Modeling in vivo recovery of intracellular pH in muscle to provide a novel index of proton handling: application to the diagnosis of mitochondrial myopathy

Mechanisms of the effect of oxidative phosphorylation deficiencies on the skeletal muscle bioenergetic system in patients with mitochondrial myopathies

Simulations carried out using a previously developed model of the skeletal muscle bioenergetic system, involving the "inorganic phosphate (P_i) double-threshold" mechanism of muscle fatigue, lead to the conclusion that a decrease in the oxidative phosphorylation (OXPHOS) activity, caused by mutations in mitochondrial or nuclear DNA, is the main mechanism underlying the changes in the kinetic properties of the system in mitochondrial myopathies (MM). These changes generally involve the very-heavy-exercise-like behavior and exercise termination because of fatigue at low work intensities. In particular, a sufficiently large (at a given work intensity) decrease in OXPHOS activity leads to slowing of the primary phase II of the oxygen uptake (V̇o₂) on-kinetics, decrease in maximal V̇o₂ (V̇o_2max), appearance of the slow component of the V̇o₂ on-kinetics, exercise intolerance, and lactic acidosis at relatively low power outputs encountered in experimental studies in patients with MM. Thus, the "P_i double-threshold" mechanism of muscle fatigue is able to account, at least semiquantitatively, for various kinetic effects of inborn OXPHOS deficiencies of the skeletal muscle bioenergetic system. Exercise can be potentially lengthened and V̇o_2max elevated in patients with MM through an increase in peak P_i (Pi_peak), at which exercise is terminated because of fatigue. Generally, a mechanism underlying the kinetic effects of OXPHOS deficiencies on the skeletal muscle bioenergetic system in MM is proposed that was absent in the literature.NEW & NOTEWORTHY A mechanism of the OXPHOS deficiencies-induced changes of the skeletal muscle bioenergetic system in patients with mitochondrial myopathies (MM), namely, appearance of the slow component of the V̇o₂ on-kinetics at relatively low work intensities, slowed primary phase II of the V̇o₂ on-kinetics, lowered V̇o_2max, and lactic acidosis is proposed. It involves a decrease in OXPHOS activity acting through the "P_i double-threshold" mechanism of muscle fatigue comprising initiation of the additional ATP usage and termination of exercise.

Quantification of skeletal muscle mitochondrial function by 31P magnetic resonance spectroscopy techniques: a quantitative review

Magnetic resonance spectroscopy (MRS) can give information about cellular metabolism in vivo which is difficult to obtain in other ways. In skeletal muscle, non-invasive (31) P MRS measurements of the post-exercise recovery kinetics of pH, [PCr], [Pi] and [ADP] contain valuable information about muscle mitochondrial function and cellular pH homeostasis in vivo, but quantitative interpretation depends on understanding the underlying physiology. Here, by giving examples of the analysis of (31) P MRS recovery data, by some simple computational simulation, and by extensively comparing data from published studies using both (31) P MRS and invasive direct measurements of muscle O2 consumption in a common analytical framework, we consider what can be learnt quantitatively about mitochondrial metabolism in skeletal muscle using MRS-based methodology. We explore some technical and conceptual limitations of current methods, and point out some aspects of the physiology which are still incompletely understood.

Blood metabolite data in response to maximal exercise in healthy subjects

Maximal exercise test with gas exchange measurement evaluates exercise capacities with maximal oxygen uptake (VO(2) max) measurement. Measurements of lactate (L), lactate/pyruvate ratio (L/P) and ammonium (A) during rest, exercise and recovery enhance interpretative power of maximal exercise by incorporating muscular metabolism exploration. Maximal exercise test with gas exchange measurement is standardized in cardiopulmonary evaluations but, no reference data of blood muscular metabolites are available to evaluate the muscular metabolism. We determined normal values of L, L/P and A during a standardized maximal exercise and recovery in 48 healthy sedentary volunteers and compared with results obtained in four patients with exercise intolerance and a mitochondrial disease. In healthy subjects, L, L/P and A rose during exercise. In 98% of them L, L/P or A decreased between the fifth and the fifteenth minutes of recovery. In mitochondrial patients, VO(2) max was normal or low, and L, L/P and A had the same evolution as normal subjects or showed no decrease during recovery. We gave normal L, L/P and A values, which establish references for a maximal exercise test with muscular metabolism exploration. This test is helpful for clinicians in functional evaluation, management and treatment of metabolic myopathy and would be a useful tool in diagnosis of metabolic myopathy.

