Acetazolamide-sensitive and resistant carbonic anhydrase activity in rat and rabbit skeletal muscles of different fiber type composition

1. Acetazolamide (ACET)-resistant and -sensitive carbonic anhydrase (CA) activity was measured in post-mitochondrial supernatants from the soleus (SOL), deep vastus lateralis (DVL) and superficial vastus lateralis (SVL) muscles of rats and rabbits. 2. The relative total CA activity in the three muscles of both species can be summarized as SOL greater than DVL greater than SVL. 3. ACET-resistant CA activity was found in the SOL and DVL muscles of both species whereas a low level of ACET-sensitive CA activity was detected in the SVL muscle. 4. ACET-sensitive CA activity was also found in sarcoplasmic reticulum preparations from rat and rabbit SOL muscles.     

Fiber type changes in rat skeletal muscle after intense interval training

Female Sprague-Dawley rats were subjected to a ten week training program to determine the influence of intense interval running on the fiber type composition of selected hindlimb muscles; soleus (S), plantaris (P), deep vastus lateralis (DVL), and superficial vastus lateralis (SVL). The muscles of one hindlimb were used for histochemical ATPase analysis to determine the distribution of fiber types and those of the contralateral hindlimb were assayed biochemically for citrate synthase activity (an aerobic marker). Training induced a significant increase in citrate synthase activity in each muscle section. The largest absolute increase occurred in the DVL and the largest relative increase occurred in the SVL. The distribution of fiber types within the S (85% slow-twitch) and SVL (100% fast-twitch) remained unchanged with training. However, significant increases in the percentage of type I (slow-twitch) fibers in both the P (2-fold) and DVL (3-fold) were observed with concomitant decreases in the type II (fast-twitch) population. In addition, training induced significant changes in the fast-twitch subtype populations of the DVL (IIB----IIA). These data suggest exercise-induced fiber type transformations occurring both within the fast-twitch population and between fast-twitch and slow-twitch fibers in certain hindlimb muscles of the rat following a high intensity interval training program.     

Fiber type changes in rat skeletal muscle after intense interval training

Summary Female Sprague-Dawley rats were subjected to a ten week training program to determine the influence of intense interval running on the fiber type composition of selected hindlimb muscles; soleus (S), plantaris (P), deep vastus lateralis (DVL), and superficial vastus lateralis (SVL). The muscles of one hindlimb were used for histochemical ATPase analysis to determine the distribution of fiber types and those of the contralateral hindlimb were assayed biochemically for citrate synthase activity (an aerobic marker). Training induced a significant increase in citrate synthase activity in each muscle section. The largest absolute increase occurred in the DVL and the largest relative increase occurred in the SVL. The distribution of fiber types within the S (85% slow-twitch) and SVL (100% fast-twitch) remained unchanged with training. However, significant increases in the percentage of type I (slow-twitch) fibers in both the P (2-fold) and DVL (3-fold) were observed with concomitant decreases in the type II (fast-twitch) population. In addition, training induced significant changes in the fast-twitch subtype populations of the DVL (IIBIIA). These data suggest exercise-induced fiber type transformations occurring both within the fast-twitch population and between fast-twitch and slow-twitch fibers in certain hindlimb muscles of the rat following a high intensity interval training program.     

The presence and binding characteristics of calmodulin in microsomal preparations enriched in sarcoplasmic reticulum from rabbit skeletal muscle

