Isoform diversity of giant proteins in relation to passive and active contractile properties of rabbit skeletal muscles

The active and passive contractile performance of skeletal muscle fibers largely depends on the myosin heavy chain (MHC) isoform and the stiffness of the titin spring, respectively. Open questions concern the relationship between titin-based stiffness and active contractile parameters, and titin's importance for total passive muscle stiffness. Here, a large set of adult rabbit muscles (n = 37) was studied for titin size diversity, passive mechanical properties, and possible correlations with the fiber/MHC composition. Titin isoform analyses showed sizes between approximately 3300 and 3700 kD; 31 muscles contained a single isoform, six muscles coexpressed two isoforms, including the psoas, where individual fibers expressed similar isoform ratios of 30:70 (3.4:3.3 MD). Gel electrophoresis and Western blotting of two other giant muscle proteins, nebulin and obscurin, demonstrated muscle type-dependent size differences of < or =70 kD. Single fiber and single myofibril mechanics performed on a subset of muscles showed inverse relationships between titin size and titin-borne tension. Force measurements on muscle strips suggested that titin-based stiffness is not correlated with total passive stiffness, which is largely determined also by extramyofibrillar structures, particularly collagen. Some muscles have low titin-based stiffness but high total passive stiffness, whereas the opposite is true for other muscles. Plots of titin size versus percentage of fiber type or MHC isoform (I-IIB-IIA-IID) determined by myofibrillar ATPase staining and gel electrophoresis revealed modest correlations with the type I fiber and MHC-I proportions. No relationships were found with the proportions of the different type II fiber/MHC-II subtypes. Titin-based stiffness decreased with the slow fiber/MHC percentage, whereas neither extramyofibrillar nor total passive stiffness depended on the fiber/MHC composition. In conclusion, a low correlation exists between the active and passive mechanical properties of skeletal muscle fibers. Slow muscles usually express long titin(s), predominantly fast muscles can express either short or long titin(s), giving rise to low titin-based stiffness in slow muscles and highly variable stiffness in fast muscles. Titin contributes substantially to total passive stiffness, but this contribution varies greatly among muscles.     

The behavior of titin and the proteins of its family from skeletal muscles of ground squirrel (Citellus undulatus) during hibernation and rats under conditions of simulated microgravity

By the use of SDS PAGE, the behavior of titin and MyBP-C in fast (m. psoas) as well as titin and MyBP-X in slow (m. soleus) muscles of ground squirrels (Citellus undulatus) during hibernation was compared with the behavior of titin and MyBP-X in rat m. soleus under conditions of simulated microgravity. A decrease in the amount of titin 1 and MyBP-C relative to that of myosin heavy chains by approximately 30% and approximately 40%, correspondingly, in muscles of hibernating and arousing ground squirrels was revealed in comparison with active animals. No differences in the relative amount of MyBP-X in m. soleus of hibernating, arousing and active ground squirrels were found. Under conditions of simulated microgravity, a decrease in the amount of titin 1 by approximately 2 times and MyBP-X by approximately1.5 times relative to that of myosin heavy chains in rat m. soleus was observed. By the method of SDS PAGE modified by us, an almost twofold decrease in the amount of short isovariants of the titin N2A isoform relative to that of myosin heavy chains was shown in muscles of hibernating and arousing ground squirrels, whereas no changes were found in the amount of long titin isovariants. The conditions of simulated microgravity resulted in a twofold decrease in the relative amount of both short and long titin isovariants in rat m. soleus. The results indicate that hibernating ground squirrels have an evolutionarily determined adaptive mechanism of selective degradation of fast muscle fibers and preservation or increase of slow fibers, as the most economic and energetically advantageous, with proteins typical of them. The microgravitation of nonhibernating animals (rats) leads to a non-selective degradation of MyBP-X and titin isovariants, which contributes to considerable atrophy of soleus fibers.     

