Heterogeneity in regard to enzymes and metabolites within individual muscle fibers

Fatigue of rat hindlimb motor units: Biochemical- physiological associations

Associations between fatigability and biochemical properties within motor unit (MU) types were explored in two hindlimb muscles of the adult rat. Type FF MUs in extensor digitorum longus and type S units in soleus were subjected either to a moderate (type FF) or severe (type S) 6-min, fatigue-inducing stimulation protocol. For both MU types, the range of values for their fatigability was considerably greater than the ranges in the activity levels of three enzymes in the units' constituent muscle fibers (MFs). These enzymes represented major energy-yielding pathways: adenylokinase, for high-energy phosphate metabolism; lactate dehydrogenase, for anaerobic glycolysis; and malate dehydrogenase, for oxidative metabolism. There were also relatively weak associations between the fatigue indices of the MUs and the activity levels of the three enzymes. Thus, this work supports previous conclusions that the force decline exhibited by MUs during electrically evoked contractions depends on both MF biochemistry and other intracellular mechanisms. Electromyographic measurements suggested that these other mechanisms are distal to the intramuscular branches of the axon in type FF units, and distal to excitation-contraction coupling in type S units.

Location along the muscle's length is a determinant of myofibril size

In a previous study of myofibril size in 'Pale' (fast-twitch-glycolytic) fibers of rabbit extraocular muscle (EOM), it was found that individual long Pale fibers demonstrate a substantial increase in the size of myofibril profiles from their proximal to their distal halves (Davidowitz et al., 1996b). That finding raised the question of whether such proximal-to-distal increase of myofibril size in the Pale fibers is determined by: (1) longitudinal position within the individual muscle fibers themselves or (2) location along the length of the muscle as a whole? This question was tested in the present study by comparing the original group of long Pale fibers, which extend the full length of the muscle, with two groups of short Pale fibers, which are respectively confined to the proximal and distal halves of the muscle. It was found that (a) in the proximal half of the muscle, the short fibers and the adjacent portions of the long fibers have the same smaller size of myofibrils, and (b) in the distal half of the muscle, the short fibers and the adjacent portions of the long fibers have the same larger size of myofibrils. This finding indicates that the proximal-to-distal increase of myofibril-profile size in these EOM Pale fibers is determined by location along the length of the muscle as a whole, and is not related to longitudinal position within the individual fibers themselves.

ATP loss with exercise in muscle fibres of the gluteus medius of the thoroughbred horse

Muscle ATP loss with exercise has implications both to the causes of fatigue and muscle damage. To study this at the single muscle fibre level, five trained thoroughbred horses performed consecutive 90 second gallops on an inclined treadmill followed by a final gallop to fatigue. Biopsies of the m. gluteus medius were taken at rest, post-exercise and during 24 hour recovery. Blood lactate was 20.0 mmol litre-1 or more, and plasma NH3 300-800 mumol litre-1, following the final gallop. Minimal changes occurred in the plasma markers, CK and AST. ATP loss with exercise was 32.2 (SD 12.2) per cent. Following exercise single fibre ATP contents showed a much broader distribution than at rest, with contents in some close to zero. Following five and 24 hour recovery, however, frequency distribution curves were close to those seen at rest. There was no difference in the ATP contents of types I, IIa and IIb at rest of with exercise or recovery. The results pointed to marked heterogeneity between individual fibres in their biochemical response with exercise, independent of fibre type.

Role of satellite cells in altering myosin expression during avian skeletal muscle hypertrophy

