Estimation of the interplay between groups of fast and slow muscle fibers of the tibialis anterior and gastrocnemius muscle while running.

Electromyograms recorded from the lower limbs of humans while running were submitted to a time/frequency analysis using wavelets. The results of the wavelet analysis yielded intensity spectra at every time point during the swing and the stance phase. It was previously shown that more or less high frequency components get activated during different periods of the movement. The purpose of this study was to test to what extent the spectra can be reconstructed by a linear superposition of two generating spectra that were associated to groups of fast and slow muscle fibers. The terms fast and slow do not only refer to the conduction velocity but also to the shape of the motor unit action potential and are used to characterize the groups in a broader sense. The principal component analysis of the spectra confirmed that a two dimensional spectral space was appropriate. A parametric spectral decomposition was used to extract the generating spectra within the two dimensional spectral space. The generating spectra were in turn used to compute the power with which the groups of muscle fibers contribute to the measured spectra and thus to the overall muscular activity. The power that was obtained for the different time points during the movement reflects the biomechanically important interplay between the groups of muscle fibers while running.     

Effect of thyroidectomy on fast and slow muscle fibres of rat gastrocnemius muscle

A combined histochemical, biochemical and electrophoretic study with respect to the enzymes succnic dehydrogenase(SDH), myofibrillar adenosine triphosphatase (m-ATPase), lactate dehydrogenase (LDH) isozymes and myosin light chains was carried out to investigate the response of rat gastrocnemius muscle (medial head). Twelve weeks after thyroidectomy, the results indicated a shift from fast to slow type pattern of LDH isozymes, fibre type transformation from Type II to Type I and a decrease in SDH and m-ATPase activity. The results suggest, possible thyroidal involvement in determining the phenotypic properties of skeletal muscle.     

Altered distribution of mitochondria in rat soleus muscle fibers after spaceflight.

The influence of spaceflight on the distribution of succinate dehydrogenase (SDH) activity throughout the cross section of fibers in the soleus was studied in five male rats and in five rats maintained under ground-based simulated flight conditions (control). The flight (COSMOS 1887) was 12.5 days in duration, and the animals were killed approximately 2 days after return to 1 G. Fibers were classified as slow-twitch oxidative or fast-twitch oxidative-glycolytic in histochemically prepared tissue sections. The distribution of SDH activity throughout the cross section of 20-30 fibers (each type) was determined using quantitative histochemical and computer-assisted image analysis techniques. In all the fibers, the distribution of SDH activity was significantly higher in the subsarcolemmal than in intermyofibrillar region. After spaceflight the entire regional distribution of SDH activity was significantly altered in the slow-twitch oxidative fibers. The fast-twitch oxidative-glycolytic fibers of the spaceflight muscles exhibited a significantly lower SDH activity only in their subsarcolemmal region. These data suggest that when determining the influence of spaceflight on muscle fiber oxidative metabolism enzymes, it is important to consider the location of the enzyme throughout the cross section of a fiber. Furthermore the functional properties of the soleus that depend on the metabolic support of mitochondria in the subsarcolemmal region may be primarily affected by exposure to microgravity.     

Miniature potentials in fast and slow muscle fibers in locusts

Excitatory miniature postsynaptic potentials were studied by an intracellular recording method in fast and slow muscle fibers ofLocusta migratorioides. Statistical analysis showed that liberation of mediator in both types of fibers can be predicted by the formula for a negative binomial distribution with a probability of 85%. This correlation is evidence of some degree of interaction between consecutive liberations of quanta of mediator by nerve endings. It is shown that the fraction of miniature potentials depending on the external calcium concentration is greater in fast muscle fibers. An increase in the magnesium ion concentration from 2 to 40 mM led to a decrease in the frequency of miniature potentials, and this decrease was greater in fast fibers; an increase in the magnesium ion concentration from 1 to 10 mM in calcium-free solutions, on the other hand, led to some increase in frequency, and this also was greater in fast muscle fibers. It is concluded that nerve endings in fast and slow muscle fibers differ in their sensitivity to changes in the ionic composition of the medium.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 13, No. 2, pp. 210–217, March–April, 1981.     

Rate of phosphate release after photoliberation of adenosine 5'-triphosphate in slow and fast skeletal muscle fibers.