Semi-automated imaging system to quantitate Her-2/neu membrane receptor immunoreactivity in human breast cancer

BACKGROUND

Immunocytochemical methods for quantitating Her-2/neu immunoreactivity rest on subjective semi-quantitative interpretations with resulting interobserver, intraobserver, and fatigue variability.

METHODS

To standardize and quantitate measurements of Her-2/neu immunoreactivity, we created epithelial-recognition and specific membrane-recognition algorithms, which could image breast cancer cells against a background of stroma, compartmentalize the cancer cell into nucleus, cytoplasm and membrane, and quantitate the degree of Her-2/neu membrane immunoreactivity based on both gray scale intensity and RGB colorimetric determinations. Image acquisition utilized either scanner or microscope with attached camera with a resolution of 20 pixels/10 microm. Areas of 150 whole slides were screened and the regions of interest manually selected for image processing. Three hundred TMA cores were directly processed. Images were acquired by jpg conversion of svs virtual slides or direct jpg photomicrograph capture. Images were then assessed for quality and processed.

RESULTS

The digital algorithms successfully created a semi-automated imaging system whose algorithm-based ordinal values for Her-2/neu both strongly correlated with the subjective measurements (intraclass correlation: 0.84; 95% confidence interval: 0.79-0.89) yet exhibited no run variability. In addition, the algorithms generated immunocytochemical measurements of Her-2/neu on an expanded continuous scale, which more reliably distinguished true Her-2/neu positivity from true Her-2/neu negativity (determined by FISH) than subjective or algorithmic ordinal scale measurements. Furthermore, the continuous scale measurements could better resolve different levels of Her-2/neu overexpression than either subjective or algorithmic ordinal interpretation. Other semi-automated analysis systems have been used to measure Her-2/neu and other cellular immunoreactivities, but these either have required proprietary hardware or have been based on luminosity differences alone. In contrast, our algorithms are independent of proprietary hardware and are based on not just luminosity but also many other imaging properties including epithelial recognition and membrane morphology.

CONCLUSION

These features provide a more accurate, versatile, and robust imaging analysis platform.

Intersubject differences in the effect of acidosis on phosphocreatine recovery kinetics in muscle after exercise are due to differences in proton efflux rates

(31)P magnetic resonance spectroscopy provides the possibility of obtaining bioenergetic data during skeletal muscle exercise and recovery. The time constant of phosphocreatine (PCr) recovery (tau(PCr)) has been used as a measure of mitochondrial function. However, cytosolic pH has a strong influence on the kinetics of PCr recovery, and it has been suggested that tau(PCr) should be normalized for end-exercise pH. A general correction can only be applied if there are no intersubject differences in the pH dependence of tau(PCr). We investigated the pH dependence of tau(PCr) on a subject-by-subject basis. Furthermore, we determined the kinetics of proton efflux at the start of recovery. Intracellular acidosis slowed PCr recovery, and the pH dependence of tau(PCr) differed among subjects, ranging from -33.0 to -75.3 s/pH unit. The slope of the relation between tau(PCr) and end-exercise pH was positively correlated with both the proton efflux rate and the apparent proton efflux rate constant, indicating that subjects with a smaller pH dependence of tau(PCr) have a higher proton efflux rate. Our study implies that simply correcting tau(PCr) for end-exercise pH is not adequate, in particular when comparing patients and control subjects, because certain disorders are characterized by altered proton efflux from muscle fibers.

31P-MRS of skeletal muscle is not a sensitive diagnostic test for mitochondrial myopathy

