Anterior Latissimus Dorsi Research Articles

Skeletal muscle Ca(2+)-independent kinase activity increases during either hypertrophy or running.

Spikes in free Ca(2+) initiate contractions in skeletal muscle cells, but whether and how they might signal to transcription factors in skeletal muscles of living animals is unknown. Since previous studies in non-muscle cells have shown that serum response factor (SRF) protein, a transcription factor, is phosphorylated rapidly by Ca(2+)/calmodulin (CaM)-dependent protein kinase after rises in intracellular Ca(2+), we measured enzymatic activity that phosphorylates SRF (designated SRF kinase activity). Homogenates from 7-day-hypertrophied anterior latissimus dorsi muscles of roosters had more Ca(2+)-independent SRF kinase activity than their respective control muscles. However, no differences were noted in Ca(2+)/CaM-dependent SRF kinase activity between control and trained muscles. To determine whether the Ca(2+)-independent and Ca(2+)/CaM-dependent forms of Ca(2+)/CaM-dependent protein kinase II (CaMKII) might contribute to some of the SRF kinase activity, autocamtide-3, a synthetic substrate that is specific for CaMKII, was employed. While the Ca(2+)-independent form of CaMKII was increased, like the Ca(2+)-independent form of SRF kinase, no alteration in CaMKII occurred at 7 days of stretch overload. These observations suggest that some of SRF phosphorylation by skeletal muscle extracts could be due to CaMKII. To determine whether this adaptation was specific to the exercise type (i.e., hypertrophy), similar measurements were made in the white vastus lateralis muscle of rats that had completed 2 wk of voluntary running. Although Ca(2+)-independent SRF kinase was increased, no alteration occurred in Ca(2+)/CaM-dependent SRF kinase activity. Thus any role of Ca(2+)-independent SRF kinase signaling has downstream modulators specific to the exercise phenotype.

Focal adhesion proteins FAK and paxillin increase in hypertrophied skeletal muscle.

Components of signaling pathways for mechanotransduction during load-induced enlargement of skeletal muscle have not been completely defined. We hypothesized that loading of skeletal muscle would result in an adaptive increase in the expression of two focal adhesion complex (FAC)-related proteins, focal adhesion kinase (FAK) and paxillin, as well as increased FAK activity. FAK protein was immunolocalized to the sarcolemmal region of rooster anterior latissimus dorsi (ALD) myofibers in the middle of the ALD muscle. FAK (77 and 81%) and paxillin (206 and 202%) protein concentrations per unit of total protein in Western blots increased significantly after 1.5 and 7 days, but not after 13 days, of stretch-induced hypertrophy-hyperplasia of the ALD muscle. FAK autokinase activity in immunoprecipitates was increased after 1.5, 7, and 13 days in stretched ALD muscles. To determine whether increased FAK and paxillin protein concentrations are associated with hypertrophy and/or new fiber formation, two additional experiments were performed. First, during formation of primary chicken myotubes (a model of new fiber formation), FAK protein concentration (63%), FAK activity (157%), and paxillin protein concentration (97%) increased compared with myoblasts. Second, FAK (112% and 611%) and paxillin (87% and 431%) protein concentrations per unit of total protein in the soleus muscle increased at 1 and 8 days after surgical ablation of the synergistic gastrocnemius muscle (a model of hypertrophy without hyperplasia). Thus increases in components of the FAC occur in hypertrophying muscle of animals and in newly formed muscle fibers in culture. Furthermore, increased FAK activity suggests a possible convergence of signaling at the FAC in load-induced growth of skeletal muscle.

SRF protein is upregulated during stretch-induced hypertrophy of rooster ALD muscle.

