Average Fiber Cross-sectional Area Research Articles

Chronic Hindlimb Ischemia Induces Alterations of Muscle Transcriptomes Based on Muscle Type

Introduction: Peripheral artery disease (PAD) is a progressive atherothrombotic disorder of the arteries supplying the lower limb and is the most prevalent cardiovascular disease, impacting over 230 million people worldwide. PAD is associated with lower extremity myopathy characterized by myofiber degeneration and fibrosis. Increased proportions of type I muscle fibers are associated with improved patient outcomes, but most studies, human and animal, only analyze the gastrocnemius (gastroc), which contains mixed amounts of type I and type II fibers. To elucidate the impacts of ischemia on muscles comprising mainly type I fibers, we evaluated the differential effect of hindlimb ischemia (HLI) on the soleus and gastroc, hypothesizing that chronic HLI would more severely impact the soleus. Methods: Chronic HLI was induced in Sprague-Dawley male rats with an endovascular iliofemoral artery occlusion approach. Time-dependent laser Doppler perfusion imaging assessed hindlimb perfusion before and after HLI up to 42 days. Myopathy was evaluated 28 days post-HLI by staining muscle cross-sections with Hematoxylin and Eosin (H&E) and Masson trichrome. Quantification of muscle fibrosis, average fiber cross-sectional area, and presence of central nuclei were used to determine muscle pathology. RNAsequencing was used to determine the differences in transcriptomes of the gastroc and soleus in both sham and HLI conditions. Results: Histological analysis revealed that there were significantly increased levels of fibrosis after HLI, but only in the soleus (18.24±7.23 vs. 7.71±2.54 %, n = 8-16, p-value <0.0001). The average fiber cross-sectional area was significantly reduced in the HLI soleus compared to the sham (1556.00±459.60 vs. 3072.00±831.50 μm, n = 8-16, p-value <0.0001), but not between HLI and sham gastroc. Conversely, HLI increased the proportions of centralized nuclei in both muscles (soleus — 22.72±12.46 vs. 1.12±1.49 %, n = 8-16, p-value = 0.0002, gastroc — 12.75±10.17 vs. 0.00±0.00 %, n = 8-16, p-value = 0.0002), indicating prior muscle damage. The HLI soleus exhibited worse outcomes in all parameters compared to the HLI gastroc. These data were substantiated by RNAseq, which showed that the HLI soleus transcriptome contained more differentially expressed genes (DEGs, ~1000 altered genes) than its sham counterpart compared to HLI gastroc, which possessed <100 DEGs (n = 5-6). Moreover, the HLI soleus transcriptome was highly fibrotic, pro-angiogenic, and promoted myoblast fusion, a combination that could explain the histological myopathy findings. Conclusions: Chronic HLI induces alterations of muscle ultrastructure and transcription. These effects occur predominantly in the soleus rather than the gastroc. The results of the present study implicate that the pathophysiology is different for distinct muscles and may result from varying fiber composition. This work can help identify the mechanisms responsible for the myopathy of PAD and may lead to the design of targeted interventions for the care of PAD patients. R01HL169205-01. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Trained Integrated Postexercise Myofibrillar Protein Synthesis Rates Correlate with Hypertrophy in Young Males and Females.

Resistance training induces skeletal muscle hypertrophy via the summated effects of postexercise elevations in myofibrillar protein synthesis (MyoPS) that persist for up to 48 h, although research in females is currently lacking. MyoPS is regulated by mTOR translocation and colocalization; however, the effects of resistance training on these intracellular processes are unknown. We hypothesized that MyoPS would correlate with hypertrophy only after training in both sexes and would be associated with intracellular redistribution of mTOR. Recreationally active males and females (n = 10 each) underwent 8 wk of whole-body resistance exercise three times a week. Fasted muscle biopsies were obtained immediately before (REST) and 24 and 48 h after acute resistance exercise in the untrained (UT) and trained (T) states to determine integrated MyoPS over 48 h (D2O ingestion) and intracellular mTOR colocalization (immunofluorescence microscopy). Training increased (P < 0.01) muscle strength (~20%-126%), muscle thickness (~8%-11%), and average fiber cross-sectional area (~15%-20%). MyoPS increased above REST in UT (P = 0.032) and T (P < 0.01), but to a greater extent in males (~23%; P = 0.023), and was positively (P < 0.01) associated with muscle thickness and fiber cross-sectional area at T only in both males and females. mTOR colocalization with the cell periphery increased (P < 0.01) in T, irrespective of sex or acute exercise. Training increased (P ≤ 0.043) total mTOR, LAMP2 (lysosomal marker), and their colocalization (P < 0.01), although their colocalization was greater in males at 24 and 48 h independent of training status (P < 0.01). MyoPS during prolonged recovery from exercise is greater in males but related to muscle hypertrophy regardless of sex only in the trained state, which may be underpinned by altered mTOR localization.

