Size and metabolic properties of fibers in rat fast-twitch muscles after hindlimb suspension

Hindlimb suspension (HS) results in whole muscle atrophic and metabolic changes that vary in magnitude in different hindlimb muscles. The present study was designed to investigate these effects in single fibers. Fiber type and size and the activities of two metabolic marker enzymes were determined in a deep (close to the bone) and a superficial (away from the bone) region of the medial gastrocnemius (MG) and the tibialis anterior (TA) of control (CON) and 28-day HS adult female rats. Fibers were classified as dark or light adenosinetriphosphatase (ATPase) based on their qualitative staining reaction for myosin ATPase following alkaline preincubation. Fiber area and succinate dehydrogenase (SDH) and alpha-glycerophosphate dehydrogenase (GPD) activities were determined in tissue sections by use of an image analysis system. After 28 days of HS, the mean body weights of the CON and HS were similar. MG atrophied 28%, whereas TA weight was maintained in the HS. Both dark and light ATPase fibers in the deep region of the MG had smaller cross-sectional areas following HS, with the atrophic response being approximately twice as great in the light ATPase fibers. No significant changes in fiber type composition in either muscle or in fiber sizes in the superficial region of the MG or in either region of the TA were observed. Mean SDH activities of both fiber types were significantly lower in the MG and TA following HS. In contrast, mean GPD activities were either increased or maintained in light and dark ATPase fibers of both muscles in HS. Changes in SDH and GPD activity could not be directly linked to changes in fiber cross-sectional area. In summary, these data suggest an independence of the mechanisms determining muscle fiber size and metabolic adaptations associated with HS.     

Enzymatic responses of cat medial gastrocnemius fibers to chronic inactivity.

The role of neuromuscular activity in maintaining the normal enzyme heterogeneity found in a predominantly fast mixed muscle was studied. Enzymatic profiles of single fibers in the adult cat medial gastrocnemius (MG) were examined after almost complete elimination of neuromuscular activity for 6 mo. Inactivity was achieved by spinal cord isolation (SI), i.e., spinal transection at T12-T13 and L7-S1 combined with bilateral dorsal rhizotomy between the two transection sites. Cross-sectional area and succinate dehydrogenase (SDH) and alpha-glycerophosphate dehydrogenase (GPD) activities were determined in a population of fibers identified in frozen serial cross sections. Each fiber was categorized as light or dark on the basis of its staining characteristics for qualitative myosin adenosinetriphosphatase (ATPase), alkaline preincubation, and its reaction to fast and slow myosin heavy chain (MHC) antibodies. SI resulted in a conversion of nearly all light (approximately 36% in the control) to dark ATPase fibers. Virtually all MG fibers in the SI cats reacted with the fast MHC antibody, whereas very few fibers reacted with slow MHC antibody. On the basis of fiber cross-sectional area, it was estimated that the MG atrophied by approximately 10% after SI. Compared with the mean of the dark and light ATPase fibers in control (weighted by the percent fiber type distribution), mean SDH activity was significantly lower (approximately 70%) and mean GPD activity was significantly higher (approximately 120%) in the SI cats. These data indicate that prolonged electrical silence of a mixed fast hindlimb extensor results in virtually all fibers expressing fast MHC as well as oxidative and glycolytic enzyme profiles normally observed in fast glycolytic fibers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of spaceflight on rat skeletal muscle

