Posterior Latissimus Dorsi Muscles Research Articles

Myocytes changes and satellite cells proliferation following denervation of ALD (Anterior Latissimus Dorsi) and PLD (Posterior Latissimus Dorsi) muscles in two strains of turkey (Meleagris gallopavo)

Morphological features and chronology of myocytes changes after denervation were studied over 35 days period in 2 heavy (HW) and light-­weight (LW) strains of 5-­week-­old male turkeys. Denervation caused progressive atrophy in posterior latissimus dorsi (PLD). By day 28, the weight of the PLD muscle had reached about 62% of the non-­denervated contralateral muscle weight in both strains. On the contrary, ALD muscle mass increase progressively after denervation. Thus the maximum hypertrophy of the ALD, expressed as a percentage of contralateral muscle, was respectively about 67% and 37%,day 21 in the HW strain and day 28 in LW strain after denervation. ALD hypertrophy ceased apparently after day 21 (HW strain) and day 28 (LW strain). Morphometric analysis revealed that fast twitch (type II) fibers were atrophied after denervation, whereas slow tonic (type III) fibers were hypertrophied from day 7.Coagulative necrosis with fragmentation and lysis associated with moderate infiltration of inflammatory cells, were similar in both strains. Irregularities in mitochondrial distribution occurred mainly in type III fibers of ALD muscle at day 7. Seven and 14 days after denervation, immunolabelling of proliferating cell nuclear antigen (PCNA) revealed satellite cell activation in denervated muscles. The number of activated satellite cells was greater in the LW than HW mainly in ALD muscles

The vitamin C transporter SVCT2 is down-regulated during postnatal development of slow skeletal muscles

Vitamin C plays key roles in cell homeostasis, acting as a potent antioxidant as well as a positive modulator of cell differentiation. In skeletal muscle, the vitamin C/sodium co-transporter SVCT2 is preferentially expressed in oxidative slow fibers. Besides, SVCT2 is up-regulated upon the early fusion of primary myoblasts. However, our knowledge of the postnatal expression profile of SVCT2 remains scarce. Here we have analyzed the expression of SVCT2 during postnatal development of the chicken slow anterior and fast posterior latissimus dorsi muscles, ranging from day 7 to adulthood. SVCT2 expression is consistently higher in the slow than in the fast muscle at all stages. After hatching, SVCT2 expression is significantly down-regulated in the anterior latissimus dorsi, which nevertheless maintains a robust slow phenotype. Taking advantage of the C2C12 cell line to recapitulate myogenesis, we confirmed that SVCT2 is expressed in a biphasic fashion, reaching maximal levels upon early myoblasts fusion and decreasing during myotube growth. Together, these findings suggest that the dynamic expression levels of SVCT2 could be relevant for different features of skeletal muscle physiology, such as muscle cell formation, growth and activity.

Differential expression of genes characterizing myofibre phenotype

Skeletal muscle is composed of metabolically heterogeneous myofibres that exhibit high plasticity at both the morphological and transcriptional levels. The objective of this study was to employ microarray analysis to elucidate the differential gene expression between the tonic-'red' anterior latissimus dorsi (ALD) muscle, the phasic-'white' posterior latissimus dorsi (PLD) and 'mixed'-phenotype biceps femoris (BF) in 1-week-and 19-week-old male turkeys. A total of 170 differentially expressed genes were identified in the muscle samples analysed (P < 0.05). Gene GO analysis software was utilized to identify top gene networks and metabolic pathways involving differentially expressed genes. Quantitative real-time PCR for selected genes (BAT2D, CLU, EGFR and LEPROT) was utilized to validate the microarray data. The largest differences were observed between ALD and PLD muscles, in which 32 genes were over-expressed and 82 genes were under-expressed in ALD1-PLD1 comparison, and 70 genes were over-expressed and 70 under-expressed in ALD19-PLD19 comparison. The largest number of genes over-expressed in ALD muscles, as compared to other muscles, code for extracellular matrix proteins such as dystroglycan and collagen. The gene analysis revealed that phenotypically 'red' BF muscle has high expression of glycolytic genes usually associated with the 'white' muscle phenotype. Muscle-specific differences were observed in expression levels of genes coding for proteins involved in mRNA processing and translation regulation, proteosomal degradation, apoptosis and insulin resistance. The current findings may have large implications in muscle-type-related disorders and improvement of muscle quality in agricultural species.