This study examined whether satellite cells express an embryonic isoform of myosin upon fusion with hypertrophying muscle fibers. Anterior latissimus dorsi (ALD) muscle hypertrophy was induced in adult chickens by weighting one wing. One and 7 days of wing-weighting produced significant increases in ALD muscle wet weight and in the number of mature fibers expressing ventricular-like embryonic (V-EMB) myosin. V-EMB myosin expression could be an event during regeneration of fibers injured by overload or part of the hypertrophy process itself. Although there was an increase in both the number of damaged fibers and the number of mature fibers expressing embryonic myosin after wing-weighting, results from this study suggest that these two events were not necessarily related. The apparent health of fibers expressing V-EMB myosin and the lack of correlation between the numbers of damaged and V-EMB myosin positive fibers (r = 0.20) suggest that embryonic myosin expression in mature fibers was likely a feature of the hypertrophy process itself. The appearance of V-EMB myosin in mature fibers 1 day after wing-weighting suggests that the change in myosin expression did not involve satellite cells since 24 hr is too short a time to permit more than limited satellite cell fusion. The relationship between satellite cells and embryonic myosin expression was examined more closely by labeling dividing satellite cells and their progeny with 5-bromo-2-deoxyuridine, and then colocalizing labeled myofiber nuclei and embryonic myosin in consecutive transverse sections of hypertrophied ALD muscle. One week of wing-weighting resulted in marked increases in myofiber nuclear labeling index and myofiber nuclear density compared to contralateral control. V-EMB myosin was not expressed uniformly throughout individual fibers, but rather in discrete regions of varying length. Many V-EMB myosin positive regions had a higher labeled nuclear density than V-EMB myosin negative regions indicating that V-EMB myosin expression was associated with an accumulation of satellite cell progeny in a restricted area. However, it was also clear that satellite cell progeny were not the sole source of V-EMB myosin since labeled nuclei were completely absent from 41% of the V-EMB positive regions. Furthermore, the presence of new nuclei did not result in obligatory expression of embryonic myosin because many V-EMB negative regions had a high labeled nuclear density. Thus, recently incorporated nuclei arising by satellite cell division are implicated as one, but not the sole source of embryonic myosin in hypertrophying muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

Enzyme activity analyses along ragged-red and normal single muscle fibres

Mitochondrial myopathies are morphologically characterized by ragged-red fibres (RRF). Serial cross-section revealed that the ragged-red appearance was only focal. This is in agreement with a partial cytochrome c oxidase (COX) deficiency in chronic progressive external ophthalmoplegia (CPEO). Since most of these patients show deletions of the mitochondrial genome single fibre analyses were performed determining COX and succinate dehydrogenase (SDH) in serial muscle sections from two patients with CPEO. High SDH activity was demonstrated in RRF; in contrast COX activity was lower in RRF in a patient, possibly representing a focal assembly of mitochondria with deletions in their genomes. The variation of enzyme activities along the muscle fibre was especially high in RRF. This study presents the first quantitative evidence that enzyme activities vary considerably along fibres in muscle from patients with a mitochondrial myopathy.

Factors causing difference in force output among motor units in the rat medial gastrocnemius muscle

1. The isometric contractile properties and morphological characteristics of the muscle unit portion of motor units were investigated in the medial gastrocnemius (MG) muscle of Fischer 344 rats. Individual motor units were functionally isolated by stimulating single MG axons in finely dissected ventral root filaments. 2. To study the mechanical properties of the motor units in the rat MG muscle, ninety-six motor units in five animals were classified into three categories (type FF, FR and S units) using two physiological criteria: presence or absence of the 'sag' property and fatigability. The relative distribution of the different motor unit types in the sample was 35.4% for type FF, 47.9% for type FR, and 16.7% for type S units. 3. There was little overlap in the distribution of twitch contraction time between type F (including types FF and FR) and type S units. The mean value was 17.1 ms for type FF, 15.7 ms for type FR, and 28.0 ms for type S units. Type FF units produced the largest tetanic tension (mean +/- S.D.; 201 +/- 75 mN). Tension output of type S units was the smallest (15 +/- 6 mN), and that of type FR units was intermediate (100 +/- 45 mN). These values were significantly different. 4. A muscle unit portion of twenty-three motor units (8 FF, 6 FR, and 9 S units) was depleted of its glycogen through repetitive stimulation after characterization of its mechanical properties. Cross-sectional areas of units fibres and innervation ratio were directly measured in sections stained for glycogen using a periodic and acid-Schiff (PAS) reaction. Specific tension of unit fibres was calculated by dividing the maximum tetanic tension of a unit by its total fibre area. 5. The number of unit fibres ranged from 44 to 77 for type S, 116 to 198 for type FR, and 221 to 356 for type FF units, and differences among their means (66, 154 and 271, respectively) were significant. Tetanic tension was correlated with innervation ratio for all of the twenty-three units, or units within a particular type. 6. Mean fibre area for type S units (1983 microns2) was significantly smaller than that for type FF units (3489 microns2). Fibres belonging to type FR units had an intermediate size (2648 microns2). Correlation between tetanic tension and fibre area was significant for either all units or units within a particular type. 7. Total cross-sectional area was significantly different among the motor unit types, and was highly correlated to the maximum tetanic tension.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolic variability within individual fibres of the cat tibialis posterior and diaphragm muscles

