Patagialis Muscle Research Articles

FOXO Proteins, Downstream of Akt Signaling, Partially Mediate both Loading Induced Hypertrophy and Subsequent Unloading Induced Atrophy In the Fast Twitch Patagialis Muscles of Middle‐Aged Japanese Quail

The aim of this study was to determine the effects of loading and subsequent unloading on AKT and FOXO protein signaling in the Patagialis (PAT) muscle of middle‐aged Japanese Quail (16–24mo). Muscle hypertrophy was induced by attaching a weight representing 10% of the animals’ body weight to one wing for 14 days. Animals were then randomly assigned to either a 0, 7, or 14 day unloading period. The contralateral limb served as the intra‐animal control. Total muscle homogenates were analyzed for the protein content of FOXO1, FOXO3a, AKT, pAKt and pGSK. Fourteen days of loading significantly increased PAT muscle mass by 22%. Muscle mass remained elevated at 7 days of unloading, however, the difference was not significant 14 days after weight removal. FOXO3a protein content was elevated after 14 days of loading (Day 0 unloading), however, there were no significant changes in the protein content of FOXO1, AKT, pAKT or pGSK. FOXO3a remained elevated at both day 7 and day 14 of unloading, although, no other measured proteins showed significant changes after 7 days of unloading. Interestingly, both pAkt and pGSK were significantly elevated after 14 days of unloading, although there were no significant changes in total Akt protein content. These results suggest that FOXO proteins, downstream of AKT, may partially mediate both skeletal muscle hypertrophy and atrophy in middle‐aged Japanese Quail. Funded by NIH: NIHROIAG021530

Interleukin-15 responses to aging and unloading-induced skeletal muscle atrophy

Interleukin-15 (IL-15) mRNA is constitutively expressed in skeletal muscle. Although IL-15 has proposed hypertrophic and anti-apoptotic roles in vitro, its role in skeletal muscle cells in vivo is less clear. The purpose of this study was to determine if skeletal muscle aging and unloading, two conditions known to promote muscle atrophy, would alter basal IL-15 expression in skeletal muscle. We hypothesized that IL-15 mRNA expression would increase as a result of both aging and muscle unloading and that muscle would express the mRNA for a functional trimeric IL-15 receptor (IL-15R). Two models of unloading were used in this study: hindlimb suspension (HS) in rats and wing unloading in quail. The absolute muscle wet weight of plantaris and soleus muscles from aged rats was significantly less when compared with muscles from young adult rats. Although 14 days of HS resulted in reduced muscle mass of plantaris and soleus muscles from young adult animals, this effect was not observed in muscles from aged animals. A significant aging times unloading interaction was observed for IL-15 mRNA in both rat soleus and plantaris muscles. Patagialis (PAT) muscles from aged quail retained a significant 12 and 6% of stretch-induced hypertrophy after 7 and 14 days of unloading, respectively. PAT muscles from young quail retained 15% hypertrophy at 7 days of unloading but regressed to control levels following 14 days of unloading. A main effect of age was observed on IL-15 mRNA expression in PAT muscles at 14 days of overload, 7 days of unloading, and 14 days of unloading. Skeletal muscle also expressed the mRNAs for a functional IL-15R composed of IL-15Ralpha, IL-2/15R-beta, and -gammac. Based on these data, we speculate that increases in IL-15 mRNA in response to atrophic stimuli may be an attempt to counteract muscle mass loss in skeletal muscles of old animals. Additional research is warranted to determine the importance of the IL-15/IL-15R system to counter muscle wasting.

Aging alters the reduction of pro-apoptotic signaling in response to loading-induced hypertrophy

This study tested the hypothesis that loading decreased apoptosis in skeletal muscle in an aging-dependent fashion. One wing of young and aged Japanese quails was loaded for 7 or 21 days to induce hypertrophy. The contralateral wing served as the intra-animal control. Loading increased fast-twitch quail patagialis muscle mass by 28 and 49%, after 7 or 21 days of loading, respectively in young adult birds. Muscle mass was not elevated after 7 days of loading, but increased by 29% after 21 days of loading in aged birds. Seven days of loading reduced DNA fragmentation and cytosolic accumulation of cytochrome c in muscles from young birds but not in muscles from aged birds. ARC protein content was lower and H 2O 2 content was higher in muscles from aged birds following 7 days of loading. The mitochondria-free cytosolic protein fraction from muscles loaded for 7 days had 41 and 29% lower AIF content than control muscles in young and aged birds, respectively. XIAP, an apoptotic suppressor protein increased after 7 days of loading in muscles from young adult but not aged birds. Our results suggest that loading suppresses pro-apoptotic signaling in quail muscle but aging delays or attenuates these anti-apoptotic changes.

