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1. Differential expression of myosin heavy chain (MHC) isoforms dramatically affects mechanical and energetic properties of skeletal muscle fibre types. As many as five different fibre types, each with different mechanical properties, have been reported in frog hindlimb muscles. However, only two frog MHC isoforms have previously been detected by SDS-PAGE and only one adult hindlimb MHC isoform has been cloned. 2. In the present study, four different fibre types (type 1, type 2, type 3 and tonic) were initially identified in adult Rana pipiens anterior tibialis muscle based on myosin ATPase histochemistry, size and location. Each fibre type exhibited unique reactivity to a panel of MHC monoclonal antibodies. Single fibre analysis using SDS-PAGE revealed that MHCs from immunohistochemically defined type 1, type 2 and type 3 fibres ran as three distinct isoform bands, while MHC of tonic fibres co-migrated with type 1 MHC. The combined data from immunohistochemistry and SDS-PAGE suggests that Rana fibre types are composed of four different MHCs. 3. Four novel MHC cDNAs were cloned and expression of the corresponding transcripts was measured in single immuno-identified fibres using specific polymerase chain reaction (PCR) primer pairs. Each of the four transcripts was found to be primarily expressed in a different one of the four fibre types. 4. Coexpression of MHC isoforms was observed only between types 1/2 and types 2/3 at both the protein and mRNA level. 5. These data provide a molecular basis for differentiation between frog fibre types and permit future molecular studies of MHC structure/function and gene regulation in this classic physiological system. 6. Comparison of sequence homology among amphibian, avian and mammalian MHC families supports the concept of independent evolution of fast MHC genes within vertebrate classes subsequent to the amphibian/avian/mammalian radiation.
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PMID:Four novel myosin heavy chain transcripts define a molecular basis for muscle fibre types in Rana pipiens. 951 24

To examine the expression patterns of myosin heavy chain (MHC) isoforms in single fibres of the soleus muscle following weightlessness, 10-week-old male Wistar rats were subjected to hindlimb suspension for 4 weeks. Hindlimb suspension resulted in reduced body weight and absolute and relative mass of the soleus muscle compared with controls (P < 0.01). A total of 975, 892 and 1098 single fibres from pre-suspended controls, age-matched controls and suspension groups, respectively, were subjected to MHC analyses using SDS-PAGE. Single fibres containing only MHC I decreased (87.9 vs. 67.9%, P < 0.05) and single fibres containing only MHC IIa disappeared after hindlimb suspension. On the contrary, single fibres containing multiple type II MHC isoforms were observed as follows: 10.1% single fibres contained MHCs IIa and IId; 14.1% contained MHCs I, IIa and IId; and some (1.4%) expressed the MHC IIb isoform with MHCs IIa and IId. The relative content (%) of each MHC isoform in MHC hybrid single fibres was calculated using densitometer scanning. The MHCs IIa and IId hybrid single fibres contained the same amount of MHC IIa (51.3 +/- 6.3%) and MHC IId (48.7 +/- 6.3%). In the MHCs I, IIa and IId hybrid single fibres, the percentage of MHC IIa was distributed in a wide range (approximately 80%), whereas the percentage of MHC IId was a relatively low range (approximately 40%), and the relative content of MHC I was inversely correlated with that of MHC IIa and MHC IId, respectively. The fibre type composition of suspended soleus muscle, analysed by histochemical myosin ATPase staining, was changed, with a decrease in the percentage of type I fibres and an increase in that of type IIA fibres. Our results indicate that hindlimb suspension induces multiple type II MHC expression in the soleus single fibres and suggest that the single fibres containing multiple type II MHC isoforms should be classified into type IIA.
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PMID:Hindlimb suspension induces the expression of multiple myosin heavy chain isoforms in single fibres of the rat soleus muscle. 955 Feb 24