Reflex control of posterior shoulder muscles from arm afferents in healthy people

In order to position the hand during functional tasks, control of the shoulder is required. Heteronymous reflexes from the upper limb to shoulder muscles are used to assist in this control. To investigate this further, the radial and ulnar nerves were stimulated at elbow level whilst surface electromyographic activity of posterior deltoid, infraspinatus and latissimus dorsi muscles were recorded. In addition, the cutaneous branch of the radial nerve and the skin of the fifth digit were stimulated in order to investigate any cutaneous contribution to reflex activity. Reflexes were evoked in all three of these shoulder muscles from hand and/or forearm afferents. However, the reflexes differed; whereas both excitatory and inhibitory reflexes were evoked in posterior deltoid and infraspinatus, the reflexes in latissimus dorsi were mainly excitatory. Cutaneomuscular reflexes were seldom evoked here, but when they were present they were generally evoked at longer latencies than the reflexes evoked by mixed nerve stimulation. The results suggest a role for reflexes originating from the forearm and/or hand in the control of the shoulder.

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.

Characterization of the myosin heavy chains of avian adult fast muscles at the protein and mRNA levels.

High resolution anion-exchange chromatography of myosin subfragment-1 in avian fast muscles revealed five fast heavy chains (I-V) expressed in muscle-specific patterns. Sequence analysis of a unique peptide established that the proteins differed in primary structure and suggested correlation with heavy chain genes identified independently by Robbins and coworkers. The identities of the isoforms and their expression patterns were confirmed at the mRNA level by a reverse-transcription, 5'-anchored PCR procedure. The fast white pectoralis major muscle possessed heavy chain I, the posterior latissimus dorsi muscle, of similar fibre type, expressed heavy chains I, III and IV. The fast red adductor superficialis muscle expressed either, or both, of heavy chains II and IV. The lateral gastocnemius muscle, of mixed fibre type, expressed heavy chains II-V. In general, heavy chains I, III and V appeared to be favoured in fast white fibres, while heavy chains II and IV were characteristic of fast red fibres. These results imply a greater subtlety of fast muscle function than has previously been appreciated.

Further observation of leucocyte migration through the endothelium of venules in the posterior latissimus dorsi muscle of the quail-chick spinal cord chimeras.

The migration of leucocytes through the walls of venules was examined in detail in the posterior latissimus dorsi muscle of quail-chick spinal cord chimeras, in which flaccid paralysis of the wings was observed. Examinations were made from the 70th to 80th day after hatching. Muscle fibers were degenerated and intramuscular nerve bundles destroyed. Massive leucocytes (almost lymphocytes) were found around the venules, depending on the passage of wandering leucocytes through the endothelium. Lymphocytes penetrated and were encased in the cytoplasm of the venular endothelial cell, and did not pass through the interendothelial junction. These findings suggest that, in the venules of the atrophied chimeric muscle, wandering leucocytes in the blood may pass through the endothelial cell body and migrate into the inflamed extravascular space.

Distribution of parvalbumin in specific fibre types of chicken skeletal muscles

1. The distribution of parvalbumin (PA), which functions as a relaxing factor in the skeletal muscles, was examined in slow anterior latissimus dorsi (ALD), fast posterior latissimus dorsi (PLD), mixed sartorius (SA), pectoralis superficialis (PS) and pectoralis profundus (PP) muscles from chickens. 2. The biochemical characteristics of these muscles were confirmed by the assay of total lactate dehydrogenase (LDH) activity as well as the LDH isozymes for anaerobic metabolism, and by the photometrical analysis of myoglobin for anaerobic metabolism, and by the photometrical analysis of myoglobin for aerobic metabolism. 3. PA in individual muscles was determined by a sandwich ELISA and was demonstrated by 2-dimensional polyacrylamide-gel electrophoresis. 4. Because of poor myoglobin and higher LDH activity or M-type isozyme pattern, the PLD was confirmed as containing primarily fast-twitch glycolytic and oxidative-glycolytic (FG/FOG) fibres, while the SA was shown to be composed mostly of FOG fibres because of the highest myoglobin content and the intermediate LDH activity or H and M-type isozyme pattern. PA content was high and variable in both PLD and SA. 5. PA was undetectable in the ALD which appears to contain exclusively slow-tonic (ST) fibres, being verified by its myoglobin-rich nature and lowest LDH activity or predominant H-type isozyme characteristics. PA was absent from the PS ad PP, which probably contain predominantly FG fibres because of negligible amounts of myoglobin and the highest LDH activities or M-type isozyme pattern.