Subcellular responses of p53 and Id2 in fast and slow skeletal muscle in response to stretch-induced overload

Tumor suppressor p53 and inhibitor of DNA-binding/differentiation Id2 were examined after 7 or 21 days of wing weighting in fast patagialis (PAT) and slow anterior latissimus dorsi (ALD) wing muscles of young adult and old Japanese quails. The contralateral wing served as the intra-animal control. Seven days of loading increased PAT and ALD muscle weight by 28 and 96%, respectively, in young birds. PAT and ALD muscle weight was 49 and 179% greater, respectively, than control muscles after 21 days of loading in young birds. In aged birds, no PAT or ALD hypertrophy was found after 7 days of loading; however, PAT and ALD muscle weight increased by 29 and 102%, respectively, after 21 days of loading. Id2 protein in the nuclear muscle fraction increased in both PAT and ALD muscles from young adult and old birds that were loaded for 7 days and in ALD muscles after 21 days of loading relative to contralateral control muscles. Nuclear p53 protein was greater in 7- or 21-day loaded PAT and ALD muscles relative to control muscles in both age groups. Cytosolic Id2 and p53 protein contents were not changed in loaded PAT or ALD muscles relative to control muscles at any time point. These data suggest that nuclear, but not cytosolic, Id2 and p53 are responsive to stretch-induced muscle overload. Moreover, the attenuated ability of the aged skeletal muscle to achieve hypertrophy does not appear to be explained by the subcellular changes in Id2 and p53 content with overload.

Id2 and p53 participate in apoptosis during unloading-induced muscle atrophy

Apoptotic signaling was examined in the patagialis (PAT) muscles of young adult and old quail. One wing was loaded for 14 days to induce hypertrophy and then unloaded for 7 or 14 days to induce muscle atrophy. Although the nuclear Id2 protein content was not different between unloaded and control muscles in either age group, cytoplasmic Id2 protein content of unloaded muscles was higher than that in contralateral control muscles after 7 days of unloading in young quails. Nuclear and cytoplasmic p53 contents and the p53 nuclear index of the unloaded muscles were higher than those in control muscles after 7 days of unloading in young quails, whereas in aged quails, the p53 and Id2 contents and p53 nuclear index of the unloaded muscles were not altered by unloading. Immunofluorescent staining indicated that myonuclei and activated satellite cell nuclei contributed to the increased number of p53-positive nuclei. Conversely, unloading in either young adult or aged PAT muscles did not alter c-Myc protein content. Although Cu-Zn-SOD content was not different in unloaded and control muscles, Mn-SOD content increased in PAT muscles after 7 days of unloading in young quails, suggesting that unloading induced an oxidative disturbance in these muscles. Moderate correlational relationships existed among Id2, p53, c-Myc, SOD, apoptosis-regulatory factors, and TdT-mediated dUTP nick end labeling index. These data indicate that Id2 and p53 are involved in the apoptotic responses during unloading-induced muscle atrophy after hypertrophy in young adult birds. Furthermore, our data suggest that there is an aging-dependent regulation of Id2 and p53 during unloading of previously hypertrophied muscles.

Differential Responses Of Apoptotic Factors To Unloading-Induced Atrophy Following Muscle Hypertrophy In Adult And Aged Quail Muscles

