.R50X/p.R50X) mouse disease model allows an investigation of the phenotypic consequences of muscle glycogen unavailability and the physiopathology of exercise intolerance. We analysed, in 2-month-old mice [wild-type (wt/wt), heterozygous (p.R50X/wt) and p.R50X/p.R50X)], maximal endurance exercise capacity and the molecular consequences of an absence of GP-MM in the main glycogen metabolism regulatory enzymes: glycogen synthase, glycogen branching enzyme and glycogen debranching enzyme, as well as glycogen content in slow-twitch (soleus), intermediate (gastrocnemius) and glycolytic/fast-twitch (extensor digitorum longus; EDL) muscles. Compared with wt/wt, exercise capacity (measured in a treadmill test) was impaired in p.R50X/p.R50X (∼48%) and p.R50X/wt mice (∼18%). p.R50X/p.R50X mice showed an absence of GP-MM in the three muscles. GP-MM was reduced in p.R50X/wt mice, especially in the soleus, suggesting that the function of 'slow-twitch' muscles is less dependent on glycogen catabolism. p.R50X/p.R50X mice showed increased glycogen debranching enzyme in the soleus, increased glycogen branching enzyme in the gastrocnemius and EDL, as well as reduced levels of mucle glycogen synthase protein in the three muscles (mean ∼70%), reflecting a protective mechanism for preventing deleterious glycogen accumulation. Additionally, glycogen content was highest in the EDL of p.R50X/p.R50X mice. Amongst other findings, the present study shows that the expression of the main muscle glycogen regulatory enzymes differs depending on the muscle phenotype (slow- vs. fast-twitch) and that even partial GP-MM deficiency affects maximal endurance capacity. Our knock-in model might help to provide insights into the importance of glycogen on muscle function.

The pathogenomics of McArdle disease--genes, enzymes, models, and therapeutic implications

Numerous biomedical advances have been made since Carl and Gerty Cori discovered the enzyme phosphorylase in the 1940s and the Scottish physician Brian McArdle reported in 1951 a previously 'undescribed disorder characterized by a gross failure of the breakdown in muscle of glycogen'. Today we know that this disorder, commonly known as 'McArdle disease', is caused by inherited deficiency of the muscle isoform of glycogen phosphorylase (GP). Here we review the main aspects of the 'pathogenomics' of this disease including, among others: the spectrum of mutations in the gene (PYGM) encoding muscle GP; the interplay between the different tissue GP isoforms in cellular cultures and in patients; what can we learn from naturally occurring and recently laboratory-generated animal models of the disease; and potential therapies.

McArdle Disease and Exercise Physiology

McArdle disease (glycogen storage disease Type V; MD) is a metabolic myopathy caused by a deficiency in muscle glycogen phosphorylase. Since muscle glycogen is an important fuel for muscle during exercise, this inborn error of metabolism provides a model for understanding the role of glycogen in muscle function and the compensatory adaptations that occur in response to impaired glycogenolysis. Patients with MD have exercise intolerance with symptoms including premature fatigue, myalgia, and/or muscle cramps. Despite this, MD patients are able to perform prolonged exercise as a result of the “second wind” phenomenon, owing to the improved delivery of extra-muscular fuels during exercise. The present review will cover what this disease can teach us about exercise physiology, and particularly focuses on the compensatory pathways for energy delivery to muscle in the absence of glycogenolysis.

Oxidative stress and Nrf2 signaling in McArdle disease

McArdle disease (MD) is a metabolic myopathy due to myophosphorylase deficiency, which leads to a severe limitation in the rate of adenosine triphosphate (ATP) resynthesis. Compensatory flux through the myoadenylate deaminase > > xanthine oxidase pathway should result in higher oxidative stress in skeletal muscle; however, oxidative stress and nuclear factor erythroid 2-related factor 2 (Nrf2) mediated antioxidant response cascade in MD patients have not yet been examined. We show that MD patients have elevated muscle protein carbonyls and 4-hydroxynonenal (4-HNE) in comparison with healthy, age and activity matched controls (P < 0.05). Nuclear abundance of Nrf2 and Nrf2-antioxidant response element (ARE) binding was also higher in MD patients compared with controls (P < 0.05). The expressions of Nrf2 target genes were also higher in MD patients vs. controls. These observations suggest that MD patients experience elevated levels of oxidative stress, and that the Nrf2-mediated antioxidant response cascade is up-regulated in skeletal muscle to compensate.

