Thyroxine induces transitions in red muscle kinetics and steady swimming kinematics in rainbow trout (Oncorhynchus mykiss)

During normal development, rainbow trout undergo a shift in red muscle contraction kinetics and swimming kinematics. Young trout parr have faster muscle kinetics and faster tailbeat frequency during swimming than older, larger juvenile trout. In this study, the thyroid hormone thyroxine (T(4)) was used to induce these changes in trout parr. This allowed a comparison of swimming kinematics, through the use of video analysis and electromyography, and red muscle contractile properties, through the use of in vitro muscle preparations, between natural parr and same-sized induced juveniles. The red muscle of natural parr has faster contractile properties than induced juveniles, including faster twitch time and a faster maximum shortening velocity (V(max)). Further, natural parr swim with faster tailbeat frequencies than induced juveniles. The results suggest that the natural shift in red muscle contraction kinetics observed during parr-smolt transfomation in trout directly affects swimming behavior in these fish. Also, thyroid hormones appear to induce a shift towards slower isoforms of the muscle protein myosin heavy chain (MHC), a result distinct from work on rats where thyroid hormones induce shifts towards faster forms of MHC. J. Exp. Zool. 290:115-124, 2001.     

Rainbow trout display a developmental shift in red muscle kinetics, swimming kinematics and myosin heavy chain isoform

Both activation and relaxation times of rainbow trout Oncorhynchus mykiss red muscle were shorter in parr than in older juveniles. Furthermore, parr red muscle had a faster maximum shortening velocity than that of older fish, as estimated with the force-clamp technique. Parr swam with higher tailbeat frequencies and lower tailbeat amplitude than did older fish across a range of length-specific steady swimming speeds. The developmental shift in contraction kinetics of red muscle and steady swimming kinematics was associated with a reduction from two or three myosin heavy chain isoforms in parr to one in older juveniles. This transition provides a mechanism to explain the variations in muscle contraction kinetics and swimming performance.     

Myosin heavy chain expression in the red, white, and ventricular muscle of juvenile stages of rainbow trout.

Juvenile stages of rainbow trout, smaller parr and older juveniles, termed smolts, show differences in red muscle contractile properties: parr red muscle has faster kinetics and a faster maximum shortening velocity than smolt red muscle. A developmental reduction in the number of MHC isoforms as detected by SDS-PAGE between parr and smolt has also been observed. To investigate whether this shift in contractile kinetics results from differential gene expression, three different MHC cDNA fragments, one each from red, white, and ventricular muscle, were identified. The red muscle and ventricular forms are novel MHCs, and the white muscle form is identical to a published MHC from adult trout white muscle. Tissue and developmental stage-specific expression patterns of these MHC isoforms were examined using isoform-specific RT-PCR. Ventricular muscle typically showed only the ventricular form; 60% parr and 80% smolts expressed the ventricular form only. Approximately half of the white muscle samples of either parr or smolts, 58% and 50%, respectively, expressed only white muscle MHC. Red muscle samples were the most heterogeneous, with red muscle MHC found in combination with either the white or ventricular form or both. Combining samples from the anterior and posterior, 8% of parr red muscle samples expressed solely the red muscle MHC form, and 30% of smolt red muscle samples expressed the red muscle form alone. Variations in the relative contribution of each MHC to the red muscle of parr and smolt may explain observed differences in protein composition and contractile properties. J. Exp. Zool. 290:751-758, 2001.     

Parvalbumin expression in trout swimming muscle correlates with relaxation rate

Rainbow trout (Oncorhynchus mykiss) display longitudinal and developmental shifts in muscle relaxation rate. This study aimed to determine the role of variations in parvalbumin content in modulating muscle relaxation. Parvalbumin is a low molecular weight protein that buffers myoplasmic Ca2+ and enhances muscle relaxation. In some fish, longitudinal variations in muscle relaxation have been linked to variations in the total amount of parvalbumin present in muscle and in the relative expression of two parvalbumin isoforms. We have demonstrated previously that anterior slow-twitch or red myotomal muscle relaxes more rapidly than that from the posterior for both rainbow and brook trout. Further, younger rainbow trout parr have faster red muscle relaxation rates than older smolts. Here we report similar results for fast-twitch or white muscle. We quantified the parvalbumin expression in red and white muscle from different body positions of rainbow trout parr and smolts and for brook trout (Salvelinus fontinalis) adults. There was a significant shift in total parvalbumin content of muscle: the faster muscle from the anterior myotome contained greater amounts of parvalbumin. For brook trout, longitudinal variation in relaxation rate was also associated with shifts in the relative expression of the two parvalbumin isoforms. The faster muscle of parr contained more parvalbumin. Lastly, trout white muscle tended to have higher levels of parvalbumin and greater levels of the Parv2 (relative to Parv1) isoform as compared to red muscle. Parvalbumin expression correlated with muscle relaxation rate in trout, although there were species-specific differences in the importance of altering total parvalbumin content versus shifts in relative parvalbumin isoform expression.     