ATP-dependent oxalate facilitated calcium transport in sarcoplasmic reticulum (SR) preparations obtained from rabbit vastus lateralis muscle (fast skeletal muscle; F_sr) and soleus (slow skeletal muscle; S_sr) was determined. Addition of exogenous calmodulin did not stimulate calcium transport in either F_sr or S_sr preparations. F_sr and S_sr previously washed in 1 mM EGTA demonstrated a reduced capacity to transport Ca²⁺; the exogenous addition of calmodulin (0.24 μM) under these conditions, did not restore uptake activity but significantly decreased the steady-state level of Ca²⁺ uptake. Extracts of skeletal SR prepared by treatment with 0.2 mM EDTA and boiling produced significantly more stimulation of red cell Ca²⁺ATPase activity than extracts prepared by boiling alone. This stimulation of red cell Ca²⁺-ATPase was inhibited to a significant extent by $\text{48}\text{80}$, a known anti-calmodulin agent. Radioimmunoassay revealed that extracts prepared by boiling or EDTA-treatment followed by boiling contained considerable amounts of calmodulin. Washing with 1 mM EGTA, though, did not release any calmodulin from SR. These studies reveal that calmodulin is present in both F_sr and S_sr and can only be removed by harsh treatments. The role of calmodulin in skeletal muscle Ca²⁺-transport remains to be determined.     

A method for measuring sarcoplasmic reticulum calcium uptake in skeletal muscle using Fura-2

We have presented an assay for measuring the rate of sarcoplasmic reticulum (SR) Ca²⁺ uptake and Ca²⁺ release in skeletal muscle homogenates using the fluorescent Ca²⁺ probe Fura-2. Using this assay, we investigated the effects of an elevated temperature (40°C) and lowered pH (6.8), two factors proposed to be involved in skeletal muscle fatigue, on SR Ca²⁺ uptake. The EDL muscle was found to have a higher rate of Ca²⁺ uptake than the soleus (34%). Exposure of the muscles to a raised temperature, but not a reduced pH, resulted in a reduction in the rate of Ca²⁺ uptake in both the EDL and soleus homogenates. This uptake process was blocked by cyclopiazonic acid (CPA) a specific inhibitor of the major transport protein of the sarcoplasmic reticulum, the Ca²⁺-ATPase. Calcium release was induced using AgNO₃ after loading of the vesicles during the uptake process. It was found that AgNO₃ was only effective in producing Ca²⁺ release in the EDL muscles. The soleus muscles did not release Ca²⁺ under varying [Mg²⁺] or with Hg²⁺ substitution for Ag⁺, suggesting that fast- and slow-twitch muscle fibres require different conditions for maximum Ca²⁺-release, or that different isoforms of the Ca²⁺ release channels are present in the different fibres.     

Seasonal changes in the isoform composition of the myosin heavy chains in skeletal muscles of hibernating ground squirrels Spermophilus undulatus

Seasonal changes of the isoform composition of myosin heavy chains in skeletal muscles (m. triceps, m. longissimus dorsi, m. soleus, m. gastrocnemius, m. vastus lateralis) of hibernating ground squirrels Spermophilus undulatus were studied. Functional properties of myosin (the actin-activated ATPase activity and its Ca²⁺-sensitivity in vitro) were also examined. It was observed that the content of slow myosin heavy chain I isoform increased and the content of fast IIx/d isoform decreased in muscles of torpid ground squirrels and animals which are active in autumn and winter. In muscles of these animals the content of N2A-titin isoform decreased although the relative content of NT-titin isoform, observed in striated muscles of mammals in our previous experimental works, increased. Actin-activated ATPase activity and Ca²⁺-sensitivity of myosin isolated from skeletal muscles of torpid and interbout ground squirrels were found to reduce. The changes observed are discussed in the context of adaptation of skeletal muscles of ground squirrels to hibernation conditions.     

Carbonic anhydrase III in skeletal muscle fibers: an immunocytochemical and biochemical study