Developmental modulation of myosin expression by thyroid hormone in avian skeletal muscle.

It is well established that a rise in circulating thyroid hormone during the second half of chick embryo development significantly influences muscle weight gain and bone growth. We studied thyroid influence on differentiation in slow anterior latissimus dorsi (ALD) and fast posterior latissimus dorsi (PLD) muscles of embryos rendered hypothyroid by hypophysectomy or administration of an anti-thyroid drug. The expression of native myosins and myosin light chains (MLCs) was studied by electrophoretic analysis, and the myosin heavy chain (MHC) was characterized by immunohistochemistry. The first effects of hypothyroid status were observed at day 21 of embryonic development (stage 46 according to Hamburger and Hamilton). Analysis of myosin isoform expression in PLD muscles of hypothyroid embryos showed persistence of slow migrating native myosins and slow MLCs as well as inhibition of neonatal fast MHC expression, indicating retarded differentiation of this muscle. In ALD muscle, hypothyroidism maintained fast embryonic MHC and induced noticeable amounts of fast MLCs, thus delaying slow muscle differentiation. Our results suggest that thyroid hormones play a role in modulating the appearance of neonatal fast MHC and the disappearance of isomyosins transiently present during embryogenesis. However, T3 supplemental treatment would seem to compensate in part for the effects of hypothyroidism induced by hypophysectomy, suggesting that thyroid hormone might interfere with other factors also accounting for the observed effects.

Research Note: Simultaneous Histochemical Determination of Three Fiber Types in Single Sections of Broiler Skeletal Muscles

A staining procedure used for simultaneously determining three different fiber types in single sections of bovine, porcine, or ovine skeletal muscle was modified for use with broiler skeletal muscle. The modification involved acid-preincubation of muscle sections at a pH of 4.15 followed by staining for reduced nicotinamide adenine dinucleotide (NADH) tetrazolium reductase enzyme and for acid-stable myosin-adenosine triphosphatase enzyme activity, respectively. Four broiler muscles were selected for fiber-type determination of fast (α), slow (β), red, aerobic (R), or white, anaerobic (W) properties. The anterior latissimus dorsi muscle was composed almost entirely of β-R fibers while the pectoralis superficialis muscle was composed almost entirely of α-W fibers. The sartorius and posterior latissimus dorsi muscles were much more heterogeneous in fiber-type distribution, exhibiting all three types of fibers (α-W, α-R, and β-R).

Effects of Electrical Stimulation on Molecular Forms of Butyrylcholinesterase in Denervated Fast and Slow Latissimus Dorsi Muscles of Newly Hatched Chicken

The effects of denervation and direct electrical stimulation upon the activity and the molecular form distribution of butyrylcholinesterase (BuChE) were studied in fast-twitch posterior latissimus dorsi (PLD) and in slow-tonic anterior latissimus dorsi (ALD) muscles of newly hatched chicken. In PLD muscle, denervation performed at day 2 substantially reduced the rate of rapid decrease of BuChE specific activity which takes place during normal development, whereas in the case of ALD muscle little change was observed. Moreover, the asymmetric forms which were dramatically reduced in denervated PLD muscle were virtually absent in denervated ALD muscle at day 14. Denervated PLD and ALD muscles were stimulated from day 4 to day 14 of age. Two patterns of stimulation were applied, either 5-Hz frequency (slow rhythm) or 40-Hz frequency (fast rhythm). Both patterns of stimulation provided the same number of impulses per day (about 61,000). In PLD muscle, electrical stimulation almost totally prevented the postdenervation loss in asymmetric forms and led to a decrease in BuChE specific activity. In ALD muscle, electrical stimulation partially prevented the asymmetric form loss which occurs after denervation. This study emphasizes the role of evoked muscle activity in the regulation of BuChE asymmetric forms in the fast PLD muscle and the differential response of denervated slow and fast muscles to electrical stimulation.