The Roles of Pink and Red Muscle in Powering Steady Swimming in Scup, Stenotomus chrysops

Fishes power steady, undulatory swimming using both red andpink muscle. In this study we examined the roles of the twofiber types in generating power for swimming by using two-steptechnique. First, in vivo data is collected from swimming fish,and second, the electrical activity and muscle length changeconditions recorded in vivo are recreated in vitro with isolatedmuscle bundles. Force production and power generation by muscleduring swimming can then be estimated. In scup, both red andpink muscle are recruited to power swimming at the maximum sustainedswimming speed. For both fiber types, the duration of electricalactivity decreases from anterior to posterior. However, theamplitude of muscle length change increases anterior to posterior.Mass-specific power production increases posteriorly for bothmuscle types. The faster contraction kinetics of pink muscletranslate to higher power production pink muscle relative tored muscle for all longitudinal positions of the fish. Determinationof absolute power production, based on mass-specific power andmuscle mass, shows that the posterior regions of the fish generatethe most power for swimming. At 20°C, red muscle generatesmore absolute power than pink due to its higher muscle mass.However, at 10°C, pink muscle generates more absolute powerthan red, because red muscle produces little or no positivepower for all longitudinal positions.     

Myosin regulatory light chain expression in trout muscle

Muscle's contractile properties can vary along different trajectories, including between muscle fiber types, along the body (within a muscle fiber type), and between developmental stages. This study explores the role of the regulatory myosin light chain (MLC2) in modulating contractile properties in rainbow trout myotomal muscle. Rainbow trout show longitudinal variations in muscle activation and relaxation, with faster contractile properties in the anterior myotome. The expression of two muscle proteins, troponin T and parvalbumin, vary along the length of trout in concert with shifts in muscle activation and relaxation. However, there is no longitudinal variation in myosin heavy chain in trout. This study explores the role of MLC2 (or regulatory light chain), part of the myosin hexamer, in contributing to longitudinal variations in contractile properties of trout swimming muscle. We cloned and sequenced two isoforms of MLC2 from trout muscle and used real-time quantitative polymerase chain reaction to assess the relative expression of these two isoforms in red and white muscle from different body positions of two ages of rainbow trout: parr and smolt. Longitudinal variations in slow (sMLC2) but not fast (fMLC2) regulatory light chain isoforms were observed in young trout parr but not older trout smolts. The differences in sMLC2 expression correlated with shifts in muscle contractile properties in the parr. J. Exp. Zool. 309A:64-72, 2008. (c) 2007 Wiley-Liss, Inc.     

Muscle Dynamics in Fish During Steady Swimming

SYNOPSIS. Recent research in fish locomotion has been dominatedby an interest in the dynamic mechanical properties of the swimmingmusculature. Prior observations have indicated that waves ofmuscle activation travel along the body of an undulating fishfaster than the resulting waves of muscular contraction, suggestingthat the phase relation between the muscle strain cycle andits activation must vary along the body. Since this phase relationis critical in determining how the muscle performs in cycliccontractions, the possibility has emerged that dynamic musclefunction may change with axial position in swimming fish. Quantificationof muscle contractile properties in cyclic contractions relieson in vitro experiments using strain and activation data collectedin vivo. In this paper we discuss the relation between theseparameters and body kinematics. Using videoradiographic datafrom swimming mackerel we demonstrate that red muscle straincan be accurately predicted from midline curvature but not fromlateral displacement. Electromyographic recordings show neuronalactivation patterns that are consistent with red muscle performingnet positive work at all axial positions. The relatively constantcross-section of red muscle along much of the body suggeststhat positive power for swimming is generated fairly uniformlyalong the length of the fish.     