[Exercise-induced muscle pain due to phosphofrutokinase deficiency: Diagnostic contribution of metabolic explorations (exercise tests, 31P-nuclear magnetic resonance spectroscopy)]

INTRODUCTION

Muscle phosphofructokinase deficiency, the seventh member of the glycogen storage diseases family, is also called Tarui's disease (GSD VII).

METHODS

We studied two patients in two unrelated families with Tarui's disease, analyzing clinical features, CK level, EMG, muscle biopsy findings and molecular genetics features. Metabolic muscle explorations (forearm ischemic exercise test [FIET]; bicycle ergometer exercise test [EE]; 31P-nuclear magnetic resonance spectroscopy of calf muscle [31P-NMR-S]) are performed as appropriate.

RESULTS

Two patients, a 47-year-old man and a 38-year-old woman, complained of exercise-induced fatigue since childhood. The neurological examination was normal or showed light weakness. Laboratory studies showed increased CPK, serum uric acid and reticulocyte count without anemia. There was no increase in the blood lactate level during the FIET or the EE although there was a light increase in the respiratory exchange ratio during the EE. 31P-NMR-S revealed no intracellular acidification or accumulated intermediates such as phosphorylated monoesters (PME) known to be pathognomic for GSD VII. Two new mutations were identified.

DISCUSSION

FIET and EE were non-contributive to diagnosis, but 31P-NMR provided a characteristic spectra of Tarui's disease, in agreement with phosphofructokinase activity level in erythrocytes. Muscle biopsy does not always provide useful information for diagnosis. In these two cases, genetic studies failed to establish a genotype-phenotype correlation.

CONCLUSION

The search for phosphofructokinase deficiency should be continued throughout life in adults experiencing fatigability or weakness because of the severe disability for daily life activities caused by the late onset form.

Contribution of creatine to protein homeostasis in athletes after endurance and sprint running

PURPOSE

Few studies have focused on the metabolic changes induced by creatine supplementation. This study investigated the effects of creatine supplementation on plasma and urinary metabolite changes of athletes after endurance and sprint running.

METHODS

Twelve male athletes (20.3 ± 1.4 y) performed two identical (65-70 % maximum heart rate reserved) 60 min running exercises (endurance trial) before and after creatine supplementation (12 g creatine monohydrate/day for 15 days), followed by a 5-day washout period. Subsequently, they performed two identical 100 m sprint running exercises (power trial) before and after 15 days of creatine supplementation in accordance with the supplementary protocol of the endurance trial. Body composition measurements were performed during the entire study. Plasma samples were examined for the concentrations of glucose, lactate, branched-chain amino acids (BCAAs), free-tryptophan (f-TRP), glutamine, alanine, hypoxanthine, and uric acid. Urinary samples were examined for the concentrations of hydroxyproline, 3-methylhistidine, urea nitrogen, and creatinine.

RESULTS

Creatine supplementation significantly increased body weights of the athletes of endurance trial. Plasma lactate concentration and ratio of f-TRP/BCAAs after recovery from endurance running were significantly decreased with creatine supplementation. Plasma purine metabolites (the sum of hypoxanthine and uric acid), glutamine, urinary 3-methylhistidine, and urea nitrogen concentrations tended to decrease before running in trials with creatine supplements. After running, urinary hydroxyproline concentration significantly increased in the power trial with creatine supplements.

CONCLUSIONS

The findings suggest that creatine supplementation tended to decrease muscle glycogen and protein degradation, especially after endurance exercise. However, creatine supplementation might induce collagen proteolysis in athletes after sprint running.

Testosterone replacement elevates the serum uric acid levels in patients with female to male gender identity disorder

Gender identity disorder (GID) results from a disagreement between a person's biological sex and the gender to which he or she identifies. With respect to the treatment of female to male GID, testosterone replacement therapy (TRT) is available. The uric acid (UA) level can be influenced by testosterone; however, the early effects and dose-dependency of TRT on the serum UA concentration have not been evaluated in this population. We herein conducted a dose-response analysis of TRT in 160 patients with female to male GID. The TRT consisted of three treatment groups who received intramuscular injections of testosterone enanthate: 125 mg every two weeks, 250 mg every three weeks and 250 mg every two weeks. Consequently, serum UA elevation was observed after three months of TRT and there was a tendency toward testosterone dose-dependency. The onset of hyperuricemia was more prevalent in the group who received the higher dose. We also demonstrated a positive correlation between increased levels of serum UA and serum creatinine. Since the level of serum creatinine represents an individual's muscle volume and the muscle is a major source of purine, which induces UA upregulation, the serum UA elevation observed during TRT is at least partially attributed to an increase in muscle mass. This is the first study showing an association between serum UA elevation and a TRT-induced increase in muscle mass. The current study provides important information regarding TRT for the follow-up and management of the serum UA levels in GID patients.