Metformin induces muscle atrophy by transcriptional regulation of myostatin via HDAC6 and FoxO3a

BackgroundSkeletal muscle atrophy is a severe condition that involves loss of muscle mass and quality. Drug intake can also cause muscle atrophy. Biguanide metformin is the first‐line and most widely prescribed anti‐diabetic drug for patients with type 2 diabetes. The molecular mechanism of metformin in muscle is unclear.MethodsMyostatin expression was investigated at the protein and transcript levels after metformin administration. To investigate the pathways associated with myostatin signalling, we used real‐time polymerase chain reaction, immunoblotting, luciferase assay, chromatin immunoprecipitation assay, co‐immunoprecipitation, immunofluorescence, primary culture, and confocal microscopy. Serum analysis, physical performance, and immunohistochemistry were performed using our in vivo model.ResultsMetformin induced the expression of myostatin, a key molecule that regulates muscle volume and triggers the phosphorylation of AMPK. AMPK alpha2 knockdown in the background of metformin treatment reduced the myostatin expression of C2C12 myotubes (−49.86 ± 12.03%, P < 0.01) and resulted in increased myotube diameter compared with metformin (+46.62 ± 0.88%, P < 0.001). Metformin induced the interaction between AMPK and FoxO3a, a key transcription factor of myostatin. Metformin also altered the histone deacetylase activity in muscle cells (>3.12‐fold ± 0.13, P < 0.001). The interaction between HDAC6 and FoxO3a induced after metformin treatment. Confocal microscopy revealed that metformin increased the nuclear localization of FoxO3a (>3.3‐fold, P < 0.001). Chromatin immunoprecipitation revealed that metformin induced the binding of FoxO3a to the myostatin promoter. The transcript‐level expression of myostatin was higher in the gastrocnemius (GC) muscles of metformin‐treated wild‐type (WT) (+68.9 ± 10.01%, P < 0.001) and db/db mice (+55.84 ± 6.62%, P < 0.001) than that in the GC of controls (n = 4 per group). Average fibre cross‐sectional area data also showed that the metformin‐treated C57BL/6J (WT) (−31.74 ± 0.75%, P < 0.001) and C57BLKS/J‐db/db (−18.11 ± 0.94%, P < 0.001) mice had decreased fibre size of GC compared to the controls. The serum myoglobin level was significantly decreased in metformin‐treated WT mice (−66.6 ± 9.03%, P < 0.01).ConclusionsOur results demonstrate that metformin treatment impairs muscle function through the regulation of myostatin in skeletal muscle cells via AMPK‐FoxO3a‐HDAC6 axis. The muscle‐wasting effect of metformin is more evident in WT than in db/db mice, indicating that more complicated mechanisms may be involved in metformin‐mediated muscular dysfunction.

Sixteen weeks of testosterone with or without evoked resistance training on protein expression, fiber hypertrophy and mitochondrial health after spinal cord injury.

