Actomyosin ATPase. I. Quantitative measurement of activity in cryostat sections

Post-exercise protein synthesis rates are only marginally higher in type I compared with type II muscle fibres following resistance-type exercise

We examined the effect of an acute bout of resistance exercise on fractional muscle protein synthesis rates in human type I and type II muscle fibres. After a standardised breakfast (31 ± 1 kJ kg⁻¹ body weight, consisting of 52 Energy% (En%) carbohydrate, 34 En% protein and 14 En% fat), 9 untrained men completed a lower-limb resistance exercise bout (8 sets of 10 repetitions leg press and leg extension at 70% 1RM). A primed, continuous infusion of l-[ring-¹³C₆]phenylalanine was combined with muscle biopsies collected from both legs immediately after exercise and after 6 h of post-exercise recovery. Single muscle fibres were dissected from freeze-dried biopsies and stained for ATPase activity with pre-incubation at a pH of 4.3. Type I and II fibres were separated under a light microscope and analysed for protein-bound l-[ring-¹³C₆]phenylalanine labelling. Baseline (post-exercise) l-[ring-¹³C₆]phenylalanine muscle tissue labelling, expressed as (∂¹³C/¹²C), averaged −32.09 ± 0.28, −32.53 ± 0.10 and −32.02 ± 0.16 in the type I and II muscle fibres and mixed muscle, respectively (P = 0.14). During post-exercise recovery, muscle protein synthesis rates were marginally (8 ± 2%) higher in the type I than type II muscle fibres, at 0.100 ± 0.005 versus 0.094 ± 0.005%/h, respectively (P < 0.05), whereby rates of mixed muscle protein were 0.091 ± 0.005%/h. Muscle protein synthesis rates following resistance-type exercise are only marginally higher in type I compared with type II muscle fibres.

Functional consequences of motor unit recruitment order reversals following spinal cord transection in cat

Motoneuron recruitment order determinations were made for acute, 2-week chronic, and 3-month chronic spinal cats by comparing cutaneous nerve stimulation thresholds for evoking single unit tibialis anterior (TA) electromyogram (EMG) spikes of different sizes. Recruitment order was largely ( approximately 80%) orderly (small spikes recruited at lower stimulus intensities than large spikes) in acute and 3-month chronic spinal animals. However, in 2-week chronic spinal animals recruitment order was reversed, with large units more often recruited at lower stimulus intensities than small units ( approximately 65%). Morphological analyses of TA muscle fibers suggested that fiber size changes were unlikely to account for the dramatic alterations in recruitment order results of the 2-week chronic spinal animals. Additional studies suggested that the recruitment order reversal in the 2-week chronic animals coincided with an enhanced reflex neural output (increased recruitment or reflex gain) for the flexion reflex which compensated for disuse atrophy related decreases in flexor muscle force generation capability in these animals. The data from 2-week chronic spinal animals represent a functionally significant example of deviation from the normal size principle of motoneuron recruitment order as the corresponding reflex gain increases can enhance the rapidity of motor function recovery (standing, locomotion) following spinal injury.

Hypoxaemia enhances peripheral muscle oxidative stress in chronic obstructive pulmonary disease

BACKGROUND

Because oxidative stress affects muscle function, the underlying mechanism to explain exercise induced peripheral muscle oxidative stress in patients with chronic obstructive pulmonary disease (COPD) is clinically relevant. This study investigated whether chronic hypoxaemia in COPD worsens peripheral muscle oxidative stress and whether an abnormal muscle inflammatory process is associated with it.

METHODS

Nine chronically hypoxaemic and nine non-hypoxaemic patients performed repeated knee extensions until exhaustion. Biopsy specimens were taken from the vastus lateralis muscle before and 48 hours after exercise. Muscle oxidative stress was evaluated by lipid peroxidation (lipofuscin and thiobarbituric acid reactive substances (TBARs)) and oxidised proteins. Inflammation was evaluated by quantifying muscle neutrophil and tumour necrosis factor (TNF)-alpha levels.