Fat and carbohydrate metabolism during exercise in late-onset Pompe disease

Pompe disease is caused by absence of the lysosomal enzyme acid alpha-glucosidase. It is generally assumed that intra-lysosomal hydrolysis of glycogen does not contribute to skeletal muscle energy production during exercise. However, this hypothesis has never been tested in vivo during exercise. We examined the metabolic response to exercise in patients with late-onset Pompe disease, in order to determine if a defect in energy metabolism may play a role in the pathogenesis of Pompe disease. We studied six adult patients with Pompe disease and 10 healthy subjects. The participants underwent ischemic forearm exercise testing, and peak work capacity was determined. Fat and carbohydrate metabolism during cycle exercise was examined with a combination of indirect calorimetry and stable isotope methodology. Finally, the effects of an IV glucose infusion on heart rate, ratings of perceived exertion, and work capacity during exercise were determined. We found that peak oxidative capacity was reduced in the patients to 17.6 vs. 38.8 ml kg(-1) min(-1) in healthy subjects (p = 0.002). There were no differences in the rate of appearance and rate of oxidation of palmitate, or total fat and carbohydrate oxidation, between the patients and the healthy subjects. None of the subjects improved exercise tolerance by IV glucose infusion. In conclusion, peak oxidative capacity is reduced in Pompe disease. However, skeletal muscle fat and carbohydrate use during exercise was normal. The results indicate that a reduced exercise capacity is caused by muscle weakness and wasting, rather than by an impaired skeletal muscle glycogenolytic capacity. Thus, it appears that acid alpha-glucosidase does not play a significant role in the production of energy in skeletal muscle during exercise.

Metabolic myopathies: functional evaluation by different exercise testing approaches

Metabolic myopathies are a clinically and etiologically heterogeneous group of disorders due to defects in muscular energy metabolism. They include glycogen storage diseases, fatty acid oxidation defects, and mitochondrial disorders. The typical manifestations of a metabolic myopathy are exercise-induced myalgias, exercise intolerance, and cramps. Evaluating subjects with such symptoms is not easy because of the frequent lack of clinical features. Exercise tests are, therefore, reliable screening tools. Here, we discuss the possible role of such exercise testing techniques in the diagnostic approach of a patient with suspected metabolic myopathy.

Breakdown of adenine nucleotide pool in fatiguing skeletal muscle in McArdle's disease: a noninvasive 31P-MRS and EMG study

Energy metabolism and electrical muscle activity were studied in the calf muscles of 19 patients with proven McArdle's disease and in 25 healthy subjects. Phosphorus magnetic resonance spectroscopy and surface electromyography (S-EMG) were performed during two isometric muscle contractions of 3 min at 30% maximum voluntary contraction, one performed during normal perfusion and the other during applied ischemia. After about 1 min of ischemic muscle contraction in diseased muscle a significant acceleration in phosphocreatine breakdown was observed, along with a significant decrease in adenosine triphosphate. During both contractions the absence of glycolysis was shown by a significant alkalinization. Furthermore, in patients we observed a greater increase in the S-EMG amplitude than in control subjects. We conclude that early on during moderate exercise, a small number of muscle fibers reach metabolic depletion, indicated by a reduction in the adenine nucleotide pool. An increasing number of motor units, which are still in a high-energy state, are continuously recruited to compensate for muscle fatigue. This functional compartmentation may contribute to the pathophysiology of exercise intolerance in McArdle's disease.

Phenotype modulators in myophosphorylase deficiency

Myophosphorylase deficiency is characterized by exercise intolerance, muscle cramps, and recurrent myoglobinuria. Some patients are severely affected, whereas others are minimally affected or asymptomatic. The molecular basis of the disease has been elucidated but does not provide an explanation for the clinical variability. In a large cohort of patients with myophosphorylase deficiency, we tested the hypothesis that polymorphic variants in either myoadenylate deaminase (MADA) or angiotensin-converting enzyme (ACE) could act as modulators of phenotype expression. Forty-seven patients were evaluated. Clinical severity was assessed according to a severity scale of four grades. MADA activity was studied by histochemical and biochemical analysis of muscle, and the Q12X mutation in the adenine monophosphate deaminase 1 gene (AMPD1) and the insertion/deletion polymorphism in the ACE gene were assessed genetically. A complete MADA defect together with the Q12X mutation was detected in one severely affected patient. Eleven patients were heterozygous for the Q12X mutation. There was no association between clinical grading and MADA status. In contrast, we found a highly significant (p < 0.01) association between ACE genotype and clinical severity, with strong correlation between severe phenotype and number of D alleles. We show that ACE insertion/deletion polymorphism may play a significant role as phenotype modulator in McArdle's disease.