Passive force generation and titin isoforms in mammalian skeletal muscle.

When relaxed striated muscle cells are stretched, a resting tension is produced which is thought to arise from stretching long, elastic filaments composed of titin (also called connectin). Here, I show that single skinned rabbit soleus muscle fibers produce resting tension that is several-fold lower than that found in rabbit psoas fibers. At sarcomere lengths where the slope of the resting tension-sarcomere length relation is low, electron microscopy of skinned fibers indicates that thick filaments move from the center to the side of the sarcomere during prolonged activation. As sarcomeres are stretched and the resting tension sarcomere length relation becomes steeper, this movement is decreased. The sarcomere length range over which thick filament movement decreases is higher in soleus than in psoas fibers, paralleling the different lengths at which the slope of the resting tension-sarcomere length relations increase. These results indicate that the large differences in resting tension between single psoas and soleus fibers are due to different tensions exerted by the elastic elements linking the end of each thick filament to the nearest Z-disc, i.e., the titin filaments. Quantitative gel electrophoresis of proteins from single muscle fibers excludes the possibility that resting tension is less in soleus than in psoas fibers simply because they have fewer titin filaments. A small difference in the electrophoretic mobility of titin between psoas and soleus fibers suggests the alternate possibility that mammalian muscle cells use at least two titin isoforms with differing elastic properties to produce variations in resting tension.     

Expression of SERCA2a is not regulated by calcineurin or upon mechanical unloading in skeletal muscle regeneration

This study investigates to what extent the expression of the slow myosin heavy chain (MyHCI) isoform and the slow type sarcoplasmic reticulum Ca2+ ATPase (SERCA2a) isoform are co-regulated in fibers of regenerating skeletal soleus muscle. Both overexpression of cain, a calcineurin inhibitor, or partial tenotomy prevented the expression of MyHCI but left SERCA2a expression unaffected in fibers of regenerating soleus muscles. These data complement those from different experimental models and clearly show that the expression of MyHCI and SERCA2a--the major proteins mediating, respectively, the slow type of contraction and relaxation--are not coregulated in regenerating soleus muscle.     

Changes in titin and myosin heavy chain isoform composition in skeletal muscles of Mongolian Gerbil (Meriones unguiculatus) after 12-day spaceflight

Changes of titin and myosin heavy chain isoform composition in skeletal muscles (m. soleus, m. gastrocnemius, m. tibialis anterior, m. psoas major) in Mongolian Gerbil (Meriones unguiculatus) were investigated after 12-day spaceflight on board of Russian space vehicle “Foton-M3.” In m. psoas and m. soleus in the gerbils from “Flight” group the expected increase in the content of fast myosin heavy chain isoforms (IIxd and IIa, respectively) were observed. No significant differences were found in the content of IIxd and IIa isoforms of myosin heavy chain in m. tibialis anterior in the gerbils from control group as compared to that in “Flight” group. An unexpected increase in the content of slow myosin heavy chain I isoform and a decrease in the content of fast IIx/d isoform in m. gastrocnemius of the gerbils from “Flight” group were observed. In skeletal muscles of the gerbils from “Flight” group the relative content of titin N2A-isoform was reduced (by 1.2–1.7 times), although the content of its NT-isoform, which was revealed in striated muscles of mammals in our experiments earlier, remained the same. When the content of titin N2A-isoform was decreased, no predictable abnormalities in sarcomeric structure and contractile ability of skeletal muscles in the gerbils from “Flight” group were found. An assumption on the leading role of titin NT-isoform in maintenance of structural and functional properties of striated muscles of mammals was made.     