RESULTS

When both groups were taken together, arterial oxygen pressure was positively correlated with quadriceps endurance time (n = 18, r = 0.57; p < 0.05). At rest, quadriceps lipofuscin inclusions were significantly greater in hypoxaemic patients than in non-hypoxaemic patients (2.9 (0.2) v 2.0 (0.3) inclusions/fibre; p < 0.05). Exercise induced a greater increase in muscle TBARs and oxidised proteins in hypoxaemic patients than in non-hypoxaemic patients (40.6 (9.1)% v 10.1 (5.8)% and 51.2 (11.9)% v 3.7 (12.2)%, respectively, both p = 0.01). Neutrophil levels were significantly higher in hypoxaemic patients than in non-hypoxaemic patients (53.1 (11.6) v 21.5 (11.2) counts per fibre x 10(-3); p < 0.05). Exercise did not alter muscle neutrophil levels in either group. Muscle TNF-alpha was not detected at baseline or after exercise.

CONCLUSION

Chronic hypoxaemia was associated with lower quadriceps endurance time and worsened muscle oxidative stress at rest and after exercise. Increased muscle neutrophil levels could be a source of the increased baseline oxidative damage. The involvement of a muscle inflammatory process in the exercise induced oxidative stress of patients with COPD remains to be shown.

Exercise-induced quadriceps oxidative stress and peripheral muscle dysfunction in patients with chronic obstructive pulmonary disease

Exercise-induced muscle oxidative stress may be involved in the myopathy associated with chronic obstructive pulmonary disease (COPD). This study was designed to look at whether local exercise induces muscle oxidative stress and whether this oxidative stress may be associated with the reduced muscle endurance in patients with COPD. Quadriceps endurance was measured in 12 patients with COPD (FEV1 = 0.96 +/- 0.14 SEM) and 10 healthy sedentary subjects by repeated knee extensions of the dominant leg. Biopsies of the vastus lateralis muscle were obtained before and 48 hours after exercise. Muscle oxidative stress was measured by lipid peroxidation and oxidized proteins. Muscle antioxidant was evaluated by peroxidase glutathion activity. Quadriceps endurance was significantly reduced in patients with COPD when compared with the healthy control subjects (p < 0.01). Forty-eight hours postexercise, only patients with COPD had a significant increase in muscle lipid peroxidation (p < 0.05) and oxidized proteins (p < 0.05), whereas increased peroxidase glutathion activity was only observed in control subjects (p < 0.05). Both increases in muscle lipid peroxidation and oxidized proteins were significantly and inversely correlated with quadriceps endurance capacity in COPD (p < 0.05). In summary, local exercise induced muscle oxidative stress in patients with COPD, whereas it failed to raise antioxidant activity. In these individuals, muscle oxidative stress was associated with a reduced quadriceps endurance.

Lipofuscin accumulation in the vastus lateralis muscle in patients with chronic obstructive pulmonary disease

Exercise-induced oxidative stress has been reported in patients with chronic obstructive pulmonary disease (COPD) and may play a role in muscle fatigue. It is speculated that oxidative stress during exercise originates from the contracting muscles but this has not been documented. The accumulation of lipofuscin, a marker of cellular oxidative damage, was evaluated in the vastus lateralis muscle in 17 patients with COPD and 10 healthy subjects of similar age. Each subject performed a stepwise exercise test up to maximal capacity during which oxygen uptake (VO(2)) was measured. Resting and peak exercise blood gases were also obtained. Two indices of lipofuscin accumulation were used: lipofuscin inclusions/fiber ratio (LI/F) and lipofuscin inclusions/fiber cross-sectional area ratio (LI/CSA). These ratios were also determined for each specific fiber-type. LI/F (P < 0.01) and LI/CSA (P < 0.01) were greater in COPD compared to healthy subjects. LI/F and LI/CSA for all fiber types were also greater in COPD (P < 0.001). In both groups, LI/F (P < 0.001) and LI/CSA (P < 0.01) were higher in type I than in type II fibers. LI/F and LI/CSA did not correlate significantly with resting PaO(2) and SaO(2), peak VO(2), and DeltaPaO(2) and DeltaSaO(2) during exercise (P > 0.05). Increased lipofuscin accumulation, a marker of oxidative damage, was found in the vastus lateralis muscle in patients with COPD compared to healthy subjects. Oxidative damage of muscle tissue may thus be involved in skeletal muscle dysfunction and wasting in COPD.