Muscle activity in steady swimming scup, Stenotomus chrysops, varies with fiber type and body position

The red and pink aerobic muscle fibers are used to power steady swimming in fishes. We examined red and pink muscle recruitment and function during swimming in scup, Stenotomus chrysops, through electromyography and high-speed ciné. Computer analysis of electromyograms (EMGs) allowed determination of initial speed of muscle recruitment and duty cycle and phase of muscle electromyographic activity for both fiber types. This analysis was carried out for three longitudinal positions over a range of swimming speeds. Fiber type and longitudinal position both affected swimming speed of initial recruitment. Posterior muscle is recruited at the lowest swimming speed, whereas more anterior muscle is not initially recruited until higher speeds. At more anterior positions, the initial recruitment of pink muscle occurs at a higher swimming speed than the recruitment of red muscle. The duty cycle of pink muscle EMG activity is significantly shorter than that of red muscle, reflecting a difference in the onset time of activation during each cycle of length change: pink muscle onset time follows that of red. The different patterns of usage of red and pink muscle reflect differences in their contraction kinetics. Because pink muscle generates force more rapidly than red muscle, it can be activated later in each tailbeat cycle. Pink muscle is used to augment red muscle power production at higher swimming speeds, allowing a higher aerobically based steady swimming speed than that possible by red muscle alone.     

Size-specific swimming behavior of Daphnia pulex

Body length affects several aspects of the behavior of quietlyswimming Daphnia pulex Swimming and sinking rates were measuredat 0.033 s intervals during the ‘hops’ characteristicof Daphnia swimming behavior Larger animals swim faster, covermore distance, and produce more powerful swimming strokes. LargerDaphnia also sink faster, but the sinking rate scales as lengthto the 0 58 power, far lower than the power of 2 00 predictedby Stokes Law considerations. The number of hops s^–1 wasindependent of body size, although a theoretical analysis predictshopping rate (antennal beat frequency) should increase as bodylength squared. Turning behavior, measured as the ratio of displacementto total distance, during 5 s, is also independent of body sizeIndependence of several parameters of body motion and body sizeimplies that factors other than simple mechanics affect Daphniaswimming behavior     

Red muscle function during steady swimming in brook trout, Salvelinus fontinalis.

Red muscle function during steady swimming in brook trout was studied through both in vivo swimming and in vitro muscle mechanics experiments. In the swimming experiments, red muscle activity was characterized through the use of electromyography and sonomicrometry, allowing the determination of several parameters such as tailbeat frequency, EMG burst duration, muscle length change patterns and relative phase of EMG activity and length change. Brook trout do show some shifts in these variables along their length during steady swimming, but the magnitude of these shifts is relatively small. In the muscle mechanics experiments, the in vivo muscle activity data were used to evaluate patterns of power production by red muscle during swimming. Unlike many fish species, the red muscle along the length of brook trout shows little change in isometric kinetic variables such as relaxation rate and twitch time. Furthermore, there is no rostral-caudal shift in red muscle mass-specific power output during steady swimming. This last result contrasts sharply with rainbow trout and with a variety of other fish species that power steady swimming primarily with the posterior red myotome.     

Swimming behavior of Daphnia: its role in determining predation risk

Individual swimming behavior of zooplankton can play an importantrole in determining how planktivorous fishselect their prey.Although several studies have documented the effect of preysize, contrast or degree of pigmentation, escape ability, encounterrate and abundance in determining predation risk, the importanceof individual behavior has received relatively little attentionby aquatic ecologists. Recent advances in the technology ofvideo recording and computer analysis of motion have allowedus to collect digitized three-dimensional videorecords of free-swimmingzooplankton such as Daphnia. We found that Daphnia clones, includingthose within a single species, exhibit a wide range of swimmingbehaviors as measured by swimming speed. The individual behaviorof a species cannot be adequately described by looking at oneclone. We alsoshow that different behavior observed in liveDaphnia can play an important role in determining attractivenessto visual predators. Given a choice between two clones of equalsize and visibility contrast, fish selected indi viduals fromthe faster swimming clone. Our results suggest that currentmodels of prey selection would be improved by the incorporationof individual swimming behavior because it is an important factordetermining overall prey visibility.     