The size, succinate dehydrogenase (SDH) and alpha-glycerolphosphate dehydrogenase (GPD) activities, and alkaline myofibrillar adenosinetriphosphatase (ATPase) staining properties were determined from quantitative histochemical analyses of single fibers from five hindlimb muscles of six male rats exposed to a 7-day National Aeronautics and Space Administration spaceflight mission (SL-3). These same properties were determined in a group of ground-based control rats housed under simulated environmental conditions. The wet weight of each of the flight muscles was significantly reduced relative to control. However, the loss of mass varied from 36% in the soleus to 15% in the extensor digitorum longus. The cross-sectional areas of fibers in the flight muscles also were reduced, except for the dark ATPase fibers in the medial gastrocnemius. The greatest relative fiber atrophy occurred in the muscles with the highest proportion of light ATPase fibers. An increase in the percentage of dark ATPase fibers also was observed in flight muscles with a predominance of light ATPase fibers. Also, there was an increase in the biochemically determined myofibrillar ATPase activity of tissue sections of the flight soleus. No changes in histochemical or biochemical measures of ATPase activity were observed in the flight extensor digitorum longus. In general, the SDH activity of flight muscles was maintained, whereas GPD activity either was maintained or increased. Based on a metabolic profile of ATPase, SDH, and GPD, there was an increase in the proportion of fast oxidative-glycolytic fibers in some muscles.     

Size and metabolic properties of single muscle fibers in rat soleus after hindlimb suspension

The effects of 28 days of hindlimb suspension (HS) and HS plus 10 daily forceful lengthening contractions on rat soleus muscle fibers were studied. Compared with age-matched controls (CON), soleus wet weights of suspended rats were significantly decreased (approximately 49%). In HS rats, the light adenosinetriphosphatase (ATPase) fibers (staining lightly for myosin ATPase, pH = 8.8) atrophied more than the dark ATPase fibers (staining darkly for myosin ATPase, pH = 8.8). Single-fiber alpha-glycerophosphate dehydrogenase (GPD) and succinate dehydrogenase (SDH) activities and the proportion of dark ATPase fibers were higher in HS than CON rats. Daily forceful lengthening contractions did not prevent the suspension-induced changes. These results considered in conjunction with a collaborative study on the mechanical properties of HS rats (Roy et al., accompanying paper) suggest a shift in the contractile potential of the muscle following HS without a deficit in SDH, a metabolic property commonly associated with resistance to fatigue. The results support the view that soleus muscle fibers can change from a slow-twitch oxidative to a fast-twitch oxidative-glycolytic profile, but rarely to a fast-twitch glycolytic one, and that SDH and GPD activity per volume of tissue can be maintained or increased even when there are severe losses of contractile proteins.     

Enzyme profiles of single muscle fibers never exposed to normal neuromuscular activity.

Do muscle fiber properties commonly associated with fiber types in adult animals and the population distribution of these properties require normal activation patterns to develop? To address this issue, the activity of an oxidative [succinic dehydrogenase (SDH)] and a glycolytic [alpha-glycerophosphate dehydrogenase (GPD)] marker enzyme, the characteristics of myosin adenosinetriphosphatase (myosin ATPase, alkaline preincubation), and the cross-sectional area of single fibers were studied. The soleus and medial gastrocnemius of normal adult cats were compared with cats that 6 mo earlier had been spinally transected at T12-T13 at 2 wk of age. In control cats, SDH activity was higher in dark than light ATPase fibers in the soleus and higher in light than dark ATPase fibers in the medial gastrocnemius. After transection, SDH activity was similar to control in both muscles. GPD activity appeared to be elevated in some fibers in each fiber type in both muscles after transection. The cross-sectional areas most affected by spinal transection were light ATPase fibers of the soleus and dark ATPase fibers of the medial gastrocnemius, the predominant fiber type in each muscle. These data demonstrate that although the muscle fibers of cats spinalized at 2 wk of age presumably were never exposed to normal levels of activation, the activity of an oxidative marker enzyme was maintained or elevated 6 mo after spinal transection. Furthermore, although the absolute enzyme activities in some fibers were elevated by transection, three functional protein systems commonly associated with fiber types, i.e., hydrolysis of ATP by myosin ATPase and glycolytic (GPD) and oxidative (SHD) metabolism, developed in a coordinated manner typical of normal adult muscles.     