Effect of muscular exercise on the concentration of uridine and purine bases in plasma--adenosine triphosphate consumption-induced pyrimidine degradation

To identify whether muscular exercise increases the plasma concentration of uridine and of purine bases, the effect of rigorous muscular exercise was determined in five healthy men with a bicycle ergometer. Twenty-five-minute muscular exercise at 65% maximum O2 consumption increased the concentration of uridine, purine bases, and inorganicphosphate in plasma and of NH3 and lactic acid in blood. These results suggest that exercise-induced excessive adenosine triphosphate (ATP) consumption enhanced not only purine degradation but also pyrimidine degradation (uridine triphosphate [UTP]-->uridine diphosphate [UDP]-->uridine monophosphate [UMP]-->uridine) in exercising muscles.

Is the plasma uridine level a marker of the overproduction of uric acid?

To determine whether the plasma level of uridine can be used to identify patients with gout, the plasma concentration of uridine was determined in patients with gout and normal subjects. Plasma uridine was significantly higher in patients with gout than in normal subjects. It was also significantly higher in patients with gout of the overexcretion (of uric acid) type than in those with gout of the underexcretion type. Plasma uridine was used to classify gout patients into underexcretion and overexcretion types, with a diagnostic accuracy of 92.5%. Results indicate that the plasma uridine concentration may be a marker of uric acid production and can be used to separate hyperuricemia into the overexcretion and underexcretion types.

IMP reamination to AMP in rat skeletal muscle fiber types

Inosine 5'-monophosphate (IMP) reamination in skeletal muscle fiber sections of the rat hindlimb was studied. High IMP concentrations were established during ischemic contractions in each fiber section: 3.1, 2.8, or 0.6 mumol/g in the fast-twitch white (FTW), fast-twitch red (FTR), and slow-twitch red (STR) muscle sections, respectively. Thereafter blood flow was restored and stimulation was discontinued to allow reamination of IMP. After 0, 2, 5, 10, 15, or 20 min of recovery, muscle sections were freeze-clamped and analyzed for metabolite contents. IMP was nearly fully reaminated after 10 and 20 min of recovery in STR and FTR muscles, respectively. Reamination in TW fibers was delayed and slower, with only 50% of the IMP reaminated after 20 min of recovery. Significant recovery (approximately 75%) of phosphocreatine occurs in each fiber section before the onset of reamination. Reamination was also evaluated after high-speed treadmill running with or without inhibition of reamination by hadacidin. Running resulted in large accumulations of IMP in FTW and FTR fibers (3.5 and 1.4 mumul/g, respectively); IMP in FTR fibers was higher with hadacidin treatment. Reamination after running was much greater in FTR than in FTW fibers and was associated with recovery of phosphocreatine. After running, the purine degradation products inosine and hypoxanthine were increased in FTW and FTR fibers in normal and hadacidin-treated animals. Plasma inosine, hypoxanthine, and urate increased after exercise; concentrations continued to increase if reamination was inhibited by hadacidin. These results demonstrate that when muscle IMP is increased, subsequent degradation and loss of purines occur. Rapid reamination should minimize the quantity of purine lost from muscle and limit the metabolic cost of replenishing purines by the de novo synthesis or salvage pathways.

Myogenic hyperuricemia: what can we learn from metabolic myopathies?

The association of muscle glycogenosis with hyperuricemia led to the identification of a unique purine disorder. Myogenic hyperuricemia is ascribed to excessive degradation of muscle purine nucleotides, secondary to impaired ATP generation. Although this pathophysiological condition has been observed not only in glycolytic defects but also in mitochondrial diseases affecting lipid and carbohydrate oxidation, it is most common and prominent in muscle phosphofructokinase deficiency, in which neither glycogen nor glucose can be used as metabolic fuels. The first key reaction of muscle purine degradation is catalysis by AMP deaminase. Numerous studies have indicated that AMP deaminase may play an important role in energy metabolism in contracting muscle. Arguments against this hypothesis have emerged through analyses on muscle AMP deaminase deficiency. According to a recent study, the mutant allele is extremely frequent among Caucasians and African-Americans, suggesting that many individuals with this enzyme defect may be clinically asymptomatic. Further study is required to explain the significance of muscle purine degradation in energy metabolism.

