Isometric training of young rats — Effects upon hind limb muscles

The soleus, rectus femoris, and gastrocnemius muscles of young rats trained isometrically for 4 weeks were studied by light and electron microscopy.--The percentage of fast-twitch oxidative muscle fibers decreased at the cost of the fast-twitch glycolytic fibers in the rectus femoris muscle. The percentages of the slow-twitch oxidative fibers did not change significantly in any of the muscles studied. The changes in the areas of the muscle fibers were specific for the muscle and the fiber type and indicate geometrical rearrangements of the fibers in the trained muscles. The Z and M lines were broader in the soleus (containing about 85% slow-twitch oxidative fibers) than in the rectus femoris muscle (containing about 90% fast-twitch glycolytic fibers), while the sarcomere length and the pseudo-H zone were similar. The length of the myosin filaments appeared to be slightly shorter in the fast rectus femoris than in the slow soleus muscle.--The hypothesis on the temporal progress of muscle adaptation to training (Müller, 1974) was substantiated. Correlations between biochemical (Exner et al., 1973a) and histochemical parameters measuring the oxidative capacity were preserved during adaptation to training. The comparison of the histochemical results with the physiological data on similar animals (Exner et al., 1973a) suggests a complex relationship between the contraction time and the percentage of fast-twitch muscle fibers.     

Effect of heavy-resistance exercise training on muscle fiber composition in young rats

The purpose of this investigation was to determine whether heavy-resistance exercise training alters the skeletal muscle fiber composition of young rats. Ten male Long Evans rats (3 wk old) were trained to lift progressively heavier weights, which were secured to the rats' tails, while they ascended a 40-cm 90 degree mesh incline 20 times/day 5 days/wk for a food reward. After 8 wk of training, they lifted 406 +/- 19 (SD) g in addition to their body weight (261 +/- 9 g). Compared with 10 sedentary pair-fed rats, no hypertrophy of forelimb muscles (biceps brachii and brachialis) was observed, but rectus femoris wet and dry weights were greater (P less than 0.01) in the trained group. In the deep region of the rectus femoris, type I fiber area was similar between groups, but the trained rats had both a lower (P less than 0.05) percentage of type I fibers and a smaller (P less than 0.05) portion of the total area occupied by type I fibers. The percentage of type IIb fibers in the deep region of the rectus femoris was also similar between groups, but the portion of the deep area composed of type IIb fibers was greater (P less than 0.05) in the trained rats. In the superficial region of the rectus femoris, the trained rats' type IIb fibers were larger (P less than 0.01) and occupied a greater (P less than 0.05) portion of the superficial muscle area.(ABSTRACT TRUNCATED AT 250 WORDS)     

Adaptations in rat skeletal muscle following long-term resistance exercise training

The purpose of this investigation was to determine whether long-term, heavy resistance training would cause adaptations in rat skeletal muscle structure and function. Ten male Wistar rats (3 weeks old) were trained to climb a 40-cm vertical ladder (4 days/week) while carrying progressively heavier loads secured to their tails. After 26 weeks of training the rats were capable of lifting up to 800 g or 140% of their individual body mass for four sets of 12–15 repetitions per session. No difference in body mass was observed between the trained rats and age-matched sedentary control rats. Absolute and relative heart mass were greater in trained rats than control rats. When expressed relative to body mass, the mass of the extensor digitorum longus (EDL) and soleus muscles was greater in trained rats than control rats. No difference in absolute muscle mass or maximum force-producing capacity was evident in either the EDL or soleus muscles after training, although both muscles exhibited an increased resistance to fatigue. Individual fibre hypertrophy was evident in all four skeletal muscles investigated, i.e. EDL, soleus, plantaris and rectus femoris muscles of trained rats, but muscle fibre type proportions within each of the muscles tested remained unchanged. Despite an increased ability of the rats to lift progressively heavier loads, this heavy resistance training model did not induce gross muscle hypertrophy nor did it increase the force-producing capacity of the EDL or soleus muscles. Accepted: 17 September 1997     