How swimming fish use slow and fast muscle fibers: implications for models of vertebrate muscle recruitment

We quantified the intensity and duration of electromyograms (emgs) from the red and white axial muscles in five bluegill sunfish (Lepomis macrochirus) which performed three categories of behavior including steady swimming and burst and glide swimming at moderate and rapid speeds. Steady swimming (at 2 lengths/s) involved exclusively red muscle activity (mean posterior emg duration = 95 ms), whereas unsteady swimming utilized red and white fibers with two features of fiber type recruitment previously undescribed for any ectothermic vertebrate locomotor muscle. First, for moderate speed swimming, the timing of red and white activity differed significantly with the average onset time of white lagging behind that of red by approximately 40 ms. The durations of these white emgs were shorter than those of the red emgs (posterior mean = 82 ms) because offset times were effectively synchronous. Second, compared to steady and moderate speed unsteady swimming, the intensity of red activity during rapid unsteady swimming decreased while the intensity of white muscle activity (mean white emg duration = 33 ms) increased. Decreased red activity associated with increased white activity differs from the general pattern of vertebrate muscle recruitment in which faster fiber types are recruited in addition to, but not to the exclusion of, slower fiber types.     

Effects of sustained swimming on rainbow trout muscle structure, blood oxygen transport, and lactate dehydrogenase isozymes: evidence for increased aerobic capacity of white muscle

Groups of rainbow trout (Salmo gairdneri, Richardson) were continuously swum at 20 cm s-1 (1.0 body lengths s-1) for 0, 3, 30, and 200 days. No significant changes in fish condition factor, combined red and white muscle mass, muscle fibre size or fibre size distribution were observed. After 200 days of swimming there was a significant 2.2 fold increase in red muscle mass. Number of capillaries per red muscle fibre increased significantly in each group by a maximum of 27% after 200 days exercise. Number of capillaries per white muscle fibre increased significantly by 95% after 200 days exercise. Blood lactate, haemoglobin (Hb) concentration haematocrit, erythrocyte adenosine triphosphate, and whole blood oxygen affinity P50 were unchanged by swimming. After 30 and 200 days swimming there was a shift in expression of white muscle lactate dehydrogenase (LDH) isozymes from LDH-A to LDH-B. Within the duplicated LDH-B isozyme complex, there was a shift in expression from LDH-B to LDH-B' subunits. These results suggest that sustained swimming at 1(-1) bl s-1 increased the aerobic capacity of red and particularly white (fast) muscle of rainbow trout but did not alter the gas transport characteristics of the blood.     

Changes in satellite cell content and myosin isoforms in low-frequency-stimulated fast muscle of hypothyroid rat

Chroniclow-frequency stimulation was used to study the effects of enhancedcontractile activity on satellite cell content and myosin isoformexpression in extensor digitorum longus muscles from hypothyroid rats.As verified by immunohistochemical staining for desmin, vimentin, andmyosin heavy chain (MHC) isoforms and by histological analysis,stimulation induced a transformation of existing fast fibers towardslower fibers without signs of fiber deterioration or regeneration.Immunohistochemically detected increases in MHC I and MHC IIa isoforms,as well as reduced numbers of fibers expressing the faster MHCisoforms, mirrored the rearrangement of the thick-filament composition.These changes, especially the upregulation of MHC IIa, were accompaniedby an induction of developmental MHC isoforms in the transforming adultfibers. Satellite cell content rose 2.6-, 3.0-, and 3.7-fold over thatof corresponding controls (P < 0.05 in all cases) in 5-, 10-, and 20-day-stimulated muscles, respectively.Hypothyroidism alone had no effect on satellite cell content butresulted in a significant reduction in fiber size. The relativesatellite cell contents increased (P < 0.05) from 3.8% in euthyroid control muscles to 7.9, 11.5, and13.8% in the 5-, 10-, and 20-day-stimulated hypothyroid muscles,respectively. In 20-day-stimulated muscles, the relative satellite cellcontent reached an almost twofold higher level than that of normalslow-twitch soleus muscle. This increase occurred concomitantly with arise in myonuclear density, most probably because of the fusion of satellite cells with existing fibers.

