Transitional Hybrid Skeletal Muscle Fibers in Rat Soleus Development

Skeletal muscles comprise hundreds of individual muscle fibers, with each possessing specialized contractile properties. Skeletal muscles are recognized as being highly plastic, meaning that the physiological properties of single muscle fibers can change with appropriate use. During fiber type transitions, one myosin heavy chain isoform is exchanged for another and over time the fundamental nature of the fiber adapts to become a different fiber type. Within the rat triceps surae complex, the soleus muscle starts out as a muscle comprised of a mixture type IIA and type I fibers. As neonatal rats grow and mature, the soleus undergoes a near complete transition into a muscle with close to 100% type I fibers at maturity. We used immunohistochemistry and single fiber SDS-PAGE to track the transformation of type IIA into type I fibers. We found that transitioning fibers progressively incorporate new myofibrils containing type I myosin into existing type IIA fibers. During this exchange, distinct type I-containing myofibrils are segregated among IIA myofibrils. The individual myofibrils within existing muscle fibers thus appear to represent the functional unit that is exchanged during fiber type transitions that occur as part of normal muscle development.

Northcroft Memorial Lecture 2005

The British Orthodontic Society invites outstanding contributors from the field of Orthodontics to give the guest lecture in memory of George Northcroft. In 2005 the guest lecturer was Professor Nigel Hunt. The article that follows was presented as the Northcroft Memorial Lecture 2005 at the World Orthodontic Congress, Paris.

[Influence of modeling of gravitational unloading on the postsynaptic acetylcholine receptor organization and acetylcholinesterase activity in neuromuscular synapses of rat fast and slow muscles]

Using immunofluorescent techniques, we have revealed that, after 35 days of rats hindlimb unloading, neuromuscular synapses of fast and slow muscles show enhanced fluorescence intensity and decreased area of fluorescent staining of acetylcholine receptors; increased fluorescent intensity and area of fluorescent staining for acetylcholinesterase. The ratio of the number of postsynaptic acetylcholine receptors and the amount of acetylcholinesterase changed as well as their spatial position in relation to each other. These rearrangements correspond to electrophysiological data on the reduction of the amplitude of the miniature endplate currents in both muscles. Identified synapses restructuring accompanied by a decrease in the volume of muscle fibers. Hindlimb unloading (simulation of hypogravity) leads to an increase in functional activity of acetylcholinesterase on the background of reduced postsynaptic membrane area occupied by acetylcholine receptors. This leads to a decrease in the amplitude of excitatory postsynaptic potentials thereby reducing the nerve-muscle excitation transmission safety factor.

Urethral striated muscle and extracellular matrix morphological characteristics among mildly diabetic pregnant rats: translational approach

INTRODUCTION AND HYPOTHESIS

Diabetes mellitus (DM) during pregnancy is associated with high levels of urinary incontinence (UI) and pelvic floor muscle dysfunction. Mild DM can lead to changes in urethral striated muscle and extracellular matrix (ECM) in pregnant rats considering both structures as an entire system responsible for urinary continence.

METHODS

Ninety-two female Wistar rats were distributed in four experimental groups: virgin, pregnant, diabetic, and diabetic pregnant. In adult life, parental nondiabetic female rats were mated with nondiabetic male rats to obtain newborns. At the first day of birth, newborns received citrate buffer (nondiabetic group) or streptozotocin 100 mg/kg body weight, subcutaneous route (mild DM group). At day 21 of the pregnancy, the rats were lethally anesthetized and the urethra and vagina were extracted as a unit. Urethral and vaginal sections were cut and analyzed by: (a) cytochemical staining for ECM and muscle structural components, (b) immunohistochemistry to identify fast- and slow-muscle fibers, and (c) transmission electron microscopy for ultrastructural analysis of urethral striated muscle.

RESULTS

In comparison with the three control groups, variations in the urethral striated muscle and ECM from diabetic pregnant rats were observed including thinning, atrophy, fibrosis, increased area of blood vessels, mitochondria accumulation, increased lipid droplets, glycogen granules associated with colocalization of fast and slow fibers, and a steady decrease in the proportion of fast to slow fibers.

CONCLUSIONS

Mild DM and pregnancy can lead to a time-dependent disorder and tissue remodeling in which the urethral striated muscle and ECM has a fundamental function.

Mitochondrial myopathy associated with a novel 5522G>A mutation in the mitochondrial tRNA(Trp) gene

We report a novel pathogenic mutation of the mitochondrial transfer RNA (tRNA) gene for tryptophan in a patient with isolated myopathy and persistently elevated creatine kinase. Muscle studies revealed ragged red fibres and decreased activity of respiratory chain complex I and cytochrome c oxidase (COX). Sequencing of the 22 mitochondrial tRNA genes revealed a mutation m.5522G>A, which alters a conserved base pairing in the D-stem of the tRNA for tryptophan. The mutation was heteroplasmic with a mutational load between 88 and 99% in COX-negative fibres. This case contributes to the genetic heterogeneity of mitochondrial diseases caused by mutations in mitochondrial tRNA genes.

Nutritional intervention restores muscle but not kidney phenotypes in adult calcineurin Aα null mice

Mice lacking the α isoform of the catalytic subunit of calcineurin (CnAα) were first reported in 1996 and have been an important model to understand the role of calcineurin in the brain, immune system, bones, muscle, and kidney. Research using the mice has been limited, however, by failure to thrive and early lethality of most null pups. Work in our laboratory led to the rescue of CnAα−/− mice by supplemental feeding to compensate for a defect in salivary enzyme secretion. The data revealed that, without intervention, knockout mice suffer from severe caloric restriction. Since nutritional deprivation is known to significantly alter development, it is imperative that previous conclusions based on CnAα−/− mice are revisited to determine which aspects of the phenotype were attributable to caloric restriction versus a direct role for CnAα. In this study, we find that defects in renal development and function persist in adult CnAα−/− mice including a significant decrease in glomerular filtration rate and an increase in blood urea nitrogen levels. These data indicate that impaired renal development we previously reported was not due to caloric restriction but rather a specific role for CnAα in renal development and function. In contrast, we find that rather than being hypoglycemic, rescued mice are mildly hyperglycemic and insulin resistant. Examination of muscle fiber types shows that previously reported reductions in type I muscle fibers are no longer evident in rescued null mice. Rather, loss of CnAα likely alters insulin response due to a reduction in insulin receptor substrate-2 (IRS2) expression and signaling in muscle. This study illustrates the importance of re-examining the phenotypes of CnAα−/− mice and the advances that are now possible with the use of adult, rescued knockout animals.

Ultrastructural differences and histochemical characteristics in swimming muscles between wild and reared Atlantic salmon

AIM

The swimming capacity of wild and reared fish differs. Whether the differences are associated with metabolic, contractile or structural variation in swimming musculature is unknown. In the present study, some aspects of contractile machinery in swimming muscles of wild and reared salmon are compared.

METHODS

Several morphological parameters and key enzyme activities were measured using electron microscopy and histochemical methods.

RESULTS

The density and size of the mitochondria was significantly higher in the muscle samples from wild fish when compared with the reared ones. Similar variability was also seen in the density of triads. Conversely, the size and density of lipid droplets was significantly lower in the red muscle of wild vs. reared salmon. The densities of two excitation contraction coupling components, dihydropyridine and ryanodine receptor, were considerably higher in swimming muscles of wild salmon than in reared fish. A similar difference was observed in the activities of aerobic enzymes. Moreover, oxygen consumption followed the same pattern, being significantly higher in the samples of wild salmon. Phosphorylase activity was, on the other hand, significantly lower in the muscles of wild fish.

CONCLUSIONS

There are significant differences in morphology, Ca(2+)-regulating capacity and enzyme activities in swimming muscles between wild and reared salmon. These results provide evidence that the prerequisites for efficient contraction of the swimming muscles are better met in wild than in reared salmon. Importantly, the results also suggest that the observed variation is a major contributing factor to the difference in the swimming capacity between wild and hatchery-reared salmon.

Rosuvastatin: characterization of induced myopathy in the rat

Rosuvastatin is a relatively new member of the statin family (HMG-CoA reductase inhibitors), with superior lipid-lowering effects and a pattern of clinical side effects, including a low incidence of myopathy, similar to other widely prescribed statins. This article describes investigations of myopathy in the rat following administration of very high doses of rosuvastatin. The nature of the changes were found to be entirely consistent with those seen with other statins, including a differential sensitivity of muscle fibers (with glycolytic fibers [type IIB] the most sensitive and oxidative fibers [type I] the least), a delay of approximately 10 days after the start of oral dosing before necrosis was apparent, and ultrastructural alterations appearing first in mitochondria. In addition, the development of myopathy was prevented by coadministration of mevalonate, the product of HMG-CoA reductase. The findings illustrate a pattern of induced myopathy in the rat directly attributable to inhibition of HMG-CoA reductase that is entirely consistent between the various statins, with the oral dose required to produce the changes being a differentiating feature (based on these new data and a previously reported study from the same laboratory): cerivastatin dose less than simvastatin, and simvastatin dose less than rosuvastatin.