The objectives of the present study were to determine if carbonic anhydrase III (CA III) demonstrated a specific association for any particular organelle or structure of the skeletal muscle cell and to quantify the activity and content of this enzyme in different types of skeletal muscle fibers. Ultrastructural localization of CA III in the soleus (SOL), deep vastus lateralis (DVL), and superficial vastus lateralis (SVL), composed of predominantly type I, IIa, and IIb fibers, respectively, was performed using a high-resolution immunocytochemical technique and antibody specific for CA III on ultra-thin sections of skeletal muscle embedded in the water-soluble medium polyvinyl alcohol (PVA). The results indicated a uniform distribution of CA III within the sarcomere. Mitochondria, nuclei, triads, Z-, and M-bands were not specifically labeled. Immunoblotting of washed myofibril preparations did not show any detectable CA III associated with this structure. In addition to quantification of the immunogold labeling, CA III activity and content were assayed in the post-mitochondrial supernatant of the three muscles. In the SOL, these values were found to be 3.6-7.6 times higher than in the DVL. The SVL showed a labeling intensity slightly higher than background level, while the enzyme activity and content were indistinguishable from background levels. We therefore conclude that CA III is randomly distributed in the cytoplasm of the three muscle fiber types and that the relative CA III content and activity in the three muscles studied is SOL greater than DVL greater than SVL approximately equal to 0.     

Glutathione and antioxidant enzymes in skeletal muscle: effects of fiber type and exercise intensity.

Glutathione status and antioxidant enzymes in various types of rat skeletal muscle were studied after an acute bout of exercise (Ex) at different intensities. Glutathione (GSH) and glutathione disulfide (GSSG) concentrations were the highest in soleus (SO) muscle, followed by those in deep (DVL) and then superficial (SVL) portions of vastus lateralis. In DVL, but not in SO or SVL, muscle GSH increased proportionally with Ex intensity and reached 1.8 +/- 0.08 mumol/g wet wt compared with 1.5 +/- 0.03 (P < 0.05) in resting controls (R). GSSG in DVL was increased from 0.10 +/- 0.01 mumol/g wet wt in R to 0.14 +/- 0.01 (P < 0.05) after Ex. Total glutathione (GSH + GSSG) contents in DVL were also significantly elevated with Ex, whereas GSH/GSSG ratio was unchanged. Activities of GSH peroxidase (GPX), GSSG reductase (GR), and catalase (CAT) were significantly higher in SO than in DVL and SVL, but there was no difference in superoxide dismutase activity between the three muscle types. Furthermore, Ex at moderate intensities elicited significant increases in GPX, GR, and CAT activities in DVL muscle. None of the antioxidant enzymes was affected by exercise in SO. It is concluded that rat DVL muscle is particularly vulnerable to exercise-induced free radical damage and that a disturbance of muscle GSH status is indicative of an oxidative stress.     

Effect of high-intensity exercise training on functional capacity of limb skeletal muscle

The purpose of this study was to determine whether a program of regular sprint exercise training alters the functional properties or protects against the development of fatigue in fast- and slow-twitch rat skeletal muscle. The training program consisted of 6 sprints of 4.5-min duration at 40 m/min and 15% slope with 2.5-min rest intervals, performed 5 days/wk for 6 wk. The exercise program significantly increased (P less than 0.05) citrate synthase activity (mumol X g-1 X min-1) in the predominantly type I soleus (SOL) from 28 +/- 2 to 44 +/- 2; the type IIb superficial region of the vastus lateralis (SVL) from 10 +/- 1 to 16 +/- 1; and the type IIa deep region of the vastus lateralis (DVL) from 34 +/- 2 to 53 +/- 2. Phosphofructokinase activity (mumol X g-1 X min-1) also increased with training in the SOL (17 +/- 1 vs. 23 +/- 1) and the DVL (64 +/- 5 vs. 79 +/- 5). Sprint training reduced (P less than 0.05) the contraction time (CT) (111 +/- 7 vs. 92 +/- 3 ms) and the one-half relaxation time (118 +/- 3 vs. 104 +/- 2 ms) in the slow-twitch soleus. The exercise program also induced a decreased CT in the fast-twitch extensor digitorum longus (EDL), but significance was limited to the P less than 0.1 level. Muscle fatigue was produced by electrical stimulation at 45 trains/min and either 15 trains/min in SOL or 10 trains/min in the EDL and SVL for 1, 5, or 10 min.(ABSTRACT TRUNCATED AT 250 WORDS)     

Atypical fast SERCA1a protein expression in slow myofibers and differential S-nitrosylation prevented by exercise during long term bed rest