1035 PURPOSE: The present study investigated the responses of Bax, a pro-apoptotic factor and Bcl-2, an anti-apoptotic factor to unloading-induced atrophy following loading-induced muscle hypertrophy in adult and aged quail muscles. METHODS: A weight corresponding to 10% of the quails' body weight was attached to one wing of 10 adult Japanese quails (2 mo of age) and 11 aged quails (2 yr of age) for 14 days to induce muscle hypertrophy. Animals were sacrificed 7 days (Adult, n = 5; Aged, n = 6) or 14 days (Adult, n = 5; Aged, n = 5) after removal of the weight. The contralateral wing served as the intra-animal control. Patagialis muscles were dissected to determine the protein and mRNA contents of Bax and Bcl-2 by western immunoblot and RT-PCR, respectively. RESULTS: 7-days of unloading did not change the protein and mRNA contents of Bax in muscles of Adult quails and protein content of Bax in muscles of Aged quails. However, 7-days of unloading resulted in 20% increase (P < 0.05) in Bax mRNA content in muscles of Aged quails. After 7-days of unloading, Bcl-2 protein content decreased by 15% and 34% (P < 0.01) in muscles of Adult and Aged quails, respectively. Bcl-2 mRNA content was trend to decrease (−29%, P = 0.055) in muscles of Adult quails but unchanged in muscles of Aged quails after 7-days of unloading. 14-days of unloading did not change the protein and mRNA contents of Bax in muscles of Adult and Aged quails and the Bcl-2 protein and mRNA contents in muscles of Adult quails. However, 14-days of unloading increased Bcl-2 protein content by 47% (P < 0.01) and there was trend to increase (54%, P = 0.056) Bcl-2 mRNA content in the muscles of Aged quails. CONCLUSIONS: We conclude that muscles from adult and aged quails have different responses in Bax and Bcl-2 to unloading-induced atrophy following muscle hypertrophy. Supported by NIH AG 17143.

Transgene expression in hypertrophied and aged skeletal muscle in vivo by lentivirus delivery.

Human immunodeficiency virus type 1 (HIV-1)-based lentiviral vectors can infect dividing and non-dividing cells. The purpose of this study was to investigate the efficacy of lentiviral transfer of the green fluorescent protein (eGFP) gene into post-mitotic quail skeletal muscle and into fibers undergoing hypertrophy and hyperplasia via addition of satellite cell nuclei. A vesicular stomatitis virus G-protein (pMFG)-pseudotyped lentiviral vector encoding the eGFP gene coding sequence, flanked by two long terminal repeats (LTRs) (pHR-CMV-eGFP-Wsin-18) and an optional plasmid (pRSVrev), was generated in 293T cells by means of the three-plasmid system. The resulting lentiviral vector, LtV/eGFP, was injected in vivo into the patagialis muscles of young (6 weeks of age) and senescent (100 weeks of age) Japanese quail. Polybrene (10 micro g/muscle) was injected in the right patagialis muscles of some birds. A weight was attached to the left wing of birds immediately following LtV/eGFP injection for 7, 14 or 21 days to induce hypertrophy in the patagialis muscle via satellite cell activation. A time-released bromodeoxyuridine (BrdU) pellet was implanted in some birds to identify satellite cells that had proliferated during stretch-overload. Control and experimental muscles were examined 7, 14 or 21 days following injection and wing loading. Seven days post-injection proved to be insufficient to visualize and quantify the eGFP signal. EGFP-positive fibers were evident 14 and 21 days after injection of LtV/eGFP. As expected, wing weighting resulted in hypertrophy of the patagialis of senescent and young birds and an increase in collagen and other connective tissue in muscles of old birds. Nevertheless, this did not interfere with expression levels or intensity of the eGFP vector because the area of eGFP-positive fibers normalized to total muscle area was similar in control and overloaded muscles, and in muscles that were injected with the lentivirus vector but not loaded. Polybrene improved eGFP expression by 41% as compared with muscles that received only the lentivirus. These findings provide the first evidence that this lentivirus vector is an efficient tool for gene transfer and expression in mature (non-growing) fibers of adult birds, sarcopenic fibers of senescent birds and hypertrophying skeletal muscle fibers of both young adult and old birds. This system could be useful for tissue engineering, or treating aging-associated muscle-wasting diseases such as sarcopenia, especially when exercise/overload are added to gene therapy approaches to combat muscle loss with aging.