The Ca(V) 1.2 Ca(2+) channel is expressed in sarcolemma of type I and IIa myofibers of adult skeletal muscle

Although Ca(2+)-dependent signaling pathways are important for skeletal muscle plasticity, the sources of Ca(2+) that activate these signaling pathways are not completely understood. Influx of Ca(2+) through surface membrane Ca(2+) channels may activate these pathways. We examined expression of two L-type Ca(2+) channels in adult skeletal muscle, the Ca(V) 1.1 and Ca(V) 1.2, with isoform-specific antibodies in Western blots and immunocytochemistry assays. Consistent with a large body of work, expression of the Ca(V) 1.1 was restricted to skeletal muscle where it was expressed in T-tubules. Ca(V) 1.2 was also expressed in skeletal muscle, in the sarcolemma of type I and IIa myofibers. Exercise-induced alterations in muscle fiber types cause a concomitant increase in the number of both Ca(V) 1.2 and type IIa-positive fibers. Taken together, these data suggest that the Ca(V) 1.2 Ca(2+) channel is expressed in adult skeletal muscle in a fiber type-specific manner, which may help to maintain oxidative muscle phenotype.

Immunohistochemical Study of Differences Between the Muscle Fiber Types in the Pars Peripheralis and Marginalis

The aim of this study was to evaluate the immunohistochemical differences between the muscular fiber types in the pars peripheralis and pars marginalis of human orbicularis oris muscle. Five upper lips of fresh human adult cadavers were used. Full thickness of the upper lip, 5 mm in width, was harvested vertically at a peak point of cupid's bow. Troponin I-SS and Troponin I-FS antibodies were used to determinate the slow and fast skeletal muscle fibers. The pars peripheralis is composed of slow fibers (22%) and fast fibers (73%). The pars marginalis is composed of slow fibers (30%) and fast fibers (66%). We assume that the pars peripheralis and pars marginalis should be repaired sortably because the muscle reaction and endurance are not the same.

Plectin defects in epidermolysis bullosa simplex with muscular dystrophy

Epidermolysis bullosa simplex with muscular dystrophy (EBS-MD, MIM 226670) is caused by plectin defects. We performed mutational analysis and immunohistochemistry using EBS-MD (n = 3 cases) and control skeletal muscle to determine pathogenesis. Mutational analysis revealed a novel homozygous plectin-exon32 rod domain mutation (R2465X). All plectin/HD1-121 antibodies stained the control skeletal muscle membrane. However, plectin antibodies stained the cytoplasm of type II control muscle fibers (as confirmed by ATPase staining), whereas HD1-121 stained the cytoplasm of type I fibers. EBS-MD samples lacked membrane (n = 3) but retained cytoplasmic HD1-121 (n = 1) and plectin staining in type II fibers (n = 3). Ultrastructurally, EBS-MD demonstrated widening and vacuolization adjacent to the membrane and disorganization of Z-lines (n = 2 of 3) compared to controls (n = 5). Control muscle immunogold labeling colocalized plectin and desmin to filamentous bridges between Z-lines and the membrane that were disrupted in EBS-MD muscle. We conclude that fiber-specific plectin expression is associated with the desmin-cytoskeleton, Z-lines, and crucially myocyte membrane linkage, analogous to hemidesmosomes in skin.

Subcellular localization of the glutamate transporters GLAST and GLT at the neuromuscular junction in rodents

The vertebrate neuromuscular junction (NMJ) is known to be a cholinergic synapse at which acetylcholine (ACh) is released from the presynaptic terminal to act on postsynaptic nicotinic ACh receptors. There is now growing evidence that glutamate, which is the main excitatory transmitter in the CNS and at invertebrate NMJs, may have a signaling function together with ACh also at the vertebrate NMJ. In the CNS, the extracellular concentration of glutamate is kept at a subtoxic level by Na(+)-driven high-affinity glutamate transporters located in plasma membranes of astrocytes and neurons. The glutamate transporters are also pivotal for shaping glutamate receptor responses at synapses. In order to throw further light on the potential role of glutamate as a cotransmitter at the NMJ we used high-resolution immunocytochemical methods to investigate the localization of the plasma membrane glutamate transporters GLAST (glutamate aspartate transporter) and GLT (glutamate transporter 1) in rat and mice NMJ regions. Confocal laser-scanning immunocytochemistry showed that GLT is restricted to the NMJ in rat and mouse skeletal muscle. Lack of labeling signal in knock-out mice confirmed that the immunoreactivity observed at the NMJ was specific for GLT. GLAST was also localized at the NMJ in rat but not detected in mouse NMJ (while abundant in mouse brain). Post-embedding electron microscopic immunocytochemistry and quantitative analyses in rat showed that GLAST and GLT are enriched in the junctional folds of the postsynaptic membrane at the NMJ. GLT was relatively higher in the slow-twitch muscle soleus than in the fast-twitch muscle extensor digitorum longus, whereas GLAST was relatively higher in extensor digitorum longus than in soleus. The findings show--together with previous demonstration of vesicular glutamate, a vesicular glutamate transporter and glutamate receptors--that mammalian NMJs contain the machinery required for synaptic release and action of glutamate. This indicates a signaling role for glutamate at the normal NMJ and provides a basis for the ability of denervated muscle to be reinnervated by glutamatergic axons from the CNS.

Short-term electrical stimulation alters tongue muscle fibre type composition

OBJECTIVE

To examine whether short-term exogenous activation of a tongue muscle induced a phenotypic shift from a fast to a slow fibre-type, and thus assess a potential therapeutic avenue to protect against obstructive sleep apnoea (OSA).

METHODS

New Zealand White rabbit genioglossus (GG) muscle, characteristically a fast muscle, was continuously stimulated at a frequency attributed to slow muscle (10Hz, 3V DC pulses) using an implanted micro-circuit for 7 days. Changes in muscle fibre types and aerobic capacity were assessed between stimulated and un-stimulated (control) groups using immunohistochemistry and electrophoresis for myosin heavy chain (MHC) and assayed for citrate synthase.

RESULTS

Compared to the un-stimulated control group, stimulated GG muscles had more (approximately 13%) type I MHC (slow-twitch) content; a proportional decrease in type II MHC (fast-twitch) isoform also occurred in the stimulated GG muscle (P<0.05). Electrophoresis analysis on whole muscle and single fibre MHC showed an increased type I expression in the stimulated GG muscle (P<0.01). A commensurate rise in citrate synthase activity, indicating a change in aerobic capacity, was also observed in the stimulated GG muscles.

CONCLUSION

Together, these results demonstrate a successful alteration in tongue muscle characteristics using exogenous electrical stimulation and perhaps a potential therapeutic application for OSA.

Immunohistochemical characterization of slow and fast myosin heavy chain composition of muscle fibres in the styloglossus muscle of the human and macaque (Macaca rhesus)

OBJECTIVE

Muscle fibre contractile diversity is thought to be increased by the hybridization of multiple myosin heavy chain (MHC) isoforms in single muscle fibres. Reports of hybrid fibres composed of MHCI and MHCII isoforms in human, but not macaque, tongue muscles, suggest a human adaptation for increased tongue muscle contractile diversity. Here we test whether hybrid fibres composed of MHCI and MHCII are unique to human tongue muscles or are present as well in the macaque.

METHODS

MHC composition of the macaque and human styloglossus was characterized with antibodies that allowed identification of three muscle fibre phenotypes, a slow phenotype composed of MHCI, a fast phenotype composed of MHCII and a hybrid phenotype composed of MHCI and MHCII.

RESULTS

The fast phenotype constitutes 68.5% of fibres in the macaque and 43.4% of fibres in the human (P<0.0001). The slow phenotype constitutes 20.2% of fibres in the macaque and 39.3% of fibres in the human (P<0.0001). The hybrid phenotype constitutes 11.2% of fibres in the macaque and 17.3% of fibres in the human (P=0.0002). Macaques and humans do not differ in fiber size (cross-sectional area, diameter). However, measures of fibre size differ by phenotype such that fast>hybrid>slow (P<0.05).

CONCLUSION

These data demonstrate differences in the relative percent of muscle fibre phenotypes in the macaque and human styloglossus but also demonstrate that all three phenotypes are present in both species. These data suggest a similar range of mechanical properties in styloglossus muscle fibres of the macaque and human.