We monitored changes in SERCA isoform specific expression and S-nitrosylation in myofibers of lower limb soleus (SOL) and vastus lateralis (VL) muscle biopsies before and after 60 days of voluntary long term bed rest (BR) without (BR-CTRL group, n = 8) and with exercise countermeasure (BR-EX group, n = 8). Before BR, a typical myofiber type-specific distribution of fast and slow SERCA1/2a isoforms was seen. After BR, a subpopulation (approx. 15%) of slow myofibers in BR-CTRL additionally expressed the fast SERCA1a isoform which was not seen in BR-EX. After BR, SERCA1a S-nitrosylation patterns analyzed by the biotin-switch assay decreased in disused SOL only but increased in both muscles following exercise. Differential SERCA1a S-nitrosylation and SERCA1a/2a co-expression in subsets of slow myofibers should be considered as signs of an altered cytosolic Ca²⁺ homeostasis following chronic muscle disuse. Exercise preserved myofiber type-specific SERCA1a expression and S-nitrosylation in VL and SOL in a different way, suggesting muscle-specific responses to the countermeasure protocol applied during bed rest.     

Inhibition of sarcoplasmic reticulum calcium pump by cytosolic protein(s) endogenous to heart and slow skeletal muscle but not fast skeletal muscle

Cytosol from rabbit heart and slow and fast skeletal muscles was fractionated using (NH₄)₂SO₄ to yield three cytosolic protein fractions, viz., CPF-I (protein precipitated at 30% saturation), CPF-II (protein precipitated between 30 and 60% saturation), and cytosol supernatant (protein soluble at 60% saturation). The protein fractions were dialysed and tested for their effects on ATP-dependent, oxalate-supported Ca²⁺ uptake by sarcoplasmic reticulum from heart and slow and fast skeletal muscles. CPF-I from heart and slow muscle, but not from fast muscle, caused marked inhibition (up to 95%) of Ca²⁺ uptake by sarcoplasmic reticulum from heart and from slow and fast muscles. Neither unfractionated cytosol nor CPF-II or cytosol supernatant from any of the muscles altered the Ca²⁺ uptake activity of sarcoplasmic reticulum. Studies on the characteristics of inhibition of sarcoplasmic reticulum Ca²⁺ uptake by CPF-I (from heart and slow muscle) revealed the following: (a) Inhibition was concentration- and temperature-dependent (50% inhibition with approx. 80 to 100 μg CPF-I; seen only at temperatures above 20°C). (b) The inhibitor reduced the velocity of Ca²⁺ uptake without appreciably influencing the apparent affinity of the transport system for Ca²⁺. (c) Inhibition was uncompetitive with respect to ATP. (d) Sarcoplasmic reticulum washed following exposure to CPF-I showed reduced rates of Ca²⁺ uptake, indicating that inhibition results from an interaction of the inhibitor with the sarcoplasmic reticulum membrane. (e) Concomitant with the inhibition of Ca²⁺ uptake, CPF-I also inhibited the Ca²⁺-ATPase activity of sarcoplasmic reticulum. (f) Heat-treatment of CPF-I led to loss of inhibitor activity, whereas exposure to trypsin appeared to enhance its inhibitory effect. (g) Addition of CPF-I to Ca²⁺-preloaded sarcoplasmic reticulum vesicles did not promote Ca²⁺ release from the vesicles. These results demonstrate the presence of a soluble protein inhibitor of sarcoplasmic reticulum Ca²⁺ pump in heart and slow skeletal muscle but not in fast skeletal muscle. The characteristics of the inhibitor and its apparently selective distribution suggest a potentially important role for it in the in vivo regulation of sarcoplasmic reticulum Ca²⁺ pump, and therefore in determining the duration of Ca²⁺ signal in slow-contracting muscle fibers.     