Histochemical, enzymatic, and contractile properties of skeletal muscle fibers in the lizard Dipsosaurus dorsalis

Lizard skeletal muscle fiber types were investigated in the iliofibularis (IF) muscle of the desert iguana (Dipsosaurus dorsalis). Three fiber types were identified based on histochemical staining for myosin ATPase (mATPase), succinic dehydrogenase (SDH), and alphaglycerophosphate dehydrogenase (alphaGPDH) activity. The pale region of the IF contains exclusively fast-twitch-glycolytic (FG) fibers, which stain dark for mATPase and alphaGPDH, light SDH. The red region of the IF contains fast-twitch-oxidative-glycolytic (FOG) fibers, which stain dark for all three enzymes, and tonic fibers, which stain light for mATPase, dark for SDH, and moderate for alphaGPDH. Enzymatic activities of myofibrillar ATPase, citrate synthase, and alphaGPDH confirm these histochemical interpretations. Lizard FG and FOG fibers possess twitch contraction times and resistance to fatigue comparable to analogous fibers in mammals, but are one-half as oxidative and several times as glycolytic as analogous fibers in rats. Lizard tonic fibers demonstrate the acetylcholine sensitivity common to other vertebrate tonic fibers.     

Histochemical heterogeneity of intrafusal muscle fibres in slow and fast skeletal muscles of the rat

Summary Intrafusal muscle fibres of the slow soleus (Sol) and fast vastus lateralis (VL) muscles of the rat were studied histochemically. Serial transverse sections were incubated for the localization of succinate dehydrogenase (SDH), alpha glycerophosphate dehydrogenase (GPD) and adenosine triphosphatase (ATPase). The latter was examined further after preincubation in acidic solution held at either low or room temperature (R_T). The bag₂ intrafusal fibres in both muscles displayed high regular and acid stable ATPase, but low SDH and GPD activities. Bag₁ intrafusal fibres showed low to moderate regular ATPase, a regional heterogeneity after R_T acid preincubation (low activity in juxtaequatorial and high in polar zones), moderate SDH, but low GPD reactions. In both muscles the chain fibres usually exhibited high ATPase for both regular and cold acid preincubated reactions, but usually low activity after R_T acid preincubation; they had high SDH but variable GPD activities. In Sol muscle, however, approximately 25% of spindles contained chain fibres that showed high acid-stable ATPase reaction after both cold and R_T acid preincubation. In contrast, chain fibres in some VL spindles had a characteristically low ATPase reaction even after cold acid preincubation. This study, therefore, has delineated the existence of an inherent heterogeneity among chain fibres (with respect to their histochemical reactions) in muscle spindles located within slow and fast muscles and also between those found within populations of either Sol or VL muscle spindles.     

Skeletal muscle fiber, nerve, and blood vessel breakdown in space-flown rats

Histochemical and ultrastructural analyses were performed postflight on hind limb skeletal muscles of rats orbited for 12.5 days aboard the unmanned Cosmos 1887 biosatellite and returned to Earth 2 days before sacrifice. The antigravity adductor longus (AL), soleus, and plantaris muscles atrophied more than the non-weight-bearing extensor digitorum longus, and slow muscle fibers were more atrophic than fast fibers. Muscle fiber segmental necrosis occurred selectively in the AL and soleus muscles; primarily, macrophages and neutrophils infiltrated and phagocytosed cellular debris. Granule-rich mast cells were diminished in flight AL muscles compared with controls, indicating the mast cell secretion contributed to interstitial tissue edema. Increased ubiquitination of disrupted myofibrils implicated ubiquitin in myofilament degradation. Mitochondrial content and succinic dehydrogenase activity were normal, except for subsarcolemmal decreases. Myofibrillar ATPase activity of flight AL muscle fibers shifted toward the fast type. Absence of capillaries and extravasation of red blood cells indicated failed microcirculation. Muscle fiber regeneration from activated satellite cells was detected. About 17% of the flight AL end plates exhibited total or partial denervation. Thus, skeletal muscle weakness associated with spaceflight can result from muscle fiber atrophy and segmental necrosis, partial motor denervation, and disruption of the microcirculation.     