Calcineurin-mediated slow-type fiber expression and growth in reloading condition

PURPOSE

Calcineurin (CaN) signaling pathway has been implicated in the transcriptional regulation of slow muscle fiber genes and in muscle hypertrophy. Our aim was to investigate the functional role of CaN as a regulator of muscle growth and/or muscle fiber type under conditions of recovery from inactivity.

METHODS

Female ICR mice (8 wk of age, 28-32 g) were used. To examine the effects of hindlimb suspension (HS) and reloading on skeletal muscle fiber size and muscle fiber type, animals were designated to 8 wk of HS and subsequent reloading for 4 wk. During reloading, animals were treated with pharmacological inhibitors for CaN (FK506) by intraperitoneal administration (3-5 mg.kg.d). After each experimental period, antigravitational soleus muscle was analyzed.

RESULTS

HS treatment resulted in obvious muscle atrophy and slow-to-fast fiber-type transformation in the soleus muscle. Subsequent reloading for 4 wk following HS induced muscle regrowth and fiber-type reversion toward a slow profile. FK506 administration prevented this kind of reloading-induced transformation of muscle fiber type. Furthermore, it was confirmed that FK506 administration attenuated maintenance of fiber cross-sectional area and reloading-induced fiber regrowth, specifically in slow-type muscle fibers.

CONCLUSION

Reloading-induced fiber-type reversion toward a slow profile is prevented by the pharmacological inhibition of CaN. Additionally, inhibition of CaN prevented maintenance and regrowth of slow-type muscle fibers. These results implicate that the CaN signaling pathway is required in the slow-type muscle fiber program under maintenance and suspension-reloading conditions.

Plasticity of neuromuscular junction architectures in rat slow and fast muscle fibers following temporary denervation and reinnervation processes

We evaluated the effects of brief, temporary denervation caused by ischiadic nerve-freezing on the processes of degeneration and regeneration of ultrastructural features in neuromuscular junction (NMJ) architecture in different types of rat skeletal muscle fibers. Nerve terminal (NT) area was decreased significantly 12 h after nerve freezing in both fast-twitch (FT) and slow-twitch (ST) fibers. One day after nerve freezing, some terminal axons were absent; decrease in NT area was remarkable in ST fibers, and there was retraction of Schwann cells and perineural epithelial cells. Fiber type-specific differences were observed in pattern of decrease in NT area between 24 h and 7 days after nerve freezing (there was significantly more decrease in FT fibers). The primary synaptic cleft became shallow, and the secondary junctional folds shorter and wider, but the basement lamina filling the subneural apparatus was unaltered. The number of secondary junctional folds decreased gradually between 6 h and 14 days after nerve freezing in both types of fiber. In control muscle fibers, synaptic vesicle density (SVD) per terminal area was significantly higher in FT fibers. The SVD densities decreased following nerve freezing-induced destruction of NMJs, and were minimal 3 days in FT fibers or 7 days ST fibers after nerve freezing. At 3 weeks, regeneration of both FT and ST fibers was well advanced, and all parameters had recovered to control values in FT fibers 28 days after nerve freezing. Severe degradation of the ultrastructural features in NMJs occurred due to temporary denervation during muscle fiber degeneration processes, and these structural changes were all reversible and fiber type-specific.

Effect of Exercise on mRNA Levels for Growth Factors in Skeletal Muscle of Hemodialysis Patients

OBJECTIVES

Muscle mass and muscle mRNA levels for certain growth factors are reduced in maintenance hemodialysis (MHD) patients. This study tested the hypothesis that in MHD patients endurance exercise training (EET) increases mRNA levels for insulin-like growth factors and reduces myostatin mRNA.

DESIGN

Biopsies of the right vastus lateralis muscle were performed before and at the end of 8.9 +/- 0.9 (SEM) weeks of EET in MHD patients. Muscle tissue was analyzed histologically by electron microscopy and for fiber cross-sectional area, and, in 8 pairs of biopsies, muscle was examined for mRNA levels for the following proteins: myostatin, insulin-like growth factor-I (IGF-I), IGF-I receptor (IGF-IR), IGF binding proteins (IGFBPs)-1, -2, -3, -4, and -5, and IGF-binding protein-related protein-1 (IGFBP-rP1).

SETTING

Outpatient MHD centers.

PATIENTS

This was a pilot study conducted in sedentary clinically stable MHD patients undergoing EET with no control group.

INTERVENTION

EET that was carefully supervised by exercise trainers.

MAIN OUTCOME MEASURE

Skeletal muscle mRNA levels, especially myostatin mRNA.

RESULTS

With EET, skeletal muscle myostatin mRNA decreased by 51%, mRNA levels increased significantly for IGF-IR (by 41%), IGFBP-2, -4, and -5, and IGFBP-rP1. IGF-I mRNA increased by 35%; this change was not significant. IGFBP-3 mRNA did not change, and IGFBP-1 mRNA was undetectable. There were mild to moderate alterations in skeletal muscle ultrastructure that did not change significantly with EET. Muscle fiber size, measured in 5 patients, did not change.

CONCLUSION

In MHD patients who undergo approximately 9 weeks of EET, skeletal muscle mRNA for myostatin decreases and mRNA for IGF-IR, IGFBPs -2, -4, and -5 and IGFBP-rP1 increases. These changes may indicate mechanisms by which EET improves muscle exercise capacity in MHD patients.

Multiple isoforms of myosin light chain 1 in pig diaphragm slow fibers: correlation with maximal shortening velocity and force generation

Pig diaphragm slow fibers exhibit heterogeneity in myosin light chain 1 (MLC1) isoform expression, with many expressing fast-type MLC1 (MLC1F), as well as two isoforms of slow-type MLC1 (MLC1Sa and MLC1Sb). The goal of this study was to test if there is a relationship between MLC1 isoform expression and contractile properties among these fibers. Maximal shortening velocity (V(max)) and maximal isometric force generation, normalized with fiber cross-sectional area (P(o)/CSA), were measured in single fibers. V(max) was inversely related to the relative level of MLC1Sa. The level of MLC1Sa was reciprocally related to the levels of MLC1Sb and of MLC1F among individual fibers. Fibers expressing MLC1Sa and in which MLC1Sb was not detected generated greater P(o)/CSA, compared to fibers expressing MLC1Sb and not MLC1Sa. The results indicate a complex pattern of MLC1 isoform expression among pig diaphragm slow fibers and suggest that shortening velocity and force generation are modulated, in these fibers, by the MLC1 isoform composition.

Mitochondrial efficiency in rat skeletal muscle: influence of respiration rate, substrate and muscle type

AIM

To investigate the hypothesis that mitochondrial efficiency (i.e. P/O ratio) is higher in type I than in type II fibres during submaximal rates of respiration.

METHODS

Mitochondria were isolated from rat soleus and extensor digitorum longus (EDL) muscles, representing type I and type II fibres, respectively. Mitochondrial efficiency (P/O ratio) was determined with pyruvate (Pyr) or palmitoyl-l-carnitine (PC) during submaximal (constant rate of adenosine diphosphate infusion) and maximal (V(max), state 3) rates of respiration and fitted to monoexponential functions.

RESULTS

There was no difference in V(max) between PC and Pyr in soleus but in EDL V(max) with PC was only 58% of that with Pyr. The activity of 3-hydroxyacyl-CoA dehydrogenase was threefold higher in soleus than in EDL. P/O ratio at V(max) was 8-9% lower with PC [2.33 +/- 0.02 (soleus) and 2.30 +/- 0.02 (EDL)] than with Pyr [2.52 +/- 0.03 (soleus) and 2.54 +/- 0.03 (EDL)] but not different between the two muscles (P > 0.05). P/O ratio was low at low rates of respiration and increased exponentially when the rate of respiration increased. The asymptotes of the curves were similar to P/O ratio at V(max). P/O ratio at submaximal respirations was not different between soleus and EDL neither with Pyr nor with PC.

CONCLUSION

Mitochondrial efficiency, as determined in vitro, was not significantly different in the two fibre types neither at V(max) nor at submaximal rates of respiration. The low V(max) for PC oxidation in EDL may relate to low activity of beta-oxidation.