Alterations in sarcoplasmic reticulum function in female vastus lateralis with eccentric exercise

This study examined the alterations in sarcoplasmic reticulum (SR) Ca²⁺ sequestration function in homogenates during eccentric exercise and recovery and following additional eccentric exercise, and correlated these alterations with changes in force output. Eight healthy, untrained females, aged 20-25 years, cycled for a total of 60 min on an eccentric cycle ergometer (30 min at 66 ± 3% O₂ peak and 30 min at 76 ± 3% O₂ peak, determined during concentric exercise). Biopsies (extracted from the vastus lateralis) were taken before and after the exercise as well as on days 2, 6 and prior to and following identical exercise on day 14. Ca²⁺-uptake (nmol/min/mg protein) was unaffected (p > 0.05) following the first session of eccentric exercise; however, by day 2 a depression in uptake (p < 0.05) was observed which persisted throughout the remainder of the experiment. Maximal Ca²⁺-ATPase activity (nmol/min/mg protein) was elevated (p < 0.05) immediately following the first exercise session, remained elevated through day 2 and returned to pre-exercise levels by day 6 of recovery and increased again by day 14. No changes in either Ca²⁺-ATPase activity or Ca²⁺-uptake were observed with exercise on day 14. Both eccentric sessions, performed on days 0 and 14, resulted in similar depressions in force (p < 0.05) immediately following exercise. By day 2 force had recovered to pre-exercise levels. The results demonstrate that a prolonged alteration in SR Ca²⁺-uptake occurs following eccentric work that is unaccompanied by parallel changes in either SR Ca²⁺-ATPase activity or mechanical performance.     

Co-Expression of SERCA Isoforms,Phospholamban and Sarcolipin in Human Skeletal Muscle Fibers

Sarcolipin (SLN) and phospholamban (PLN) inhibit the activity of sarco(endo)plasmic reticulum Ca²⁺-ATPases (SERCAs) by reducing their apparent affinity for Ca²⁺. A ternary complex between SLN, PLN, and SERCAs results in super-inhibition of SERCA activity. Analysis of skeletal muscle homogenate has limited our current understanding of whether SLN and PLN regulate SERCA1a, SERCA2a, or both in skeletal muscle and whether SLN and PLN are co-expressed in skeletal muscle fibers. Biopsies from human vastus lateralis were analyzed through single fiber Western blotting and immunohisto/fluorescence staining to circumvent this limitation. With a newly generated SLN antibody, we report for the first time that SLN protein is present in human skeletal muscle. Addition of the SLN antibody (50 µg) to vastus lateralis homogenates increased the apparent Ca²⁺ affinity of SERCA (K _Ca, pCa units) (-Ab, 5.85 ± 0.02 vs. +Ab, 5.95 ± 0.02) and maximal SERCA activity (μmol/g protein/min) (-Ab, 122 ± 6.4 vs. +Ab, 159 ± 11) demonstrating a functional interaction between SLN and SERCAs in human vastus lateralis. Specifically, our results suggest that although SLN and PLN may preferentially regulate SERCA1a, and SERCA2a, respectively, physiologically they both may regulate either SERCA isoform. Furthermore, we show that SLN and PLN co-immunoprecipitate in human vastus lateralis homogenate and are simultaneously expressed in 81% of the fibers analyzed with Western blotting which implies that super-inhibition of SERCA may exist in human skeletal muscle. Finally, we demonstrate unequivocally that mouse soleus contains PLN protein suggesting that super-inhibition of SERCA may also be important physiologically in rodent skeletal muscle.     

Comparative effects of hindlimb suspension and exercise on skeletal muscle myosin isozymes in rats