Variance in succinate dehydrogenase activity along the transverse and longitudinal axes of fibres from the cat tibialis posterior and diaphragm muscles was determined in order to estimate the three-dimensional distribution of mitochondria within single fibres. The variance (coefficient of variation) in succinate dehydrogenase activity along the transverse fibre axis was greatest in type IIB fibres from both muscles. Intracellular compartmentalization (i.e. subsarcolemmal vs central core differences in succinate dehydrogenase activity) was observed only in type II fibres from the tibialis posterior; the succinate dehydrogenase activity of the subsarcolemmal region was significantly greater than that of the central core. The extent of succinate dehydrogenase variance along the longitudinal fibre axis was dependent on the total length of the fibre segment analyzed, the muscle, and fibre type. The coefficient variation for short fibre segments, i.e. 40 microns, was significantly lower than that for much longer fibre segments (840 microns). Significant differences in the coefficient variation for 840 microns fibre segments were observed between the diaphragm and tibialis posterior muscles. The longitudinal variance in succinate dehydrogenase activity was higher in diaphragm muscle fibres. The succinate dehydrogenase variance along the longitudinal axis of type II fibres was higher in diaphragm muscle fibres. The succinate dehydrogenase variance along the longitudinal axis of type II fibres was significantly greater than that of the type I fibre population. These results indicate that mitochondria are heterogeneously distributed within muscle fibres. Possible functional implications of such intrafibre metabolic variance are discussed.

A luminometric method for the determination of ATP and phosphocreatine in single human skeletal muscle fibres

A sensitive method for the analysis of ATP and phosphocreatine (PCr) in single human skeletal muscle fibres is described. Muscle tissue was freeze-dried and single fibres were dissected free with the aid of low-power microscopy. The fibres were then extracted in trichloroacetic acid and neutralized with KHCO3. The assay is based on the continuous monitoring of light produced as a result of ATP degradation in the firefly luciferase reaction. PCr is measured as the amount of ATP formed in the creatine kinase reaction. The coefficient of variation was less than 4% for both ATP and PCr determination. The amount of tissue required for the assay is approximately 0.5 microgram (dry weight). The assay showed good agreement with spectrophotometric and high-performance liquid chromatographic (HPLC) measurements made upon extracts of whole muscle tissue.

Metabolic and contractile uniformity of isolated motor unit fibres of snake muscle

1. Motor units in the thin transversus abdominis muscle of the garter snake were identified and physiologically characterized in the living state. Motor unit fibres, and fibres chosen randomly to serve as controls, were subsequently excised and subjected to biochemical analyses. 2. The metabolic capacity of fibres was assessed by measuring activities of three enzymes, each representing a different metabolic pathway. The microchemical enzyme assays were performed using enzyme extraction preparations of whole single fibres. 3. Metabolic capacity ranged widely among the muscle's entire fibre population, even among fibres of the same type. In contrast, enzyme activities of twitch fibres belonging to individual motor units were, within analytical error, identical. 4. Twitch contraction times of individual fibres within one motor unit were similar, compared to a wide range of contraction times observed among fibres of the same type but belonging to different motor units. 5. When several motor units were studied in one muscle, a systematic relationship was observed among motor unit tension, enzymatic profile and contraction time. As motor unit tension increased, fibres exhibited greater capacities for glycolytic and high-energy phosphate metabolism, diminished capacity for oxidative metabolism, and faster twitch contraction times. 6. Given the great diversity of metabolic and contractile properties exhibited within the fibre population, the uniformity of such properties within motor units indicates that neural influence dominates over other extrinsic factors present in the microenvironment of the muscle fibres.