Parvalbumin deficiency in fast-twitch muscles leads to increased 'slow-twitch type' mitochondria, but does not affect the expression of fiber specific proteins

Parvalbumin (PV), a small cytosolic protein belonging to the family of EF-hand calcium-binding proteins, is highly expressed in mammalian fast-twitch muscle fibers. By acting as a 'slow-onset' Ca2+ buffer, PV does not affect the rapid contraction phase, but significantly contributes to increase the rate of relaxation, as demonstrated in PV-/- mice. Unexpectedly, PV-/- fast-twitch muscles were considerably more resistant to fatigue than the wild-type fast-twitch muscles. This effect was attributed mainly to the increased fractional volume of mitochondria in PV-/- fast-twitch muscle, extensor digitorum longus, similar to levels observed in the slow-twitch muscle, soleus. Quantitative analysis of selected mitochondrial proteins, mitochondrial DNA-encoded cytochrome oxidase c subunit I and nuclear DNA-encoded cytochrome oxidase c subunit Vb and F1-ATPase subunit beta revealed the PV-/- tibialis anterior mitochondria composition to be almost identical to that in wild-type soleus, but not in wild-type fast-twitch muscles. Northern and western blot analyses of the same proteins in different muscle types and in liver are indicative of a complex regulation, probably also at the post-transcriptional level. Besides the function in energy metabolism, mitochondria in both fast- and slow-twitch muscles act as temporary Ca2+ stores and are thus involved in the shaping of Ca2+ transients in these cells. Previously observed altered spatio-temporal aspects of Ca2+ transients in PV-/- muscles are sufficient to up-regulate mitochondria biogenesis through the probable involvement of both calcineurin- and Ca2+/calmodulin-dependent kinase II-dependent pathways. We propose that 'slow-twitch type' mitochondria in PV-/- fast muscles are aimed to functionally replace the slow-onset buffer PV based on similar kinetic properties of Ca2+ removal.

Effect of satellite cell ablation on low-frequency-stimulated fast-to-slow fibre-type transitions in rat skeletal muscle

The purpose of this study was to determine whether satellite cell ablation within rat fast-twitch muscles exposed to chronic low-frequency stimulation (CLFS) would limit fast-to-slow fibre-type transitions. Twenty-nine male Wistar rats were randomly assigned to one of three groups. Satellite cells of the left tibialis anterior were ablated by weekly exposure to a 25 Gy dose of gamma-irradiation during 21 days of CLFS (IRR-Stim), whilst a second group received only 21 days of CLFS (Stim). A third group received weekly doses of gamma-irradiation (IRR). Non-irradiated right legs served as internal controls. Continuous infusion of 5-bromo-2'-deoxyuridine (BrdU) revealed that CLFS induced an 8.0-fold increase in satellite cell proliferation over control (mean +/-s.e.m.: 23.9 +/- 1.7 versus 3.0 +/- 0.5 mm(-2), P < 0.0001) that was abolished by gamma-irradiation. M-cadherin and myogenin staining were also elevated 7.7- and 3.8-fold (P < 0.0001), respectively, in Stim compared with control, indicating increases in quiescent and terminally differentiating satellite cells; these increases were abolished by gamma-irradiation. Myonuclear content was elevated 3.3-fold (P < 0.0001) in Stim, but remained unchanged in IRR-Stim. Immunohistochemical analyses revealed attenuation of fast-to-slow fibre-type transitions in IRR-Stim compared with Stim. Comparable changes were observed at the protein level by SDS-PAGE. It is concluded that although considerable adaptive potential exists within myonuclei, satellite cells play a role in facilitating fast-to-slow fibre-type transitions.

The extraocular motor nuclei: organization and functional neuroanatomy

The organization of the motoneuron subgroups in the brainstem controlling each extraocular eye muscle is highly stable through the vertebrate species. The subgroups are topographically organized in the oculomotor nucleus (III) and are usually considered to form the final common pathway for eye muscle control. Eye muscles contain a unique type of slow non-twitch, fatigue-resistant muscle fiber, the multiply innervated muscle fibers (MIFs). The recent identification the MIF motoneurons shows that they too have topographic organization, but very different from the classical singly innervated muscle fiber (SIF) motoneurons. The MIF motoneurons lie around the periphery of the oculomotor nucleus (III), trochlear nucleus (IV), and abducens nucleus (VI), slightly separated from the SIF subgroups. The location of four different types of neurons in VI are described and illustrated: (1) SIF motoneurons, (2) MIF motoneurons, (3) internuclear neurons, and (4) the paramedian tract neurons which project to the flocculus. Afferents to the motoneurons arise from the vestibular nuclei, the oculomotor and abducens internuclear neurons, the mesencephalic and pontine burst neurons, the interstitial nucleus of Cajal, nucleus prepositus hypoglossi, the supraoculomotor area and the central mesencephalic reticular formation and the pretectum. The MIF and SIF motoneurons have different histochemical properties and different afferent inputs. The hypothesis that SIFs participate in moving the eye and MIFs determine the alignment seems possible but is not compatible with the concept of a final common pathway.

Muscular design in the equine interosseus muscle

We studied the forelimb interosseus muscle in horses, Equus caballus, to determine the muscular properties inherent in its function. Some authors have speculated that the equine interosseus contains muscle fibers at birth only to undergo loss of these fibers through postnatal ontogeny. We describe the muscle fibers in eight interosseus specimens from adult horses. These fibers were studied histochemically using myosin ATPase studies and immunocytochemically using several antibodies directed against type I and type II myosin heavy chain antibodies. We determined that 95% of the fibers were type I, presumed slow-twitch fibers. All fibers exhibited normal morphological appearance in terms of fiber diameter and cross-sectional area, suggesting that the muscles are undergoing normal cycles of recruitment. SDS-PAGE studies of myosin heavy chain isoforms were consistent with these observations of primarily slow-twitch muscle. Fibers were determined to be approximately 800 microm long when studied using nitric acid digestion protocols. Short fiber length combined with high pinnation angles suggest that the interosseus muscle is able to generate large amounts of force but can produce little work (measured as pulling the distal tendon proximally). While the equine interosseus muscle has undergone a general reduction of muscle content during its evolution, it remains composed of a significant muscular component that likely contributes to forelimb stability and elastic storage of energy during locomotion.

Roles of exercise and pharmacokinetics in cerivastatin-induced skeletal muscle toxicity

Three-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors are associated with adverse skeletal muscle effects, but the underlying mechanisms remain unclear. To determine whether toxicity involves the level of drug exposure in muscle tissue and to test the effect of exercise on cerivastatin (CVA)-induced skeletal muscle damage, female rats were administered vehicle or CVA at 0.1, 0.5, and 1.0 mg/kg/day by gavage for two weeks and exercised or not on treadmills for 20 min/day. Clinical chemistry and plasma and tissue pharmacokinetics were evaluated; light and transmission electron microscopy (TEM) of Type I and Type II fiber-predominant skeletal muscles were performed. Serum levels of AST, ALT, CK, and plasma lactic acid were significantly elevated dose-dependently. CVA treatment decreased psoas and quadriceps weights. At 1 mg/kg all muscles except soleus demonstrated degeneration. Exercise-exacerbated severity of CVA-induced degeneration was evident in all muscles sampled except soleus and quadriceps. Early mitochondrial involvement in toxicity is suggested by the numerous membranous whorls and degenerate mitochondria observed in muscles at 0.5 mg/kg. No significant differences in CVA concentrations between either EDL and soleus or plasma and muscle were found. We found that CVA had no effect on cleaved caspase 3. In summary, we found that treadmill exercise exacerbated the incidence and severity of CVA-induced damage in Type II fiber-predominant muscles. Tissue exposure is likely not the key factor mediating CVA-induced skeletal muscle toxicity.

Histochemical and stereological analysis of the levator ani (pubocaudal) muscle in nulliparous and multiparous beagles

Fiber-type composition and several stereological parameters of the levator ani (pubocaudal) muscle were evaluated in five nulliparous and five multiparous beagles using myosin ATPase-histochemistry and systematically selected muscle cross-sections. With respect to the narrow canine pelvic cavity, this study was also undertaken to determine whether vaginal birth of at least seven litters causes similar neuromuscular changes in the canine levator ani (pubocaudal) muscle analoguous to those seen in the pelvic floor muscles of women after vaginal delivery. The canine pubocaudal muscle is comprised of approximately equal amounts of slow twitch type I and fast twitch type II (IIA, IIS) fibers. The muscles of both the nulliparous and multiparous beagles did not display any signs indicative of denervation or myopathology. The multiparous dogs exhibited significantly increased mean absolute muscle (1720 mm(3)) and total fiber-type I volumes (850 mm(3)) as well as relevantly increased mean diameter of type I fibers (72.0 microm) when compared with the nulliparous group. The canine levator ani (pubocaudal) muscle is not pathologically affected by vaginal deliveries and seems to adapt to numerous successive pregnancies and births through fiber-type I hypertrophy.