Serum response element 1 has previously been reported to be necessary and sufficient for activation of the skeletal alpha-actin promoter during hypertrophy of the anterior latissimus dorsi (ALD) muscle of roosters [J. A. Carson, R. J. Schwartz, and F. W. Booth. Am. J. Physiol. 270 (Cell Physiol. 39): C1624-C1633, 1996]. Serum response factor (SRF) protein is the transcription factor that binds as a homodimer to serum response element 1 and activates the skeletal alpha-actin promoter. An increased expression of exogenous SRF protein in replicating C2C12 myoblasts induced a three- to fourfold activation of the skeletal alpha-actin promoter (L. Wei, W. Zhou, J. D. Croissant, F.-E. Johansen, R. Prywes, A. Balasubramamyan, and R. J. Schwartz. J. Biol. Chem. 273: 30287-30294, 1998). Thus we hypothesized that SRF protein concentration would be increased during hypertrophy of skeletal muscle. In the present study, 10% of the rooster's body weight was attached to the left wing to induce enlargement of the ALD muscle compared with the contralateral muscle. With Western analysis, a significant increase in SRF protein per gram of wet weight of the ALD muscle was noted at 7 and 13 days of hypertrophy. Furthermore, the increase in SRF protein occurred in both crude nuclear protein and cytoplasmic fractions in 7-day stretched ALD muscles. This is the first report showing increased protein concentration for a transcription factor whose regulatory element in the skeletal alpha-actin promoter has previously been shown to be required for the transduction of a hypertrophy signal in overloaded skeletal muscle of an animal.

Denervation of Chicken Skeletal Muscle Causes an Increase in Acetylcholinesterase mRNA Synthesis

We have examined the changes in enzymatic activity and the levels of transcripts for AChE following denervation of chicken skeletal muscle. Quantitation of RNA blots indicates that AChE transcripts are increased following denervation. AChE transcripts increased ∼17-fold in the fast-twitch posterior latissimus dorsi muscle and ∼4-fold in the tonic anterior latissimus dorsi muscle 10 days after denervation of adult chickens. Both AChE transcript levels and enzyme activity increased in parallel for the two muscles. AChE transcripts also increased ∼4-fold in the shank muscles of 2-day-old chicks following denervation. Transcript synthesis, measured by run-on transcription, increased ∼3-fold in these denervated muscles. These results suggest that the increase in AChE transcripts following denervation in the chicken is due, at least in part, to an increase in the rate of its synthesis.

Stretch-induced myogenin, MyoD, and MRF4 expression and acute hypertrophy in quail slow-tonic muscle are not dependent upon satellite cell proliferation.

The objectives of these studies were to determine if (1) hypertrophy-stimulated myogenic regulatory factor (MRF) mRNA increases occur in the absence of proliferating satellite cells, and (2) acute hypertrophy occurs without satellite cell proliferation. Adult and aged quails were exposed to 0 or 2500 Rads gamma irradiation, and then wing muscles were stretch-overloaded for 3 or 7 days. MRF mRNA levels in stretch-overloaded and contralateral anterior latissimus dorsi (ALD) muscles were determined after 3 days; hypertrophy was determined after 7 days. The elimination of proliferating cells in irradiated muscles was verified histologically by bromodeoxyuridine incorporation. Relative levels of MRF4, MyoD, and myogenin mRNA were elevated 100%-400% in stretch-overloaded ALD muscles from irradiated adult quails indicating that satellite cell proliferation was not a prerequisite for MRF mRNA increases. Myogenin was the only MRF that exhibited mRNA increases that were lowered by irradiation. This suggests that satellite cells contribute only to myogenin mRNA increases in non-irradiated adult muscles following 3 days of stretch-overload. Stretch-overloaded ALD muscles from aged quails had a relative increase in myogenin mRNA of approximately 150%. The myogenin increase was the same in non-irradiated and irradiated aged animals and also the same as that in stretch-overloaded muscles from irradiated adult quails. Together, these data indicate that attenuated increases in MRF expression in muscles from aged animals are attributable to lower satellite cell MRF expression. ALD muscle masses and protein contents in adult irradiated quails approximately doubled after 7 days of stretch-overload demonstrating hypertrophy despite the elimination of satellite cell proliferation.