Characterization of C-Protein Isoforms Expressed in Developing, Denervated, and Dystrophic Chicken Skeletal Muscles by Two-Dimensional Gel Electrophoresis1

C-Proteins in developing, denervated, and dystrophic chicken skeletal muscles were examined by means of two-dimensional (2D) gel electrophoresis in combination with immunoblotting. In this analysis, the electrophoresis system which was devised by Hirabayashi (Anal. Biochem. 117, 443-451, 1981) provided excellent resolution; three C-protein variants, one fast-type (Cf) and two slow-types (CS3 and CS4) with different Mrs and pIs, were distinguished on a 2D gel. In the neonatal breast muscle, both Cf and CS3 were detected, but during postnatal development, CS3 disappeared from this muscle and Cf became only the C-protein isoform in the adult muscle. In posterior latissimus dorsi (PLD) muscle, both Cf and CS3 were similarly detected at the neonatal stage, but CS3 was replaced by CS4 as this muscle developed. When the breast and PLD muscles were denervated or suffered from muscular dystrophy, both CS3 and CS4 were co-expressed in these muscles in addition to Cf. These results definitely show that the C-protein isoform pattern varies during development and degeneration of chicken skeletal muscles, and in addition the dystrophic or denervated muscle differs from the neonatal muscle with regard to C-protein isoform expression. We suggest that chicken skeletal muscle degenerating due to denervation or muscular dystrophy does not simply recapture the nature of the neonatal muscle, but shifts in a somewhat different direction.

Scanning electron microscope study of neuromuscular junctions in different muscle fiber types in the zebra finch and rat.

Examination by scanning electron microscopy revealed differences between neuromuscular junctions in the muscle fibers of the zebra finch (bird) and rat. The neuromuscular junctions between the anterior and posterior latissimus dorsi muscles of the zebra finch were compared. The junctions of the former, exclusively slow tonic fibers, were small and numerous along the long axis of a single muscle fiber. The synaptic depressions per junction were few. The junctions of the latter, exclusively fast twitch fibers, were large and consisted of more synaptic depressions than the former. Junctional folds were occasionally found in some depressions. The neuromuscular junctions between the extensor digitorum longus and soleus muscles of the rat were also compared. The former consisted almost entirely of fast twitch muscle fibers, whereas the latter consisted of both slow twitch fibers (75%) and fast twitch fibers (25%). The junctions in the extensor digitorum longus muscle were almost all labyrinthine gutters containing exclusively slit-like junctional folds. In the soleus muscle, two types of junctions were observed. One type was similar to that of the extensor digitorum longus muscle; the other was characterized by labyrinthine gutters containing sparse, narrow slit-like and pit-like junctional folds. We suggest from these structural differences of the subneural apparatuses that the junction of the fast twitch muscle is characterized by the subneural apparatus containing numerous slit-like junctional folds, and that of the slow twitch muscle fiber characterized by the apparatus containing sparse, narrow slit-like and pit-like junctional folds.

Ventricular myosin expression in developing and regenerating muscle, cultured myotubes, and nascent myofibers of overloaded muscle in the chicken

In embryonic chicken skeletal muscle, the presence of a ventricular myosin heavy chain (MHC) is demonstrated by reactivity with an anti-ventricular MHC antibody. Developmental repression of the ventricular MHC expression occurs earlier in embryonic fast twitch posterior latissimus dorsi (PLD) than in embryonic slow-tonic anterior latissimus dorsi (ALD) muscles. Ventricular MHC expression also occurs in developing myotube cultures and in regenerating ALD muscles. Following the application of a weight overload, a population of nascent myofibers emerges in ALD but not PLD muscles which express a ventricular MHC. Localization of cells reactive with anti-ventricular and anti-fast MHC antibodies suggests that satellite cells participate in nascent myofiber formation. In addition, some mature ALD muscle fibers demonstrate reactivity with these antibodies, suggesting that satellite cell fusion with mature myofibers also occurs in overloaded ALD muscles and results in the reinitiation of the embryonic phenotype in hypertrophying fibers. The presence of cells reacting exclusively with the anti-fast antibody after removal of the overload for 9 wk indicates that nascent myofiber formation results in the establishment of a new myofiber population with an abnormal phenotype in overloaded ALD muscles. These cells form a lattice-like network around normal fascicles.

Size and charge heterogeneity of C-protein isoforms in avian skeletal muscle. Expression of six different isoforms in chicken muscle.