Attenuation of Ca 2+-activated ATPase and shortening velocity in hypertrophied fast twitch skeletal muscle from aged Japanese quail

The effect of aging on the in vitro contractile properties of the patagialis (PAT) muscle of 35 young adult (YA; 8 weeks of age) and 35 aged adult (AA, 110 weeks of age) Coturnix quails was examined after 0–30 days of stretch-overload. Overload was achieved by placing a weight equivalent to 12% of the birds' body weight on one wing. The contralateral wing served as the intra-animal control. Overload increased the weight of the PAT by 45.1±2.1% in YA, and 24.1±2.6% in AA. Twitch contraction time increased with loading from 43.2±1.2 to 67.3±2.2 ms in YA birds and 57.2±1.7 to 77.4±1.9 ms in AA birds. Unloaded shortening velocity (Vo) decreased by 40.1±2.2 and 38.8±3.2% in YA and aged birds, respectively. The decrease in fast myosin expression was greater in overloaded muscles of YA (20%) as compared to AA birds (12%). However, this was accompanied by a greater decrease in total muscle ATPase activity in aged birds (61%) compared to YA birds (40%). Myosin isozyme Ca 2+-ATPase activity was 26% lower in FM1 but not other fast myosins in YA birds, but it was ∼30% lower in all fast myosins in PAT muscles of aged birds. These data show that the reduction of Vo and the increase in twitch duration with aging may be due in part to reductions in ATPase activity in all myosin isoforms, as compared to myosin isoforms isolated from YA birds.

Serum response factor mRNA induction in the hypertrophying chicken patagialis muscle.

Gene expression in the stretched chicken patagialis (Pat) muscle has not been extensively examined. This study's purpose was to determine the Pat muscle's expression pattern of serum response factor (SRF), skeletal alpha-actin, and MyoD mRNAs after 3 days (onset of stretch), 6 days (end of first week of rapid growth), and 14 days (slowed rate of stretch-induced growth) of stretch. SRF mRNA demonstrated two species (B1 and B2), with B2 being more prevalent in the predominantly fast-twitch Pat muscle, compared with the slow-tonic muscle. Stretch overload increased B1 and B2 SRF mRNA concentrations, and the increase in B1 SRF mRNA concentration was greater at day 6 compared with days 3 or 14. MyoD mRNA concentration was greater in 3-day-stretched Pat muscles, compared with days 6 or 14. Skeletal alpha-actin mRNA concentration was not changed during the study. Gel mobility shift assays demonstrated that SRF binding with serum response element 1 of the skeletal alpha-actin promoter had no altered binding patterns from 6-day-stretched Pat nuclear extracts. It appears that SRF and MyoD mRNAs are induced in the stretch-overloaded Pat muscle but at different time points.

Hypertrophy-stimulated myogenic regulatory factor mRNA increases are attenuated in fast muscle of aged quails.

Myogenic regulatory factors (MRFs) are a family of skeletal muscle-specific transcription factors that regulate the expression of several muscle genes. This study was designed to determine whether MRF transcripts were increased in hypertrophy-stimulated muscle of adult quails and whether equivalent increases occurred in muscles of older quails. Slow-tonic anterior latissimus dorsi and fast-twitch patagialis muscles of adult, middle-aged, aged, and senescent quails were stretch overloaded for 6, 24, or 72 h, with contralateral muscles serving as controls. RNase protection assays showed that MRF4 and MyoD transcript levels were increased and myogenin and Myf5 transcripts were induced in stretch-overloaded muscles. However, MRF4 and MyoD increases were significantly attenuated in patagialis muscles of older quails. RT-PCR analyses of three MRF-regulated genes showed that increases in the transcription of these genes occurred with stretch overload, but the increases were less in muscles of older quails. In summary, attenuated MRF responses in muscles from aged animals may partially explain why muscles from older animals do not hypertrophy to the same extent as muscles from younger animals.

Adaptations of myonuclei to hypertrophy in patagialis muscle fibers from aged quail