We investigated the effects of testosterone replacement therapy (TRT) with and without evoked resistance training (RT) on protein expression of key metabolic and hypertrophy regulators, muscle fiber cross-sectional area (CSA), and markers of mitochondrial health after spinal cord injury (SCI). Twenty-two men with chronic motor complete SCI were randomly assigned to either TRT + RT (n = 11) or TRT (n = 11) for 16 wk. TRT + RT men underwent twice weekly progressive RT using electrical stimulation with ankle weights. TRT was administered via testosterone patches (2-6 mg/day). Muscle biopsies were obtained before and after 16 wk from the right vastus lateralis. Expression of proteins associated with oxidative muscles and mechanical loading (PGC-1α and FAK), muscle hypertrophy (total and phosphorylated Akt, total and phosphorylated mTOR), and cellular metabolism (total and phosphorylated AMPK and GLUT4) were evaluated. Immunohistochemistry analysis was performed to measure fiber CSA and succinate dehydrogenase (SDH) activity as well as mitochondrial citrate synthase (CS) activity and complex III (CIII) activities. TRT + RT demonstrated a robust 27.5% increase in average fiber CSA compared with a -9% decrease following TRT only (P = 0.01). GLUT4 protein expression was elevated in the TRT + RT group compared with TRT only (P = 0.005). Total Akt (P = 0.06) and phosphorylated Akt Ser389 (P = 0.049) were also elevated in the TRT + RT group. Mitochondrial activity of SDH (P = 0.03) and CS (P = 0.006) increased in the TRT + RT group, with no changes in the TRT-only group. Sixteen weeks of TRT with RT resulted in fiber hypertrophy and beneficial changes in markers of skeletal muscle health and function.NEW & NOTEWORTHY Fiber cross-sectional area (CSA), protein expression, mitochondrial citrate synthase (CS), and succinate dehydrogenase (SDH) were measured following 16 wk of low-dose testosterone replacement therapy (TRT) with and without electrically evoked resistance training (RT) in men with spinal cord injury (SCI). Fiber CSA and protein expression of total GLUT4, total Akt, and phosphorylated Akt increased following TRT + RT but not in the TRT-only group. Mitochondrial CS and SDH increased after TRT + RT but not in TRT-only group.

Soleus muscle stability in wild hibernating black bears.

Based on studies of fast skeletal muscles, hibernating black and brown bears resist skeletal muscle atrophy during months of reduced physical activity and not feeding. The present study examined atrophy sparing in the slow soleus muscle, known to be highly prone to disuse atrophy in humans and other mammals. We demonstrated histochemically that the black bear soleus is rich in slow fibers, averaging 84.0 ± 6.6%. The percentages of slow fibers in fall (87.3 ± 4.9%) and during hibernation (87.1 ± 5.6%) did not differ ( P = 0.3152) from summer. The average fiber cross-sectional area to body mass ratio (48.6 ± 11.7 µm2/kg) in winter hibernating bears was not significantly different from that of summer (54.1 ± 11.8 µm2/kg, P = 0.4186) and fall (47.0 ± 9.7 µm2/kg, P = 0.9410) animals. The percentage of single hybrid fibers containing both slow and fast myosin heavy chains, detected biochemically, increased from 2.6 ± 3.8% in summer to 24.4 ± 24.4% ( P = 0.0244) during hibernation. The shortening velocities of individual hybrid fibers remained unchanged from that of pure slow and fast fibers, indicating low content of the minority myosins. Slow and fast fibers in winter bears exhibited elevated specific tension (kN/m2; 22%, P = 0.0161 and 11%, P = 0.0404, respectively) and maintained normalized power. The relative stability of fiber type percentage and size, fiber size-to-body mass ratio, myosin heavy chain isoform content, shortening velocity, power output, and elevated specific tension during hibernation validates the ability of the black bear to preserve the biochemical and performance characteristics of the soleus muscle during prolonged hibernation.

Exercise conditioning in old mice improves skeletal muscle regeneration.