A histochemical study of postnatal differentiation of skeletal muscle with reference to functional overload.

Postnatal differentiation and growth of the fibers comprising the tonic soleus and phasic plantaris muscles, were investigated histochemically in kittens. Compensatory hypertrophy was induced by ablation of the synergistic gastrocnemius muscle.At birth both muscles consist of relatively homogeneous fiber populations as demonstrated by myosin ATPase and succinic dehydrogenase activities and glycogen content (PAS method). Differential myosin ATPase activities become evident during the first week (soleus and plantaris), while diversification of fiber types according to SDH and glycogen develop gradually and independently during the first 2 mo of life (plantaris).Compensatory hypertrophy is associated with a substantial enlargement of both dark and light fibers (incubated for myosin ATPase) and, with increases in SDH activity which are most notable in fibers that normally are low in this enzyme. The normal growth associated reduction in the percentage of fibers with high myosin ATPase activity is significantly accelerated in the hypertrophic soleus, while the hypertrophic plantaris, which normally undergoes only a slight reduction in the percentage of such fibers, is unaffected. These results underline the paramount role of the nerve fiber in the process of differentiation but also indicate that functional overload exerts a modifying influence on this process.     

A histochemical and enzymatic study of the muscle fiber types in the water monitor, Varanus salvator

Histochemical analysis of five muscles from the water monitor, Varanus salvator, identified three major classes of fibers based on histochemical activities of the enzymes myosin ATPase (mATPase), succinic dehydrogenase (SDH), and alpha-glycerophosphate dehydrogenase (alpha GPDH). Fast-twitch, glycolytic (FG) fibers were the most abundant fiber type and exhibited the following reaction product intensities: mATPase, dark; SDH, light; alpha GPDH, moderate to dark. Fast-twitch, oxidative, glycolytic (FOG) fibers were characteristically mATPase, dark; SDH, light; alpha GPDH, moderate to dark. The third class of fibers had the following histochemical characteristics: mATPase, light; SDH, moderate to dark; alpha GPDH, light. These fibers were considered to be either slow twitch, or tonic, and oxidative (S/O). Pyruvate kinase (PK), alpha GPDH, and citrate synthase (CS) activities were measured in homogenates of the same muscles studied histochemically. There was a positive relationship between both PK and alpha GPDH activities and the percentage of glycolytic fiber types within a muscle. Likewise, CS activities were greater in muscles high in FOG and S/O content. Based on CS activities, Varanus S/O fibers were eight-fold more oxidative than FG fibers within the same muscle. PK/CS ratios suggested that FG fibers possess high anaerobic capacity, similar to the iguanid lizard Dipsosaurus. The fiber type composition of the gastrocnemius muscle, relative to that of other lizard species, suggests that varanid lizards may possess a greater proportion of FOG and S/O fibers than other lizards.     

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.     

Variation and limitations in fiber enzymatic and size responses in hypertrophied muscle.

The present study was designed to determine whether the degree and kind of adaptation of a muscle fiber to a functional overload (FO) are determined by properties that are intrinsic to that fiber. The study also addresses the question of the capability of fibers to maintain a normal level of coordination of proteins per fiber as fiber volume changes dramatically. The plantaris muscle of six adult female cats was overloaded for 12 wk by bilateral synergist removal. Plantaris muscle fiber mean size doubled after FO, although some very small fibers that stained dark for adenosinetriphosphatase (ATPase) were observed in some of the FO muscles. There appeared to be no change in total succinate dehydrogenase activity per fiber. A reduction in succinate dehydrogenase activity per unit volume was observed in a substantial number of fibers, reflecting a disproportionate increase in fiber volume relative to mitochondrial volume. In contrast, total alpha-glycerophosphate dehydrogenase activity and actomyosin ATPase activity increased as fiber size increased, whereas there was no change in alpha-glycerophosphate dehydrogenase and ATPase activities per unit volume. Control and FO muscle fibers generally expressed either a fast or slow myosin heavy chain type, but in some cases FO muscle fibers expressed both fast and slow myosin heavy chains. The persistence of variability in fiber sizes and enzyme activities in fibers of overloaded muscles suggests a wide range in the adaptive potential of individual fibers to FO. These data indicate that a severalfold increase in cell size may occur without significant qualitative changes in the coordination of protein regulation associated with metabolic pathways and ATP utilization.     