C-protein is an abundant protein, of unknown function, found in the striated muscles of all vertebrates (Offer et al., 1973). Based on differences in size, charge, antigenicity and sarcomere distribution, at least three different isoforms of this protein have been identified (Callaway & Bechtel, 1981; Yamamoto & Moos, 1983; Reinach et al., 1982; Dhoot et al., 1985). These have been termed fast-, slow- and cardiac-type isoforms, relative to their distribution in adult striated muscles. Each of these isoforms appears to be expressed sequentially during the development of the chicken pectoralis muscle (Obinata et al., 1984; Obinata, 1985). To better characterize the various isoforms of C-protein, we have reexamined its in vivo expression during avian myogenesis using a combination of 1- and 2-dimensional gel electrophoresis, cell-free translation and immunoblotting procedures. In this manuscript we demonstrate for the first time that at least four major C-protein isoforms can be distinguished in adult chicken muscles. These include a fast-type isoform in the pectoralis (PECT) muscle (Cf), a slow-type isoform in the anterior latissimus dorsi (ALD) muscle (Cs3), a second slow-type isoform in the posterior latissimus dorsi (PLD) muscle (Cs4) and a cardiac-type in the ventricle (Cc). During embryonic development of the PECT muscle two additional isoforms can be resolved. These are both slow-type isoforms based on their reactivities with ALD66, a monoclonal antibody specific for adult slow-type C-protein. These latter isoforms have been termed Cs1 and Cs2. Several of the isoforms, particularly Cs1 ands Cs3, exhibit two or more spots of different charge but identical molecular weight on 2-D gels. This observation suggests the possibility that these isoforms are post-translationally modified and possibly phosphorylated. Our data show the C-protein family in avian striated muscles to be highly complex. Additional genetic analyses and primary sequence studies will be required to distinguish transcriptional from post-transcriptional variants.

Acetylcholinesterase in chick embryo latissimus dorsii muscles: effects of curarization and electrical stimulation

Abstract The accumulation of acetylcholinesterase (AChE), the changes in AChE-specific activity and in AChE molecular form distribution were studied in slowtonic anterior latissimus dorsi (ALD) and in fast-twitch posterior latissimus dorsi (PLD) muscles of the chick embryo. From stage 36 (day 11) to stage 42 (day 17) of Hamburger and Hamilton, the AChE-specific activity decreased, while the relative proportion of asymmetric A 12 and A 8 forms increased. Repetitive injection of curare resulted at stage 42 (day 17) in a decrease in AChE-specific activity, in the accumulation of the synaptic AChE and in the expression of AChE asymmetric forms. Electrical stimulation at a relatively high frequency (40 Hz) of curarized ALD and PLD muscles resulted in a normal increase in AChE asymmetric forms, whereas a lower frequency (5 Hz) resulted in a dominance of globular forms. Both patterns of stimulation partly prevented the loss in synaptic AChE accumulations. These results suggest that in chick embryo muscles, muscle activity and its rhythms are involved in the normal evolution of AChE.

Synthesis and assembly of native myosin on muscle polyribosomes

We have used the overload-induced growth of avian muscles to study the assembly of the newly synthesized myosins which were separated by non-denaturing pyrophosphate-polyacrylamide gel electrophoresis. Using this model, we have observed the appearance of fast-like isomyosins in polyribosomes prepared from slow anterior latissimus dorsi muscle after 72 h of overload. These new isoforms comigrating with native myosin from fast posterior latissimus dorsi muscle were not yet present in cellular extracts from the same muscle. The in vitro translation system utilizing muscle specific polyribosomes directs the synthesis of the corresponding myosin isoforms. Under denaturing conditions, myosin heavy chains and light chains dissociate to the expected subunit composition of each specific isoform. The synthesis and assembly of native myosin on polyribosomes indicate that myosin exists as a single mature protein prior to the incorporation in the thick filament.

Effects of a monoclonal anti-acetylcholine receptor antibody on the avian end-plate.