Skeletal muscle possesses the ability to regenerate after injury, but this ability is impaired or delayed with aging. Regardless of age, muscle retains the ability to positively respond to stimuli, such as exercise. We examined whether exercise is able to improve regenerative response in skeletal muscle of aged mice. Twenty-two-month-old male C57Bl/6J mice (n = 20) underwent an 8-wk progressive exercise training protocol [old exercised (O-Ex) group]. An old sedentary (O-Sed) and a sedentary young control (Y-Ctl) group were included. Animals were subjected to injections of cardiotoxin into the tibialis anterior muscle. The tibialis anterior were harvested before [O-Ex/O-Sed/Y-Ctl control (CTL); n = 6], 10 d (O-Ex/O-Sed/Y-Ctl d 10; n = 8), and 28 d (O-Ex/O-Sed/Y-Ctl d 28; n = 6) postinjection. Average fiber cross-sectional area was reduced in all groups at d 10 (CTL: O-Ex: 2499 ± 140; O-Sed: 2320 ± 165; Y-Ctl: 2474 ± 269; d 10: O-Ex: 1191 ± 100; O-Sed: 1125 ± 99; Y-Ctl: 1481 ± 167 µm(2); P < 0.05), but was restored to control values in O-Ex and Y-Ctl groups at d 28 (O-Ex: 2257 ± 181; Y-Ctl: 2398 ± 171 µm(2); P > 0.05). Satellite cell content was greater at CTL in O-Ex (2.6 ± 0.4 satellite cells/100 fibers) compared with O-Sed (1.0 ± 0.1% satellite cells/100 fibers; P < 0.05). Exercise conditioning appears to improve ability of skeletal muscle to regenerate after injury in aged mice.-Joanisse, S., Nederveen, J. P., Baker, J. M., Snijders, T., Iacono, C., Parise, G. Exercise conditioning in old mice improves skeletal muscle regeneration.

Muscle Fiber Cross-sectional Area Is Unaffected 14 Days Following A Clinical Dose Of Radiation

Table 1. Mean for RLV (L) across trials on land and submerged in water, and of FIR, ROW, WIN, LOW, and trials to criteria in each condition. RLV Land RLV Submerged Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 FIR Trial to Criteria ROW Trial to Criteria WIN Trial to Criteria LOW Trial to Criteria CONCLUSION: For healthy, young male subjects the number of trials needed to determine RLV depends upon the criteria selected. Based on means and standards deviations, it is recommended that at least 4 trials be used for ROW and WIN, and at least 5 trials for LOW. Additionally, FIR is not an appropriate criterion due to changes in RLV in subsequent trials. C-26 Free Communication/Poster - Cancer, Exercise, and Muscle Thursday, June 2, 2016, 7:30 AM - 12:30 PM Room: Exhibit Hall A/B Board #13 June 2, 9:00 AM - 10:30 AM Muscle Fiber Cross-sectional Area Is Unaffected 14 Days Following A Clinical Dose Of Radiation Krishan Bhakta 1 , Vinny Alvionita 1 , Michael J. Baker 2 , Lewis Akers 3 , Munjal M. Acharya 4 , Charles L. Limoli 4 , Vincent J. Caiozzo, FACSM 4 , Joshua A. Cotter 1 . 1 California State University, Long Beach, Long Beach, CA. 2 Fullerton College, Fullerton, CA. 3 Irvine Valley College, Irvine, CA. 4 University of California, Irvine, Irvine, CA. (Sponsor: Vincent Caiozzo, FACSM) Email: kbhakta12@gmail.com (No relationships reported) Skeletal muscle regeneration has previously been shown to be blunted following a dose of 7 Gy gamma irradiation with cardiotoxin injury. It has yet to be established if the same dose of radiation effects non-injured skeletal muscle. PURPOSE: The objective of the current study was to investigate the effects of a 7 Gy dose of irradiation on the cross-sectional area (CSA) of the tibialis anterior (TA). METHODS: Adult male mice (C57BL/J6) were assigned to one of 6 groups: 1) 4 day control, 2) 4 day irradiated (IRR), 3) 7 day control, 4) 7 day IRR, 5) 14 day control, 6) 14 day IRR. Each mouse was injected with saline before irradiation as part of a larger study. Following injection, mice in the irradiation groups were exposed to a dose of 7 Gy of gamma irradiation which is considered to be a clinically relevant dose. Following each time period the TA was removed and prepared for histological analysis by hematoxylin and eosin staining. Approximately 100 fibers of each muscle sample were analyzed with ImageJ software to estimate average fiber CSA. Data were analyzed by one-way ANOVA in GraphPad Prism. RESULTS: No differences were found between control and IRR mean fiber size at any of the time periods. CONCLUSIONS: These results indicate that a dose of 7 Gy of gamma irradiation may blunt myogenesis following cardiotoxin injury but does not alter fiber CSA up to 14 days following exposure. Board #14 June 2, 9:00 AM - 10:30 AM Role of Testosterone on Muscle Protein Syntheis during Prostate Cancer Treatment Erik D. Hanson 1 , Andre R. Nelson 2 , Daniel W.D. West 3 , John Violet 4 , Lannie O’Keefe 2 , Stuart M. Phillips, FACSM 3 , Alan Hayes 2 . 1 University of North Carolina at Chapel Hill, Chapel Hill, NC. 2 Victoria University, Melbourne, Australia. 3 McMaster University, Hamilton, ON, Canada. 4 Peter MacCallum Cancer Centre, Melbourne, Australia. (Sponsor: Stuart Phillips, FACSM) Email: edhanson@email.unc.edu (No relationships reported) Androgen deprivation therapy (ADT) is a common treatment for prostate cancer (PCa), but low testosterone levels result in muscle loss. Muscle protein synthesis (MPS) stimulated after resistance training may help to partially reverse muscle loss. However, some studies show attenuated load-mediated hypertrophy during ADT whereas others suggest that testosterone is not essential for normal accretion of lean mass. Copyright © 2016 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.