Expression of fibroblast growth factor family during postnatal skeletal muscle hypertrophy.

The potential role of the fibroblast growth factor (FGF) family during stretch-induced postnatal skeletal muscle hypertrophy was analyzed by using an avian wing-weighting model. After 2 or 11 days of weighted stretch, anterior latissimus dorsi (ALD) muscles were, on average, 34 (P < 0.01) and 85% (P < 0.01) larger, respectively, than unweighted ALD control muscles. By using quantitative RT-PCR, FGF-1 mRNA expression was found to be significantly decreased in ALD muscles stretched for 2 or 11 days. In contrast, FGF-4 and FGF-10 mRNA expression was significantly increased 2 days after initiation of stretch. FGF-2, FGF-10, fibroblast growth factor receptor 1, and FREK mRNA expression was significantly increased at 11 days poststretch. Increases in FGF-2 and FGF-4 protein could be detected throughout the myofiber periphery after 11 days of stretch. On a cellular level, FGF-2 and FGF-4 proteins were differentially localized. This differential expression pattern and protein localization of the FGF family in response to stretch-induced hypertrophy suggest distinct roles for individual FGFs during the postnatal hypertrophy process.

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.

Myogenin mRNA is elevated during rapid, slow, and maintenance phases of stretch-induced hypertrophy in chicken slow-tonic muscle.

Biochemical changes that are associated with the growth phase of stretch-induced skeletal muscle hypertrophy are better understood than events that maintain the increased muscle mass. One purpose of this study was to determine whether changes that occur during the period of rapid muscle hypertrophy persist during periods when muscle growth plateaus or the rate of enlargement slows. Serum response factor (SRF), myogenin, MyoD, and actin mRNA expression patterns were examined. SRF protein interactions with serum response element-1 (SRE1) of the chicken skeletal alpha-actin gene were also characterized. Anterior latissimus dorsi (ALD) wet weight (132% and 122%) and total RNA concentration (29% and 19%) increased after 2 and 3 weeks of stretch overload, respectively. Myogenin mRNA per microgram RNA increased after 3 (775%), 6 (1073%), 14 (227%), and 21 days (133%) of stretch overload. At 6 days, myogenin mRNA levels were increased in the distal, middle and proximal regions of the ALD. Serum response factor (SRF) mRNA per microgram total RNA was not increased after 2 or 3 weeks of stretch overload. MyoD and skeletal alpha-actin mRNAs per microgram total RNA were also unchanged after 2 and 3 weeks of stretch. Gel mobility shift assays demonstrated that SRF bound to SRE1 from 14-day-stretched ALD nuclear extracts had an increased mobility compared to control, and this difference in mobility was maintained in nuclear extracts from ALD muscle whose mass was declining. These results indicate that the expression of myogenin mRNA and total RNA remains elevated during either slow or maintenance periods of stretch-induced increases in ALD mass, when SRF mRNA has returned to control levels. Additionally, stretch-induced alterations in SRF binding to SRE1, from the skeletal alpha-actin promoter, occur regardless of the rate of stretch-induced growth.

Overload-induced C-Myc oncoprotein is reduced in aged skeletal muscle.

The C-Myc oncoprotein was examined in anterior latissimus dorsi (ALD) muscles of young (6 weeks) and aged (90 weeks) quail after 0.5 hours to 14 days of stretch. Western analyses of nuclear extracts showed an increase in the C-Myc oncoprotein after 1 h of stretch in young adult birds, and C-Myc remained elevated for 3 days of stretch. The onset and total accumulation of the C-Myc oncoprotein was less in muscles from aged quail as compared to muscles from young adult birds. Immunocytochemical analyses showed that C-Myc was localized in nuclei and averaged 0.2 +/- 0.04 nuclei/muscles fiber cross-section (n/f) in control muscles. C-Myc immunopositive nuclei were more numerous in muscles from young adult birds (1.7 +/- 0.2 n/f) compared to aged birds (1.1 +/- 0.1 n/f) after 2-12 h of stretch. C-Myc positive nuclei declined to 0.7 +/- 0.1 n/f after 3 days of stretch, in muscles from young adult birds; however, this was greater than in muscles from aged birds at the same time point (0.3 +/- 0.04 n/f). Many nuclei that were associated with muscle fibers expressed the C-Myc oncoprotein but did not incorporate bromodeoxyuridine, a marker of DNA synthesis and activated satellite cells. These data show a decreased ability of skeletal muscles from aged quails to initiate a program inclusive of early C-Myc oncoprotein accumulation in response to stretch.