Glucose infusion paradoxically accelerates degradation of adenine nucleotide in working muscle of patients with glycogen storage disease type VII

We investigated the effect of glucose infusion on adenosine triphosphate degradation in skeletal muscle of patients with glycogen storage disease type VII. Three patients and six healthy subjects exercised on a bicycle ergometer twice, once with 20% glucose infusion and once with saline infusion. The glucose infusion increased plasma glucose levels to 170 to 182 mg/dl and serum insulin levels to 30 to 50 microU/ml, while it markedly decreased plasma free fatty acid levels. The exercise-induced increases in plasma ammonia, inosine, and hypoxanthine were much larger with glucose than with saline infusion in the patients. Urinary excretion of inosine and hypoxanthine with glucose infusion was twice as high as that with saline infusion. No such differences were present between glucose and saline infusion in the healthy subjects. Glucose infusion therefore accelerates the energy crisis in working muscle of patients with glycogen storage disease type VII, probably due to a decrease in fatty acid utilization.

A new variant case of muscle phosphofructokinase deficiency, coexisting with gastric ulcer, gouty arthritis, and increased hemolysis

Muscle phosphofructokinase (PFK) deficiency includes both clinically and genetically heterogeneous conditions. A 22-year-old man with muscle PFK deficiency due to previously unrecognized mutation was admitted because of gastric ulcer. He had noticed mild fatigability on vigorous exercise, but had never experienced painful cramps and myoglobinuria. His history included five time relapses of gastric ulcer and gouty arthritis at ages 19 and 21 years. His laboratory data showing impaired muscle glycolysis, increased hemolysis, and myogenic hyperuricemia had aspects in common with those reported for the classic form of this disease, except that lactate concentrations in his blood increased considerably after exercise. The mutant PFK enzyme of this patient, who was demonstrated to have a missense mutation, could exert some catalytic activity that permitted glycolytic flux in vivo, thus leading to the absence of typical myopathic symptoms. The association of relapsing gastric ulcer with muscle PFK deficiency was detected for the first time. There is a possibility that oxygen radical-induced tissue damage resulting from increased hypoxanthine on exertion plays a role in the pathogenesis of ulceration, since the patient is more tolerant to exercise than reported cases with the classic form of muscle PFK deficiency.

Influence of daily drinking habits on ethanol-induced hyperuricemia

We examined the influence of alcohol drinking habits on the serum uric acid level after the ingestion of a small amount of ethanol. Subjects were divided into two groups according to their alcohol drinking habits--regular drinkers, who consume more than 60 g ethanol every day, and nondrinkers/occasional drinkers, who consume less than 20 g ethanol occasionally. Drinking 0.5 g ethanol/kg increased serum uric acid levels in regular drinkers by 52.6 +/- 26.3 mumol/L (0.8 +/- 0.4 mg/dL), whereas it did not in nondrinkers/occasional drinkers. Urinary excretion of uric acid was unaltered in both groups. Hypoxanthine and xanthine in both plasma and urine and serum acetate were increased more in regular drinkers than in nondrinkers/occasional drinkers. Accelerated adenine nucleotide degradation secondary to enhanced ethanol oxidation likely explains the ethanol-induced hyperuricemia in regular drinkers.

Clinical and biochemical aspects of uric acid overproduction

Purine nucleotides are synthesized and degraded through a regulated series of reactions which end in the formation of uric acid. Increased uric acid synthesis may be the result of two major pathophysiological disorders: increased de novo purine synthesis and enhanced purine nucleotide degradation, both of which may be the result of an increased or decreased enzyme activity. In addition, some conditions and disorders associated with uric acid overproduction have been recognized as the result of increased ATP degradation or decreased synthesis of ATP. The clinical manifestations of the diseases leading to excess uric acid synthesis are heterogenous, but symptoms related to uric acid overproduction are always secondary to the precipitation of crystals in soft tissues, joints, and the kidney excretory system. In clinical practice, serum urate concentration and urinary uric acid excretion are used to assess uric acid synthesis, taking into account that a purine-rich diet can be a confounding variable. Quantification of uric acid precursors, such as adenosine, inosine, guanosine, hypoxanthine, and xanthine, in biological fluids and intracellular nucleotides has provided further insight into the metabolic disturbances underlying disorders associated with uric acid overproduction. Additional studies are necessary to define precisely the metabolic derangement in idiopathic uric acid overproduction and to assess fully the consequences of increased purine nucleotide degradation, such as free-radical formation, increased adenosine synthesis, and reduced synthesis of signal transducers.