Muscle fiber-type distribution as a predictor of blood pressure: a 19-year follow-up study

The known association between physical activity and low blood pressure may be influenced by inherited characteristics. Skeletal muscle consists of type I (slow-twitch) and type II (fast-twitch) muscle fibers, with proportions highly variable between individuals and mostly determined by genetic factors. A high percentage of type I fibers (type I%) has been associated with low blood pressure in cross-sectional studies. We investigated whether type I percentage predicts future blood pressure levels and explains part of the association between physical activity and blood pressure. At baseline, in 1984, muscle fiber-type distribution, physical activity, and body mass index (BMI) were determined in 64 healthy men (age, 32 to 58 years). At follow-up, in 2003, blood pressure, physical activity, and BMI were determined in these men. In subjects without antihypertensive medication (n=43), type I percentage accounted for 5%/18% of the variation in systolic/diastolic blood pressure. A high type I percentage predicted, independent of both baseline (in 1984) and follow-up (in 2003), physical activity, BMI, and low systolic and diastolic blood pressure. Adjusted for all baseline covariates, a 20-unit higher type I percentage predicted a 11.6-mm Hg lower systolic blood pressure (P=0.018) and a 5.0-mm Hg lower diastolic blood pressure (P=0.018). High levels of physical activity in 1984 predicted low diastolic blood pressure, but this association was lost when type I percentage was included into the model. A high proportion of type I fibers in skeletal muscle is an independent predictor of low blood pressure and explains part of the known association between high levels of physical activity and low blood pressure.

STRUCTURAL ASSESSMENT OF THE URETHRAL SPHINCTER IN WOMEN WITH URINARY RETENTION

PURPOSE

The pathophysiology of urinary retention in women is generally unknown but a subgroup of women with urinary retention have been diagnosed as having so-called primary disorder of sphincter relaxation on the basis of an abnormal urethral sphincter electromyogram. It was suggested this sphincter overactivity could lead to work hypertrophy of the urethral rhabdosphincter and in this study we looked for any evidence of such muscle fiber hypertrophy.

MATERIALS AND METHODS

In 9 women 18 to 45 years old (mean age 31.6) with urinary retention and overactive urethral sphincter electromyogram, light and electron microscopy were used to examine core needle biopsies of the urethral rhabdosphincter taken under transvaginal ultrasound control. Of the 9 patients only 5 biopsies processed for light microscopy and 4 processed for electron microscopy contained striated urethral muscle fibers. The results of these biopsies were compared to the morphology of a control specimen from a postmenopausal woman without a history of urinary retention.

RESULTS

On light microscopy the urethral rhabdosphincter fiber diameter did not differ among patients (mean average 7.6 mum), was less than that reported in the literature (15 to 20), but did not differ from that of the control (mean 9.9). In all patients electron microscopy showed excessive peripheral sarcoplasm with lipid and glycogen deposition, and sarcoplasmic accumulation of normal mitochondria. These ultrastructural abnormalities were not seen in the control.

CONCLUSIONS

To our knowledge this is the first morphological description of the urethral rhabdosphincter in a subgroup of women with urinary retention. Mean rhabdosphincter fiber diameter was approximately the same in patients and controls. This study does not support the previous theory that urethral sphincter overactivity in a subgroup of women with urinary retention leads to work hyperplasia of urethral rhabdosphincter fibers. An alternative hypothesis is suggested.

Transformation of Skeletal Muscle from Fast- to Slow-Twitch during Acquisition of Cold Tolerance in the Chick

Although birds lack brown adipose tissue, a thermogenic organ found in mammals, they possess other thermogenic mechanisms. In the current studies, we examined the molecular mechanisms of avian thermogenesis by studying how chicks acquire cold tolerance. We found that the acquisition of cold tolerance corresponded with an increase in the redness of the skeletal muscle, suggesting an increase in slow-twitch muscle fiber. This was confirmed by histological analysis. In addition, in chicks acquiring cold tolerance, there was an enhanced expression of the chicken homologue of peroxisome proliferator-activated receptor-gamma coactivator-1alpha, a protein involved in adaptive thermogenesis in mammalian brown adipose tissue and in slow-twitch fiber formation in mammalian skeletal muscle. Subtraction and differential display techniques further showed that, when chicks acquired cold tolerance, the expression of genes associated with slow-twitch fibers increased, whereas those associated with fast-twitch fibers decreased. There was also an enhanced expression of mitochondrial oxidative genes. Together, these results suggest that transformation of skeletal muscle fiber from fast-twitch to slow-twitch is involved in the acquisition of thermogenesis in chicks.

Desmin filaments influence myofilament spacing and lateral compliance of slow skeletal muscle fibers

Intermediate filaments composed of desmin interlink Z-disks and sarcolemma in skeletal muscle. Depletion of desmin results in lower active stress of smooth, cardiac, and skeletal muscles. Structural functions of intermediate filaments in fast (psoas) and slow (soleus) skeletal muscle were examined using x-ray diffraction on permeabilized muscle from desmin-deficient mice (Des-/-) and controls (Des+/+). To examine lateral compliance of sarcomeres and cells, filament distances and fiber width were measured during osmotic compression with dextran. Equatorial spacing (x-ray diffraction) of contractile filaments was wider in soleus Des-/- muscle compared to Des+/+, showing that desmin is important for maintaining lattice structure. Osmotic lattice compression was similar in Des-/- and Des+/+. In width measurements of single fibers and bundles, Des-/- soleus were more compressed by dextran compared to Des+/+, showing that intermediate filaments contribute to whole-cell compliance. For psoas fibers, both filament distance and cell compliance were similar in Des-/- and Des+/+. We conclude that desmin is important for stabilizing sarcomeres and maintaining cell compliance in slow skeletal muscle. Wider filament spacing in Des-/- soleus cannot, however, explain the lower active stress, but might influence resistance to stretch, possibly minimizing stretch-induced cell injury.

Alterations in enzyme histochemical characteristics of the masseter muscle caused by long-term soft diet in growing rabbits

OBJECTIVE

Recently young people have an increasing tendency to intake an easily chewable diet and spend less time on mastication. The aim of the present study was to investigate the histochemical effects of long-term soft diet on the masseter muscle in growing rabbits.

MATERIALS AND METHODS

Twelve young male Japanese white rabbits were divided into two groups (n = 6 each) at weaning (1 month after birth) and fed a solid diet (control group) or a powder diet (soft-diet group). The duration of the experimental period was 6 months. Masseter fibers from the superficial and the deep portions were histochemically defined as type 1, 2A, 2B, or 2C fibers.

RESULTS

As compared with that of the control, the deep masseter of the soft-diet group showed a significantly lower ratio of type 1 fiber cross-sectional area to total area (6.3 and 10.1% for the soft-diet and control group, respectively), significantly more type 2A fibers (74.0%vs 50.3%) and significantly fewer type 2B fibers (4.3%vs 12.5%). However, fiber size did not differ between the two groups. NADH-tetrazolium-reductase (NADH-TR) of the masseter was less reactive in the soft-diet group, reflecting a lower oxidative capacity.

CONCLUSIONS

These findings indicate that the alteration of the functional activities contributed to selective disuse influences on the type 1 and type 2B fibers, and a resultant increase in type 2A fibers. This study suggests that long-term alteration of jaw function induced by a soft diet can lead to adaptations of the masseter muscle.

Plasticity of the transverse tubules following denervation and subsequent reinnervation in rat slow and fast muscle fibres

We have studied the effects of short term denervation followed by reinnervation on the ultrastructure of the membrane systems and on the content of and distribution of key proteins involved in calcium regulation of fast-twitch (FT) extensor digitorum longus (EDL) and slow-twitch (ST) soleus (SOL) muscle fibres. Ischiadic nerve freezing resulted in total lack of neuromuscular transmission for 3 days followed by a slow recovery, but no decline in twitch force elicited by direct stimulation. The latter measurements indicate no significant atrophy within this time frame. The membrane systems of skeletal muscle fibres were visualized using Ca92+)-K3Fe(CN)6-OsO4 techniques and observed using a high voltage electron microscope. [3H]nitrendipine binding was used to detect levels of dihydropyridine receptor (DHPR) expression. The Ca2+ pumping free sarcoplasmic reticulum domains were not affected by the denervation, but the Ca2+ release domains were dramatically increased, particularly in the FT-EDL muscle fibres. The increase is evidenced by a doubling up of the areas of contacts between SR and transverse (t-) tubules, so that in place of the normal triadic arrangement, pentadic and heptadic junctions, formed by multiple interacting layers of ST and t-tubules are seen. Frequency of pentads and heptads increases and declines in parallel to the denervation and reinnervation but with a delay. Immunofluorecence and electron microscopy observations show presence of DHPR and ryanodine receptor clusters at pentads and heptads junctions. A significant (P < 0.01) positive correlation between the level of [3H]nitrendipine binding component and the frequency pentads and heptads was observed in both the FT-EDL and ST-SOL muscle fibres indicating that overexpression of DHPRs accompanies the build up extra junctional contacts. The results indicate that denervation reversibly affects the domains of the membrane systems involved in excitation-contraction coupling.