Expression of fast myosin heavy chain transcripts in developing and dystrophic chicken skeletal muscle

The expression of fast myosin heavy chain (MyHC) genes was examined in vivo during fast skeletal muscle development in the inbred White Leghorn chicken (line 03) and in adult muscles from the genetically related dystrophic White Leghorn chicken (line 433). RNA dotblot and northern hybridization was employed to monitor MyHC transcript levels utilizing specific oligonucleotide probes. The developmental pattern of MyHC gene expression in the pectoralis major (PM) and the gastrocnemius muscles was similar during embryonic development with three embryonic MyHC isoform genes, Cemb1, Cemb2, and Cemb3, sequentially expressed. Following hatching, MyHC expression patterns in each muscle differed. The expression of MyHC genes was also studied in muscle cell cultures derived from 12-day embryonic pectoralis muscles. In vitro, Cvent, Cemb1, and Cemb2 MyHC genes were expressed; however, little if any Cemb3 MyHC gene expression could be detected, even though Cemb3 was the predominant MyHC gene expressed during late embryonic development in vivo. In most adult muscles other than the PM and anterior latissimus dorsi (ALD), the Cemb3 MyHC gene was the major adult MyHC isoform. In addition, two general patterns of expression were identified in fast muscle. The fast muscles of the leg expressed neonatal (Cneo) and Cemb3 MyHC genes, while other fast muscles expressed adult (Cadult) and Cemb3 MyHC genes. MyHC gene expression in adult dystrophic muscles was found to reflect the expression patterns found in corresponding normal muscles during the neonatal or early post-hatch developmental period, providing additional evidence that avian muscular dystrophy inhibits muscle maturation.

Expression of fast myosin heavy chain transcripts in developing and dystrophic chicken skeletal muscle

The expression of fast myosin heavy chain (MyHC) genes was examined in vivo during fast skeletal muscle development in the inbred White Leghorn chicken (line 03) and in adult muscles from the genetically related dystrophic White Leghorn chicken (line 433). RNA dot-blot and northern hybridization was employed to monitor MyHC transcript levels utilizing specific oligonucleotide probes. The developmental pattern of MyHC gene expression in the pectoralis major (PM) and the gastrocnemius muscles was similar during embryonic development with three embryonic MyHC isoform genes, Cemb1, Cemb2, and Cemb3, sequentially expressed. Following hatching, MyHC expression patterns in each muscle differed. The expression of MyHC genes was also studied in muscle cell cultures derived from 12-day embryonic pectoralis muscles. In vitro, Cvent, Cemb1, and Cemb2 MyHC genes were expressed; however, little if any Cemb3 MyHC gene expression could be detected, even though Cemb3 was the predominant MyHC gene expressed during late embryonic development in vivo. In most adult muscles other than the PM and anterior latissimus dorsi (ALD), the Cemb3 MyHC gene was the major adult MyHC isoform. In addition, two general patterns of expression were identified in fast muscle. The fast muscles of the leg expressed neonatal (Cneo) and Cemb3 MyHC genes, while other fast muscles expressed adult (Cadult) and Cemb3 MyHC genes. MyHC gene expression in adult dystrophic muscles was found to reflect the expression patterns found in corresponding normal muscles during the neonatal or early post-hatch developmental period, providing additional evidence that avian muscular dystrophy inhibits muscle maturation. Dev. Dyn. 208:491–504, 1997. © 1997 Wiley-Liss, Inc.