Hypertrophic responses to stretch are attenuated in slow-tonic muscles from old quail, relative to young birds. It is not known if the age-associated differences would be similar in fast-twitch muscles after stretch. This study compared the effect of 30 days of stretch overload on slow β fibers and fast-α fibers in patagialis (PAT) muscles from young adult (YA) quail aged 12 weeks, old adult (OA) quail aged 52 weeks and the oldest (OO) quail, aged 90 weeks. The PAT muscle was stretched for 7, 14, 21, or 30 days by a sleeve that was attached to one wing. Birds received a subcutaneously implanted 5′-bromo-2′-deoxyuridine (BrdU) pellet that provided a constant release of 0.22mg BrdU/g body wt/day to label nuclei that had undergone DNA synthesis during stretch. Fiber cross-sectional area (CSA) was determined by planimetry from slow β and fast-α fibers after identification by myosin ATPase. Harris hematoxylin was used to identify muscle nuclei. Muscle mass increased by 44.1 ± 3.1, 32.6 ± 3.9 and 25.7 ± 4.3% in YA, OA and OO birds, respectively. Slow-β fiber hypertrophy was observed at day 7 of stretch in all birds; however, YA birds had greater hypertrophy than the older birds. Fast-α fibers were unaffected by stretch. No significant difference was observed between PAT muscles from OA and OO birds. Total fiber number was not significantly elevated in the PAT muscle from any age group after 30 days of stretch. Activated satellite cells were not detected in stretched muscles of either young or old birds. This suggests that the increase in mean slow-β fiber CSA was potentiated by existing myonuclei rather than by recruiting additional nuclei.

Modulation of IGF mRNA abundance during stretch-induced skeletal muscle hypertrophy and regression.

Increased load on a muscle (synergistic overload or stretch) results in muscle hypertrophy. The expression of insulin-like growth factor I (IGF-I) mRNA in rat skeletal muscle is increased during synergistic overload-induced hypertrophy. Although it has also been established that fasting animals lose muscle protein, it has been shown that compensatory muscle hypertrophy occurs in adult fasting rats that are undergoing a net loss of body weight. The purpose of this investigation was to determine whether a relationship exists between IGF-I mRNA levels and muscle growth and regression. This was accomplished by examining whether IGF-I mRNA levels were altered during muscle hypertrophy after stretch and regression and the effect of fasting on IGF-I mRNA levels during stretch-induced hypertrophy. Patagialis (PAT) muscle weights increased 13 and 44% at 2 and 11 days of stretch, respectively. However, after removal of the stretch stimulus on day 11, PAT weights began to decrease, reaching control weights by 18 days. During the first time point (2 days), PAT muscle IGF-I mRNA remained constant. IGF-I mRNA abundance was threefold greater than contralateral control levels by 11 days of stretch. IGF-I mRNA levels decreased but remained significantly above control levels throughout the regression of hypertrophy (13, 18, and 25 days). Fasting did not alter PAT muscle response to stretch. After 11 days of stretch, PAT muscle weight increased 60% compared with contralateral control muscles and IGF-I mRNA levels increased three-fold. This study supports a role for IGF-I in muscle hypertrophy but not muscle atrophy.

Expression of embryonic myosin heavy chain mRNA in stretched adult chicken skeletal muscle

Chronic stretch of the chicken fast-twitch patagialis muscle increases the rate of growth and percentage of fast-twitch oxidative fibers. We have analyzed the effects of stretch on the expression of two previously identified "embryonic" myosin heavy chain (MHC) mRNAs (p251 and p110). Both MHC mRNAs were expressed in the patagialis at their highest levels in the embryo and 1 wk after hatching. During posthatch development (7-52 wk), the p110 mRNA was expressed in only trace quantities while the p251 mRNA was not detectable. After 2 wk of stretch of the patagialis in 7- or 38-wk-old birds, the p110 mRNA was increased to levels similar to that found in patagialis of newly hatched chicks, whereas expression of the p251 transcript was not affected. The existence of two other MHC mRNAs homologous to the p110 mRNA was suggested by the S1 mapping analysis, one of which was expressed at dramatically reduced levels in the stretched patagialis. It is concluded that stretch can cause selective alterations in the expression of developmentally regulated MHC isoforms in chicken fast-twitch muscle.