Cellular concentrations of enzymes and their substrates

The activity of crude and pure enzyme preparations as well as the molecular weight of these enzymes were obtained from the literature for several organisms. From these data enzyme concentrations were calculated and compared to the concentration(s) of their substrates in the same organism. The data are expressed as molar ratios of metabolite concentration to enzyme site concentration. Of the 140 ratios calculated, 88% were one or greater, indicating that in general substrates exceed their cognate enzyme concentrations. Of the 17 cases where enzyme exceeds metabolite concentration, 16 were in glycolysis. The data in general justify the use of enzyme kinetic mechanisms determined in vitro in the construction of dynamic models which simulate in vivo metabolism.

Coordination of electromechanical and metabolic properties of cat soleus motor units

Motor units were studied in the soleus muscle of normal adult cats and adult cats that had undergone complete spinal cord transection approximately 4 mo earlier. Intracellular recording and stimulation techniques were used to study selected electrical properties of the motoneuron and isometric contractile properties of the muscle unit. Motor unit fibers were depleted of their glycogen through repetitive stimulation of the motoneuron and identified by a quantitative histochemical determination of glycogen. A sample of muscle fibers from the glycogen-depleted unit and from fibers not depleted of glycogen were analyzed for cross-sectional area, succinate dehydrogenase (SDH), alpha-glycerolphosphate dehydrogenase (GPD), and alkaline myofibrillar adenosine triphosphatase. It was observed that the fiber-to-fiber variability in cross-sectional area and SDH and GPD activity within units of normal and transected cats was significantly larger than that measured in repeated samples from a single fiber. Additionally, for each of these properties, the range found among fibers within a unit was similar to that found among nondepleted fibers of the same myosin type. The influence of spinal cord transection on some muscle fibers seemed to result in a metabolic shift from the generalized category of slow-oxidative toward fast-oxidative glycolytic. This shift in metabolic properties appeared to be coupled with a similar shift in the physiological properties of the muscle unit and motoneuron from slow to fast.

Metabolic and fiber size properties of cat tibialis anterior motor units

The variability among single muscle fiber enzymatic activities and fiber size within a motor unit was studied in the cat tibialis anterior (TA) muscle. Fourteen units were isolated for physiological testing using standard ventral root filament stimulation techniques, and the muscle fibers of these units were identified by glycogen depletion. The cross-sectional areas, succinate dehydrogenase (SDH) and alpha-glycerolphosphate dehydrogenase (GPD) activities, and the relative alkaline myofibrillar adenosine triphosphate staining densities of a sample of glycogen-depleted and -nondepleted muscle fibers were determined using quantitative histochemical techniques. Each of the unit types previously identified to be present in the TA, based on physiological criteria, were represented by the sample population. The variability among the fibers of a unit was significantly more than the variability among repeated measures on a single fiber for cross-sectional area and SDH and GPD activities. The mean coefficients of variation for SDH and GPD activity within motor unit fibers were 29 and 56%, respectively, whereas the variability between fibers of different units within a muscle was significantly greater (53 and 69%, respectively). Additionally, the mean coefficient of variation for cross-sectional area among motor unit fibers was less than that among fibers not depleted of glycogen (25 vs. 46%). These data suggest that although there is clear evidence for some level of neural control of the properties of a muscle unit (variation within a unit was less than the variation across units), this control is not complete, since the variability among fibers of a single unit was significantly more than the variability found between repeated measurements on a single fiber.

Spatial heterogeneity of metabolism in skeletal muscle in vivo studied by 31P-NMR spectroscopy

Phase-modulated rotating-frame imaging, a localization technique for phosphorus nuclear magnetic resonance spectroscopy, has been applied to obtain information on heterogeneity of phosphorus-containing metabolites in skeletal muscle of the rat in vivo. The distal muscles of the rat hindlimb have been studied at rest and during steady-state isometric twitch contraction; the use of a transmitter surface coil and an electrically isolated, orthogonal receiver Helmholtz coil ensure accurate spatial assignment (1 mm resolution). At rest, intracellular pH was higher and PCr/(PCr + Pi) was lower in deeper muscles compared with superficial muscle of the distal hindlimb. Upon steady-state stimulation, the relatively more alkaline pH of deep muscle was maintained, whereas greater changes in PCr/(PCr + Pi) and Pi/ATP occurred in the superficial muscle layer. This method allows rapid (75 min for each spectral image) acquisition of quantitative information on metabolic heterogeneity in vivo.