Human skeletal muscle fibres: molecular and functional diversity

Contractile and energetic properties of human skeletal muscle have been studied for many years in vivo in the body. It has been, however, difficult to identify the specific role of muscle fibres in modulating muscle performance. Recently it has become possible to dissect short segments of single human muscle fibres from biopsy samples and make them work in nearly physiologic conditions in vitro. At the same time, the development of molecular biology has provided a wealth of information on muscle proteins and their genes and new techniques have allowed analysis of the protein isoform composition of the same fibre segments used for functional studies. In this way the histological identification of three main human muscle fibre types (I, IIA and IIX, previously called IIB) has been followed by a precise description of molecular composition and functional and biochemical properties. It has become apparent that the expression of different protein isoforms and therefore the existence of distinct muscle fibre phenotypes is one of the main determinants of the muscle performance in vivo. The present review will first describe the mechanisms through which molecular diversity is generated and how fibre types can be identified on the basis of structural and functional characteristics. Then the molecular and functional diversity will be examined with regard to (1) the myofibrillar apparatus; (2) the sarcolemma and the sarcoplasmic reticulum; and (3) the metabolic systems devoted to producing ATP. The last section of the review will discuss the advantage that fibre diversity can offer in optimizing muscle contractile performance.

Histochemical and morphological characteristics of the vastus lateralis muscle in patients with chronic obstructive pulmonary disease

PURPOSE AND METHODS

In this study, we examined the fiber-type proportions, cross-sectional areas (CSA), and capillarization from needle biopsies of the vastus lateralis muscle in 20 patients with chronic obstructive pulmonary disease (COPD) (FEV1 = 37 +/- 11% predicted, peak VO2 = 13 +/- 4 mL.min-1.kg-1) and nine age-matched normal subjects (peak VO2 = 33 +/- 7 mL.min-1.kg-1). The effects of endurance training on these parameters were also evaluated in 11 of the 20 patients with COPD.

RESULTS

The proportion of Type I fiber was smaller in COPD than normals (34 +/- 14% vs 58 +/- 16 in normals, P < 0.0005) with a corresponding increase in Type IIb fiber (P = 0.015). The CSA of Type I, IIa, and IIab fibers was also smaller in COPD. The capillary to fiber ratio tended to be reduced in patients, but this difference did not reach statistical significance (P = 0.15). The number of capillary contact for Type I, IIa, and IIab fibers was significantly reduced in COPD compared with normal subjects (P < 0.05). When corrected for the CSA, this parameter was similar for both groups. After training, peak VO2 increased by 11% (P < 0.05), the fiber-type proportion remained unchanged, and the CSA of Type I and IIa fibers increased by 31 and 21%, respectively (P < 0.05). Although the number of capillary contact for each fiber types increased with training, the capillary to fiber ratio and the number of capillary contact for the different fiber types relative to their CSA remain unchanged.

CONCLUSIONS

We conclude that in COPD, 1) the vastus lateralis muscle is characterized by a marked decrease in Type I fiber proportion, an increase in Type IIb fiber proportion, a decrease in Type I, IIa, and IIab fiber CSA and by a relatively preserved capillarization; and 2) a 12-wk training program induces a significant increase in Type I and IIa CSA.