Fiber number, area, and composition of mouse soleus muscle following enlargement

Muscle fiber number, cross-sectional area, and composition were studied in response to enlargement produced by synergistic ablation in the mouse soleus muscle. The effect of the location of a histological section on the number of fibers that appear in the section was also studied using the mouse soleus muscle. Enlargement was produced in the soleus muscle of 15 male and 15 female mice by ablation of the ipsilateral gastrocnemius muscle. Fiber counts, using the nitric acid digestion method, revealed no difference between control and enlarged muscles in male and female mice. Mean fiber area, determined by planimetry, was 49.1 and 34.5% greater following enlargement in male and female mice, respectively. Increase in muscle weight could be totally accounted for by the increase in fiber area following enlargement. A transformation of type II to type I fibers occurred following enlargement for both sexes. Counts of fibers from histological sections revealed that there was a progressive decrease in the fiber number as the section was moved from the belly to the distal end of the muscle. The results of these studies indicate that muscle enlargement in the mouse soleus muscle is due to hypertrophy of the existing muscle fibers.     

Denervation and reinnervation of fast and slow muscles. A histochemical study in rats.

A histochemical study, using myosin-adenosine triphosphatase activity at pH 9.4, was conducted in soleus and plantaris muscles of adult rats, after bilateral crushing of the sciatic nerve at the sciatic notch. The changes in fiber diameter and per cent composition of type I and type II fibers plus muscle weights were evaluated along the course of denervation-reinnervation curve at 1, 2, 3, 4 and 6 weeks postnerve crush. The study revealed that in the early denervation phase (up to 2 weeks postcrush) both the slow and fast muscles, soleus and plantaris, resepctively, atrophied similarly in muscle mass. Soleus increased in the number of type II fibers, which may be attributed to "disuse" effect. During the same period, the type I fibers of soleus atrophied as much or slightly more than the type II fibers; whereas the type II fibers of plantaris atrophied significantly more than the type I fibers, reflecting that the process of denervation, in its early stages, may affect the two fiber types differentially in the slow and fast muscles. It was deduced that the type I fibers of plantaris may be essentially different in the slow (soleus) and fast (plantaris) muscles under study. The onset of reinnervation, as determined by the increase in muscle weight and fiber diameter of the major fiber type, occurred in soleus and plantaris at 2 and 3 weeks postcrush, respectively, which confirms the earlier hypotheses that the slow muscles are reinnervated sooner than the fast muscles. It is suggested that the reinnervation of muscle after crush injury may be specific to the muscle type or its predominant fiber type.     

Temporal progress of muscle adaptation to endurance training in hind limb muscles of young rats

Summary The soleus, rectus femoris and gastrocnemius muscles of young rats were studied after 3, 6 and 12 weeks of treadmill training. The muscle fibers were characterized histochemically by their succinate dehydrogenase (SDH) and myofibrillar ATPase activity, and morphometrically by their cross-sectional areas, which were corrected for different body weights of control and trained animals.After 12 weeks of training the mean area of fibers in the muscles studied was not significantly different from the controls, as expected. In the soleus muscle the percentage of the fast-twitch fibers was decreased as a result of their transformation into slow-twitch fibers. Trained soleus muscles were the only muscles showing pathologically altered fibers, suggesting overload. The percentages of fiber types and their areas exhibited changes specific for the muscles and muscle regions studied.From these results it is concluded that the adaptation follows the sequence proportional adaptation of morphometrical parameters, disproportional adaptation of the areas of fiber types, and disproportional adaptation of the percentages and/or the areas of the fiber types. It is shown by comparison with the literature that this sequence may be generalized to a sequence of increasing expense necessary for the adaptation to increasing stimuli, and that the most decisive factors for adaptation are work load, frequency of exercise, period of training, and the age of the subject at the initiation of the training.     

Running during recovery from hindlimb suspension induces transient muscle injury

The objectives were to study morphological adaptations of soleus muscle to decreased loading induced by hindlimb suspension and the effect of run training during the subsequent recovery period. Adult female Wistar rats were kept for 28 days with hindlimbs suspended. For the next 28 days, rats were assigned to a cage-sedentary or daily running group. Compared with control soleus muscles, 28 days of hindlimb suspension reduced the mass and fiber cross-sectional area to 58 and 53% of control values, respectively, and decreased type I fibers from 92 +/- 2 to 81 +/- 2%. During recovery, clusters of damaged fibers were observed in the soleus muscle, and this observation was more pronounced in trained animals. Type IIc fibers appeared transiently during recovery, and their presence was exacerbated with training, as IIc fibers increased to approximately 20% of the total by day 14 of recovery and were no longer evident at day 28. Although muscle wet mass does not differ as a result of mode of recovery at day 14, training transiently decreased the overall fiber area compared with sedentary recovery at this point. By day 28 of recovery the morphological characteristics of soleus muscle in the trained group did not differ from control muscle, whereas in the sedentary group muscle mass and overall fiber cross-sectional area were approximately 14% less than control values.     