Disruption of muscle renin-angiotensin system in AT1a−/−mice enhances muscle function despite reducing muscle mass but compromises repair after injury

The role of the renin-angiotensin system (RAS) in vasoregulation is well established, but a localized RAS exists in multiple tissues and exerts diverse functions including autonomic control and thermogenesis. The role of the RAS in the maintenance and function of skeletal muscle is not well understood, especially the role of angiotensin peptides, which appear to contribute to muscle atrophy. We tested the hypothesis that mice lacking the angiotensin type 1A receptor (AT(1A)(-/-)) would exhibit enhanced whole body and skeletal muscle function and improved regeneration after severe injury. Despite 18- to 20-wk-old AT(1A)(-/-) mice exhibiting reduced muscle mass compared with controls (P < 0.05), the tibialis anterior (TA) muscles produced a 25% higher maximum specific (normalized) force (P < 0.05). Average fiber cross-sectional area (CSA) and fiber oxidative capacity was not different between groups, but TA muscles from AT(1A)(-/-) mice had a reduced number of muscle fibers as well as a higher proportion of type IIx/b fibers and a lower proportion of type IIa fibers (P < 0.05). Measures of whole body function (grip strength, rotarod performance, locomotor activity) were all improved in AT(1A)(-/-) mice (P < 0.05). Surprisingly, the recovery of muscle mass and fiber CSA following myotoxic injury was impaired in AT(1A)(-/-) mice, in part by impaired myoblast fusion, prolonged collagen infiltration and inflammation, and delayed expression of myogenic regulatory factors. The findings support the therapeutic potential of RAS inhibition for enhancing whole body and skeletal muscle function, but they also reveal the importance of RAS signaling in the maintenance of muscle mass and for normal fiber repair after injury.

Intramuscular injection of the β‐agonist formoterol enhances early functional repair after myotoxic injury in rat skeletal muscles

Systemic administration of β-adrenoceptor agonists (β-agonists) to rats exerts an anabolic action on skeletal muscle and hastens recovery of function after muscle injury. However, their clinical application has so far been limited by side effects, such as cardiac hypertrophy. We tested the hypothesis that intramuscular (i.m.) injection of the β-agonist, formoterol, could improve functional muscle repair after bupivacaine-induced injury in rats without causing deleterious effects on the heart. Adult male rats were deeply anesthetized and the right EDL muscle was surgically exposed and injected with bupivacaine to destroy all muscle fibers. The left EDL muscle served as the uninjured control. At 5 days post-injury, rats received either an i.m. injection of formoterol (100 μg) into the regenerating muscle, or allowed to recover without treatment. Following myotoxic injury, functional recovery was assessed in vitro at 7, 10, 14, and 21 days post-injury. Heart mass was also determined at each time point. At 7 days post-injury, formoterol treated muscles had 19% greater mass, 50% greater maximum isometric force, and 51% greater average fiber cross-sectional area, than untreated muscles. There were no differences in these parameters at the later time points. A single i.m. injection of formoterol enhanced early regeneration after injury and the absence of cardiac hypertrophy suggested that this was a safe mode of administration with potential therapeutic application. Treatment efficacy will likely depend on multiple i.m. injections during recovery. Supported by the Muscular Dystrophy Association (USA) and the National Health & Medical Research Council (Australia).