Periodic weight support effects on rat soleus fibers after hindlimb suspension

The morphological and histochemical properties of the rat soleus were studied after 1 wk of hindlimb suspension, one model that removes the weight-bearing function of the hindlimbs. To examine the effectiveness of weight support activity in maintaining soleus mass, fiber size, and succinate dehydrogenase (SDH) activity, the hindlimbs of adult male Sprague-Dawley rats were suspended (HS) and half of these rats were walked on a treadmill for 40 min/day (10 min every 6 h) at 5 m/min and a 19 degree grade (HS-WS). Significant reductions in soleus mass and fiber size were found after 1 wk of HS. Weight support activity decreased the atrophic response by approximately 50%. In the alkaline myofibrillar adenosine triphosphatase (ATPase) dark-staining fibers, SDH activity was higher in the HS than control rats, whereas it was similar to control in the HS-WS rats. Total SDH activity (SDH activity X cross-sectional area) in fibers staining lightly for ATPase in HS and HS-WS rats was lower than in control rats, whereas in the darkly stained ATPase fibers it was similar among the three groups. No changes were observed in fiber type percentages after 1 wk of HS or HS-WS. The results suggest that short-duration, daily weight support activity can ameliorate, but not prevent, soleus atrophy induced by HS. Furthermore, fiber cross-sectional area is more responsive to periodic weight support in dark than light ATPase fibers. These results also demonstrate that muscle fiber atrophy need not be associated with a loss in SDH activity.     

Effects of periodic weight support on medial gastrocnemius fibers of suspended rat

Based on the current view that muscle fiber types reflect, at least to some degree, the probability of excitation of motor units in most normal movements, it was hypothesized that brief moderate periods of weight support would have little effect on a muscle that consists predominantly of high-threshold motor units. To test this hypothesis, the effects of 7 days of hindlimb suspension (HS) and HS plus intermittent weight support activity on the size and metabolic properties of individual fibers in the medial gastrocnemius (MG) were studied. HS resulted in a 40% decrease in the mean cross-sectional area of fibers that stain either dark or light for myosin adenosinetriphosphatase (ATPase) at an alkaline preincubation and are located in the deep region (i.e., close to the bone) of the MG. Dark ATPase fibers located in the superficial region were 17% smaller than controls (P greater than 0.05). Although the mean succinate and alpha-glycerophosphate dehydrogenase activities (optical density/min) per muscle fiber were not significantly (P greater than 0.05) affected by HS, it appeared that selected fibers of the deep MG region of HS rats had elevated enzyme activities. HS plus walking on a treadmill for 10 min every 6 h at 5 m/min and at a 19 degrees incline (total of 40 min/day) resulted in mean fiber cross-sectional area and enzyme activities nearer to control than the HS values. All adaptations were much less obvious in the fibers in a superficial (i.e., away from the bone) MG region.(ABSTRACT TRUNCATED AT 250 WORDS)     

Direct correlation of parvalbumin levels with myosin isoforms and succinate dehydrogenase activity on frozen sections of rodent muscle