Inorganic phosphate (Pi) release was determined by means of a fluorescent Pi-probe in single permeabilized rabbit soleus and psoas muscle fibers. Measurements of Pi release followed photoliberation of approximately 1.5 mM ATP by flash photolysis of NPE-caged ATP in the absence and presence of Ca2+ at 15 degrees C. In the absence of Ca2+, Pi release occurred with a slow rate of 11 +/- 3 microM . s-1 (n = 3) in soleus fibers and 23 +/- 1 microM . s-1 (n = 10) in psoas fibers. At saturating Ca2+ concentrations (pCa 4.5), photoliberation of ATP was followed by rapid force development. The initial rate of Pi release was 0.57 +/- 0.05 mM . s-1 in soleus (n = 13) and 4.7 +/- 0.2 mM . s-1 in psoas (n = 23), corresponding to a rate of Pi release per myosin head of 3.8 s-1 in soleus and 31.5 s-1 in psoas. Pi release declined at a rate of 0.48 s-1 in soleus and of 5.2 s-1 in psoas. Pi release in soleus was slightly faster in the presence of an ATP regenerating system but slower when 0.5 mM ADP was added. The reduction in the rate of Pi release results from an initial redistribution of cross-bridges over different states and a subsequent ADP-sensitive slowing of cross-bridge detachment.     

Controlled differentiation of myoblast cells into fast and slow muscle fibers

Skeletal muscles are classified into fast and slow muscles, which are characterized by the expression of fast-type myosin heavy chains (fMyHCs) or slow-type myosin heavy chains (sMyHCs), respectively. However, the mechanism of subtype determination during muscle fiber regeneration is unclear. We have analyzed whether the type of muscle is determined in the myoblast cells or is controlled by the environment in which the muscle fibers are formed from myoblast cells. When myoblast cells from 7-day-old chick embryo were cultured and formed into muscle fibers, more than half of the fibers produced only fMyHCs, and the remaining fibers produced both fMyHCs and sMyHCs. However, when myoblast cells were cultured in medium supplemented with a small amount of slow muscle extract, the expression of sMyHCs in muscle fibers increased, whereas the expression of fMyHCs increased in the group supplemented with fast muscle extract compared with the control group. The same results were obtained when cloned mouse myoblast cells (C₂C₁₂ cells) were cultured and formed into muscle fibers. The data presented here thus show that the subtype differentiation of muscle fiber is controlled by the environment in which the muscle fiber forms. This work was funded by the Sasakawa Scientific Research Grant of the Japan Science Society.     

Effects of pH on contraction of rabbit fast and slow skeletal muscle fibers.

We have investigated (a) effects of varying proton concentration on force and shortening velocity of glycerinated muscle fibers, (b) differences between these effects on fibers from psoas (fast) and soleus (slow) muscles, possibly due to differences in the actomyosin ATPase kinetic cycles, and (c) whether changes in intracellular pH explain altered contractility typically associated with prolonged excitation of fast, glycolytic muscle. The pH range was chosen to cover the physiological pH range (6.0-7.5) as well as pH 8.0, which has often been used for in vitro measurements of myosin ATPase activity. Steady-state isometric force increased monotonically (by about threefold) as pH was increased from pH 6.0; force in soleus (slow) fibers was less affected by pH than in psoas (fast) fibers. For both fiber types, the velocity of unloaded shortening was maximum near resting intracellular pH in vivo and was decreased at acid pH (by about one-half). At pH 6.0, force increased when the pH buffer concentration was decreased from 100 mM, as predicted by inadequate pH buffering and pH heterogeneity in the fiber. This heterogeneity was modeled by net proton consumption within the fiber, due to production by the actomyosin ATPase coupled to consumption by the creatine kinase reaction, with replenishment by diffusion of protons in equilibrium with a mobile buffer. Lactate anion had little mechanical effect. Inorganic phosphate (15 mM total) had an additive effect of depressing force that was similar at pH 7.1 and 6.0. By directly affecting the actomyosin interaction, decreased pH is at least partly responsible for the observed decreases in force and velocity in stimulated muscle with sufficient glycolytic capacity to decrease pH.     

Metabolic and morphologic properties of single muscle fibers in the rat after spaceflight, Cosmos 1887