Effect of anabolic steroids on skeletal muscle mass during hindlimb suspension

The efficacy of anabolic steroid treatment [0.3 or 0.9 mg nandrolone decanoate (Deca-Durabolin) per day] was examined in the context of sparing rodent fast-twitch plantaris and slow-twitch soleus muscle weight, sparing subcellular protein, and altering isomyosin expression in response to hindlimb suspension. Female rats were assigned to four groups (7 rats/group for 6 wk): 1) normal control (NC), 2) normal steroid (NS), 3) normal suspension (N-SUS), and 4) suspension steroid (SUS-S). Compared with control values for the plantaris and soleus muscles, suspension induced 1) smaller body and muscle weight (P less than 0.05), 2) losses in myofibril content (mg/muscle, P less than 0.05), and 3) shifts in the relative expression (expressed as %of total isomyosin) of isomyosins which favored lesser slow myosin and greater fast myosin isotypes (P less than 0.05). Steroid treatment of suspended animals (SUS-S vs. N-SUS) partially spared body and muscle weight (P less than 0.05) and spared plantaris but not soleus myofibril content (mg/muscle, P less than 0.05). However, steroid treatment did not modify the isomyosin pattern induced by suspension. In normal rats (NS vs. NC), steroid treatment enhanced body and plantaris muscle weight but not soleus weight (P less than 0.05) and did not alter isomyosin expression in either muscle type. Collectively these data suggest that in young female rats anabolic steroids 1) enhance the body weight and the weight of a fast-twitch ankle extensor in normal rats, 2) ameliorate the loss in body weight, fast-twitch muscle weight and protein content and slow-twitch muscle weight associated with hindlimb suspension.(ABSTRACT TRUNCATED AT 250 WORDS)     

Death receptor-associated pro-apoptotic signaling in aged skeletal muscle

Tumor necrosis factor-alpha (TNF-α) is elevated in the serum as a result of aging and it promotes pro-apoptotic signaling upon binding to the type I TNF receptor. It is not known if activation of this apoptotic pathway contributes to the well-documented age-associated decline in muscle mass (i.e. sarcopenia). We tested the hypothesis that skeletal muscles from aged rodents would exhibit elevations in markers involved in the extrinsic apoptotic pathway when compared to muscles from young adult rodents, thereby contributing to an increased incidence of nuclear apoptosis in these muscles. The plantaris (fast) and soleus (slow) muscles were studied in young adult (5–7 mo, n=8) and aged (33 mo, n=8) Fischer₃₄₄ × Brown Norway rats. Muscles from aged rats were significantly smaller while exhibiting a greater incidence of apoptosis. Furthermore, muscles from aged rats had higher type I TNF receptor and Fas associated death domain protein (FADD) mRNA, protein contents for FADD, BCL-2 Interacting Domain (Bid), FLICE-inhibitory protein (FLIP), and enzymatic activities of caspase-8 and caspase-3 than muscles from young adult rats. Significant correlations were observed in the plantaris muscle between caspase activity and muscle weight and the apoptotic index, while similar relationships were not found in the soleus. These data demonstrate that pro-apoptotic signaling downstream of the TNF receptor is active in aged muscles. Furthermore, our data extend the previous demonstration that type II fibers are preferentially affected by aging and support the hypothesis that type II fiber containing skeletal muscles may be more susceptible to muscle mass loses via the extrinsic apoptotic pathway.     