Myosin heavy chain isoforms in postnatal muscle development of mice

In this study, using a high-resolution gel electrophoresis technique, we have characterized the myosin heavy chain composition in different skeletal muscle of the mouse during postnatal development. The pattern of myosin heavy chain expression was studied in four hind limb muscles, the diaphragm, the tongue and the masseter. All of these muscles displayed the usual sequential transitions from embryonic to neonatal and to adult myosin heavy chain isoforms but more interestingly these transitions occur with a distinct chronology in the different muscles. In addition, our results demonstrated a transitory pattern of expression for certain adult myosin heavy chain isoforms in the soleus and the tongue. In the soleus muscle IIB and in the tongue IIA myosin heavy chain isoforms were detected only for a short time during postnatal life. Our results demonstrate that muscles of the mouse with different functions are subjected to a distinct programs of myosin isoform transitions during postnatal muscle development. This study describes new data which will help us to understand both postnatal muscle development in transgenic mouse muscles as well as in muscle pathology.     

Passive tension and stiffness of vertebrate skeletal and insect flight muscles: the contribution of weak cross-bridges and elastic filaments.

Tension and dynamic stiffness of passive rabbit psoas, rabbit semitendinosus, and waterbug indirect flight muscles were investigated to study the contribution of weak-binding cross-bridges and elastic filaments (titin and minititin) to the passive mechanical behavior of these muscles. Experimentally, a functional dissection of the relative contribution of actomyosin cross-bridges and titin and minititin was achieved by 1) comparing mechanically skinned muscle fibers before and after selective removal of actin filaments with a noncalcium-requiring gelsolin fragment (FX-45), and 2) studying passive tension and stiffness as a function of sarcomere length, ionic strength, temperature, and the inhibitory effect of a carboxyl-terminal fragment of smooth muscle caldesmon. Our data show that weak bridges exist in both rabbit skeletal muscle and insect flight muscle at physiological ionic strength and room temperature. In rabbit psoas fibers, weak bridge stiffness appears to vary with both thin-thick filament overlap and with the magnitude of passive tension. Plots of passive tension versus passive stiffness are multiphasic and strikingly similar for these three muscles of distinct sarcomere proportions and elastic proteins. The tension-stiffness plot appears to be a powerful tool in discerning changes in the mechanical behavior of the elastic filaments. The stress-strain and stiffness-strain curves of all three muscles can be merged into one, by normalizing strain rate and strain amplitude of the extensible segment of titin and minititin, further supporting the segmental extension model of resting tension development.     

Single-fiber myosin heavy chain polymorphism: how many patterns and what proportions?

Previous studies have reported the existence of skeletal muscle fibers that coexpress multiple myosin heavy chain isoforms. These surveys have usually been limited to studying the polymorphic profiles of skeletal muscle fibers from a limited number of muscles (i.e., usually <4). Additionally, few studies have considered the functional implications of polymorphism. Hence, the primary objective of this study was to survey a relatively large number of rat skeletal muscle/muscle regions and muscle fibers (n approximately 5,000) to test the hypothesis that polymorphic fibers represent a larger fraction of the total pool of fibers than do so-called monomorphic fibers, which express only one myosin heavy chain isoform. Additionally, we used Hill's statistical model of the force-velocity relationship to differentiate the functional consequences of single-fiber myosin heavy chain isoform distributions found in these muscles. The results demonstrate that most muscles and regions of rodent skeletal muscles contain large proportions of polymorphic fibers, with the exception of muscles such as the slow soleus muscle and white regions of fast muscles. Several muscles were also found to have polymorphic profiles that are not consistent with the I<-->IIA<-->IIX<-->IIB scheme of muscle plasticity. For instance, it was found that the diaphragm muscle normally contains I/IIX fibers. Functionally, the high degree of polymorphism may 1) represent a strategy for producing a spectrum of contractile properties that far exceeds that simply defined by the presence of four myosin heavy chain isoforms and 2) result in relatively small differences in function as defined by the force-velocity relationship.     