The molecular regulation of exercised-induced muscle fibre hypertrophy in the common carp: Expression of MyoD, PCNA and components of the calcineurin-signalling pathway

Growth was investigated over 16 d in juvenile common carp ( Cyprinus carpio L.) held in either static water (tank rested, TR 16) or exercised in a flume at 2.5–3.2 body lengths s − 1 for 18 h a day (exercised, E 16). Relative to the start of the experiment (TR 0), the TR 16 group showed a 31% increase in body mass (specific growth rate, 1.57% d − 1 ), whereas there was no net change in the E 16 group. There was, however, a significant exercise-induced hypertrophy of slow muscle fibres with average fibre cross-sectional area ( F CSA) increasing by 35% in the E 16 group, compared with 11% in the TR 16 group. In contrast, F CSA of fast muscle fibres increased by 34% in the TR 16 group compared to just 18% in the E 16 group. The relative concentrations and subcellular localisation of proteins hypothesised to play a role in the regulation of muscle growth were measured. MyoD concentration was similar in the TR 0, TR 16 and E 16 groups in both slow and fast muscle. However, there was a small (5%–10%) but statistically significant increase in nuclear localisation of MyoD in those groups showing a significant increase in F CSA over the time course of the experiment. PCNA concentration was 31% and 12% higher in the TR 16 than in either the TR 0 or E 16 groups for slow and fast muscle, respectively. Exercise resulted in a ∼10% increase in nuclear factor of T-cells (NFAT2) concentration in slow muscle but no change in NFAT2 localisation. Calcineurin B concentration was similar in tank rested and exercised groups. The results do not support a major role for the calcineurin-signalling pathway in the regulation of muscle hypertrophy in the common carp.

Changes In Soleus Myosin Heavy Chain Expression In Response To Training Following Spinal Cord Injury

PURPOSE To study the effect of treadmill vs cycling training on soleus myosin heavy chain (MHC) expression following spinal cord injury (SCI). METHODS: Contusion injuries were produced using a MASCIS impactor and protocol. Sprague Dawley rats were randomly assigned into a treadmill training, cycling training, or untrained control group. Rats in both training groups were trained from 1-week post SCI continuously for 3 months, 5 days a week, 20minutes/trail, 2 trails/day. Soleus MHC isoform expression was determined using immunofluorescence techniques with monoclonal antibodies (BA-D5, SC-71, BF-F3, and BF-35). RESULTS: Following 3 months of contusion injury, soleus MHC isoforms shifted from MHCI towards MHCII, with approx. 77.9% (vs 85.1% in the uninjured animal), 19.9% (vs 15.0% in the uninjured animal), and 4.3% (vs 0% in the uninjured animal) of the total fibers containing MHCI, MHC-IIa and MHC-IIx, respectively. Both treadmill and cycling training reversed this shift. Following 3 months treadmill training, the soleus contained 89.5 ± 2.7% type I fibers and 9.4 ± 1.5% type IIx fibers. No type IIx fibers were observed. Similar results were noted following cycling training. In addition, both training paradigms significantly increased the soleus fiber cross-sectional area (CSA), with an average fiber CSA of 2,452.9 ± 67.7mm2 in the untrained group, 2,804.1 ± 99.0mm2 and 2,779.0 ± 109.5mm2 in the treadmill and cycling group, respectively. CONCLUSION: This study demonstrates that both cycling and treadmill training paradigms can effectively reverse both the fiber type shift and decrease in fiber CSA following SCI. ACKNOWLEDGEMENTS: This study was supported by Christopher Reeve Paralysis Foundation (CRPF) # BA2-0202-2 and RO1HD37645, RO1HD40850.