Patterns of myosin isoforms in mammalian skeletal muscle fibres

The present article attempts to combine existing information on the distribution of fast and slow myosin isoforms in histochemically distinct muscle fibres. Four myosin heavy chain (MHC) isoforms, MHCI, MHCIIa, MHCIIb, and MHCIId(x), have been identified in small mammals and have been assigned to the histochemically defined fibre types I, IIA, IIB, and IID(X), respectively. These fibres express only one MHC isoform and are called pure fibre types. Hybrid fibres expressing two MHC isoforms are regarded as transitory between respective pure fibre types. The existence of pure and hybrid fibres even in normal muscles under steady state conditions creates a spectrum of fibre types. The multiplicity of fibre types is even greater when myosin light chains are taken into account. A large number of isomyosins results from the combinatorial patterns of various myosin light and heavy chains isoforms, further increasing the diversity of muscle fibres. As shown by comparative studies, the distribution of different fibre types varies in a muscle-specific, as well as a species-specific manner.

Physiologic, metabolic, and muscle fiber type characteristics of musculus uvulae in sleep apnea hypopnea syndrome and in snorers

Upper airway dilator muscles play an important role in the pathophysiology of sleep apnea hypopnea syndrome (SAHS). The mechanical and structural characteristics of these muscles remain unknown. The aim of this study was to compare the physiologic, metabolic, and fiber type characteristics of one upper airway dilator muscle (musculus uvulae, MU) in 11 SAHS and in seven nonapneic snorers. The different analyses were done on MU obtained during uvulo-palato-pharyngoplasty. Snorers and SAHS differed only in their apnea + hypopnea indices (11.5 +/- 5.9 and 34.2 +/- 14.6/h, respectively, mean +/- SD). Absolute twitch and tetanic tension production of MU was significantly greater in SAHS than in snorers while the fatigability index was similar in the two groups. Protein content and anaerobic enzyme activities of MU were significantly greater in SAHS than in snorers; no difference was observed for aerobic enzyme activities. The total muscle fiber cross-sectional area of MU was significantly higher in SAHS (2.2 +/- 0.9 mm2) than in snorers (1.1 +/- 0.7 mm2). The surface occupied by type IIA muscle fibers of MU was larger in SAHS (2.00 +/- 0.96) than in snorers (0.84 +/- 0.63 mm2). We conclude that the capacity for tension production and the anaerobic metabolic activity of MU are greater in SAHS than in snorers.

Myofibrillar ATPase activity during isometric contraction and isomyosin composition in rat single skinned muscle fibres

1. Myofibrillar ATPase activity, isometric tension (Po) and unloaded shortening velocity (Vo) were determined in single skinned fibres isolated from rat hindlimb muscles during maximal calcium activation at 12 degrees C. In each fibre, myosin heavy chain (MHC) isoforms were identified using electrophoresis and immunocytochemistry. ATPase activity was determined spectrophotometrically from NADH oxidation in a coupled enzyme assay. 2. On the basis of their MHC isoform composition, the fibres (n = 102) were divided into five groups containing the slow isoform, I MHC, or one of the fast isoforms, IIB MHC, IIA MHC, IIX MHC, or a mixture of the latter three. ATPase activity was significantly higher in IIB than in 2X and IIA fibres (0.230 +/- 0.010, 0.178 +/- 0.023 and 0.168 +/- 0.026 nmol mm-3 s-1, respectively). Mixed fibres had intermediate values. ATPase activity in slow fibres was considerably less (0.045 +/- 0.006 nmol mm-3 s-1). 3. The ratio between ATPase activity and Po, i.e. tension cost, was found to be 2.90 +/- 0.09, 2.56 +/- 0.14, 1.89 +/- 0.22, 1.52 +/- 0.13 and 0.66 +/- 0.004 pmol ATP nM-1 mm-1 s-1 in IIB, mixed, IIX, IIA and slow fibres, respectively. All the differences were statistically significant except that between IIA and IIX fibres. 4. Within each group of fibres with the same MHC composition, ATPase activity was found to correlate with Po, but not Vo. However, ATPase activity was found to correlate with Vo when all the fibre types were pooled together. 5. In thirty-seven fast fibres the MLC ratio, i.e. the proportion of the fast alkali light chain isoform, MLC3f, to the amount of the regulatory light chain, MLC2f, was determined. IIB fibres had the highest proportion of MLC3f and IIA fibres, the lowest. 6. A multiple regression analysis, used to distinguish between the effects of MHC and MLC composition, showed that ATPase activity was insensitive to the MLC ratio, whereas it had a significant impact on Vo. 7. The results obtained in this study indicate that in rat skeletal muscle fibres: (a) ATPase activity during isometric contractions and tension cost are strongly dependent on MHC isoform composition, and (b) there is no evidence that the alkali MLC ratio is a determinant of ATPase activity.