Expression of titin isoforms in red and white muscle fibres of carp (Cyprinus carpio L.) exposed to different sarcomere strains during swimming

Titin (also known as connectin) is a striated-muscle-specific protein that spans the distance between the Z- and M-lines of the sarcomere. The elastic segment of the titin molecule in the I-band is thought to be responsible for developing passive tension and for maintaining the central position of thick filaments in contracting sarcomeres. Different muscle types express isoforms of titin that differ in their molecular mass. To help to elucidate the relation between the occurrence of titin isoforms and the functional properties of different fibre types, we investigated the presence of different titin isoforms in red and white fibres of the axial muscles of carp. Gel electrophoresis of single fibres revealed that the molecular mass of titin was larger in red than in white fibres. Fibres from anterior and posterior axial muscles were also compared. For both white and red fibres the molecular mass of titin in posterior muscle fibres was larger than in anterior muscle fibres. Thus, the same fibre type can express different titin isoforms depending on its location along the body axis. The contribution of titin to passive tension and stiffness of red anterior and posterior fibres was also determined. Single fibres were skinned and the sarcomere length dependencies of passive tension and passive stiffness were determined. Measurements were made before and after extracting thin and thick filaments using relaxing solutions with 0.6 mol · l⁻¹ KCl and 1 mol · l⁻¹ KI. Tension and stiffness measured before extraction were assumed to result from both titin and intermediate filaments, and tension after extraction from only intermediate filaments. Compared to mammalian skeletal muscle, intermediate filaments developed high levels of tension and stiffness in both posterior and anterior fibres. The passive tension-sarcomere length curve of titin increased more steeply in red anterior fibres than in red posterior fibres and the curve reached a plateau at a shorter sarcomere length. Thus, the smaller titin isoform of anterior fibres results in more passive tension and stiffness for a given sarcomere strain. During continuous swimming, red fibres are exposed to larger changes in sarcomere strain than white fibres, and posterior fibres to larger changes in strain than anterior fibres. We propose that sarcomere strain is one of the functional parameters that modulates the expression of different titin isoforms in axial muscle fibres of carp. Accepted: 7 May 1997     

Fiber types in canine muscles: myosin isoform expression and functional characterization

This study was aimed to achieve a definitive and unambiguous identification of fiber types in canine skeletal muscles and of myosin isoforms that are expressed therein. Correspondence of canine myosin isoforms with orthologs in other species as assessed by base sequence comparison was the basis for primer preparation and for expression analysis with RT-PCR. Expression was confirmed at protein level with histochemistry, immunohistochemistry, and SDS-PAGE combined together and showed that limb and trunk muscles of the dog express myosin heavy chain (MHC) type 1, 2A, and 2X isoforms and the so-called "type 2dog" fibers express the MHC-2X isoform. MHC-2A was found to be the most abundant isoform in the trunk and limb muscle. MHC-2X was expressed in most but not all muscles and more frequently in hybrid 2A-2X fibers than in pure 2X fibers. MHC-2B was restricted to specialized extraocular and laryngeal muscles, although 2B mRNA, but not 2B protein, was occasionally detected in the semimembranosus muscle. Isometric tension (P(o)) and maximum shortening velocity (V(o)) were measured in single fibers classified on the basis of their MHC isoform composition. Purified myosin isoforms were extracted from single muscle fibers and characterized by the speed (V(f)) of actin filament sliding on myosin in an in vitro motility assay. A close proportionality between V(o) and V(f) indicated that the diversity in V(o) was due to the different myosin isoform composition. V(o) increased progressively in the order 1/slow < 2A < 2X < 2B, thus confirming the identification of the myosin isoforms and providing their first functional characterization of canine muscle fibers.     