The purpose of this study was to ascertain the time course of changes, whilst suspending the hindlimb and physical exercise training, of myosin light chain (LC) isoform expression in rat soleus and vastus lateralis muscles. Two groups of six rats were suspended by their tails for 1 or 2 weeks, two other groups of ten rats each were subjected to exercise training on a treadmill for 9 weeks, one to an endurance training programme (1-h running at 20 m.min-1 5 days.week-1), and the other to a sprint programme (30-s bouts of running at 60 m.min-1 with rest periods of 5 min). At the end of these experimental procedures, soleus and vastus lateralis superficialis muscles were removed for myosin LC isoform determination by two-dimensional gel electrophoresis. Hindlimb suspension for 2 weeks significantly increased the proportion of fast myosin LC and decreased slow myosin LC expression in the soleus muscle. The pattern of myosin LC was unchanged in the vastus lateralis muscle. Sprint training or endurance training for 9 weeks increased the percentage of slow myosin LC in vastus lateralis muscle, whereas soleus muscle myosin LC was not modified. These data indicate that hindlimb suspension influences myosin LC expression in postural muscle, whereas physical training acts essentially on phasic muscle. There were no differences in myosin LC observed under the influence of sprint- or endurance-training programme.     

Rat skeletal muscle triacylglycerol utilization during exhaustive swimming

The utilization of triacylglycerol in slow oxidative, fast oxidative-glycolytic, and fast glycolytic skeletal muscle fiber types was examined in rats subjected to a prolonged exhaustive swim. Significant reductions of intramuscular triacylglycerol occurred following 2 h and 40 min of swimming in all muscles containing a predominance of slow oxidative and fast oxidative-glycolytic fibers, which possess a high capacity for free fatty acid oxidation. Triacylglycerol content in the soleus decreased by 48%, and reductions of 41, 29, and 27% were measured in the red vastus lateralis, red gastrocnemius, and plantaris muscles, respectively. In the white vastus lateralis and white gastrocnemius muscles (fast glycolytic fibers) triacylglycerol concentrations were unaffected. In all muscles the variability of intramuscular triacylglycerol measurements between animals was 20-50% and the within animal variance (right vs. left hindlimb) was similar. Analytical repeatability was approximately 10% in all muscles and significantly less than the between- and within-animal variances. It was concluded that a real biological variation exists in the triacylglycerol content of all rat skeletal muscles and that intramuscular triacylglycerol is an important energy source during prolonged exercise of moderate intensity.     

Sarcoplasmic reticulum calcium uptake in whole muscle homogenates

It is often desirable to determine calcium uptake by the sarcoplasmic reticulum in specific muscles. By employing the procedure described in this communication the investigator can determine sarcoplasmic reticulum calcium uptake rates using very small amounts of tissue. Ca²⁺ uptake values are reported for whole homogenates of heart, soleus and epitrochlearis muscles from adult rats.     

Capillary supply and cross-sectional area of slow and fast twitch muscle fibres in man

Summary The muscles triceps brachii, quadriceps femoris (part vastus lateralis) and soleus were analysed in 6 men and 6 women for fibre composition (% slow twitch, ST-fibres and % fast twitch, FT-fibres), fibre cross sectional areas, and capillarization. Also the fraction of fibres enclosed by their own fibre type was analysed together with the capillary supply of these fibres. Fibre composition was 39(19–60)% ST in m. triceps brachii, 60(29–78)% ST in m. vastus lateralis and 73(49–88)% ST in m. soleus. Fibre areas ranged from 2,320 to 16,667 m² being smallest in m. triceps brachii and largest in m. soleus (p<0.05) and with ST fibres being significantly smaller than FT fibres in some of the muscles. In all muscles the shape of the fibres was elliptical with the larger diameter being about twice the smaller diameter. Capillary density per cross sectional muscle area was not related to the fibre composition and was 379(302–500) cap/mm² in m. triceps brachii, 404(284–529) cap/mm² in m. vastus lateralis and 417(333–592) cap/mm² in m. soleus. However, capillary supply expressed as fibre type area per capillary was up to 40% larger for FT-fibres than for ST-fibres within the same muscle (p<0.05). The capillary supply of enclosed fibres was not different from that of fibres surrounded also by the other fibre type. The results demonstrate that the difference in capillary supply to ST and FT-fibres is less distinct in humans than in other mammals, which is consistent with the metabolic potentials also being more alike.     

Muscle fibre type populations of human leg muscles.