Myosin expression in hypertrophied fast twitch and slow tonic muscles of normal and dystrophic chickens

Disruption of the development program of myosin gene expression has been reported in chicken muscular dystrophy. In the present report, the relationship between muscular dystrophy and the ability of muscle to respond to an increased work load with a transition in the myosin phenotype has been investigated. Hypertrophy of slow tonic anterior latissimus dorsi (ALD) and fast twitch patagialis (PAT) muscles was induced by overloading for 35 days and myosin expression was analyzed by electrophoresis and immunocytochemistry. Normal and dystrophic chicken ALD muscles have nearly identical proportions of SM-1 and SM-2 isomyosins and both exhibit an age-related repression of the SM-1 isomyosin which is enhanced and accelerated by overloading. Immunocytochemistry with anti-myosin heavy chain (MHC) antibodies demonstrates the appearance of nascent myofibers in overloaded ALD muscles from both normal and dystrophic chickens. A minor fast twitch fiber population is also identified which doubles in number with overloading in normal ALD muscles. There are only half as many fast twitch fibers in control dystrophic ALD muscles and this number does not increase with overloading. In contrast to ALD muscles, the isomyosin profile of normal and dystrophic PAT muscles is quite different. There is significantly more FM-3 and significantly less FM-1 isomyosin in the dystrophic PAT muscle. However, both normal and dystrophic PAT muscles exhibit an overload-induced accumulation of the FM-3 isomyosin. Immunocytochemistry reveals that, unlike the normal PAT muscle, the dystrophic PAT muscle contains a population of myofibers which express slow MHCs. As in the ALD muscle, overload-induced hypertrophy is associated with a repression of the SM-1 MHC in these fibers. Nascent myofiber formation does not occur in either normal or dystrophic overloaded PAT muscles.

Varying amounts of stretch stimulus regulate stretch-induced muscle hypertrophy in the chicken

1. 1. The effects of different amounts of passive stretch per day and number of days of stretch on muscle hypertrophy in the chicken patagialis (PAT) muscle were determined. 2. 2. Stretch for 24 hr per day (h/d) resulted in a more rapid hypertrophy both on a wet and dry tissue basis ( P < 0.001) than stretch for 4 h/d. 3. 3. Stretch increased PAT weight 43% and 25% in 24 h/d and 4 h/d treatments, respectively, after 10 days of stretch, but by day 25 of stretch there was no difference between treatments. 4. 4. In a second experiment, the PAT muscle was hypertrophied and then the effects of intermittent stretch (4 h/d) on regression of hypertrophy (muscle atrophy) were investigated. 5. 5. Intermittent stretch (4 h/d) for 5 and 10 d significantly ( P < 0.001) inhibited regression of hypertrophied muscle. 6. 6. The results of the present study indicate that stretch-induced hypertrophy can be modulated by varying the amount of stretch applied per day. 7. 7. Intermittent stretch can be used to inhibit the regression which occurs when a continuous stretch stimulus is removed. 8. 8. Intermittent stretch is a useful model for investigating mechanisms of muscle hypertrophy and inhibition of muscle atrophy.

Effects of food restriction on stretch induced muscle hypertrophy in chickens of various ages

1. 1. The patagialis muscle of three groups of different aged chickens was subjected to passive stretch by placing an inflexible sleeve over the left wing, and the right wing served as the control. 2. 2. Food was removed from the animals 2 days prior to applying passive stretch. After 6 days of passive stretch on young 7-week-old animals, the stretched muscle was 200% larger than the control. 3. 3. During the restriction, control muscle lost 69% of its weight and the stretched muscle maintained its original weight. 4. 4. Changes in muscle DNA and hydroxyproline content were similar to changes in muscle weight but hydroxyproline content lagged changes in muscle weight. 5. 5. Restricted 10-month-old chickens responded to passive stretch with an absolute increase in muscle mass which was accompanied by increases in protein, DNA and hydroxyproline content. 6. 6. Muscle from the 28-month-old chickens did not respond to either 11 days of restriction or passive stretch. 7. 7. The results of the present study indicate that as the chicken grows older, the ability of the muscle to respond to stimuli, stretch and/or starvation, is reduced.