Muscle atrophy and hypoplasia with aging: impact of training and food restriction

The purpose of this work is to study the influence of aging, training, and food restriction on skeletal muscle mass and fiber number. Male Fischer 344 rats (n = 49) at 3 mo postpartum were assigned to three groups: 1) sedentary control (confined to cage), 2) exercise trained (18 m/min, 8 degrees grade, 20 min/day, 5 days/wk), or 3) food restricted (alternate days of free access and no access to food). At 12 and 27 mo postpartum the soleus and extensor digitorum longus (EDL) muscles were excised, weighed, and fiber number was quantified after HNO3 digestion. At 27 mo the masses of soleus and EDL muscles of sedentary control rats were 83 and 70%, respectively, of 12-mo values (138 +/- 5 and 151 +/- 4 mg). At 27 mo, soleus muscle mass of trained rats was 113% of sedentary control values, whereas EDL muscle mass was unaffected by training. At 27 mo, food restriction had no effect on the mass of both muscles compared with 27-mo sedentary control values. Fiber number was not affected by training or food restriction in both muscles. Fiber number for soleus and EDL muscles of combined groups declined with age by 5.6 and 4.2%, respectively. With aging, the small loss of muscle fibers can account at most for approximately 25% of the observed skeletal muscle atrophy.     

Mechanical properties of fast and slow skeletal muscle with special reference to collagen and endurance training

The mechanical properties of the slow soleus and the fast rectus femoris muscle under passive stretching were studied in endurance trained, untrained and lathyritic rats, aged 3 months. The soleus muscle with more abundant and cross-linked collagen had higher ultimate tensile strength and tangent modulus compared to the fast rectus femoris muscle which, on the other hand, had higher maximum strain. The inhibition of collagen cross-linking by lathyrism resulted in decreased tensile strength and stiffness, especially in the soleus muscle, whereas endurance training showed the opposite effects. It is supposed that the properties of collagen partly explain the capacity of slow muscles to maintain posture and to perform prolonged dynamic work. The effects of training on the tensile properties further indicate the close relationship between intramuscular collagen and the endurance capacity of muscles.     

PGC-1α and FOXO1 mRNA levels and fiber characteristics of the soleus and plantaris muscles in rats after hindlimb unloading

Fifteen-week-old rats were subjected to unloading induced by hindlimb suspension for 3 weeks. The peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) and forkhead box-containing protein O1 (FOXO1) mRNA levels and fiber profiles of the soleus and plantaris muscles in rats subjected to unloading (unloaded group) were determined and compared with those of age-matched control rats (control group). The body weight and both the soleus and plantaris muscle weights were lower in the unloaded group than in the control group. The PGC-1α mRNA was downregulated in the soleus, but not in the plantaris muscle of the unloaded group. The FOXO1 mRNA was upregulated in both the soleus and plantaris muscles of the unloaded group. The oxidative enzyme activity was reduced in the soleus, but not in the plantaris muscle of the unloaded group. The percentage of type I fibers was decreased and the percentages of type IIA and IIC fibers were increased in the soleus muscle of the unloaded group, whereas there was no change in fiber type distribution in the plantaris muscle of the unloaded group. Atrophy of all types of fibers was observed in both the soleus and plantaris muscles of the unloaded group. We conclude that decreased oxidative capacity and fiber atrophy in unloaded skeletal muscles are associated with decreased PGC-1α and increased FOXO1 mRNA levels.     

Isoproterenol hydrochloride augments collagen proliferation and ATPase activity in mice soleus and EDL muscles

Aim of this study is to analyze the effect of chronic administration of beta agonist isoproterenol hydrochloride (60 mg kg(-1) day(-1); 30 days) on soleus (a slow type) and extensor digitorum longus (EDL, a fast type) muscles in young mice. Isoproterenol resulted in significant increase in muscle weight to whole body weight ratio with no increase in hypertrophy index in soleus muscle. A significant increase in noncontractile protein collagen is also observed in both muscles but more prominent in soleus muscle. Collagen proliferation is also analyzed on sodium dodecyle sulphate polyacrylamide gel electrophoresis (SDS-PAGE) of pepsin soluble and Cyanogen Bromide (CN Br) treated pepsin insoluble collagen. Isoproterenol remolded the myofibrillar proteins in both muscles but significant increase in myofibrillar ATPase activity occurred only in soleus muscle. It is concluded that growth stimulatory effect of isoproterenol hydrochloride is more prominent in soleus than FDL muscle. Isoproterenol augmented the proliferation of non-contractile protein collagen in soleus and EDL muscles. The transformation in myofibrillar proteins caused by isoproterenol might lead to an enhancement of contractile performance.     