Type-selective muscular degeneration promotes infiltrative growth of intramuscular lipoma

Background

Intramuscular lipoma is a relatively common benign neoplasm that is occasionally described as an infiltrating lipoma. Typical benign tumors show a clear margin, however, the infiltrative growth pattern of this lipoma mimics that of a malignant tumor. Although its growth has an effect on muscle bundles and it is known to never metastasize, the mechanism of infiltrative growth is not well understood. Previously, little attention has been paid to pathogenic features of muscle fibers around an intramuscular lipoma.

Methods

In the present study, we focused on pathologic changes of the surrounding skeletal muscles especially to the degenerative features of involving muscular types, and evaluate the role of type-selective muscular degeneration for the infiltrative growth of intramuscular lipomas. Following a review of the medical records in our institute, 17 lesions containing muscle tissues in their specimens (15 infiltrating lipomas, 2 well-circumscribed lipomas) were analyzed immunohistochemically. The tumor from the most recent case was also subjected to ultrastructural analysis. Two cases of the traumatic muscle damage were also evaluated as the control experiments.

Results

These analyses revealed type-selective muscle involution in 11 of 17 intramuscular lipomas and in 10 of 11 of the infiltrative type, with an involving pattern that resembled that of a neurogenic or myogenic disorder. Immunoreactivity to cathepsin-D, a lysosomal catabolic enzyme, was increased in the involved muscle fibers. Subsarcolemmal vacuoles in the muscle fibers of the peripheral areas were also positive for cathepsin-D, while degenerative findings were not visually apparent in these areas. Ultrastructural analysis revealed degenerative changes in those fibers. Neither positive staining for cathepsin-D nor type-selective atrophy was detected in the sections of traumatic muscle damage.

Conclusions

Our findings suggest that type-selective muscular degeneration and endomysial fatty growth as a result of atrophy may modulate the infiltrating growth characteristic of intramuscular lipoma.

Differential expression of nitric oxide synthases (NOS 1-3) in human skeletal muscle following exercise countermeasure during 12 weeks of bed rest

Adaptive changes of major body systems in astronauts during spaceflight can be simulated by strict anti-orthostatic head-down tilt (HDT) bed rest (BR), a ground-based microgravity (microG) model that provides a meaningful opportunity to study atrophy mechanisms and possible countermeasures under controlled experimental conditions. As nitric oxide (NO) signaling is linked to muscle activity, we investigated altered expression of the three major isoforms of nitric oxide synthase (NOS 1-3) at cellular compartments during prolonged HDT BR without (control group) and with resistance exercise interventions (exercise group) using a flywheel ergometer (FWE). Atrophy detected in mixed (fast-slow) m. vastus lateralis (VL) and slow-type m. soleus (SOL) myofiber Types I and II (minus 35-40% of myofiber cross-sectional area) was prevented by FWE training. Concomitant to muscle atrophy, reduced NOS 1 protein and immunostaining was found in VL not in SOL biopsies. In trained VL, NOS 1 protein and immunostaining at myofibers II were significantly increased at the end of BR. Exercise altered NOS 2/caveolin 3 co-immunostaining patterns of subsarcolemmal focal accumulations in VL or SOL myofibers, which suggests reorganization of sarcolemmal microdomains. In trained VL, increased capillary-to-fiber (C/F) ratio and NOS 3 protein content were documented. Activity-linked NO signaling may be widespread in skeletal muscle cellular compartments that may be directly or indirectly impacted by adequate exercise countermeasure protocols to offset the negative effects induced by disuse, immobilization, or extended exposure to microgravity.

Chronic low back pain-associated paraspinal muscle dysfunction is not the result of a constitutionally determined "adverse" fiber-type composition

STUDY DESIGN

Investigative case control study.

OBJECTIVES

To determine whether excessive paraspinal muscle fatigue in chronic low back pain results from a paucity of muscle type I fiber content.

SUMMARY OF BACKGROUND DATA

Paraspinal muscle function is vital for spinal protection. Prospective studies suggest that excessive paraspinal muscle fatigability may increase risk of first-time low back pain. As contractile performance of the paraspinal muscles is governed by their constitutionally determined fiber composition, the question arises whether a constitutionally determined "adverse" composition could predispose to low back pain through impaired spinal protection.

METHODS

Thirty-five male patients with chronic low back pain were compared with 32 male control patients of similar age and anthropometry. During Sorensen and 60% of maximum voluntary isometric contraction fatigue tests, median frequency declines in the paraspinal muscle surface electromyograph signal were monitored and correlated with muscle histomorphometry.

RESULTS

Patients were weaker than controls during maximum voluntary isometric contractions (84.47 [28.44]vs. 98.74 [18.11] kg, respectively; P = 0.02) and more fatigable during their Sorensen tests (endurance time 105.29 [28.53]vs. 137.50 [40.38] sec, respectively; P < 0.01). There were no between-group differences in median frequency declines during the Sorensen (-0.37 [0.16]vs. -0.36 [0.12]%.sec) or 60% maximum voluntary isometric contraction (-0.42 [0.31]vs. -0.51 [0.29]%.sec) tests, for patients and controls, respectively. There were no between-group differences in the percent number of paraspinal muscle type I fibers (64 [11]vs. 64 [9]%) or the percent area occupied by type I fibers (67 [11]vs. 69 [9]%), for patients and controls, respectively. Type I and II muscle fiber narrow diameters were similar for both groups.

CONCLUSION

In the patients with chronic low back pain tested, their associated paraspinal muscle dysfunction was not the result of a constitutionally determined "adverse" fiber type composition.

Craniofacial morphology and growth in the ferret: effects from alteration of masticatory function

INTRODUCTION

Our hypothesis is that any effects on craniofacial growth and morphology induced by altering the masticatory function will vary according to the species involved. The general aim of this thesis was to test the possible effects of altering the masticatory function on craniofacial growth and morphology, as well as features of masticatory muscles in ferrets. To better elucidate these mechanisms involved, the anatomy of the skull, craniofacial growth pattern and development of occlusion in the ferret were investigated to serve as baseline data.

MATERIALS AND METHODS

The description of the anatomy and the radiographic features of the skull was based on observations of 100 skulls of adult ferrets of both sexes. The craniofacial growth pattern was studied in 16 young ferrets of both sexes using serial cephalograms taken on 5 occasions according to age: 25 days, 35 days, 55 days, 80 days and 300 days. To follow the development of occlusion, these animals underwent dental examinations every other day. Forty male ferrets were divided into two groups from the age of 5 weeks. One group was fed hard pellets (hard-diet group), and the other the same diet but softened with water (soft-diet group). After six months, specimens of masticatory muscles were dissected and analysed histochemically, and cephalograms were analysed cephalometrically.

RESULTS

The ferret skull is relatively elongated with a short facial region, being longer and broader in males than in females. The viscero- and neurocranium follow an orderly pattern of expansive growth. The growth of the mandible is mainly characterized by an anteroposterior body elongation, and enlargement of the coronoid process. Craniofacial growth in ferrets ceases earlier in female than in male animals. The deciduous teeth erupted between the 19th and 31st postnatal day, and exfoliated approximately between days 51 and 76. The time of eruption of the permanent teeth ranged from 42 to 77 days. The female ferrets were generally ahead of the males regarding the eruption age of permanent teeth. The alteration of the masticatory function by feeding the animals a soft diet caused a hard palate plane more distant from the cranial base plane, smaller inter-frontal and inter-parietal widths, and a slenderer zygomatic arch, as well as a generally shorter and narrower coronoid process. Variations of most variables are greater in animals fed on soft diet. The mean cross-sectional area of type I and type II fibres in the temporalis and the masseter, and type II fibres in the digastricus, as well as the number of capillaries per fibre were significantly smaller in the soft-diet animals.

CONCLUSION

Altered masticatory function has induced changes in masticatory muscles and certain effects on craniofacial growth in ferrets. These changes are not the same as those reported in other species (e.g. rats). It appears that reduced masticatory function leads to smaller fibres in the elevators and transverse skull dimensions. The different effects are presumably related to the differences in craniofacial anatomy and growth pattern, as well as the masticatory system, in the various species studied previously.