A physiological basis for variation in the contractile properties of isolated rat heart

1. The maximum Ca(2+)-activated force, maximum velocity of unloaded shortening and both Ca(2+)- and actin-activated ATPase activities of myosin have been measured in detergent-skinned preparations of isolated bundles of rat right ventricle after exposure of the intact tissue to different conditions of superfusion, mechanical activity and temperature. 2. Maximum Ca(2+)-activated force per unit cross-sectional area decreases with increasing cross-sectional area, and, in the absence of electrical stimulation, with the duration of superfusion. Maximum velocity of unloaded shortening is not influenced by these differences. 3. Actin-activated ATPase activity of myosin decreases as cross-sectional area increases and duration of superfusion increases, but the extent of the decrease in enzymatic activity is less than that of developed force. Ca(2+)-activated ATPase activity is independent of these differences. 4. Actin-activated ATPase activity in cryostatic sections of quickly frozen tissue is not uniform across the transverse section. In thin bundles, it is highest in the centre and lowest at the edge of the section, which correspond, respectively, to the centre and the surface of the tissue bundle. Exposure of the tissue section to 1 microM-cyclic AMP increases the actin-activated ATPase activity of myosin with the largest increase in activity occurring at or near the surface of the bundle. 5. Ca(2+)-activated ATPase activity of myosin is uniform across the transverse section and is not changed by cyclic AMP. 6. Electrical stimulation, elevated Ca2+ concentration in the superfusion medium, or isoprenaline partially or completely reverse the decline in maximum Ca(2+)-activated force produced by prolonged superfusion of the bundle before its skinning. 7. These observations are similar in many ways to those made on frog skeletal muscles by Elzinga, Howarth, Rull, Wilson & Woledge (1989a). An explanation based on the existence of a physiological mechanism for regulating the properties of force generators is proposed. Regulation of the attachment of the cross-bridge to an actin filament may be the basis for the regulatory mechanism.

Activity dependent characteristics of fast and slow muscle: biochemical and histochemical considerations

The effects of denervation and hindlimb suspension induced disuse on concentrations of ATP, phosphocreatine (PC), and fiber type profile were investigated in slow twitch soleus and fast twitch extensor digitorum longus (EDL) muscles. The results show that the soleus and EDL muscles differ in their dependency on loadbearing as a stimulus for maintaining normal energy metabolism and the biochemical and morphological characteristics of muscle fibers. As determined by R-P methodology, suspension reduced ATP and PC concentrations of the soleus to 26% and 56%, respectively, while, in EDL only, PC is reduced to 71% of control with no change in ATP. Both muscles, however, show identical losses in ATP and PC following denervation. The energy charge, an indicator of Pi availability in muscle was reduced significantly in both denervated muscle to 82% and 85% in soleus and EDL, respectively. No significant reduction of the energy charge was seen in the muscles from suspended rats. Thus, in parallel with the indirect regulation through muscle loadbearing, the nerve can effectively modulate the levels of high-energy phosphates more directly by some regulatory mechanisms independent of muscle type. Denervation and suspension disuse increased the proportion of type 2 fibers in the soleus with a concomitant decrease in type 1 fibers and a relative rise in the number of very small diameter fibers. The EDL showed only variation in fiber size.