Effects of gender and functional overload on plantaris muscle morphology in the dwarf (HsdOla:dw-4) Lewis rat

To investigate relationships between pituitary function and gender on skeletal muscle growth and hypertrophy, fiber cross sectional area (CSA) and type were assessed in the plantaris muscle of normal and dwarf (Dw) male and female Lewis rats after 6 weeks of functional overload (FO). Serum growth hormone levels were 70-80% less in Dw rats of both genders, and body mass was 62% greater in normal rats when compared to their Dw counterparts. Muscle weight was affected by gender, dwarfism, and FO as well as a significant gender*Dw*FO interaction. FO increased Type I, IIA, and IIX/B fiber CSA 120%, 102%, and 75%, respectively. Only type 1H fibers exhibited a reduction in CSA as a function of gender or dwarfism. Both type IIA and IIX/B fibers were affected by a significant gender*Dw*FO interaction. Our results suggest that the growth of type II fibers is sensitive to gender and pituitary function, while hypertrophy of type II muscle fibers is a function of the interaction between mechanical load, gender, and pituitary function.     

Rectus femoris and vastus intermedius fiber excursions predicted by three-dimensional muscle models

Computer models of the musculoskeletal system frequently represent the force-length behavior of muscle with a lumped-parameter model. Lumped-parameter models use simple geometric shapes to characterize the arrangement of muscle fibers and tendon; this may inaccurately represent changes in fiber length and the resulting force-length behavior, especially for muscles with complex architecture. The purpose of this study was to determine the extent to which the complex features of the rectus femoris and vastus intermedius architectures affect the fiber changes in length ("fiber excursions"). We created three-dimensional finite-element models of the rectus femoris and vastus intermedius muscles based on magnetic resonance (MR) images, and compared the fiber excursions predicted by the finite-element models with fiber excursions predicted by lumped-parameter models of these muscles. The finite-element models predicted rectus femoris fiber excursions (over a 100 degrees range of knee flexion) that varied from 55% to 70% of the excursion of the muscle-tendon unit and vastus intermedius fiber excursions that varied from 55% to 98% of the excursion muscle-tendon unit. In contrast, the lumped-parameter model of the rectus femoris predicted fiber excursions that were 86% of the excursion of the muscle-tendon unit and vastus intermedius fiber excursions that were 97% of the excursion of the muscle-tendon unit. These results suggest that fiber excursions of many fibers are overestimated in lumped-parameter models of these muscles. These new representations of muscle architecture can improve the accuracy of computer simulations of movement and provide insight into muscle design.     

Seven skeletal muscles rich in slow muscle fibers may function to sustain neutral position in the rodent hindlimb

Skeletal muscles consist of slow-twitch and fast-twitch muscle fibers, which have distinct physiological and biochemical properties. The muscle fiber composition determines the contractile velocity and fatigability of a particular skeletal muscle. We analyzed the systemic distribution of slow muscle fibers in all rodent skeletal muscles by myosin ATPase staining and found that only seven hindlimb skeletal muscles were extremely rich in slow muscle fibers. These included the mouse piriformis (56.5%), gluteus minimus (35.7%), vastus intermedius (24.7%), quadratus femoris (69.9%), adductor brevis (44.3%), gracilis (24.6%), and soleus muscles (35.1%). In mice, the relative proportion of slow muscle fibers did not exceed 15% in skeletal muscles in other regions. The distribution of slow muscle fibers was well conserved in rats and rabbits. The soleus muscle is an important antigravity muscle in both rodents and humans; therefore, these skeletal muscles rich in slow muscle fibers might play an important role in sustaining neutral alignment of the lower extremity.     