O2 uptake kinetics in response to exercise. A measure of tissue anaerobiosis in heart failure

Oxygen uptake (VO2) reflects the rate of aerobic regeneration of high-energy phosphate compounds (primarily adenosine triphosphate [ATP]). Since lactate increase is thought to result from an inadequate rate of aerobic ATP regeneration, it might be expected that lactate increase would be associated with a delayed attainment of steady state for VO2 in response to constant load exercise. Similarly if mitochondrial ATP regeneration during exercise is inadequately supported by O2 transport mechanisms, adenosine diphosphate (ADP) and purine nucleotide by-products, such as hypoxanthine, should increase. This study investigated the relationship between VO2 kinetics during exercise and accompanying changes in blood lactate and hypoxanthine values in heart failure patients, as a model of compromised O2 transport. Twenty-five patients with chronic heart failure performed cycle ergometry for 6 min at 25 W and at a work rate midway (50 percent delta) between their lactic acidosis threshold (LAT) and peak VO2. Ventilation and gas exchange were measured breath by breath, and venous lactate, hypoxanthine, norepinephrine, and epinephrine were determined at rest and 2 min after each test. The slow component of VO2 kinetics was quantified as the rise in VO2 from the third to the sixth minute of exercise (delta VO2 [6-3]). Ten age- and size-matched normal subjects served as control subjects. delta VO2 (6-3) was correlated with the increase in lactate (r = 0.71, p < 0.001), hypoxanthine (r = 0.61, p < 0.001), and norepinephrine (r = 0.41, p < 0.01) but not epinephrine in response to exercise in the heart failure patients. The delta VO2 (6-3) and delta lactate were both greater in the patients than in the control subjects at similar absolute work rates (54 +/- 20 and 60 W, respectively). However, the slope of the relationship between delta La and delta VO2 (6-3) for the patient and normal groups was indistinguishable. The lactate increase was correlated with hypoxanthine increase (r = 0.66, p < 0.001), but not norepinephrine or epinephrine. In summary, VO2 kinetics in response to exercise reflects delayed attainment of the steady state in heart failure patients, which is correlated with increases in lactate and hypoxanthine, markers of increased anaerobic metabolism.

Relative hypoxia of the extremities in Fabry disease

A purine degradation study, thermography and near infrared spectroscopy of the extremities were performed on 2 young males with Fabry disease and 2 healthy controls. Two-minute semi-ischemic forearm exercise caused a distinct increase in lactate in all subjects, but venous hypoxanthine and ammonia were greatly increased only in the Fabry patients, suggesting a relatively hypoxic state of the extremities. Limb thermograms of the patients revealed glove and stocking type disturbance at rest. Poor recovery of the skin temperature of the hands and forearms after exercise was observed in the patients, but the sharp increase in oxygenated hemoglobin after total ischemia was found to be normal or near infrared spectroscopy. Neurotropin showed an analgesic effect, i.e. a strong and selective heat-productive action on the painful lesions, and suppressed the hypoxanthine level after exercise in 1 patient. Although the pathophysiology of the pain in Fabry disease has not been clearly elucidated, a relatively hypoxic state with peripheral hypothermia might play an important role in triggering of a painful attack or chronic burning paresthesia.

Rhabdomyolysis in childhood. A primer on normal muscle function and selected metabolic myopathies characterized by disordered energy production

Patients with rhabdomyolysis present an important clinical problem. In acute episodes immediate treatment may be necessary to prevent significant morbidity and mortality. Evaluation of affected patients necessitates an understanding of basic muscle pathophysiology and of the variety of disturbances that can interfere with muscle energy metabolism. The physician must then pursue a systematic stepwise evaluation (Table 6) that includes obtaining relevant history and laboratory studies, as well as arranging for appropriate provocative testing and muscle biopsy. Once the diagnosis is established, patient and family counseling is necessary, particularly in genetic disorders. Unfortunately, specific therapies have not proven entirely successful, and treatment generally has been directed at reducing the severity of rhabdomyolytic episodes.

McArdle's disease and gout

We report the first case of McArdle's disease (muscle phosphorylase deficiency) and tophaceous gout. To examine the contribution of adenine nucleotide degradation to the disturbance of uric acid metabolism, we labeled the adenine nucleotide pool with [8-14C]adenine, and measured plasma and urine purines following vigorous exercise tests. Plasma and urinary hypoxanthine and xanthine concentrations and the specific radioactivity of urinary purines increased markedly, but plasma urate levels and uric acid excretion were not substantially modified. We suggest that, in this patient, the association of McArcle's disease with gout is coincidental.