Disuse and passive stretch cause rapid alterations in expression of developmental and adult contractile protein genes in skeletal muscle

Contractile proteins exist as a number of isoforms that show a developmental and tissue-specific pattern of expression. Using gene-specific cDNA probes, the expression of the sarcomeric myosin heavy chain (MHC) multi-gene family and of cardiac (foetal) alpha-actin was analysed in three different rat hindlimb muscles immobilised for 5 days in either the shortened or lengthened positions. For each of the MHC genes normally expressed in adult muscle (slow, IIA and IIB), the effect of disuse alone (immobilisation in the shortened position) upon expression was markedly different to that of passive stretch (immobilisation in the lengthened position) in each of the three muscles. However, the same adult sarcomeric myosin heavy chain gene can be affected in a different, or even opposite, manner by either disuse or passive stretch depending on the muscle in which it is being expressed. The fast IIB MHC gene, for example, exhibits a rapid induction in the slow postural soleus muscle, in response to disuse but no such induction occurs in the faster plantaris and gastrocnemius muscles. Furthermore, the induction of this gene in the soleus was prevented by passive stretch. The MHC gene, normally only expressed in embryonic skeletal muscle, showed a similar response in all three muscles and was reinduced in adult muscle in response to passive stretch but not by disuse alone. In contrast, the isoform of alpha-actin which is normally only present in significant quantities in embryonic skeletal muscle and which is reduced postnatally, is not reinduced by passive stretch but is reduced still further by immobilisation in the shortened position.     

Endogenous expression and developmental changes of HSP72 in rat skeletal muscles

The purpose of the present study was to determine whether endogenous factor(s) contributes to the expression of heat shock proteins (HSPs) during the early developmental stages of rat skeletal muscles. HSP72 was expressed in both the soleus and plantaris muscles at embryonic day 22 (E22). On the basis of myosin heavy chain (MHC) immunohistochemistry, HSP72 was specifically expressed in slow type I fibers in both muscles. These slow fibers were observed throughout the entire cross section of the soleus muscle and only in the deep region (close to the bone) of the plantaris muscle. These results indicate that the expression of HSP72 is related to endogenous factors associated with type I fibers, because E22 rats have minimal exogenous influences and the soleus and plantaris muscles of E22 rats have similar metabolic and contractile profiles at this stage of development. We then examined the changes in HSP72 and heat shock cognate (HSC) 73 in the same two muscles from E22 to postnatal day 56 via Western blotting. The level of HSP72 in the soleus muscle gradually increased in parallel with the increment in the type I MHC isoform. Compared with the soleus, only a small amount of HSP72 could be detected in the plantaris muscle throughout the developmental period. For both muscles, HSC73 reached levels observed in adult muscles at postnatal day 3, and these levels were unchanged thereafter. These results indicate that the expression of HSP72, but not HSC73, is influenced by both endogenous and exogenous factors during the embryonic and early developmental periods.     

Effects of sensitization of the guinea pig slow muscle in disorders of the neurotrophic control]

The immunohistochemical profile of intact and denervated soleus muscle of guinea pigs after sensibilization was studied. It is shown, that intact soleus muscle consists of slow fibers, which have low ATP-ase activity and don't react with monoclonal antibodies against fast myosin heavy chain. No changes of immunohistochemical profile were found after denervation or sensibilization. At the same time, the fibers, reacting with monoclonal antibodies against fast myosin heavy chain and having low ATP-ase activity, were found in denervated muscles after sensibilization. It is concluded, that the synthesis of fast myosin is induced after sensibilization of denervated muscles. Validity of myosin ATP-ase histochemistry for muscle fibers typing is discussed.     