     

Kinetic properties of myosin heavy chain isoforms in mouse skeletal muscle: comparison with rat, rabbit, and human and correlation with amino acid sequence

Stretch activation kinetics were investigated in skinned mouse skeletal muscle fibers of known myosin heavy chain (MHC) isoform content to assess kinetic properties of different myosin heads while generating force. The time to peak of stretch-induced delayed force increase (t₃) was strongly correlated with MHC isoforms [t₃ given in ms for fiber types containing specified isoforms; means ± SD with n in parentheses: MHCI 680 ± 108 (13), MHCIIa 110.5 ± 10.7 (23), MHCIIx(d) 46.2 ± 5.2 (20), MHCIIb 23.5 ± 3.3 (76)]. This strong correlation suggests different kinetics of force generation of different MHC isoforms in the following order:MHCIIb > MHCIIx(d) > MHCIIa >> MHCI. For rat, rabbit, and human skeletal muscles the same type of correlation was found previously. The kinetics decreases slightly with increasing body mass. Available amino acid sequences were aligned to quantify the structural variability of MHC isoforms of different animal species. The variation in t₃ showed a correlation with the structural variability of specific actin-binding loops (so-called loop 2 and loop 3) of myosin heads (r = 0.74). This suggests that alterations of amino acids in these loops contribute to the different kinetics of myosin heads of various MHC isoforms. isoform structure-function relationship; stretch activation; muscle mechanics     

Neuromuscular electrical stimulation training induces atypical adaptations of the human skeletal muscle phenotype: a functional and proteomic analysis

The aim of the present study was to define the chronic effects of neuromuscular electrical stimulation (NMES) on the neuromuscular properties of human skeletal muscle. Eight young healthy male subjects were subjected to 25 sessions of isometric NMES of the quadriceps muscle over an 8-wk period. Needle biopsies were taken from the vastus lateralis muscle before and after training. The training status, myosin heavy chain (MHC) isoform distribution, and global protein pattern, as assessed by proteomic analysis, widely varied among subjects at baseline and prompted the identification of two subgroups: an "active" (ACT) group, which performed regular exercise and had a slower MHC profile, and a sedentary (SED) group, which did not perform any exercise and had a faster MHC profile. Maximum voluntary force and neural activation significantly increased after NMES in both groups (+～30% and +～10%, respectively). Both type 1 and 2 fibers showed significant muscle hypertrophy. After NMES, both groups showed a significant shift from MHC-2X toward MHC-2A and MHC-1, i.e., a fast-to-slow transition. Proteomic maps showing ～500 spots were obtained before and after training in both groups. Differentially expressed proteins were identified and grouped into functional categories. The most relevant changes regarded 1) myofibrillar proteins, whose changes were consistent with a fast-to-slow phenotype shift and with a strengthening of the cytoskeleton; 2) energy production systems, whose changes indicated a glycolytic-to-oxidative shift in the metabolic profile; and 3) antioxidant defense systems, whose changes indicated an enhancement of intracellular defenses against reactive oxygen species. The adaptations in the protein pattern of the ACT and SED groups were different but were, in both groups, typical of both resistance (i.e., strength gains and hypertrophy) and endurance (i.e., a fast-to-slow shift in MHC and metabolic profile) training. These training-induced adaptations can be ascribed to the peculiar motor unit recruitment pattern associated with NMES.     

Myosin heavy chain mRNA transform to faster isoforms in immobilized skeletal muscle: a quantitative PCR study

Jänkälä, Heidi, Veli-Pekka Harjola, NielsErik Petersen, and Matti Härkönen. Myosin heavy chainmRNA transform to faster isoforms in immobilized skeletal muscle: aquantitative PCR study. J. Appl.Physiol. 82(3): 977-982, 1997.A quantitative polymerase chain reaction (PCR) method was used to measure the quantities of type I, IIa, IIx, and IIb myosin heavy chain (MHC) mRNAin total RNA preparations of the soleus, gastrocnemius, and plantarismuscles of normal and hindlimb-immobilized rats. Type IIx and even typeIIb MHC mRNA were demonstrated at extremely low levels in normalsoleus, 2.1 ± 0.4 × 10⁵and 5.0 ± 0.2 × 10⁵molecules of mRNA per microgram total RNA, respectively. Immobilization for 1 wk significantly altered the gene expression of MHC isoforms. Insoleus, both type IIx and IIb MHC genes became significantly upregulated, 24-fold (P < 0.005) and 2.6-fold (P < 0.05),respectively. In gastrocnemius, the level of type IIa MHC mRNAdecreased by 51% (P < 0.01) and thelevel of type IIx MHC mRNA increased by 140%(P < 0.05). In plantaris, the levelof type IIa MHC mRNA decreased by 58%(P < 0.005). In conclusion,immobilization changed the MHC mRNA profile in three different types ofskeletal muscle toward faster isoforms. The quantitative results permitreliable evaluation of changes in mRNA levels.