The extensor carpi radialis muscle of the horse is deceptive at first appearance. It has a fusiform shape similar to other forearm extensor muscles. The fiber arrangement also appears long and relatively parallel. However, it may contain two or more compartments that correlate with differing functional roles. Histochemical and immunocytochemical analysis of proximal and distal regions of the muscle (n = 9) demonstrate that the proximal portion of the muscle is composed of a mean of 13% type I, presumed slow twitch, and 61% type IIb, presumed fast twitch fibers. In contrast, the distal compartment is composed of a mean of about 43% type I and only 22% type IIB fibers. The type I and IIa fibers are all highly aerobic based on nicotinamide dinucleotide tetrazolium reductase reactions. Correlative data regarding the myosin isoforms has been obtained with 4% SDS-PAGE analysis of myosin heavy chain isoforms which demonstrate isoforms migrating at rates similar to rat type I, IIa, and IIx. The latter has been referred to as type IIB/X in a study of the horse's gluteus medius muscle. We propose that the in-series 'compartmentalization' of the muscle, while not conforming strictly to the definitions of neuromuscular compartments, relates to the insertion of the lacertus fibrosus, a distal slip of the biceps brachii, upon the extensor carpi radialis. Earlier studies demonstrated a high proportion of type I fibers in the equine lateral biceps brachii which were thought to stabilize the shoulder during long periods of quiet standing. Because of action imposed on the distal compartment by the biceps brachii, slow and fatigue-resistant functions are part of the limb's passive stay apparatus to effect long-term standing by the horse. Thus, the fatigue-resistant compartments of biceps brachii and extensor carpi radialis may constitute an in-series arrangement of the two muscles. The proximal compartment is suited to provide powerful, more fatigable contractions during locomotion and likely affects stress or strain within the distal postural compartment.
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PMID:Architecture and the division of labor in the extensor carpi radialis muscle of horses. 957 63

In striated muscle, chronic increases in workload result in changes in myosin phenotype. The aim of this study was to determine whether such changes occur in the diaphragm of patients with severe chronic obstructive pulmonary disease, a situation characterized by a chronic increase in respiratory load and lung volume. Diaphragm biopsies were obtained from 22 patients who underwent thoracic surgery. Myosin was characterized with electrophoresis in nondenaturing conditions, SDS-glycerol PAGE, and Western blotting with monoclonal antibodies specific for slow and fast myosin heavy chain (MHC) isoforms. Flow volume curves, total lung capacity, and functional residual capacity were measured before surgery in 20 patients. We found that the human diaphragm is composed of at least four myosin isoforms, one slow and three fast, resulting from the combination of three MHC species. Chronic overload was associated with an increase in the slow beta-MHC species at the expense of the fast species (beta-MHC, 78.2 +/- 4.6 and 50.0 +/- 6.5% in emphysematous and control patients, respectively; P < 0.005). Linear correlations were found between beta-MHC percentage and forced expiratory volume in 1 s (r = -0.52; P < 0.02), total lung capacity (r = 0.44; P < 0.05), and functional residual capacity (r = 0.65; P < 0.003). The human adult diaphragm is composed of a balanced proportion of slow and fast myosin isoforms. In patients with chronic obstructive pulmonary disease, the proportion of fast myosins decreases, whereas that of slow myosin increases. This increase appears to be closely related to lung hyperinflation and may reflect an adaptation of the diaphragm to the new functional requirements.
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PMID:Myosin heavy chain gene expression changes in the diaphragm of patients with chronic lung hyperinflation. 957 70

Although prolonged diaphragm denervation (DNV) produces myofiber atrophy and a loss of type I myosin heavy chain (MHC) expression, short-term DNV leads to significant diaphragm hypertrophy. The purpose of this study was to explore the regulation of MHC isoform expression and muscle remodeling during DNV hypertrophy of the diaphragm. Both unilateral and bilateral DNV led to similar changes, with a significant increase in total RNA content and muscle mass but no change in dry-to-wet weight ratio. Sarcomere number was also increased in diaphragm myofibers after DNV ( approximately 20%), suggesting an adaptive response to muscle stretch. There was hypertrophy of type I myofibers and increased coexpression of type I and type II MHCs within single myofibers by immunocytochemistry as well as increased type I MHC (25-46%) and decreased type IIb MHC (14-39%) by SDS-PAGE. Contractility parameters were also consistent with a type II-to-type I MHC phenotype transformation. Importantly, DNV-induced modulation of MHC isoform mRNA transcript levels did not correspond to changes in their cognate proteins, suggesting a major degree of posttranscriptional control. We conclude that DNV hypertrophy of the diaphragm is associated with reciprocal changes in type I and type II MHC isoforms that are directly opposed to the type I-to-type II MHC phenotype transformation reported in the diaphragm DNV atrophy model. Furthermore, in contradistinction to most hypertrophy models, control of MHC gene expression and myofibrillar remodeling after short-term DNV appears to entail major involvement of posttranscriptional regulatory mechanisms.
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PMID:Regulation of myosin heavy chain gene expression after short-term diaphragm inactivation. 960 37