Parvalbumin (PV) is a soluble Ca++ binding protein which is particularly concentrated in fast muscles of rodents. We have developed a new protocol to fix frozen sections of muscle by formaldehyde vapor, which enabled us to immunochemically stain serial frozen sections for PV. Fiber types were defined on the basis of myosin ATPase stability, and of isomyosins identified by a variety of antibodies because ATPase stability alone yielded ambiguous results in the mouse. Slow Type I fibers in mouse and rat were devoid of PV and had intermediate to high SDH levels. Fast fiber subtypes IIA, IIB, and IIX-like were defined in the mouse on the basis of the similarity of their myosin heavy chain immunoreactivity to these types in the rat. The soleus muscle was usually PV negative, but a small population of strongly PV-positive IIX-like fibers was present in the mouse. In mouse fast muscle, small diameter IIA fibers were PV negative with high SDH activity. In both mouse and rat, PV reactivities of IIB and IIX fibers were higher than those of IIA and I, whereas SDH levels of IIA, IIX, and I fibers were higher than those of IIB. Thus, PV content correlated with the type of myosin ATPase but not with SDH levels. The method described for immunocytochemistry of PV may be applicable to other highly soluble proteins.     

Fiber type composition of the plantarflexors of giraffes (Giraffa camelopardalis) at different postnatal stages of development

1. A sample of fibers from deep (close to the bone) and superficial (away from the bone) regions of the plantaris (PLT) and medial (MG) and lateral (LG) gastrocnemius muscles of a neonatal, a 17-day-old and an adult giraffe were typed qualitatively as dark or light based on alkaline preincubation myosin ATPase staining properties and then sized. 2. Each muscle at all ages showed a higher percentage and a larger cross-sectional area (CSA) or light ATPase fibers in the deep than the superficial region. This relationship was qualitatively, although not quantitatively, similar to that reported in hindlimb muscles of other mammals. 3. At all ages, the PLT, the deepest muscle in the synergistic group, had the highest relative total CSA of light ATPase fibers among the muscles sampled. 4. At birth, the PLT had an unusually high percentage of light ATPase fibers in comparison to that found in the same muscle of other mammals. With age, the total CSA of light ATPase fibers increased dramatically in the PLT and decreased slightly in the MG and LG. 5. These data suggest that the PLT, especially the deep portion, may functionally replace the soleus muscle which is absent in the giraffe. In addition, the fiber type results demonstrate that the changes in the fiber type composition of individual muscles observed at different postnatal ages in the giraffe are relatively similar to that reported in smaller mammals, suggesting the existence of similar regulatory mechanisms.     

Spaceflight effects on single skeletal muscle fiber function in the rhesus monkey

The purpose of this investigation was to understand how 14 days of weightlessness alters the cellular properties of individual slow- and fast-twitch muscle fibers in the rhesus monkey. The diameter of the soleus (Sol) type I, medial gastrocnemius (MG) type I, and MG type II fibers from the vivarium controls averaged 60 +/- 1, 46 +/- 2, and 59 +/- 2 microm, respectively. Both a control 1-G capsule sit (CS) and spaceflight (SF) significantly reduced the Sol type I fiber diameter (20 and 13%, respectively) and peak force, with the latter declining from 0.48 +/- 0.01 to 0.31 +/- 0.02 (CS group) and 0.32 +/- 0.01 mN (SF group). When the peak force was expressed as kiloNewtons per square meter (kN/m(2)), only the SF group showed a significant decline. This group also showed a significant 15% drop in peak fiber stiffness that suggests that fewer cross bridges were contracting in parallel. In the MG, SF but not CS depressed the type I fiber diameter and force. Additionally, SF significantly depressed absolute (mN) and relative (kN/m(2)) force in the fast-twitch MG fibers by 30% and 28%, respectively. The Ca(2+) sensitivity of the type I fiber (Sol and MG) was significantly reduced by growth but unaltered by SF. Flight had no significant effect on the mean maximal fiber shortening velocity in any fiber type or muscle. The post-SF Sol type I fibers showed a reduced peak power and, at peak power, an elevated velocity and decreased force. In conclusion, CS and SF caused atrophy and a reduced force and power in the Sol type I fiber. However, only SF elicited atrophy and reduced force (mN) in the MG type I fiber and a decline in relative force (kN/m(2)) in the Sol type I and MG type II fibers.     