Histochemical properties of fibre types in muscles of wild and domestic pigs and the effect of growth rate on muscle fibre properties

This study reports the results of a comparison of the histochemical properties of various types of porcine muscles in wild and domestic pigs. In domestic pigs, the influence of growth rates on these properties were studied in five muscles: longissimus dorsi (LD), semimembranosus (SM), gluteus superficialis (GS), infra spinam (IS) and masseter (MAS). The growth rate was expressed as: (i) live weight (LW) at 165 ± 2 days and (ii) daily LW gain from day 88 to day 165 (DG) on the experimental diet. LD, SM, GS and IS of wild pigs were found to contain a higher area percentage of oxidative type IIA fibres (type IIA% area) and a lower percentage area of glycolytic type IIB fibres (type IIB% area) than the same muscles of domestic pigs. The capillary density in the light muscles (LD, SM, GS) of wild pigs was twice that of domestic pigs, indicating higher oxidative capacity. In domestic pigs the cross sectional area of type IIB fibres (CSA IIB) was markedly larger than the cross sectional area of type I (CSA I) and IIA (CSA IIA) fibres. The average fibre cross sectional area (CSA fibre) was about the same in the muscles of wild and domestic pigs except in LD and SM, where the average fibre cross sectional area was approximately 25% smaller in wild pigs than in domestic pigs. This difference was caused by the large cross sectional area of type IIB fibres in the light muscles of domestic pigs. In the light muscles of domestic pigs, the cross sectional area of type IIA fibres increased most with increasing growth rate. Growth rate influences muscle fibre properties only in light muscles, not in dark muscles.

Capillarization and vascular endothelial growth factor expression in hypertrophying anterior latissimus dorsi muscle of the Japanese quail.

Hypertrophy may increase the diffusion distances from capillaries to the interior of the muscle fibers. We hypothesized that capillary proliferation occurs during hypertrophy, which is accompanied by an up-regulation of vascular endothelial growth factor (VEGF). Hypertrophy of the left anterior latissimus dorsi muscle of Japanese quail (2-3 months old) was induced by 1-4 week stretch-overload. Capillarization was analyzed in cross-sections stained for ATPase. VEGF expression was determined with RT-PCR. Initially, hypertrophy was not accompanied by increases in fiber cross-sectional area (FCSA), but after 1 week the average FCSA did increase. The capillary to fiber ratio was decreased after 1 week, but returned to control values in subsequent weeks. This indicates that capillary proliferation occurred, because this model is characterized by extensive fiber proliferation. The absence of any significant change in VEGF mRNA levels indicates that increased levels of VEGF mRNA are not crucial for capillary proliferation during muscle hypertrophy.

Aging does not attenuate plantaris muscle hypertrophy in male Fischer 344 rats.

The present study examined the effect of extreme old age on the plasticity of the rat plantaris muscle in response to an increase in mechanical load. Male Fischer 344 rats, aged 7 months (adult) and 25 months (old) underwent bilateral surgical ablation of the gastrocnemius muscle to functionally overload (OV) the fast-twitch plantaris muscle for 8 wk At 27 months of age, plantaris wet weight and cross-sectional area (CSA) were unaffected by age, but aging decreased peak isometric tension (Po) 27% (P < 0.05). Plantaris muscle myosin heavy chain composition indicated a loss of faster myosin heavy chains (MHC) isoforms with concomitant increases in slower MHC in old rats (P < 0.05). In adult rats, OV increased muscle CSA and Po 72% and 83%, respectively (P < 0.05). Similarly, OV increased CSA and Po 69% and 73%, respectively, in old rats (P < 0.05). Average fiber CSA increased 57% and 68% in adult-OV and old-OV rats, respectively (P < 0.05). Collectively, our data indicate that plantaris muscle mass and plasticity in response to increased mechanical load are well conserved in very aged male Fischer 344 rats.

IGF-I treatment improves the functional properties of fast- and slow-twitch skeletal muscles from dystrophic mice.