1. The effects of anti-acetylcholine receptor (AChR) monoclonal antibodies (mAbs) 370 and 132A on miniature end-plate potentials (MEPPs) and end-plate currents (EPCs) in the posterior latissimus dorsi muscle of adult chickens were investigated. 2. After incubation of the electrophysiological preparation with mAb 370 (5-50 micrograms/ml), which blocks both agonist (carbamylcholine) and alpha-bungarotoxin (alpha-BTX) binding and induces a hyperacute form of experimental autoimmune myasthenia gravis (EAMG), MEPP and EPC amplitudes were irreversibly reduced. 3. This effect was not associated with any significant change in the time constant describing EPC decay (tau EPC), current reversal potential, or the voltage dependence of tau EPC. The tau EPC at -80 mV was 5.9 +/- 0.6 ms before incubation with mAb 370 (50 micrograms/ml) and 6.0 +/- 0.9 ms afterwards. Current reversal potential was -3.9 +/- 0.4 mV before mAb incubation and -4.8 +/- 1.5 mV afterwards. The change in membrane potential required to produce an e-fold change in tau EPC was 128 +/- 2.3 mV before antibody incubation compared to 125 +/- 6.6 mV after incubation. 4. A second anti-AChR mAb, 132A (50 micrograms/ml), which is capable of inducing the classically described form of EAMG without blocking agonist or alpha-BTX binding, or inducing hyperacute EAMG, produced no significant change in MEPP amplitude, EPC amplitude, tau EPC or EPC reversal potentials. 5. The mAb 370 (50 micrograms/ml) induced a partially reversible decrease of the quantal content of the neurally evoked end-plate potential (EPP). This effect was not observed with mAb 132A, (+)tubocurarine (10(-7)-10(-5) g/ml) or an irrelevant anti-oestrogen receptor mAb. 6. These data suggest that the rapid onset of weakness observed in chicken hatchlings after the injection of mAb 370 (Gomez & Richman, 1983) can be attributed to a combined effect of a block of acetylcholine (ACh)-induced ion channel activity in the postsynaptic membrane and a reduction of the neurally evoked release of acetylcholine from the nerve terminal.

The distribution of intracellular acetylcholine receptors and nuclei in developing avian fast-twitch muscle fibres during synapse elimination.

The spatial distribution of intracellular acetylcholine receptors along the length of fibres from the avian posterior latissimus dorsi muscle has been investigated during embryonic development, when distributed synaptic sites are eliminated from the muscle fibres. Cell surface AChR were irreversibly blocked with unlabelled alpha-bungarotoxin (alpha-BGT). Muscles were then fixed and ultrasonically dissociated into fibre fragments, treated with 0.5% saponin and stained with 125I-alpha-BGT. This revealed an intracellular pool of curare sensitive binding sites equivalent to about 10% of total cell AChR. The spatial distribution of this pool was studied by autoradiography. Large (longer than 2 microns) AChR-clusters (AChR-C) characteristic of neuromuscular contacts were localized on the same fibres by immunofluorescence with an anti-AChR antibody. At E11, relatively high levels of intracellular AChR were observed throughout the length of fibres. Between E11 and E18 intracellular AChR declined (19 fold) in extrajunctional parts of fibres but remained high in segments of fibre corresponding to AChR-clusters. Treatment of E14 embryos with an inhibitor of protein synthesis (cycloheximide) reduced intracellular AChR to 22 +/- 6% (mean +/- SE) of control levels, suggesting that most of the intracellular binding represented newly-synthesized AChR. Between E11 and E18 cell nuclei were found to accumulate beneath AChR-C. The mean density of nuclei in segments of fibre corresponding to AChR-C increased 5 fold between E11 and E18, but remained unchanged in extrajunctional segments. It is suggested that the elimination of excess distributed AChR-C may be due to the preferential accumulation of nuclei at a single AChR-C on each fibre accompanied by the down regulation of AChR synthesis associated with nuclei at the remaining AChR-C.

Complexity of myosin species in the avian posterior latissimus dorsi muscle.

The myosin content of the avian posterior latissimus dorsi muscle, a small fast-twitch muscle similar in fibre type to the much-studied pectoralis major muscle (type IIB), has been explored using high resolution chromatography of the proteolytic fragment known as subfragment-1 and of the products of its limited tryptic digestion, followed by N-terminal sequencing of selected peptides. The complexity of species found greatly exceeds that anticipated from the fibre-type homogeneity of the muscle and from previous studies (Bandman et al., Cell 29 (1982) 645-50; Lowey et al., J. Musc. Res. Cell Motility 4 (1983) 695-716; Crow & Stockdale Dev. Biol. 118 (1986) 333-42). A minimum of four heavy chain species were identified. One form, approximately 40% of the heavy chain complement, appears to be identical to the well-characterized type IIB isoform of the pectoralis major muscle. The remaining species differ from the pectoralis major form in primary sequence. None is identical to the post-hatch isoform of the pectoralis major muscle.