The aim of this study was to evaluate the cellular response of the diaphragm, extensor digitorum longus (EDL), and soleus (Sol) muscles to clinically relevant doses of cyclosporine administered to male rats over 4 wk. Control rats were provided with vehicle only. Muscle fiber types, cross-sectional areas, indexes of capillarity, and succinate dehydrogenase (SDH) activity were determined by quantitative histochemistry. Myosin heavy chain isoforms were identified by SDS-PAGE, and their proportions were measured by scanning densitometry. Serum cyclosporine level, 20-24 h after the last dose of cyclosporine, was 145 +/- 81 ng/ml. Final body weight and muscle mass were similar between the cyclosporine and control groups. In the diaphragm, EDL, and Sol, no differences were observed between the groups with regard to fiber type proportions, fiber cross-sectional areas, and proportions of myosin heavy chain isoforms. In the EDL, reductions, both in SDH activity in type I, IIx, and IIb fibers (-26 to -37%) and in indexes of capillarity (-18 to -37%), were noted. In the Sol, SDH activity and capillarity were similar between the groups. In the diaphragm of cyclosporine-treated rats, there was significant reduction in the number of capillaries around individual fibers (-5%), whereas levels of SDH activity tended to be lower. This suggests that activation history may in part determine muscle-specific responses to cyclosporine. We speculate that reduced oxidative activity and capillarity of some limb muscles contribute to reduced exercise capacity and the "deconditioned state" observed in patients receiving cyclosporine after successful solid-organ transplantation.
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PMID:Cellular adaptations of skeletal muscles to cyclosporine. 960 91

Male Sprague-Dawley rats (350-500 g) were made septic by intraperitoneal injection of 200 mg/kg cecal material in 5% dextrose in water (D5W; 5 ml/kg). Control rats (n = 11) received D5W. Preparations were studied on days 1 (n = 7), 3 (n = 7), and 7 (n = 8) of sepsis. In isolated hearts, ventricular function was depressed on days 3 and 7 of sepsis. Densitometric analysis of myofilament proteins from septic rats separated by SDS-PAGE showed no differences in relative amounts of actin, troponin, tropomyosin and myosin light chains compared to control. Myofilament function, assessed by measuring ATPase activities, was altered during sepsis. CA(2+)-independent Mg-ATPase activity was elevated on days 1 and 3 of sepsis, returning toward control by day 7. Maximal ATPase activity was unchanged on day 1, but was increased on days 3 and 7 sepsis. Myofibrillar myosin K(EDTA)-, Ca(2+)-, and Mg(2+)-ATPase activities were not altered, nor were there any apparent changes in myosin heavy chain isoform populations. Our data are the first to demonstrate alterations in minimal and maximal ATPase activities and myofilament CA(2+)-sensitivity during chronic peritoneal sepsis. These alterations may contribute to observed changes in ventricular function.
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PMID:Cardiac myofilament protein function is altered during sepsis. 961 37

Long-standing heavy alcohol consumption acts as a chronic stress on the heart. It is thought that alcohol-induced changes of contractility are due to altered Ca2+ handling, but no measurements of cytosolic Ca2+ ([Ca2+]c) after chronic alcohol exposure have been made. Therefore experiments were performed to determine whether alcohol-induced changes in contractility are due to altered Ca2+ handling by measuring [Ca2+]c (indo 1) in hearts from rats drinking 36% ethanol for 7 mo and age-matched controls. Peak left ventricular pressure was depressed (-16%), whereas rates of contraction (12%) and relaxation (14-20%) were faster in alcohol-exposed hearts. Systolic [Ca2+]c (808 +/- 45 vs. 813 +/- 45 nM), diastolic [Ca2+]c (195 +/- 11 vs. 193 +/- 10 nM), and rates of [Ca2+]c rise and decline were the same in alcohol-exposed and control hearts. Protein levels of Ca2+-handling proteins, sarcoplasmic reticulum Ca2+-ATPase and phospholamban, were the same in myocytes isolated from alcohol-exposed and control hearts (SDS-polyacrylamide gel). These data suggest that chronic alcohol-induced contractile changes are not due to altered Ca2+ handling but may be due to changes at the level of the myofilament. As a first step in elucidating the mechanism(s) of alcohol-induced changes at the myofilament, we assessed myosin heavy chain (MHC) isoform content (SDS-polyacrylamide gel). alpha-MHC was decreased relative to beta-MHC (a/a + b = 0.55 +/- 0.03 vs. 0.66 +/- 0.02; P < 0.02) in alcohol-exposed hearts, which cannot account for the observed alcohol-induced contractile changes. In conclusion, changes of myocardial contractility due to chronic alcohol exposure do not result from altered Ca2+ handling but from changes at the level of the myofilament that do not involve MHC isoform shifts.
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PMID:Chronic alcohol-induced changes in cardiac contractility are not due to changes in the cytosolic Ca2+ transient. 968 4