[Screening tests for malignant hyperthermia susceptibility]

The ideal screening test for malignant hyperthermia susceptibility (MHS) has yet to be discovered. It should be simple noninvasive, yet totally specific and sensitive. Until such an ideal test becomes available, allowing simple routine preoperative screening, tests should only be used in certain specific situations. These include: patients in whom a clinical crisis was suspected; the members of the family of a subject labeled MHS because of a fatal, or otherwise, crisis, or in whom tests were positive; patients with other pathological conditions which could be linked to malignant hyperthermia (MH) (some myopathies, effort or stress MH, neuroleptic malignant syndrome). The various tests proposed in the literature aim at revealing MHN subjects, using or not a triggering agent, halothane most often. However, detecting these abnormalities sometimes gives greater insight into the physiopathology of MH than in the detection of an individual patient's susceptibility. The tests have been classified as in vivo, electrophysiological, blood, and in vitro muscle biochemical, morphological, and pharmacological tests. The discovery of new tests gives renewed hope: CPK levels, platelet tests, calcium sarcoplasmic reticular reuptake, lymphocyte Quin 2 test, nuclear magnetic resonance spectroscopy. However, experts worldwide agree that the only reference test to this day remains the in vitro halothane caffeine contracture tests. These tests have shown their reliability; they must be performed on muscle strips obtained from surgically removed muscle biopsies, by laboratories used to this technique and who have at their disposal a sufficiently large group of MHS subjects with a clear-cut clinical crisis, as well as controls. The patients must therefore travel to these laboratories. The design of common protocols for European laboratories on one hand, and the North American laboratories on the other, is a good guarantee of the reliability of these tests.

Calcium-stimulated myofibrillar ATPase activity correlates with shortening velocity of muscle fibres in Xenopus laevis

The iliofibularis muscle of Xenopus laevis is reported to contain five types of fibres which have different force-velocity relationships. Ten fibres of each type were selected on the basis of succinate dehydrogenase activity, cross-sectional area and location in the muscle, in order to assess the validity of the fibre type classification. Maximum calcium-stimulated myofibrillar ATPase activity (Vmax) and apparent Michaelis constant (Km) for ATP were determined for these 50 fibres from serial sections. The values obtained varied according to the type of fibre. Type 1 had the highest and type 5 the lowest values for Km and Vmax. In a separate experiment, single freeze-dried fibres were used to determine the relationship between their ATP content and apparent Km for ATP. There was a tendency for high ATP concentrations in fibres with high Km values. When myofibrillar ATPase activity was related to the maximum velocity of shortening of the five fibre types, a significant correlation was found. It is concluded that calcium-stimulated myofibrillar ATPase histochemistry allows an estimate of the maximum shortening velocity of muscle fibres from Xenopus laevis.