Nebulin isoforms of extraocular muscle

The extraocular muscles (EOMs), which are responsible for reflexive and voluntary eye movements, have many unique biochemical, physiological, and ultrastructural features that set them apart from other skeletal muscles. For example, rodent EOMs lack M-lines and express EOM-specific myosin heavy chain (MYH13) and α-cardiac myosin heavy chain. Recent gene-expression profiling studies indicate the presence of other cardiac-specific proteins in adult EOMs. This interesting mixture of myofibrillar and cytoskeletal proteins poses the questions as to whether nebulette, as opposed to nebulin, might be expressed in EOM, and what isoforms of titin are expressed in the EOM. We have performed gel electrophoresis and immunological analyses to determine the titin and nebulin isoforms expressed in the EOM. We have found that the mass of the titin isoforms expressed in the EOM most closely resemble those found in the skeletal muscles tested, viz., the soleus and extensor digitorum longus (EDL). We also demonstrate that, although the EOM expresses cardiac isoforms of myosin, it does not express nebulette and contains a nebulin isoform with a mass consistent with that found in the prototypical fast hindlimb muscle EDL. This work was supported by grants from NIH-NHLB HL073089 to C.L.M. and NEI/NIH EY12998 to F.H.A.     

Enhancement of hybrid-fiber types in rat soleus muscle after clenbuterol administration during hindlimb unloading

Our objective was to determine the effects of a clenbuterol (CB) treatment orally administered (2 mg per kg) to rats submitted to 14 days of hindlimb unloading (HU). The morphological and the contractile properties as well as the myosin heavy chain isoforms contained in each fiber type were determined in whole soleus muscles. As classically described after HU, a decrease in muscle wet weight and in body mass associated with a loss of muscular force, an evolution of the contractile parameters towards those of a fast muscle type, and the emergence of fast myosin heavy chain isoforms were observed. The CB treatment in the HU rats helped reduce the decrease in 1) muscle and body weights, 2) force and 3) the proportion of slow fibers, without preventing the emergence of fast myosin isoforms. Clenbuterol induced a complex remodelling of the muscle typing promoting the combination of both slow and fast myosin isoforms within one fiber. To conclude, our data demonstrate that CB administration partially counteracts the effects produced by HU, and they allow us to anticipate advances in the treatment of muscular atrophy.     

The effect of neonatal deafferentation or defferentation on myosin heavy chain expression in intrafusal muscle fibers of the rat

Muscle spindles were either deafferented or deefferented by selectively severing the sensory or motor nerve supply to neonatal soleus muscles of rats at a time when spindles are formed but when intrafusal muscle fibers are structurally and immunocytochemically immature. Experimental muscles were excised two months after nerve section. Control and experimental spindles were examined using monoclonal antibodies specific for myosin heavy chains of slow-tonic (ALD58) and fast-twitch (MF30) chicken muscles. Only intrafusal fibers bound these antibodies in intact soleus muscles. The deefferented spindles exhibited a pattern of ALD58 and MF30 binding similar to that of normal adult intrafusal fibers, whereas deafferented intrafusal fibers were unreactive with the two antibodies. Thus intact sensory innervation is essential for myosin heavy chain expression in intrafusal muscle fibers during postnatal development of rat spindles.     

Differential Expression of PTP1D, a Protein Tyrosine Phosphatase with Two SH2 Domains, in Slow and Fast Skeletal Muscle Fibers

We show that PTP1D, a protein tyrosine phosphatase that contains two SH2 domains, is preferentially expressed in slow skeletal muscle fibers. Immunohistochemical staining using polyclonal antibodies against PTP1D demonstrated that PTP1D was expressed in a subpopulation of rodent muscle fibers. These fibers were identified as slow Type I fibers based on histochemical ATPase assays and slow myosin heavy chain expression. Northern and Western analyses showed that PTP1D levels were higher in predominantly slow muscles than in predominantly fast muscles. This differential expression of PTP1D in slow muscle fibers appeared by birth. In cultures of mouse myogenic cells, PTP1D was expressed after MyoD and myogenin and appeared in myotubes derived from embryonic, fetal, and postnatal myoblasts. Remarkably, PTP1D was organized into sarcomeres in a pattern coincident with myosin heavy chain, suggesting that PTP1D associates with a component of the thick filament. These results show that PTP1D is preferentially expressed in slow muscle fibers. We speculate that PTP1D may play a role in slow muscle fiber function and differentiation.     