Resting tension characteristics in differentiating intact rat fast- and slow-twitch muscle fibers

The postnatal changes in resting muscle tension were investigated at 20 degrees C by using small muscle fiber bundles isolated from either the extensor digitorum longus or the soleus of both neonatal (7-21 days old) and adult rats. The results show that the tension-extension characteristics of the bundles depended on the age of the rats. For example, both the extensor digitorum longus and soleus bundles of rats older than 14 days showed characteristic differences that were absent in bundles from younger rats. Furthermore, the tension-extension relation of the adult slow muscle fiber bundles were similar to those of the two neonatal muscles and were shifted to longer sarcomere lengths relative to those of the adult fast-fiber bundles. Thus, at the extended sarcomere length of 2.9 microm, the adult fast muscle fiber bundles developed higher resting tensions (5.6 +/- 0.5 kN/m2) than either the two neonatal ( approximately 3 kN/m2) or the adult slow (3.1 +/- 0.4 kN/m2) muscle fiber bundles. At all ages examined, the resting tension responses to a ramp stretch were qualitatively similar and consisted of three components: a viscous, a viscoelastic, and an elastic tension. However, in rats older than 14 days, all three tension components showed clear fast- and slow-fiber type differences that were absent in younger rats. Bundles from 7-day-old rats also developed significantly lower resting tensions than the corresponding adult ones. Additionally, the resting tension characteristics of the adult muscles were not affected by chemical skinning. From these results, we conclude that in rats resting muscle tension, like active tension, differentiates within the first 3 wk after birth.

Myosin storage myopathy associated with a heterozygous missense mutation in MYH7

Myosin constitutes the major part of the thick filaments in the contractile apparatus of striated muscle. MYH7 encodes the slow/beta-cardiac myosin heavy chain (MyHC), which is the main MyHC isoform in slow, oxidative, type 1 muscle fibers of skeletal muscle. It is also the major MyHC isoform of cardiac ventricles. Numerous missense mutations in the globular head of slow/beta-cardiac MyHC are associated with familial hypertrophic cardiomyopathy. We identified a missense mutation, Arg1845Trp, in the rod region of slow/beta-cardiac MyHC in patients with a skeletal myopathy from two different families. The myopathy was characterized by muscle weakness and wasting with onset in childhood and slow progression, but no overt cardiomyopathy. Slow, oxidative, type 1 muscle fibers showed large inclusions consisting of slow/beta-cardiac MyHC. The features were similar to a previously described entity: hyaline body myopathy. Our findings indicate that the mutated residue of slow/beta-cardiac MyHC is essential for the assembly of thick filaments in skeletal muscle. We propose the term myosin storage myopathy for this disease.

Functional differences in type-I fibres from two slow skeletal muscles of rabbit

The present study addressed the question of whether the slow fibres of mammalian skeletal muscle, containing the myosin heavy chain MHCI (type-I fibres), are a functionally homogeneous population. We compared various properties of Ca(2+)-activated, skinned, type-I fibres from the soleus and semitendinosus muscles of a rabbit. Soleus type-I fibres showed significantly faster kinetics of stretch activation, measured as the time-to-peak of the stretch-induced, delayed force increase, t(3), than semitendinosus fibres (1239+/-438 ms, n=136, vs. 1600+/-409 ms, n=208 respectively) (means+/-SD, 22 degrees C). Similarly, the speed of unloaded shortening at 15 degrees C was faster in soleus than in semitendinosus fibres [0.79+/-0.16 fibre lengths (FL) s(-1), n=44, vs. 0.65+/-0.15 FL s(-1), n=35 respectively]. The kinetics of stretch activation were more temperature sensitive in semitendinosus than in soleus fibres. Finally, the generation of steady-state isometric force was more sensitive to Ca(2+) in semitendinosus than in soleus fibres: [pCa(50) (-log [Ca(2+)] for half-maximal activation) at 22 degrees C: 6.29+/-0.15, n=28, vs. 6.19+/-0.10, n=18 respectively]. These results suggest strongly that there is no functional homogeneity within type-I fibres of different muscles. The observed differences might reflect the existence of more than one functionally different slow myosin heavy chain isoforms or other modifications of contractile proteins.

Nandrolone decanoate treatment affects sarcoplasmic reticulum Ca(2+) ATPase function in skinned rat slow- and fast-twitch fibres

The effects of anabolic-androgenic steroid administration on the function of the sarcoplasmic reticulum (SR) pump were investigated in chemically skinned fibres from the extensor digitorum longus (EDL) and soleus muscles of sedentary rats. Twenty male rats were divided into two groups, one group received an intramuscular injection of nandrolone decanoate (15 mg x kg(-1)) weekly for 8 weeks, the second received similar weekly doses of vehicle (sterile peanut oil). Compared with control muscles, nandrolone decanoate treatment reduced SR Ca(2+) loading in EDL and soleus fibres by 49% and 29%, respectively. In control and treated muscles, the rate of Ca(2+) leakage depended on the quantity of Ca(2+) loaded. Furthermore, for similar SR Ca(2+) contents, the Ca(2+) leakage rate was not significantly modified by nandrolone decanoate treatment. Nandrolone decanoate treatment thus affects Ca (2+) uptake by the SR in a fibre-type dependent manner.

Comparison of collagen fibre architecture between slow-twitch cranial and fast-twitch caudal parts of broiler M. latissimus dorsi

1. Collagen fibre architectures of perimysium and endomysium in the slow-twitch cranial and fast-twitch caudal parts of broiler M. latissimus dorsi were compared. 2. Type I and III collagens were distributed in both perimysium and endomysium as indicated by their positive immunohistochemical reactions to polyclonal antibodies. 3. Cells invested by endomysium with no myofibres were larger in the cranial part because of the presence of larger slow-twitch myofibres. The honeycomb structure of endomysium was divided into several parts by thick perimysium. 4. The thick perimysial collagen fibres with parallel fibrils, which were interconnected by the loose reticular fibrils and thin fibres, were more numerous and thicker in the cranial part than the caudal. 5. Thick endomysial sidewall of cells in the cranial part was composed of a rougher reticulum of slightly thicker collagen fibrils compared with the thin sidewall in the caudal part. 6. These results indicated that both perimysial constitutions of collagen fibres and endomysial collagen fibrils had attained much larger growth in the slow-twitch cranial part than the fast-twitch caudal in broiler latissimus dorsi muscle.

Differences in muscle fiber growth in slow-twitch muscles of the forelimb and hindlimb of the rat: role of the pituitary and food intake

This investigation tested the hypothesis that differences in the growth of fore- and hindlimb muscles in the rat are regulated by the pituitary and food intake. Using morphometric techniques, the growth of muscle fibers was compared in two slow-twitch muscles, the flexor carpi ulnaris (FCU) of the forelimb, and the soleus of the hindlimb, in male Wistar rats fed ad libitum, food restricted (FR) or hypophysectomized (hypox) from age 60 days. Growth was defined as an increase in fiber diameter and/or type 1 fiber percentage. The soleus had larger diameter fibers than the FCU in controls and FR, but not hypox rats. The growth in diameter, between 60 and 180 days, of both types 1 and 2 fibers in the soleus and type 2 fibers in the FCU was inhibited by hypox and, to a lesser extent, FR. Neither type 1 fiber diameter nor percentage of type 1 fibers in the FCU increased with age nor was it affected by hypox or FR. The percentage of type 1 fibers was higher in the soleus than the FCU and was further increased in the soleus of hypox rats. Food restriction produced a smaller rise than hypox in type 1 fiber percentage in the soleus. Thus, differences in fore- and hindlimb muscle fiber growth are modulated by pituitary hormones and, to a lesser extent, by food intake.

[Morphological and biochemical changes in red and white muscle fibers in skeletal musculature ischemia]

Structural reorganizations of red and white muscular fibres and peculiarities of changes in phosphoinositides in conditions of total ischemia of skeletal musculature were studied. On the whole morphological and biochemical changes were of the same type. However, decrease of phosphoinositides was more significant in white fibres than in red ones. In white muscular fibres deformations concerned myofibrillar apparatus while in red fibres mitochondrial apparatus was damaged.