The adaptation of a slow (soleus, Sol) and a fast (medial gastrocnemius, MG) skeletal muscle to spaceflight was studied in five young male rats. The flight period was 12.5 days and the rats were killed approximately 48 h after returning to 1 g. Five other rats that were housed in cages similar to those used by the flight rats were maintained at 1 g for the same period of time to serve as ground-based controls. Fibers were classified as dark or light staining for myosin adenosine triphosphatase (ATPase). On the average, the fibers in the Sol of the flight rats atrophied twice as much as those in the MG. Further, the fibers located in the deep (close to the bone and having the highest percentage of light ATPase and high oxidative fibers in the muscle cross section) region of the MG atrophied more than the fibers located in the superficial (away from the bone and having the lowest percentage of light ATPase and high oxidative fibers in the muscle cross-section) region of the muscle. Based on quantitative histochemical assays of single muscle fibers, succinate dehydrogenase (SDH) activity per unit volume was unchanged in fibers of the Sol and MG. However, in the Sol, but not the MG, the total amount of SDH activity in a 10-microns-thick section of a fiber decreased significantly in response to spaceflight. Based on population distributions, it appears that the alpha-glycerophosphate dehydrogenase (GPD) activities were elevated in the dark ATPase fibers in the Sol, whereas the light fibers in the Sol and both fiber types in the MG did not appear to change. The ratio of GPD to SDH activities increased in the dark (but not light) fibers of the Sol and was unaffected in the MG. Immunohistochemical analyses indicate that approximately 40% of the fibers in the Sol of flight rats expressed a fast myosin heavy chain compared with 22% in control rats. Further, 31% of the fibers in the Sol of flight rats expressed both fast and slow myosin heavy chains compared with 8% in control rats. Immunohistochemical changes in the MG were minimal. These data suggest that the magnitude and direction of enzymatic activity and cell volume changes are dependent on the muscle, the region of the muscle, and the type of myosin expressed in the fibers. Further, the ability of fibers to maintain normal or even elevated activities per unit volume of some metabolic enzymes is remarkable considering the marked and rapid decrease in fiber volume.     

Endothermic force generation in fast and slow mammalian (rabbit) muscle fibers.

Isometric tension responses to rapid temperature jumps (T-jumps) of 3-7 degrees C were examined in single skinned fibers isolated from rabbit psoas (fast) and soleus (slow) muscles. T-jumps were induced by an infrared laser pulse (wavelength 1.32 microns, pulse duration 0.2 ms) obtained from a Nd-YAG laser, which heated the fiber and bathing buffer solution in a 50-microliter trough. After a T-jump, the temperature near the fiber remained constant for approximately 0.5 s, and the temperature could be clamped for longer periods by means of Peltier units assembled on the back trough wall. A T-jump produced a step decrease in tension in both fast and slow muscle fibers in rigor, indicating thermal expansion. In maximally Ca-activated (pCa approximately 4) fibers, the increase of steady tension with heating (3-35 degrees C) was approximately sigmoidal, and a T-jump at any temperature induced a more complex tension transient than in rigor fibers. An initial (small amplitude) step decrease in tension followed by a rapid recovery (tau(1); see Davis and Harrington, 1993) was seen in some records from both fiber types, which presumably was an indirect consequence of thermal expansion. The net rise in tension after a T-jump was biexponential, and its time course was characteristically different in the two fibers. At approximately 12 degrees C the reciprocal time constants for the two exponential components (tau(2) and tau(3), respectively, were approximately 70.s(-1) and approximately 15.s(-1) in fast fibers and approximately 20.s(-1) and approximately 3.s(-1) in slow fibers. In both fibers, tau(2) ("endothermic force regeneration") became faster with an increase in temperature. Furthermore, tau(3) was temperature sensitive in slow fibers but not in fast fibers. The results are compared and contrasted with previous findings from T-jump experiments on fast fibers. It is observed that the fast/slow fiber difference in the rate of endothermic force generation (three- to fourfold) is considerably smaller than the reported differences in the "phosphate release steps" (> 30-fold).     

Development of ionic currents of zebrafish slow and fast skeletal muscle fibers

Voltage-gated Na+ and K+ channels play key roles in the excitability of skeletal muscle fibers. In this study we investigated the steady-state and kinetic properties of voltage-gated Na+ and K+ currents of slow and fast skeletal muscle fibers in zebrafish ranging in age from 1 day postfertilization (dpf) to 4-6 dpf. The inner white (fast) fibers possess an A-type inactivating K+ current that increases in peak current density and accelerates its rise and decay times during development. As the muscle matured, the V50s of activation and inactivation of the A-type current became more depolarized, and then hyperpolarized again in older animals. The activation kinetics of the delayed outward K+ current in red (slow) fibers accelerated within the first week of development. The tail currents of the outward K+ currents were too small to allow an accurate determination of the V50s of activation. Red fibers did not show any evidence of inward Na+ currents; however, white fibers expressed Na+ currents that increased their peak current density, accelerated their inactivation kinetics, and hyperpolarized their V50 of inactivation during development. The action potentials of white fibers exhibited significant changes in the threshold voltage and the half width. These findings indicate that there are significant differences in the ionic current profiles between the red and white fibers and that a number of changes occur in the steady-state and kinetic properties of Na+ and K+ currents of developing zebrafish skeletal muscle fibers, with the most dramatic changes occurring around the end of the first day following egg fertilization.     