Existence of parvalbumin and biochemical characterization in quail and pigeon skeletal muscles with different fiber type compositions.

The localization of parvalbumin was determined in skeletal muscles with different fiber type compositions from quails (Coturnix japonica) and pigeons (Columba livia) by sandwich ELISA. The biochemical profiles of these muscles were evaluated by the assay of total lactate dehydrogenase (LDH) activity as well as the LDH isozymes for anaerobic metabolism, and by the analysis of myoglobin for aerobic metabolism. The quail fast posterior latissimus dorsi (PLD) with a lower myoglobin content and higher LDH activity or M-type isozyme pattern was assessed as containing primarily fast-twitch glycolytic and oxidative-glycolytic (FG/FOG) fibers, and the mixed sartorius (SA) in quails and pigeons was shown to be composed mostly of FOG fibers because of the intermediate myoglobin content and LDH activity or H/M-type isozyme pattern. Parvalbumin, which functions as a relaxing factor in the fast-twitch fibers, was present in PLD and SA in significant amounts, whereas it was undetectable in quail anterior latissimus dorsi (ALD) and pigeon latissimus dorsi (LD) which contain exclusively slow-tonic (ST) fibers with the lowest LDH activity or predominant H-type isozyme characteristics. The pectoralis superficialis (PS) and pectoralis profundus (PP) muscles from quails and pigeons seem to consist mostly of FG/FOG fibers because of the highest myoglobin contents and LDH activities or M-type and H/M-type isozyme patterns. Despite fast-twitch fiber compositions, parvalbumin was absent from these pectoralis muscles. The tetanic contraction induced in avian fast-twitch pectoralis fibers during flapping flight might be independent of a function of parvalbumin as a relaxing factor.

Developmental shift of myosin heavy chain mRNA expression due to neural factor(s) and muscle activity.

The adult ventricular isoform of chicken myosin heavy chain (MHC-V) is transiently expressed in all skeletal muscle primordia analyzed and is completely repressed around embryonic days 10-12, when functional innervation is established. By ribonuclease protection assay, we demonstrated that denervation of the adult anterior latissimus dorsi muscle resulted in reexpression of MHC-V mRNA. In contrast, treatment of primary cultures of fetal breast or leg muscles with embryonic brain extract or conditioned media from glial or neuroblastoma cell lines, but not from a myogenic cell line or primary muscle cell cultures, led to inhibition of MHC-V expression. This inhibitory activity was abolished by heating and increased with protein concentration. The acquisition of both brain inhibitory activity and the competence of myogenic cells to downregulate MHC-V mRNA expression were age dependent. Furthermore, either paralysis of muscle in ovo by curare or contraction arrest of cultured myotubes resulted in persistent expression of MHC-V mRNA. Thus a putative soluble factor(s) of nerve origin as well as muscle activity are involved in the developmental downregulation of MHC-V expression in muscle primordia.

Innervation regulates myosin heavy chain isoform expression in developing skeletal muscle fibers

The influence of innervation on primary and secondary myogenesis and its relation to fiber type diversity were investigated in two specific wing muscles of quail embryo, the posterior (PLD) and anterior latissimus dorsi (ALD). In the adult, these muscles are composed almost exclusively of pure populations of fast and slow fibers, respectively. When slow ALD and fast PLD muscles developed in ovo in an aneurogenic environment induced after neural tube ablation, the cardiac ventricular myosin heavy chain (MHC) isoform was not expressed. The adult slow MHC isoform, SM2, appeared by embryonic day 7 (ED 7) in normal innervated slow ALD but was not expressed in denervated muscle. Analysis of in vitro differentiation of myoblasts from fast PLD and slow ALD muscles isolated from ED 7 control and neuralectomized quail embryos showed no fundamental differences in the pattern of MHC isoform expression. Newly differentiated fibers accumulated cardiac ventricular, embryonic fast, slow SM1 and SM3 MHC isoforms. Nevertheless, the expression of slow SM2 isoform in myotubes formed from slow ALD myoblasts only occurred when myoblasts were cultured in the presence of embryonic spinal cord. Our studies demonstrate that the neural tube influences primary as well as secondary myotube differentiation in avian forelimb and facilitates the expression of different MHC, particularly slow SM2 MHC gene expression in slow myoblasts.