Assembly of thick filaments and myofibrils occurs in the absence of the myosin head

We investigated the importance of the myosin head in thick filament formation and myofibrillogenesis by generating transgenic Drosophila lines expressing either an embryonic or an adult isoform of the myosin rod in their indirect flight muscles. The headless myosin molecules retain the regulatory light-chain binding site, the alpha-helical rod and the C-terminal tailpiece. Both isoforms of headless myosin co-assemble with endogenous full-length myosin in wild-type muscle cells. However, rod polypeptides interfere with muscle function and cause a flightless phenotype. Electron microscopy demonstrates that this results from an antimorphic effect upon myofibril assembly. Thick filaments assemble when the myosin rod is expressed in mutant indirect flight muscles where no full-length myosin heavy chain is produced. These filaments show the characteristic hollow cross-section observed in wild type. The headless thick filaments can assemble with thin filaments into hexagonally packed arrays resembling normal myofibrils. However, thick filament length as well as sarcomere length and myofibril shape are abnormal. Therefore, thick filament assembly and many aspects of myofibrillogenesis are independent of the myosin head and these processes are regulated by the myosin rod and tailpiece. However, interaction of the myosin head with other myofibrillar components is necessary for defining filament length and myofibril dimensions.     

The roles of myosin ATPase activity and myosin light chain relative content in the slowing of type IIB fibers with hindlimb unweighting in rats

We tested the hypothesis that slowing of shortening velocity generated by type IIB fibers from hindlimb-unweighted (HU) rats resulted from a reduced ATPase activity and/or a reduction in the relative content of myosin light chain 3f isoform content (MLC_3f). After 2, 3, and 4 wk of HU, maximal unloaded shortening velocity (V_o) of single permeabilized semimembranosus muscle fibers was determined by the slack test. Subsequently, the myosin heavy chain and the relative content of MLC were determined by SDS-PAGE. The ratio of MLC_3f to MLC_2f was determined by densitometric analysis. In addition, myofibrils were prepared from permeabilized fibers (soleus and semimembranosus muscles) and assayed for resting myosin ATPase and Ca²⁺-activated myosin ATPase. After HU, V_o declined by 28–40% and the MLC_3f/MLC_2f ratio decreased by 32 to 48%. A significant correlation between the relative amount of MLC_3f and V_o was found (r = 0.48, P < 0.05). Resting myosin ATPase rates were not different between myofibrils prepared from corresponding muscles of control and HU rats (P = 0.86). Ca²⁺-activated myosin ATPase activities also were not different between myofibrils prepared from corresponding muscles of control and HU rats (P = 0.13). These data suggest that the slowing of maximal unloaded shortening velocity in type IIB fibers with HU is, at least in part, due to a relative change in the essential light chain composition, a decrease in the relative amount of MLC_3f and most likely a concomitant increase in MLC_1f. However, this reduction in V_o is independent of myosin ATPase activity. unloading shortening velocity; myosin light chain 3f     

The effect of neonatal deafferentation or deefferentation on myosin heavy chain expression in intrafusal muscle fibers of the rat

Summary Muscle spindles were either deafferented or deefferented by selectively severing the sensory or motor nerve supply to neonatal soleus muscles of rats at a time when spindles are formed but when intrafusal muscle fibers are structurally and immunocytochemically immature. Experimental muscles wereexcised two months after nerve section. Control and experimental spindles were examined using monoclonal antibodies specific for myosin heavy chains of slow-tonic (ALD58) and fast-twitch (MF30) chicken muscles. Only intrafusal fibers bound these antibodies in intact soleus muscles. The deefferented spindles exhibited a pattern of ALD58 and MF30 binding similar to that of normal adult intrafusal fibers, whereas deafferented intrafusal fibers were unreactive with the two antibodies. Thus intact sensory innervation is essential for myosin heavy chain expression in intrafusal muscle fibers during postnatal development of rat spindles.