Functional properties of human muscle fibers after short-term resistance exercise training

The aim of this study was to assess the relationships between human muscle fiber hypertrophy, protein isoform content, and maximal Ca(2+)-activated contractile function following a short-term period of resistance exercise training. Six male subjects (age 27 +/- 2 yr) participated in a 12-wk progressive resistance exercise training program that increased voluntary lower limb extension strength by >60%. Single chemically skinned fibers were prepared from pre- and posttraining vastus lateralis muscle biopsies. Training increased the cross-sectional area (CSA) and peak Ca(2+)-activated force (P(o)) of fibers containing type I, IIa, or IIa/IIx myosin heavy chain by 30-40% without affecting fiber-specific force (P(o)/CSA) or unloaded shortening velocity (V(o)). Absolute fiber peak power rose as a result of the increase in P(o), whereas power normalized to fiber volume was unchanged. At the level of the cross bridge, the effects of short-term resistance training were quantitative (fiber hypertrophy and proportional increases in fiber P(o) and absolute power) rather than qualitative (no change in P(o)/CSA, V(o), or power/fiber volume).     

Decreased thin filament density and length in human atrophic soleus muscle fibers after spaceflight.

Soleus muscle fibers were examined electron microscopically from pre- and postflight biopsies of four astronauts orbited for 17 days during the Life and Microgravity Sciences Spacelab Mission (June 1996). Myofilament density and spacing were normalized to a 2. 4-microm sarcomere length. Thick filament density ( approximately 1, 062 filaments/microm(2)) and spacing ( approximately 32.5 nm) were unchanged by spaceflight. Preflight thin filament density (2, 976/microm(2)) decreased significantly (P < 0.01) to 2,215/microm(2) in the overlap A band region as a result of a 17% filament loss and a 9% increase in short filaments. Normal fibers had 13% short thin filaments. The 26% decrease in thin filaments is consistent with preliminary findings of a 14% increase in the myosin-to-actin ratio. Lower thin filament density was calculated to increase thick-to-thin filament spacing in vivo from 17 to 23 nm. Decreased density is postulated to promote earlier cross-bridge detachment and faster contraction velocity. Atrophic fibers may be more susceptible to sarcomere reloading damage, because force per thin filament is estimated to increase by 23%.     

Contractile proteins of fast and slow fibers during differentiation of lobster claw muscle

Contractile protein populations were determined, using gel electrophoresis, during development of the claw closer muscles of the lobster Homarus americanus. In the adult the paired claw closer muscles are asymmetric, consisting of a crusher muscle with all slow fibers and a cutter muscle with a majority of fast and a few slow fibers. The electrophoretic banding pattern of these adult fast and slow fibers shows a similarity in the major proteins including myosin, actin, and tropomyosin which are common to both fiber types. Paramyosin is slightly heavier in fast fibers than in slow. However, fast fibers have three proteins and slow fibers have four proteins which are unique to themselves. Several of these unique proteins belong to the regulatory troponin complexes. In juvenile 4th stage lobster, where the paired closer muscles are undifferentiated, the banding pattern reveals the presence of proteins common to both fiber types including myosin, actin, and tropomysin but the conspicuous absence of all unique fast fiber proteins as well as one unique slow fiber protein. By the juvenile 10th stage most of these unique proteins are present except for one unique slow fiber protein. Thus lobster fast and slow fiber differentiation entails coordinate gene activation to add unique contractile proteins.     

Distinction of fast fibers and slow fibers in frog abdominal muscle using certain choline esters]

We have registered the contractions of the frog's abdominal muscle in Ringer's solution. The contractions were produced either by acetylcholin or by succinyldicholin. The graphics showed that the succinyldicholin contractions were much slower than the acetylcholin ones. The tracings were not equalized by a preliminary addition of eserin. We conclude that acetylcholine excites the fast fibers and that succinyldicholin excites the slow fibers of the muscle.     

Postsynaptic end-plate ionic currents in fast and slow chick muscle fibers. Effect of cholinesterase inhibition

Miniature end-plate potentials (MEPPs) were recorded in fast and slow chick muscle fibers extracellularly (focally) and under voltage clamp conditions. The duration of the MEPPs in synapses of slow fibers was on average 2.5 times longer than their duration in synapses of fast fibers. Inhibition of acetylcholinesterase (AChE) lengthened MEPPs by a varied degree: by 1.5 times in synapses of slow fibers and by 3.5 times in those of fast fibers. As a result the difference in the decay time of MEPPs in synapses of these types disappeared almost completely, and only a small difference remained in the rise time of the MEPPs. The MEPP decay time during hyperpolarization in the slow fiber synapse was rather less dependent on potential than in synapses of fast fibers; after inhibition of AChE it became even less dependent. Similar changes in potential dependence were found after lengthening of the MEPP by the action of ethanol. The functional significance of differences in AChE activity and in the activating effect of the mediator for the kinetics of MEPPs in synapses of fast and slow fibers is discussed.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 13, No. 4, pp. 390–397, July–August, 1981.     