Expression of chicken troponin T isoforms in cultured muscle cells.

Cells prepared from chicken skeletal muscles of different developmental stages were cultured to study their troponin T isoform expression, using antisera specific to the fast- and slow-muscle-type isoforms. We found that the cultured myogenic cells from chickens and chick embryos were classified into two types, fast type and fast/slow type in which fast- and slow-muscle-type isoforms were coexpressed. Cells expressing only slow-muscle-type troponin T isoforms could not be found. Most cells prepared from pectoralis major (fast muscle) and gastrocnemius (mixed muscle) of 11-day old embryos belonged to the latter, with only a small fraction belonging to the former. The percentage of fast type cells in those cells prepared from pectoralis major increased along development to over 90% by the 17th day of incubation, while, in the cells prepared from gastrocnemius, it reached a plateau of 30-40% by the 13th day of incubation. All the cells from anterior latissimus dorsi (slow muscle) belonged to the fast/slow type. Ratios of these two types of muscle cells varied depending on their origins and stages. The in vitro expression of troponin T isoforms was different from the in vivo expression, and each muscle seems to be determined differently in the composition of cell types during the developmental course.

Relative responses of protein turnover in three different skeletal muscles to dietary lysine deficiency in chicks.

1. The effect of lysine deficiency was analysed on muscle protein turnover in 2-, 3- and 4-week-old growing broilers. Protein fractional synthesis rates (FSR, in %/d) were measured by a reliable in vivo technique (flooding dose of L-[4-3H] phenylalanine) in the Pectoralis major (PM), the Anterior latissimus dorsi (ALD) and the Sartorius (SART) muscles. Protein fractional breakdown rates (FBR, in %/d) were estimated as the difference between the synthesis rates and the growth rates of tissue protein. 2. Lysine deficiency resulted in significant increases in muscle FSR and FBR. When expressed in absolute rates (g/d), tissue protein deposition was reduced whatever the tissue. This phenomenon was accompanied by decreased protein synthesis (ASR). 3. The protein turnover responsiveness to the lysine deficiency appeared to depend on the studied muscle, since the PM muscle was the most sensitive whereas the SART and ALD muscles presented a lower sensitivity.

SRF and TEF-1 control of chicken skeletal alpha-actin gene during slow-muscle hypertrophy.

The purpose of this study was to delineate the alpha-actin regulatory elements and transcription factors that are responsible for conferring stretch-overload responsiveness during hypertrophy of the anterior latissimus dorsi (ALD) muscle of young chickens by weighting one wing. Minimal promoter constructs were evaluated by direct injection into the ALD, which demonstrated that both serum response element 1 (SRE1) and the transcriptional enhancer factor 1 (TEF-1) elements were sufficient for increased expression during stretch overload. A mutated SRE1 prevented expression in both basal and stretched ALD muscles, whereas a mutated TEF-1 element reduced actin promoter function in both control and stretched muscles. The serum response factor (SRF)-SRE1 binding complex demonstrated faster migration in mobility shift assays from day 3-and day 6-stretched ALD nuclear extracts relative to their control. TEF-1 binding was qualitatively increased in stretched extracts at day 3 but not day 6 of stretch overload. Skeletal alpha-actin mRNA accumulated from day 3 to day 6 of stretch overload. These data demonstrate that SRE1 is necessary and sufficient for stretch-overload responsiveness from the skeletal alpha-actin promoter and that the SRF-SRE1 binding complex migrates faster in stretched nuclear extracts of hypertrophied relative to control extracts from intact ALD muscles of chickens.

Contractile properties and protein isoforms of single fibres from the chicken pectoralis red strip muscle.