Although insulin-like growth factor-I (IGF-I) has been proposed for use by patients suffering from muscle wasting conditions, few studies have investigated the functional properties of dystrophic skeletal muscle following IGF-I treatment. 129P1 ReJ-Lama2(dy) (129 ReJ dy/dy) dystrophic mice suffer from a deficiency in the structural protein, laminin, and exhibit severe muscle wasting and weakness. We tested the hypothesis that 4 weeks of IGF-I treatment ( approximately 2 mg/kg body mass, 50 g/h via mini-osmotic pump, subcutaneously) would increase the mass and force producing capacity of skeletal muscles from dystrophic mice. IGF-I treatment increased the mass of the extensor digitorum longus (EDL) and soleus muscles of dystrophic mice by 20 and 29%, respectively, compared with untreated dystrophic mice (administered saline-vehicle only). Absolute maximum force (P(o)) of the EDL and soleus muscle was increased by 40 and 32%, respectively, following IGF-I treatment. Specific P(o) (sP(o)) was increased by 23% in the EDL muscles of treated compared with untreated mice, but in the soleus muscle sP(o) was unchanged. IGF-I treatment increased the proportion of type IIB and type IIA fibres and decreased the proportion of type I fibres in the EDL muscles of dystrophic mice. In the soleus muscles of dystrophic mice, IGF-I treatment increased the proportion of type IIA fibres and decreased the proportion of type I fibres. Average fibre cross-sectional area was increased in the EDL and soleus muscles of treated compared with untreated mice. We conclude that IGF-I treatment ameliorates muscle wasting and improves the functional properties of skeletal muscles of dystrophic mice. The findings have important implications for the role of IGF-I in ameliorating muscle wasting associated with the muscular dystrophies.

Lack of skeletal muscle hypertrophy in very aged male Fischer 344 x Brown Norway rats.

To examine the effect of extreme old age on muscle plasticity, 6- (adult) and 36-mo-old (old) male Fischer 344 x Brown Norway hybrid rats underwent bilateral surgical ablation of the gastrocnemius muscle to functionally overload (OV) the fast-twitch plantaris muscle for 8 wk. Plantaris muscle wet weight, muscle cross-sectional area (CSA), and average fiber CSA decreased by 44, 42, and 40%, respectively, in old compared with adult rats, and peak isometric tetanic tension decreased by 83%. Compared with muscles in age-matched controls, plantaris muscle mass increased by 53% and type I, IIA, and IIX/IIB CSA increased by 91, 76, and 103%, respectively, in adult-OV rats, but neither wet mass nor fiber CSA increased in old-OV rats. OV decreased type I, IIA, and IIX/IIB mean fiber CSA by 31, 35, and 30%, respectively, in old-OV rats. Collectively, our data indicate that in extreme old age the plantaris muscle undergoes significant loss of mass, fiber CSA, and contractile function, as well as its capacity to undergo hypertrophy in response to a chronic increase in mechanical load.

Skeletal muscle responses to lower limb suspension in humans.

Eight subjects participated in a 6-wk unilateral lower limb suspension (ULLS) study to determine the influence of reduced weight bearing on human skeletal muscle morphology. The right shoe was outfitted with a platform sole that prevented the left foot from bearing weight while walking with crutches, yet it allowed freedom of movement about the ankle, knee, and hip. Magnetic resonance images pre- and post-ULLS showed that thigh muscle cross-sectional area (CSA) decreased (P less than 0.05) 12% in the suspended left lower limb, whereas right thigh muscle CSA did not change. Likewise, magnetic resonance images collected post-ULLS showed that muscle CSA was 14% smaller (P less than 0.05) in the left than in the right leg. The decrease in muscle CSA of the thigh was due to a twofold greater response of the knee extensors (-16%, P less than 0.05) than knee flexors (-7%, P less than 0.05). The rectus femoris muscle of the knee extensors showed no change in CSA, whereas the three vastus muscles showed similar decreases of approximately 16% (P less than 0.05). The apparent atrophy in the leg was due mainly to reductions in CSA of the soleus (-17%) and gastrocnemius muscles (-26%). Biopsies of the left vastus lateralis pre- and post-ULLS showed a 14% decrease (P less than 0.05) in average fiber CSA. The decrease was evident in both type I (-12%) and II (-15%) fibers. The number of capillaries surrounding the different fiber types was unchanged after ULLS.(ABSTRACT TRUNCATED AT 250 WORDS)