Adrenergic regulation of myosin adenosine triphosphatase activity

The amount of inorganic phosphate liberated by the adenosine triphosphatase activity of myosin in a thin section of cardiac tissue can be measured quantitatively by precipitation with calcium in an alkaline medium under a defined set of conditions. Specificity of the procedure for myosin adenosine triphosphatase has been confirmed by the response to inhibitors and to different degrees of contractile filament overlap. Precise quantitation of adenosine triphosphatase activity has been demonstrated by (1) constant rate over time, (2) linearity with amount of enzyme, (3) correct values for the Km of adenosine triphosphate, and (4) a similar value for Vmax to those determined by more traditional procedures. Stimulation of the beta-adrenergic system by the release of catecholamines following injection of the animal with 6-hydroxydopamine causes a rise and then a fall of both calcium- and actin-activated adenosine triphosphatase in parallel with the changes in blood levels of the transmitter. Tyramine injection of rats produces a dose related increase in myosin adenosine triphosphatase. Perfusion of isolated hearts with isoproterenol increases myosin adenosine triphosphatase in dose-related manner. Addition of cyclic adenosine monophosphate and phosphodiesterase inhibitor to the solution bathing frozen, dried sections of heart increases both calcium- and actin-activated adenosine triphosphatase activity by almost 150%. The data show that the beta-adrenergic system, through cyclic adenosine monophosphatate, regulates the enzymatic activity of myosin, independent of the concentration of calcium. The possible role of this regulatory mechanism in the physiological modulation of cardiac contractility is discussed.

Changes in the cholinergic system of rat sciatic nerve and skeletal muscle following suspension-induced disuse

Muscle disuse-induced changes in the cholinergic system of sciatic nerve, slow-twitch soleus (SOL), and fast-twitch extensor digitorum longus (EDL) muscles were studied in rats. Rats with hind limbs suspended for 2 to 3 weeks showed marked elevation in the activity of choline acetyltransferase in sciatic nerve (38%), in the SOL (108%), and in the EDL (67%). Acetylcholinesterase (AChE) activity in the SOL increased 163% without changing the molecular forms pattern of 4S, 10S, 12S, and 16S. No significant (P greater than 0.05) changes in the activity and molecular forms pattern of AChE were seen in the EDL or in AChE activity of sciatic nerve. Nicotinic receptor binding of [3H]acetylcholine was increased in both muscles. When measured after 3 weeks of hind limb suspension the normal distribution of type I fibers in the SOL (87%) was reduced (to 58%) and a corresponding increase in types IIa and IIb fibers occurred. In the EDL no significant change in fiber proportion was observed. Muscle activity, such as loadbearing, appeared to have a greater controlling influence on the characteristics of the slow-twitch SOL muscle than on the fast-twitch EDL muscle.

Quantitative histochemistry of three mouse hind-limb muscles: the relationship between calcium-stimulated myofibrillar ATPase and succinate dehydrogenase activities

A quantitative modification of Meijer 's calcium-lead capture method, for the demonstration of calcium-stimulated myofibrillar ATPase activity at physiological pH, is described. A range of myofibrillar ATPase activities has been found among fast muscle fibres in two mouse hind-limb muscles. The myofibrillar ATPase activity of fast muscle fibres is 1.5-3 times higher than the myofibrillar ATPase activity of slow muscle fibres. Myofibrillar ATPase activities and succinate dehydrogenase activities of individual muscle fibres have been determined in serial sections. Activities of the two enzymes are correlated positively in soleus (fast and slow fibres), and negatively in plantaris (almost all fast) and extensor digitorum longus muscle (all fast). However, this correlation is not significant among the oxidative fibres in the extensor digitorum longus. The fibres of the latter muscle cannot be classified satisfactorily into two sub-types.

Myocardial cell heterogeneity in the human heart with respect to myosin ATPase activity

The PH sensitivity of the Ca2+-activated myosin ATPase in atrial, ventricular and conduction tissue of human hearts has been established. Heterogeneity with respect to ATPase activity is shown not only to exist between the atrial, the ventricular myocardium and the conduction system but also within both the ordinary atrial and ventricular myocardium and within the conduction system. These observations are related to the polymorphism of the myosin molecule and suggest that fibre types with different contractile properties co-exist in the human heart.

Actomyosin ATPase. II. Fiber typing by histochemical ATPase reaction

A new method for fiber typing based on staining for actomyosin Ca,Mg-ATPase is presented. Inclusion of ethanol in the medium enhanced the differentiation of type 2A and 2B fibers. With this technique, type 1, 2A, and 2B fibers can be distinguished in a single-step procedure for human biopsy samples.