Sarcomere length in normal and dystrophic chick muscles

An electron microscope study of 2- and 8-week-old normal and dystrophic chickens compared sarcomere lengths in relaxed and passively extended Patagialis (PAT) muscles. Sarcomeres were measured in dystrophic muscles only in fibers which exhibited no morphological signs of degeneration. Sarcomere lengths were not different from each other in normal muscles of 2- and 8-week-old chickens. Passive extension of the normal wing increased mean sarcomere length by 44%. Sarcomere lengths in unstretched dystrophic PAT muscles were 22 and 25% longer than unstretched normal sarcomeres at 2 and 8 weeks of age. Passive extension of the wing further increased sarcomere length of 2-week-old dystrophic muscles to the length of stretched sarcomeres in 2-week-old normal muscles. In 8-week-old dystrophic chickens, the wings could be passively extended to only 134°, rather than the normal range of 180°. In this case, passive extension of the wings did not further increase the length of sarcomeres. Increased sarcomere lengths in dystrophic muscles may indicate that dystrophic muscle fibers are being subjected to greater degrees of passive tension than normal muscle fibers during early stages of growth. Passive tension is known to promote fiber hypertrophy, nuclear proliferation, and increased oxidative metabolism in normal muscle. These responses to passive tension are also characteristic of prenecrotic stages of muscular dystrophy in chickens.

Passive stretch of adult chicken muscle produces a myopathy remarkably similar to hereditary muscular dystrophy

The wings of 10 chickens between 1 and 5 years of age were passively extended. An increase in plasma creatine phosphokinase activity was observed in 30 min, continued to rise for 24 h, and then declined, suggesting mechanically induced damage to muscle fibers. Wing muscles were removed and examined histologically at various times after stretch. Patagialis muscles, but not biceps brachii, showed the development of muscle fiber pathology. The patagialis muscle is less active than the biceps brachii and is stretched to a greater degree by wing extension. Susceptibility of muscles to development of pathology appeared to be correlated with the age of the chickens. Pathology was remarkably similar to that observed in young chickens with hereditary muscular dystrophy. Necrotic fibers exhibiting segmental necrosis, abnormal shapes, enlargement, splitting, vacuolation, and phagocytosis were evident. Of particular interest was the appearance of abnormal clusters of acetylcholinesterase activity along the sarcolemma. These sites were shown to appear on fibers of 2-week-old dystrophic chicks prior to necrosis and increase in plasma creatine phosphokinase activity. It is suggested that aging of inactive muscles may promote adhesions between muscle fibers rendering them susceptible to damage when stretched and that necrosis of dystrophic fibers may be initiated by a similar mechanism. Such could occur if the genetic defect resulted in interfiber adhesions. Support for this hypothesis by other reports in the literature is discussed.

Effects of passive stretch on growth and regression of muscle from chickens of various ages

1. 1. A non-invasive procedure was used to determine the effect of animal age on the growth response of muscle to passive stretch. 2. 2. Stretch increased patagialis muscle weight 61% in 6-week-old chicks and 34% in 10-month-old chicks, 28-month-old animals had an 18% loss of muscle mass during passive stretch. 3. 3. Removal of the stretch stimulus was followed by a rapid return of patagialis weight to control values in 6-week and 10-month animals, while muscle size of 28-month-old animals had not returned to control levels by 22 days, following removal of the stretch. 4. 4. The stretch-induced changes in muscle wet weight could, in part, be attributed to changes in muscle protein. 5. 5. Total muscle DNA content increased during rapid growth in 6-week- and 10-month-old chickens, and returned to control levels during muscle regression. 6. 6. Muscle hydroxyproline content increased in parallel with increases in muscle mass but did not return to control levels during muscle regression in 6-week-old animals. 7. 7. Results of the present study indicate that there was an effect of animal age on stretch-induced hypertrophy and regression of the patagialis muscle.

Effects of inhibiting DNA synthesis with hydroxyurea on stretch-induced skeletal muscle growth

Growth of chicken patagialis muscle in response to passive stretch for 7 days was studied in the absence and presence of the DNA synthesis inhibitor hydroxyurea. In the absence of hydroxyurea, the DNA content and concentration of the stretched muscle increased 101% and 40%, respectively, relative to the unstretched contralateral control. Stretch-induced growth was characterized by large increases in muscle wet weight (56%), protein content (44%), RNA concentration (63%), cross-sectional area (67%), and mean fiber cross-sectional area (50%). In the presence of hydroxyurea, DNA content increased only 22% whereas DNA concentration decreased 11% in response to stretch. However, except for RNA concentration, which increased 84%, stretch-induced growth in the hydroxyurea-treated birds exhibited the same characteristics as those in the sham-treated animals. Therefore, the large increase in DNA content and concentration observed after 7 days of stretch-induced growth in the chicken patagialis muscle was not necessary for the growth to occur.