Accelerated purine nucleotide degradation by anaerobic but not by aerobic ergometer muscle exercise

The exact conditions under which exercise causes purine nucleotide degradation are not well understood. We determined plasma hypoxanthine and uric acid levels serially in eight individuals during ergometer muscle exercise. When the load was increased gradually by 15 W/min, plasma hypoxanthine was elevated only after the status exceeded the anaerobic threshold (AT), as determined by analysis of expired gas. Nonstrenuous ergometer exercise, which kept the status continuously below the AT, induced neither blood lactic acid nor plasma hypoxanthine elevation. These results suggest that the AT is also the threshold for the acceleration of purine nucleotide degradation. Muscle exercise to a degree that does not exceed the AT does not cause major purine nucleotide degradation, and, therefore, is expected to be beneficial for patients with gout and/or hyperuricemia.

Basal and insulin-mediated carbohydrate metabolism in human muscle deficient in phosphofructokinase 1

Biopsies were obtained from the quadriceps femoris muscle of two male patients deficient in phosphofructokinase (PFK) 1. In the basal state the patients had markedly higher contents of UDP-glucose (approximately 5-fold), hexose monophosphates (approximately 7- to 13-fold), inosine monophosphate (IMP) (approximately 15-fold), and fructose 2,6-bisphosphate (F-2,6-P2; approximately 6-fold) than controls. Fructose 1,6-bisphosphate was not detectable, and phosphocreatine was lower (33 and 54 mmol/kg dry wt) than in controls [72 +/- 4 (SD)]. Patients had normal fasting plasma glucose and insulin levels and basal glucose turnover rates and responded normally to a 75-g oral glucose challenge. Patients were also studied during euglycemic hyperinsulinemia (approximately 95 mg/dl; 40 and 400 mU.m-2.min-1). Whole body glucose disposal rates were normal during both insulin infusion rates. Biopsies taken after the 400 mU insulin infusion showed decreases in acetylcarnitine and citrate and increases in the fractional activity of glycogen synthase. It is suggested that the high basal levels of F-2,6-P2 are, at least partly, a consequence of the high levels of fructose 6-phosphate, which will stimulate flux through PFK-2 and inhibit fructose-2,6-bisphosphatase. The low phosphocreatine and high IMP contents indicate that carbohydrate availability is important for control of high-energy phosphate metabolism, even in the basal state. The insulin-mediated decreases in acetylcarnitine and citrate suggest an activation of the tricarboxylic acid cycle in skeletal muscle but an absence of the normal response to replenish these intermediates.

Low glucose-1, 6-bisphosphate and high fructose-2, 6-bisphosphate concentrations in muscles of patients with glycogenosis types VII and V

The level of glucose-1, 6-bisphosphate, a potent allosteric activator of phosphofructokinase, was markedly decreased in muscles of patients with glycogenosis type VII (muscle phosphofructokinase deficiency) and type V (muscle phosphorylase deficiency). Glucose-1-phosphate kinase activity in muscle was virtually absent in a patient with glycogenosis type VII, whereas it was normal in a patient with type V glycogenosis. Glucose-1-phosphate level was increased in type VII glycogenosis, whereas it was decreased in type V glycogenosis. Another activator of phosphofructokinase, fructose-2, 6-bisphosphate was increased in muscles of patients with both types of glycogenosis although it was much higher in type VII than in type V. This finding may be partly related to the difference of fructose-6-phosphate concentrations. The results suggest that phosphofructokinase would contribute to the major glucose-1-phosphate kinase activity in normal human muscle and would also form a kind of self-activating system.

Glucose infusion abolishes the excessive ATP degradation in working muscles of a patient with McArdle's disease

A 23-year-old woman with McArdle's disease performed mild leg exercise on a bicycle ergometer. Saline or 10% glucose solution was infused throughout exercise. After exercise with saline infusion, her plasma concentrations of ammonia, hypoxanthine and creatine kinase increased greatly. Conversely, after exercise with glucose infusion, there were no appreciable changes in these plasma substances. In addition, she noticed that glucose infusion relieved her from muscle symptoms during exercise. These findings suggest that glucose infusion to patients with McArdle's disease ameliorates excessive ATP degradation in exercising muscles.