An alternative domain near the ATP binding pocket of Drosophila myosin affects muscle fiber kinetics

We examined the importance of alternative versions of a region near the ATP binding site of Drosophila myosin heavy chain for muscle mechanical properties. Previously, we exchanged two versions of this region (encoded by alternative exon 7s) between the indirect flight muscle myosin isoform (IFI) and an embryonic myosin isoform (EMB) and found, surprisingly, that in vitro solution actin-activated ATPase rates were increased (higher Vmax) by both exon exchanges. Here we examined the effect of increased ATPase rate on indirect flight muscle (IFM) fiber mechanics and Drosophila locomotion. IFM expressing EMB with the exon 7a domain replaced by the IFM specific exon 7d domain (EMB-7d) exhibited 3.2-fold greater maximum oscillatory power (Pmax) and 1.5-fold greater optimal frequency of power generation (fmax) versus fibers expressing EMB. In contrast, IFM expressing IFI with the exon 7d region replaced by the EMB exon 7a region (IFI-7a), showed no change in Pmax, fmax, step response, or isometric muscle properties compared to native IFI fibers. A slight decrement in IFI-7a flight ability was observed, suggesting a negative influence of the increased ATPase rate on Drosophila locomotion, perhaps due to energy supply constraints. Our results show that exon 7 plays a substantial role in establishing fiber speed and flight performance, and that the limiting step that sets ATPase rate in Drosophila myosin has little to no direct influence in setting fmax for fast muscle fiber types.     

Myogenic and neurogenic contributions to the development of fast and slow twitch muscles in rat.

The histochemical ATPase activity and the myosin light chains of a rat fast muscle (extensor digitorum longus, EDL) and a rat slow muscle (soleus) during development have been investigated. Both muscles initially synthesize fast myosin light chains and show the intense histochemical ATPase activity characteristic of adult fast muscle fibers. After birth, the soleus begins to accumulate slow fibers with their characteristic low histochemical ATPase activity, and slow myosin light chains begin to appear. Sciatic neurectomy prevents the development of slow fibers and the synthesis of slow myosin light chains in the soleus, while the EDL is unaffected. Similarly, cordotomy of an adult rat results, in the soleus, in the appearance of fibers with more intense staining for ATPase and an increase in fast myosin light chains. The EDL is unchanged by cordotomy. As a result, we suggest that slow muscle development, but not fast muscle development, is dependent upon the functional activity of the nervous system.     

Heterogeneity of fast-oxidative muscle fibers of chicken demonstrated by anti-myosin monoclonal antibodies

Summary The fiber type composition of two fast muscles of the chicken, namely, adductor superficialis (AS) and pectoralis major (PM) was examined by the histochemical myosin ATPase staining and immunochemical techniques using monoclonal antibodies (McAbs). Two new McAbs produced against the myosin of the anterior latissimus dorsi (ALD) muscle of the chicken and named ALD-122 and ALD-83 were characterized to be specific for myosin heavy chain (MHC) and for myosin light chain-1 respectively. They were used in conjunction with previously reported McAbs specific for slow MHC (ALD-47), fast MHC (MF-14) and fast light chain-2 (MF-5). By the histochemical ATPase test most muscle fibers of AS and PM muscles reacted as IIA and IIB respectively. By immunofluorescent staining with the anti-MHC McAbs, ALD-122, and MF-14, the fibers of AS, muscle showed remarkable heterogeneity whereas PM muscle fibers reacted, uniformly. Differences in the myosin light chain composition of AS and PM muscles were also found by SDS-gel electrophoresis and immunoblot analysis with the anti-light chain McAb, ALD-83. The study clearly indicated that the histochemically homogenous (type IIA) AS muscle is composed of several subpopulations of fibers which differ in their myosin composition and that this heterogeneity of the muscle is not simply due to presence of variable amounts of slow myosin in its fibers.