1. The contractile properties of single muscle fibres of the red strip region of adult chicken pectoralis major (PM) muscle, some of which are known to express an embryonic isoform of myosin heavy chain (MHC), were determined and compared with the properties of the fast white fibres of the PM and the slow tonic fibres of the anterior latissimus dorsi (ALD) muscle. 2. The red strip fibres could be classified into two groups, fast and slow. The mean velocity of unloaded shortening (Vmax) in fast red strip fibres was approximately half the Vmax of fast white fibres. Vmax of slow red strip fibres was less than 20% of the value for fast red strip fibres and was not different from Vmax of ALD fibres. 3. The tension-generating ability, i.e. the maximal isometric tension/fibre cross-sectional area (P0/CSA), was the same in fast red strip fibres and fast white fibres. P0/CSA was approximately 30% lower in slow red strip fibres compared with fast red strip fibres but was 70% greater in slow red strip fibres compared with ALD fibres. 4. The tension-pCa relation of fast red strip fibres was shifted to lower pCa values, indicating a lower calcium sensitivity compared with fast white fibres, and this difference was associated with a difference in troponin T isoform composition. The tension-pCa relation of slow red strip fibres was not different from that in ALD fibres. 5. The difference in Vmax between fast red strip fibres and fast white fibres was associated with different MHC compositions of these fibres. 6. The myofibrillar protein isoform composition of slow red strip fibres was identical to that of the slow tonic fibres of ALD muscle and these two groups of fibres had very similar contractile properties.

Age-Related Changes of Protein Turnover in Specific Tissues of the Chick

The effect of age on muscle and liver protein turnover was analyzed in growing broilers. Protein turnover was compared in the Pectoralis major (PM), the Anterior Latissimus dorsi (ALD), and the Sartorius (SART) muscles, as well as in the liver in broilers at three ages (2, 3, and 4 wk). Protein fractional synthesis rates (FSR, in percentage per day) were measured by a flooding dose of L-[4-3 H] Phe. Protein fractional breakdown rates (FBR, in percentage per day) were estimated as the difference between FSR and the gain rates of tissue protein. When expressed in absolute rates (grams per day), tissue protein deposition increased during chick growth (approximately 1.7-fold, whatever the tissue). This phenomenon was accompanied by increased protein synthesis (ASR) and proteolysis (ABR) in the PM and in the ALD muscles as well as in the liver. However, ASR and ABR did not seem to be significantly modified in SART. The FSR in skeletal muscles significantly decreased with age. This developmental decline was associated with decreases in RNA content and RNA:protein ratio (Cs, capacity for protein synthesis). In contrast to muscle, we found no significant variation in liver FSR between 2 and 4 wk of age. In addition, liver Cs and translational efficiencies were not decreased. Finally, in muscle as in liver, FBR did not show any clear age-related pattern.

Stretch overload-induced satellite cell activation in slow tonic muscle from adult and aged Japanese quail.

Stretch overload-induced activation of satellite cells in the anterior latissimus dorsi (ALD) muscle was examined in full-grown adult (12 wk old) and aged (90 wk old) Japanese quail. 5'-Bromo-2'-deoxyuridine (BrdU) constant-release pellets (0.22 mg BrdU.g body wt-1.day-1) were implanted subcutaneously before weighting the left wing of each bird. Nuclei that incorporated BrdU were localized by immunohistochemistry after 1 or 2 wk of stretch overload. Total fiber number was quantified by counting all fibers in a histological cross section from the midbelly of the ALD. Aging reduced increases in ALD mass and fiber number during 2 wk of stretch overload. Fiber proliferation in the ALD of aged birds also demonstrated an altered time course. The percentage of BrdU-positive nuclei associated with muscle fibers and the percentage of fibers associated with BrdU-positive nuclei did not differ between age groups. The altered time course of new fiber formation in the ALD of aged birds during 2 wk of stretch overload does not appear to be related to the capability to activate satellite cells.