Four selected leg muscles (gastrocnemius, soleus, vastus lateralis and intermedius) from thirty-two humans were autopsied within 25 hr of death and examined histochemically.The results of histochemical myofibrillar adenosine triphosphatase activity demonstrated that the soleus and vastus intermedius muscles have a higher proportion of slow twitch fibres (70%, 47%) than their synergists, gastrocnemius and vastus lateralis, respectively.The gastrocnemius contains about 50% slow twitch fibres and the vastus lateralis about 32%. Similar proportions of slow and fast twitch fibres have been reported for these hindlimb muscles in other mammals. Human muscles, however, differ from other mammalian muscles in that the proportion of slow and fast twitch fibres were similar in the superficial and deep regions of the muscles examined. Fast twitch oxidative glycolytic fibres in sedentary humans were observed less frequently, and they are less prominent in terms ofoxidative enzymatic activity when compared to similar fibres of several laboratory mammals studied previously.     

Isolation and characterization of the surface membranes of fast and slow mammalian skeletal muscle

Fast (extensor digitorum longus) and slow (soleus) rat skeletal muscles served as the source for isolation and biochemical comparison of two distinct surface membrane fractions with properties of the sarcolemma and transverse tubular system. Enriched sarcolemmal membrane from soleus demonstrated a lighter density after sucrose density centrifugation. Sialic acid content was 1.5-fold higher in soleus (62 nmol/mg) than extensor (40 nmol/mg). The specific activity of (Na⁺ + K⁺ + Mg²⁺)-ATPase was similar (1.40 and 1.65 μmol P_i/mg per 5 min) with the soleus enzyme displaying a (1) greater resistance to inhibition by ouabain, and (2) broader ionic ratio ($\text{Na}{}^{\mathrm{+}}\text{K}{}^{\mathrm{+}}$) requirement than extensor enzyme. The polypeptide and phospholipid composition showed no major differences between the two muscle types.The second surface membrane fraction, tentatively identified as transverse tubule, differed in membrane composition. The major polypeptide of extensor was of 95 000 molecular weight whereas for soleus a M_r = 28 000 species was dominant. Total phospholipid content of soleus was 1.5-fold greater than extensor due mostly to increased levels of phosphatidylcholine and phosphatidylethanolamine. Endogenous membrane protein kinase for the 28 000 molecular weight polypeptide was found exclusively in this membrane. The reaction conditions were identical for extensor and soleus since both required divalent cations (Ca²⁺ and Mg²⁺) and neither was affected by cyclic AMP. Soleus showed a 2-fold higher capacity for phosphate incorporation than extensor.These studies show that surface membrane fractions derived from fast and slow muscles differ in terms of functional and compositional properties. These differences are specific not only for the surface membrane but for the muscle type and may relate to the known physiological differences observed between fast and slow mammalian muscle.     

Is the VO2 slow component dependent on progressive recruitment of fast-twitch fibers in trained runners?

The goal of this study was to use spectral analysis of EMG data to test the hypothesis that the O2 uptake VO2) slow component is due to a recruitment of fast fibers. Thirteen runners carried out a treadmill test with a constant speed, corresponding to 95% of the velocity associated with maximal VO2. The VO2 response was fit with the classical model including three exponential functions. Electrical activity of six lower limb muscles (vastus lateralis, soleus, and gastrocnemius of both sides) was measured using electromyogram surface electrodes. Mean power frequency (MPF) was used to study the kinetics of the electromyogram discharge frequency. Three main results were observed: 1) a common pattern of the MPF kinetics in the six muscles studied was noted; 2) MPF decreased in the first part of the exercise, followed by an increase for all the muscles studied, but only the vastus lateralis, and gastrocnemius muscles of both sides increased significantly (P < 0.05); and 3) the beginning of the MPF increase of the four muscles mentioned above corresponded with the beginning of the slow component. Our results suggest a progression in the average frequency of the motor unit discharge toward the high frequencies, which coheres with the hypothesis of the progressive recruitment of fast-twitch fibers during the VO2 slow component. However, this interpretation must be taken with caution because MPF is the result of a balance between several phenomena.