Clinical phenotypes of persons with mitochondrial DNA (mtDNA) mutations vary considerably. Therefore, diagnosing mitochondrial myopathy (MM) patients can be challenging and warrants diagnostic guidelines. (31)phosphorous magnetic resonance spectroscopy ((31)P-MRS) have been included as a minor diagnostic criterion for MM but the diagnostic strength of this test has not been compared with that of other commonly used diagnostic procedures for MM. To investigate this, we studied seven patients with single, large-scale deletions-, nine with point mutations of mtDNA and 14 healthy subjects, who were investigated for the following: 1) (31)P-MRS of lower arm and leg muscles before and after exercise, 2) resting and peak-exercise induced increases of plasma lactate, 3) muscle morphology and -mitochondrial enzyme activity, 4) maximal oxygen uptake (VO(2max)), 5) venous oxygen desaturation during handgrip exercise and 6) a neurological examination. All MM patients had clinical symptoms of MM, > 2% ragged red fibers in muscle, and impaired oxygen desaturation during handgrip. Fourteen of 16 patients had impaired VO(2max), 10/16 had elevated resting plasma lactate, and 10/11 that were investigated had impaired citrate synthase-corrected complex I activity. Resting PCr/P(i) ratio and leg P(i) recovery were lower in MM patients vs. healthy subjects. PCr and ATP production after exercise were similar in patients and healthy subjects. Although the specificity for MM of some (31)P-MRS variables was as high as 100%, the sensitivity was low (0-63%) and the diagnostic strength of (31)P-MRS was inferior to the other diagnostic tests for MM. Thus, (31)P-MRS should not be a routine test for MM, but may be an important research tool.

Supplemental oxygen and muscle metabolism in mitochondrial myopathy patients

Patients with mitochondrial myopathy (MM) have a reduced capacity to perform exercise due to a reduced oxidative capacity. We undertook this study to determine whether skeletal muscle metabolism could be improved with oxygen therapy in patients with MM. Six patients with MM and six controls, matched for age, gender and physical activity, underwent (31)P-magnetic resonance spectroscopy ((31)P-MRS) examination. (31)P-MR spectra were collected at rest and in series during exercise and recovery whilst breathing normoxic (0.21 O(2)) or hyperoxic (1.0 O(2)) air. At rest, MM showed an elevated [ADP] (18 +/- 3 micromol/l) and pH (7.03 +/- 0.01) in comparison to the control group (12 +/- 1 micromol/l, 7.01 +/- 0.01) (P < 0.05) consistent with mitochondrial dysfunction. Oxygen supplementation did not change resting metabolites in either MM or the control group (P > 0.05). Inferred maximal ATP synthesis rate improved by 33% with oxygen in MM (21 +/- 3 vs. 28 +/- 5 mmol/(l min), P < 0.05) but only improved by 5% in controls (40 +/- 3 vs. 42 +/- 3 mmol/(l min), P > 0.05). We conclude that oxygen therapy is associated with significant improvements in muscle metabolism in patients with MM. These data suggest that patients with MM could benefit from therapies which improve the provision of oxygen.

Aerobic exercise and muscle metabolism in patients with mitochondrial myopathy

Exercise therapy improves mitochondrial function in patients with mitochondrial myopathy (MM). We undertook this study to determine the metabolic abnormalities that are improved by exercise therapy. This study identified metabolic pathology using (31)P-magnetic resonance spectroscopy and magnetic resonance imaging (MRI) in a group of patients with MM compared to a control group matched for age, gender, and physical activity. We also observed the effect of exercise therapy for 12 weeks on muscle metabolism and physical function in the MM group. During muscle activity, there was impaired responsiveness of the mitochondria to changes in cytosolic adenosine diphosphate concentration, increased dependence on anaerobic energy pathways, and an adaptive increase in proton efflux in patients with MM. Following exercise therapy, mitochondrial function and muscle mass improved without any change in proton efflux rate. These metabolic findings were accompanied by improvements in functional ability. We conclude that there are significant metabolic differences between patients with MM and a control population, independent of age, gender, and physical activity. Exercise therapy can assist in improving mitochondrial function in MM patients.

Phenotypic variability in myotonia congenita

Myotonia congenita is a hereditary chloride channel disorder characterized by delayed relaxation of skeletal muscle (myotonia). It is caused by mutations in the skeletal muscle chloride channel gene CLCN1 on chromosome 7. The phenotypic spectrum of myotonia congenita ranges from mild myotonia disclosed only by clinical examination to severe and disabling myotonia with transient weakness and myopathy. The most severe phenotypes are seen in patients with two mutated alleles. Heterozygotes are often asymptomatic but for some mutations heterozygosity is sufficient to cause pronounced myotonia, although without weakness and myopathy. Thus, the phenotype depends on the mutation type to some extent, but this does not explain the fact that severity varies greatly between heterozygous family members and may even vary with time in the individual patient. In this review, existing knowledge about phenotypic variability is summarized, and the possible contributing factors are discussed.