     

Mutually exclusive muscle designs: the power output of the locomotory and sonic muscles of the oyster toadfish (Opsanus tau)

Animals perform a vast array of motor activities. Although it has generally been accepted that muscles are well suited to the function that they must perform, specialization for performing one function may compromise their ability for carrying out another. We examined this principle in the toadfish muscular system: slow-twitch red and fast-twitch white myotomal muscles are used for powering swimming at relatively low frequencies, while the superfast swimbladder muscle powers mating calls by contracting at 100 Hz. We measured muscle power output over a wide range of frequencies. The red and white locomotory muscles could not generate power over ca. 2.2 and 12 Hz, respectively and, hence, could not power sound production. In contrast, the swimbladder muscle has many specializations that permit it to generate power at frequencies in excess of 100 Hz. However, these specializations drastically reduce its power output at low frequencies: the swimbladder muscle generated only one-twentieth of the power of the red muscle and one-seventh of the power of the white muscle at the frequencies used during swimming. To generate the same total power needed for swimming would require unfeasibly large amounts of swimbladder muscle that could not fit into the fish. Hence, the designs of the swimbladder and locomotory muscles are mutually exclusive.     

Function of the medial red muscle during sustained swimming in common thresher sharks: Contrast and convergence with thunniform swimmers

Through convergent evolution tunas and lamnid sharks share thunniform swimming and a medial position of the red, aerobic swimming musculature. During continuous cruise swimming these muscles move uniformly out of phase with local body curvature and the surrounding white muscle tissue. This design results in thrust production primarily from the caudal fin rather than causing whole-body undulations. The common thresher shark (Family Alopiidae) is the only other fish known to share the same medial red muscle anatomy as the thunniform swimmers. However, the overall body shape and extremely heterocercal caudal fin of the common thresher is not shared with the thunniform swimmers, which have both fusiform bodies and high aspect-ratio, lunate caudal fins. Our study used sonomicrometry to measure the dynamics of red and white muscle movement in common thresher sharks swimming in the ocean to test whether the medial position of red muscle is associated with uncoupling of muscle shortening and local body bending as characteristic of thunniform swimmers. Common threshers (～ 60–100 kg) instrumented with sonomicrometric and electromyographic (EMG) leads swam alongside of the vessel with a tail-beat frequency of ～ 0.5 Hz. EMG signals confirmed that only the red muscle was active during sustained swimming. Despite the more medial position of the red muscle relative to the white muscle, its strain was approximately 1.5-times greater than that of the overlying white muscle, and there was a notable phase shift between strain trajectories in the red muscle and adjacent white muscle. These results suggest an uncoupling (shearing) of the red muscle from the adjacent white muscle. Although the magnitude of the phase shift between red and white muscle strain was relatively constant within individuals, it varied among sharks, ranging from near zero (red and white in phase) to almost 180° out of phase. This extent in variability has not been documented previously for thunniform swimmers with a medial red muscle position and may be a characteristic of the thresher's unique body and caudal fin morphology. Nonetheless, the uncoupling of red and white muscle strain remains a consistent character associated with fishes having a medially positioned red muscle.     

Effects of isometric training on skeletal myosin heavy chain expression

This studytested the hypothesis that an isometric resistance-training programinduces upregulation of slow myosin heavy chain (MHC) expression in afast-twitch skeletal muscle. Thus we studied the effects of tworesistance-training programs on rodent medial gastrocnemius (MG) musclethat were designed to elicit repetitive isometric contractions(10-12 per set; 4 sets per session) of different duration (8 vs. 5 s) and activation frequency (100 vs. 60 Hz) per contraction during eachtraining session (total of 6 and 12 sessions). Results showed that bothtraining paradigms produced significant increases in muscle weight(~11-13%) after completion of training(P < 0.05). Significanttransformations in MHC expression occurred and involved specifically adecrease in the relative expression of the fast type IIb MHC andconcomitant increased expression of the fast type IIx MHC.These adaptations were observed in both the "white" and"red" regions of the MG, and they occurred at both the mRNA andprotein levels. These adaptations were detected after onlysix training sessions. Neither of the training programs produced anychange in the relative expression of either the slow type I MHC or themoderately fast type IIa MHC, which can be upregulated in the red MG bychronic functional overload. These findings show that theisometric protocols used in this investigation were not sufficient toinduce the hypothesized changes in the myosin heavy chain isoformexpression in rodent skeletal muscle.