Association between biochemical and physiological properties in single motor units

Motor units from the cat tibialis posterior muscle were examined for an association between physiological and biochemical properties. Functionally isolated motor units were categorized on the basis of their physiological properties. This was followed by quantitative microbiochemical analysis of single muscle fibers from each unit, identified in cross sections using the glycogen-depletion method. The activities of malate dehydrogenase and beta-hydroxyacyl-CoA dehydrogenase distinguished between fatigable (type FF) and fatigue-resistant (types FR and S) units. The activities of both lactate dehydrogenase and adenylokinase were higher in fast- than in slow-contracting units. Cluster analyses, based on both physiological and biochemical properties or on biochemical properties alone, produced groupings identical to types FF, FR, and S. The association between physiological and biochemical properties substantiates the idea that biochemically distinct groups of motor units correspond to physiologically identifiable groups.

Nonuniform myosin expression along single fibers of chronically stimulated and contralateral rabbit tibialis anterior muscles

Using a combined histochemical and biochemical technique, single fiber analyses were performed on chronically stimulated and contralateral tibialis anterior (TA) muscles of the rabbit. The major fiber population (60%) in 30 days stimulated TA was transforming fibers (type IIC). Some of these fibers displayed a nonuniform distribution of histochemically assessed myofibrillar actomyosin ATPase (mATPase) activity. This heterogeneity of mATPase activity along the fibers was verified in longitudinal sections and by microphotometric evaluation of mATPase staining intensities in serial cross-sections. Biochemical analyses of single fiber segments revealed that these C fibers not only coexpressed fast- and slow-myosin subunits but did so nonuniformly along their length. The distribution of fast- and slow-myosin subunits in these fibers was not random but focal. Variations in myosin expression were also observed in some of the C fibers in the contralateral TA. As opposed to the transforming fibers in the stimulated TA, heterogeneities of mATPase activity and myosin subunits in these contralateral C fibers were less focal and more gradual. These findings suggest that muscle fibers in chronically stimulated TA and the contralateral muscle do not transform synchronously or uniformly along their length.

The time course of glycogen depletion in single fibers of chronically stimulated rabbit fast-twitch muscle

A time course study was conducted to investigate the possibility of a relationship between fiber degeneration and glycogen depletion in chronically nerve-stimulated extensor digitorum longus muscle of the rabbit. Muscles were stimulated 12 h daily at 10 Hz using alternating one-hour periods of stimulation and rest. When measured for the first time after 3 h (1 h stimulation, 1 h rest, 1 h stimulation), microphotometry revealed complete glycogen depletion of all fiber types (fast glycolytic, FG; fast oxidative glycolytic, FOG; slow oxidative, SO). Different responses were noted beginning at day 4. At this time point, all FOG and SO fibers recovered their glycogen stores with some of the FOG population attaining levels higher than the FOG fibers in the unstimulated, contralateral muscle. Approximately 28% of the FG fibers recovered to normal glycogen values, whereas 58% remained depleted and 14% displayed "overshooting glycogen" levels. Fifteen percent of all fibers were glycogen-depleted after 12 days of stimulation. At this time, classic fiber types could no longer be distinguished. Fiber degeneration, which was recognized by the invasion of nonmuscle cells, began after 6 days and was restricted to the glycogen-depleted fibers. By this time, there was also a significant increase in DNA content. Exhaustion of glycogen, the main fuel of the FG fibers, is believed to cause a collapse of energy-supply and ATP-driven ionic pumps. The latter could be the initial step of fiber deterioration.

Control of enzyme activities in individual myotubes cultured without nerve

The activities of lactate dehydrogenase, malate dehydrogenase, phosphorylase, and adenylate kinase were measured in single myotubes dissected from primary cultures of rat skeletal muscle. For a given enzyme, activities among the spontaneously contracting cells varied as much as eightfold. When the myotubes were paralyzed with tetrodotoxin, the variability in enzyme levels was markedly decreased. These and other findings suggest that differences in enzyme levels among individual myotubes may arise as a result of differences in their pattern of contractile activity.