Denervation differs from other models of reduced neuromuscular activation due to the absence of a nerve-muscle connection and limited data exists regarding the effects of denervation on myosin heavy chain (MHC) expression. Thus, adult MHC expression (I, IIa, IIx, IIb) was studied in the rat soleus and tibialis anterior (TA) at the mRNA and protein levels 2, 4, 7, 10, 14, and 30 days following sciatic nerve transection. MHC protein content was quantified with SDS/PAGE and mRNA levels with the RNase-protection assay. Control soleus consisted predominately of type I MHC mRNA and protein, however, 4 days after denervation type I MHC mRNA was significantly decreased to 41+/-8% of control and continued to remain below control values. Soleus IIa mRNA was significantly elevated 7 and 10 days after denervation while IIx mRNA remained relatively constant until 30 days when it increased to 197+/-23% of control. At the protein level, soleus I MHC significantly decreased to 80% of the total while IIa MHC significantly increased to 20% of the total. At 30 days, Hx MHC protein accounted for 9.4+/-1.6% of the total soleus MHC protein. In the TA, IIb mRNA was significantly decreased to 57% of control by day 4 and remained significantly decreased for up to a month. TA IIx mRNA was also significantly decreased at 10 and 30 days after denervation. Similar to the soleus, TA Ha mRNA was significantly increased over control 7-14 days after denervation. There were no significant changes in TA MHC protein profile during one month of denervation. In both the soleus and TA, denervation significantly shifted the MHC mRNA profile as early as 4 days following denervation without any corresponding changes at the protein level. Significant mRNA changes without large changes in MHC protein composition continued throughout the denervation period suggesting that the muscle may be prevented from premature functional transitions by mechanisms such as decreased mRNA stability, translational block, or increased turnover of newly synthesized proteins.
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PMID:Changes in myosin mRNA and protein expression in denervated rat soleus and tibialis anterior. 974 44

To investigate the design of the frog muscular system for jumping, fibre type distribution and myosin heavy chain (MHC) isoform composition were quantified in the hindlimb muscles of Rana pipiens. Muscles were divided into two groups: five large extensor muscles which were predicted to shorten and produce mechanical power during jumping (JP), and four much smaller muscles commonly used in muscle physiology studies, but that do not shorten or produce power during jumping (NJP). fibres were classified as one of four different types (type 1, 2, 3 or tonic) or an intermediate type (type 1-2) based on their relative myosin-ATPase reactivity and MHC immunoreactivity in muscle cross-sections according to previous nomenclature established for amphibian skeletal muscle. Type 1 fibres correspond to the fastest and most powerful of the twitch fibres, and type 3 fibres are the slowest and least powerful. Myosin-ATPase histochemistry revealed that the JP muscles were composed primarily of type 1 fibres (89%) with a small percentage of type 2 (7%) and intermediate type 1-2 fibres (4%). The fibre type composition of NJP muscles was more evenly distributed between type 1 (29%), type 2 (46%) and type 1-2 (24%) fibres. Tonic fibres comprised less than 2% of the muscle cross-section in both JP and NJP groups. Similarly, MHC composition determined by quantitative SDS-PAGE revealed that JP muscles were composed predominantly of type 1 MHC (86%), with a balance of type 2 MHC (14%). The opposite pattern was found for MHC composition in the NJP muscles: type 1 (28%), type 2 (66%) and type 3 (6%). These results demonstrate that the large extensor muscles that produce the power required for jumping have a fibre type distribution that enables them to generate high levels of mechanical power, with the type 1 isoform accounting for 85-90% of the total MHC content.
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PMID:Quantitative analysis of muscle fibre type and myosin heavy chain distribution in the frog hindlimb: implications for locomotory design. 983 43


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