Decreased xanthine oxidase activities and increased urinary oxypurines in heterozygotes for hereditary xanthinuria

Two brothers with hereditary xanthinuria (xanthine oxidase deficiency) and several members of their family were studied. In both subjects, plasma and urinary concentrations of uric acid were low whereas xanthine and hypoxanthine concentrations were markedly elevated. Xanthine oxidase activity was virtually absent in the patients' duodenal mucosa, a finding that established the diagnosis of hereditary xanthinuria. In their parents (obligate heterozygotes), the duodenal xanthine oxidase activity was about 50% of that in control subjects (father 9.3 and mother 12.8 mU/g tissue compared with 21.3 +/- 5.0 mU/g tissue, mean +/- SD). The residual xanthine oxidase from the parents exhibited normal kinetics with respect to hypoxanthine. The parents' urinary xanthine and hypoxanthine concentrations were significantly greater than those of control subjects, while their plasma concentrations of oxypurines were normal. Similar findings were observed in at least 6 other relatives, a finding that suggested that they were heterozygotes. This study suggests that obligate hereditary xanthinuria heterozygotes have only 50% of the xanthine oxidase activity of controls; this deficiency results in a partial metabolic blockage at this enzymatic step in heterozygotes.

Tissue release of adenosine triphosphate degradation products during shock in dogs

Clinical monitoring of cellular metabolism during shock, based largely on traditional metabolic indicators, remains unsatisfactory. The purpose of this study was to compare venous oxygen tension and blood lactate gradients with blood gradients of purine nucleotide degradation products which are derived from tissue ATP catabolism during hypovolemic shock. Sixteen dogs were instrumented to sample arterial and venous blood. Measurements of arteriovenous lactate and PNDP gradients during spontaneous respiration were examined at four tissue sites: gut, kidney, hindlimb, and diaphragm. Hypovolemic shock (mean arterial blood pressure 35 to 40 mm Hg) was induced and maintained for one hour. The above parameters were remeasured at 30 and 60 minutes after induction of shock. Hypoxanthine gradients were greater than that of other PNDP, and so were used as the primary indicator of tissue ATP metabolism. In the hindlimb, the mean AV gradients for hypoxanthine (1 +/- 1 microM) were not significantly greater than baseline, while the lactate gradient (700 +/- 300 microM) rose markedly. In contrast, across the kidney there was a significantly greater AV hypoxanthine gradient (16 +/- 3 microM, p less than 0.002) but no lactate gradient (-400 +/- 200 microM). Both the hypoxanthine and lactate AV gradients were significantly elevated across the diaphragm and gut. Venous PO2 values less than 35 mm Hg predicted an increased hypoxanthine gradient across the kidney, but not across the hindlimb. We conclude that the metabolic response to hypovolemic shock as assessed by PNDP gradients, lactate gradients, and venous PO2 differs among tissues. Although resting muscle such as the hindlimb may be an important source of blood lactate, the viscera and working skeletal muscle (the diaphragm) are major contributors to circulating PNDP.

Muscle bio-energetics in acute glycolytic block: in vivo phosphorus-nuclear magnetic resonance study of iodo-acetate injected rats

In vivo phosphorus nuclear magnetic resonance spectroscopy of muscle was performed at rest, during work and during postexercise recovery in rats injected with iodo-acetate (IA) (35-40 mg.kg-1, intra-arterially), in order to follow bio-energetic changes in muscle with acute glycolytic block. Three animals with contracture had very low ratios of phosphocreatine:inorganic phosphate (PCr:Pi) at rest (0.5-0.9). The PCr:Pi were normal at rest (6.9 +/- 2.0, +/- 2 SD) in all other rats. Exercise-induced continuous accumulation of phosphomonoesters (PME), the characteristic finding of glycolytic block, was observed. The end-exercise levels of PME correlated with the degree of block measured in vitro. During steady-state work, induced by nerve stimulation at four frequencies, PCr:Pi values were significantly lower (p less than 0.02) than the control values at 0.25, 1.0 and 2.0 Hz. The ATP levels fell during exercise to reach 75% +/- 7% of initial values. The recovery of PCr:Pi from exercise and the disappearance of PME were slow. Two animals which survived the IA injection demonstrated much lower PME accumulation 18 h later. It is concluded that in acute muscle glycolytic block: (1) energy metabolism is impaired during exercise and also at rest, (2) accumulating PME can serve as an indicator of the degree of glycolytic block, (3) ATP levels fall during work, and (4) postexercise recovery is slow. The findings are compared with 31P-NMR observations in chronic muscle glycolytic disorders.