Effect of endurance running on cardiac and skeletal muscle in rats

We studied the effect of resistance running on left cardiac ventricle size and rectus femoris muscle fiber composition. Ten male Wistar rats were trained on a treadmill 6 days per week for 12 weeks. Ten rats remained sedentary and served as controls. A higher endurance time (40%) and cardiac hypertrophy in the trained animals were indicators of training efficiency. Morphometric analysis of the left ventricle cross-sectional area, left ventricular wall, and left ventricular cavity were evaluated. The endurance-running group demonstrated a hypertrophy of the ventricular wall (22%) and an increase in the ventricular cavity (25%); (p<0.0001). Semi-quantitative analysis of rectus femoris fiber-type composition and of the oxidative and glycolytic capacity was histochemically performed. Endurance running demonstrated a significant (p<0.01) increase in the relative frequency of Type I (24%), Type IIA (8%) and Type IIX (16%) oxidative fibers, and a decrease in Type IIB (20%) glycolytic fibers. There was a hypertrophy of both oxidative and glycolytic fiber types. The relative cross-sectional area analysis demonstrated an increase in oxidative fibers and a decrease in glycolytic fibers (p<0.0001). Changes were especially evident for Type IIX oxidative-glycolytic fibers. The results of this study indicate that the left ventricle adapts to endurance running by increasing wall thickness and enlargement of the ventricular cavity. Skeletal muscle adapts to training by increasing oxidative fiber Type. This increase may be related to fiber transformation from Type IIB glycolytic to Type IIX oxidative fibers. These results open the possibility for the use of this type of exercise to prevent muscular atrophy associated with age or post-immobilization.     

Proximal skeletal muscle alterations in streptozotocin-diabetic rats: a histochemical and morphometric analysis.

The response of rat quadriceps muscle fibers to chronic streptozotocin (STZ) diabetes was studied. Transverse sections of rectus femoris muscle from diabetic and weight-matched control rats were assayed for myofibrilar adenosine triphosphatase (ATPase) and nicotinamide adenine dinucleotide-tetrazolium reductase (NADH-TR). A quantitative analysis was carried out by an automatic interactive analysis system focused on the fiber type size and distribution. STZ-induced diabetes caused important effects in this muscle, with changes in the distribution of oxidative enzyme reactions, type I fiber hypertrophy, and type II fiber atrophy, which was greater in type IIB than in type IIA. It is concluded that hypoinsulinism produces morphological alterations in proximal skeletal muscle fibers that are similar to those of neurogenic myopathy. Thus the pathological changes in these mammalian muscle fibers could explain the clinical syndrome seen in diabetic patients called "diabetic symmetrical proximal motor neuropathy," perhaps the least understood of the major neuropathic complications of diabetes.     

Age- and training-related changes in the collagen metabolism of rat skeletal muscle

The effects of ageing and life-long endurance training on the collagen metabolism of skeletal muscle were evaluated in a longitudinal study. Wistar rats performed treadmill running 5 days a week for 2 years. The activities of collagen biosynthesis enzymes, prolyl-4-hydroxylase and galactosylhydroxylysyl glucosyltransferase, were highest in the muscles of the youngest animals, decreased up to the age of 2 months and from then on remained virtually unchanged. The enzyme activity in young animals was higher in the slow collagenous soleus muscle than in the rectus femoris muscle. The enzyme activity in the soleus muscle was higher for older trained rats than older untrained rats. The relative proportion of type I collagen increased and that of type III collagen decreased with age, suggesting a more marked contribution by type I collagen to the age-related accumulation of total muscular collagen. The results show that collagen biosynthesis decreases with maturation and that life-long endurance training maintains a higher level of biosynthesis in slow muscles.     

Effects of aging and life-long physical training on collagen in slow and fast skeletal muscle in rats

Summary Intramuscular collagen in a slow (m. soleus) and a fast (m. rectus femoris) skeletal muscle was studied by biochemical, morphometric, and immunohistochemical methods. Wistar white rats of 1, 4, 10, and 24 months were used as experimental animals. Our aim was to evaluate the effects of life-long physical training (treadmill running, 5 days a week for 1, 3, 9, and 23 months depending on the age attained). The biochemical concentration of collagen was higher in m. soleus than in m. rectus femoris and it increased in youth and in old age in m. soleus. The trained rats had higher concentrations of collagen than the untrained rats at 10 and 24 months. The morphometrically measured area-fractions of both the endomysium and perimysium were higher in m. soleus than in m. rectus femoris. The age-related increase in intramuscular connective tissue was of endomysial origin. The immunohistochemical staining of type-I, -III, and -IV collagens indicated the more collagenous nature of m. soleus as compared with m. rectus femoris for all major collagen types; this was most marked for type-IV collagen of basement membrane. The results indicate that both age and endurance-type physical training further distinguish the slow and fast muscles with respect to their connective tissue.