Substrate and enzyme profile of fast and slow skeletal muscle fibers in rhesus monkeys

Results from the Russian Cosmos program suggest that the rhesusmonkey is an excellent model for studying weightlessness-induced changes in muscle function. Consequently, the purpose of this investigation was to establish the resting levels of selected substrateand enzymes in individual slow- and fast-twitch muscle fibers of therhesus monkey. A second objective was to determine the effect of an18-day sit in the Spacelab experiment-support primate facility[Experimental System for the Orbiting Primate (ESOP)].Muscle biopsies of the soleus and medial gastrocnemius muscles wereobtained 1 mo before and immediately after an 18-day ESOP sit. Thebiopsies were freeze-dried, and individual fibers were isolated andassayed for the substrates glycogen and lactate and for the high-energyphosphates ATP and phosphocreatine. Fiber enzyme activity was alsodetermined for the glycolytic enzymes phosphofructokinase and lactatedehydrogenase (LDH) and for the oxidative markers 3-hydroxyacyl-CoAdehydrogenase (-OAC) and citrate synthase. Consistent with otherspecies, the fast type II fibers contained higher glycogen content thandid the slow type I fibers. The ESOP sit had no significant effects onthe metabolic profile of the slow fibers of either muscle or the fast fibers of the soleus. However, the fast gastrocnemius fibers showed asignificant decline in phosphocreatine and an increase in lactate. Also, similar to other species, the fast fibers contained significantly higher LDH activities and lower 3-hydroxyacyl-CoA dehydrogenase activities. For the muscle enzymes, the quantitatively most important effect of the ESOP sit occurred with LDH where activities increased inall fiber types postsit except the slow type I fiber of the medial gastrocnemius.

     

From single muscle fiber to whole muscle mechanics: a finite element model of a muscle bundle with fast and slow fibers

Muscles exhibit highly complex, multi-scale architecture with thousands of muscle fibers, each with different properties, interacting with each other and surrounding connective structures. Consequently, the results of single-fiber experiments are scarcely linked to the macroscopic or whole muscle behavior. This is especially true for human muscles where it would be important to understand of how skeletal muscles disorders affect patients’ life. In this work, we developed a mathematical model to study how fast and slow muscle fibers, well characterized in single-fiber experiments, work and generate together force and displacement in muscle bundles. We characterized the parameters of a Hill-type model, using experimental data on fast and slow single human muscle fibers, and comparing experimental data with numerical simulations obtained from finite element (FE) models of single fibers. Then, we developed a FE model of a bundle of 19 fibers, based on an immunohistochemically stained cross section of human diaphragm and including the corresponding properties of each slow or fast fiber. Simulations of isotonic contractions of the bundle model allowed the generation of its apparent force–velocity relationship. Although close to the average of the force–velocity curves of fast and slow fibers, the bundle curve deviates substantially toward the fast fibers at low loads. We believe that the present model and the characterization of the force–velocity curve of a fiber bundle represents the starting point to link the single-fiber properties to those of whole muscle with FE application in phenomenological models of human muscles.     

Influence of inorganic phosphate and pH on ATP utilization in fast and slow skeletal muscle fibers.

The influence of P(i) and pH was studied on myofibrillar ATP turnover and force development during maximally activated isometric contractions, in skinned single fibers from rabbit soleus and psoas muscle. ATP hydrolysis was coupled to the breakdown of NADH, which was monitored photometrically at 340 nm. In psoas the depression by phosphate of force is twice that of ATP turnover, but in soleus force and ATP turnover are depressed equally by P(i). Most, but not all, of the ATPase and force values observed for a combination of high P(i) and low pH could be explained by independent effects of P(i) and pH. The effects of P(i) and pH on ATP turnover can be understood by a three-state cross-bridge scheme. Mass action of phosphate on the reaction from the actomyosin(AM).ADP state to the AM.ADP.P(i) state may largely account for the phosphate dependencies of ATPase activity found. Protons affect cross-bridge detachment from the AM.ADP state and the rate of the AM.ADP.P(i)-to-AM.ADP transition. In this scheme, the effects of P(i) and pH on cross-bridge kinetics appeared to be largely independent.