Human and rat skeletal muscle adaptations to spinal cord injury

Results of studies of rodent skeletal muscle plasticity are often extrapolated to humans. However, responses to "disuse" may be species specific, in part because of different inherent properties of anatomically similar muscles. Thus, this study quantified human and rat m. vastus lateralis (VL) fiber adaptations to 11 weeks of spinal cord injury (SCI). The m. VL was taken from 8 young (54 d) male Charles River rats after T-9 laminectomy (n = 4) or sham surgery (n = 4). In addition, the m. VL was biopsied in 7 able-bodied and in 7 SCI humans (31.3 +/- 4.7 years, mean +/- SE). Samples were sectioned and fibers were analyzed for type (I, IIa, IIb/x), cross-sectional area (CSA), succinate dehydrogenase (SDH), alpha-glycerol-phosphate dehydrogenase (GPDH), and actomyosin adenosine triphosphatase (qATPase) activities. Rat fibers had 1.5- to 2-fold greater SDH and GPDH activities while their fibers were 60% the size of those in humans. The most striking differences, however, were the absence of slow fibers in the rat and its four-fold greater proportion of IIb/x fibers (80% vs. 16% of the CSA) compared to humans. SCI decreased SDH activity more in rats whereas atrophy and IIa to IIb/x fiber shift occurred to a greater extent in humans. It is suggested that the rat is a reasonable model for studying the predominant response to SCI, atrophy. However, its high proportion of IIb/x fibers limits evaluation of the mechanical consequences of shifting to "faster" contractile machinery after SCI.

Myosin heavy-chain isoform composition of human single jaw-muscle fibers

Diversity in muscle contractile properties is based on the variability of contractile properties of single muscle fibers which in turn is related to the presence of different myosin heavy-chain (MyHC) isoforms. Human jaw muscles are featured by many hybrid fibers expressing more than one MyHC isoform. The purpose of this study was to determine the proportion of each isoform within these fibers for evaluation of the fiber's capacity of producing a large diversity in contractile properties. Electrophoretic separation of MyHC isoforms was performed on 218 single fibers of the temporalis and digastric muscles. Of these fibers, 100 were classified as hybrid fibers. Most hybrid fibers co-expressed MyHC-IIA and -IIX (n = 62); a smaller number co-expressed MyHC-I and -IIA (n = 14), MyHC-I and -IIX (n = 12), and MyHC-I, -IIA, and -IIX (n = 12). The proportions of the individual MyHC isoforms in the hybrid fibers varied highly, suggesting a large range of contractile properties among these fibers.

Choline acetyltransferase mutations in myasthenic syndrome due to deficient acetylcholine resynthesis

The myasthenic syndrome due to abnormal acetylcholine resynthesis is characterized by early onset, recessive inheritance, and recurrent episodes of potentially fatal apnea. Mutations in the gene encoding choline acetyltransferase (CHAT) have been found to account for this condition. We have identified five patients from three independent families with features of this disease including, in four patients, a paradoxical worsening of symptoms with cold temperatures. Electrodiagnostic studies demonstrated impaired neuromuscular transmission in all patients. In vitro microelectrode studies performed in the anconeus muscle biopsies of two patients showed moderate reduction of quantal release. Electron microscopy of the neuromuscular junction was normal in both patients. Each patient had two heterozygous CHAT mutations including L210P and P211A (family 1), V194L and V506L (family 2), and R548stop and S694C (family 3). Three of these mutations have previously been reported and suggest that, in this syndrome, some molecular defects may be more prevalent than others.

Selective deafferentation of hand cutaneous territory is followed by changes in fibre type distribution of a forearm muscle in the horse

Based on previous observations that capsaicin can selectively damage group III and IV afferents and induce muscle fibre transformation, we hypothesized that eliminating, by means of capsaicin, the group III and IV afferents of a peripheral territory it could lead to a fibre transformation in a muscle involved in the flexor reflexes of the same peripheral territory. Therefore, capsaicin was injected into the palmar nerves of the forelimb of the horse to investigate if eliminating group III and IV afferents from the hand of the horse a muscle fibre transition would occur in the flexor carpi radialis (FCR) muscle, which is involved in the flexor reflexes of the finger itself. 120 days after capsaicin injection, type I slow fibres increased and type IIA fast fibres decreased. We presume that the long lasting deafferentation of the ergo-nociceptive fibres causes a plastic remodelling in the central nervous system and indirectly influences the motoneuron excitability via short or long loop-pathways enhancing their tonic discharge.

Muscle fibre composition and electromyographic features of cervical muscles following prolonged head extension in growing rats

Soft tissue stretching has been proposed as one of the control factors in craniofacial morphogenesis. However, its mechanism remains unclear. The present study investigated electromyographic (EMG) activity and muscle fibre composition of cervical muscles following prolonged head extension in growing rats. Thirty-six male Wistar rats were divided into two experimental (E1, E2) and one control (C) group at 25 days of age. To induce head extension, the experimental rats were raised in cylindrical cages, which were positioned horizontally for group E1 and tilted upward at 45 degrees for group E2. At 55 days of age, EMG activity was recorded from the anterior digastricus (AD), sternohyoideus (SH), sternomastoideus (SM), longus capitis (LC), and biventer cervicis (BC) muscles in the rest position and passive head extension. EMG activity was analysed on its integrated values (IEMG), and composition of muscle fibres was evaluated by myosin ATPase reaction and fibre cross-sectional areas were calculated. Group E1 showed a higher percentage of type I fibres and lower IEMG during passive head extension in AD. In group E2 there was a higher percentage of type I fibres, a higher IEMG at rest, and a lower IEMG during passive head extension in BC. The experimental groups demonstrated altered proportions of type IIA and IIB fibres in SM and LC. They also showed higher percentages of subtype fibres and reduced cross-section areas of type II fibres in most of the muscles investigated. These findings suggest that head extension affects fibre transition, distribution, cross-section area, and recruitment pattern in cervical muscles.

Exercise induces an increase in muscle UCP3 as a component of the increase in mitochondrial biogenesis

Previous studies have indicated that exercise acutely induces large increases in uncoupling protein-3 (UCP3) in skeletal muscle, whereas endurance training results in marked decreases in muscle UCP3. Because UCP3 expression appears to be regulated by the same mechanism as other mitochondrial constituents, it seemed unlikely that exercise would result in such large and divergent changes in mitochondrial composition. The purpose of this study was to test the hypothesis that major changes in UCP3 protein concentration do not occur independently of mitochondrial biogenesis and that UCP3 increases as a component of the exercise-induced increase in mitochondria. We found a large increase in UCP3 mRNA immediately and 3 h after a bout of swimming. UCP3 protein concentration was increased approximately 35% 18 h after a single exercise bout, approximately 63% after 3 days, and approximately 84% after 10 days of exercise. These increases in UCP3 roughly paralleled those of other mitochondrial marker proteins. Our results are consistent with the interpretation that endurance exercise induces an adaptive increase in mitochondria that have a normal content of UCP3.

Morphometric evaluation of muscle fiber types in different skeletal muscles of rats

The purpose of the present work was to study some morphological differences of similar muscle fiber types--classified by ATPase reactions in different muscles of rats. Morphological parameters were used as stereological techniques at light and electron microscopic level. There was a great variation in the diameter of each muscle fiber type of different muscles. The smallest diameter of type 1 myofibers of the soleus was greater than the diameter of type 1 myofibers of other muscles. The diameter of type 1 myofibers of the soleus and of the lateral part of the gastrocnemius was almost twice the diameter of type 1 myofibers of sternocleidomastoid. The lateral and medial parts of gastrocnemius had the largest 2A and 2B muscle fibers. As a whole, among the studied muscles, myofibers of postural muscles of the posterior parts of the posterior limbs had the greatest diameter. Stereological analysis at electron microscopic level revealed that there were differences in the volume density of mitochondria in the different muscles. The quantity of mitochondria was greater in the diaphragm than in the gastrocnemius, soleus and sternocleidomastoid muscles. Our results suggested that the diameter of muscle fibers is more related to the resistance the muscle is submitted than to the continuous necessity of contraction. However, the quantity of mitochondria of oxidative fibers of the diaphragm would be related to continuous necessity of contraction and high oxidative necessity of this muscle.

GLUT4 expression at the plasma membrane is related to fibre volume in human skeletal muscle fibres

In this study we examined the relationship between GLUT4 expression at the plasma membrane and muscle fibre size in fibre-typed human muscle fibres by immunocytochemistry and morphometry in order to gain further insight into the regulation of GLUT4 expression. At the site of the plasma membrane, GLUT4 was more abundantly expressed in slow as compared to fast fibres at the same fibre diameter (p < 0.01) and the GLUT4 expression increased with increasing fibre radius independently of fibre type (p < 0.01). The GLUT4 density at the surface of slow fibres of both diabetic and obese was reduced compared to control subjects at the same diameter (p < 0.001). Fast fibres in obese and type 2 diabetic subjects expressed a fibre-volume-dependent GLUT4 expression (p < 0.001), while this did not reach significance in slow fibres (obese p = 0.18 and diabetic p = 0.06). Our results show that increasing fibre volume is associated with increasing GLUT4 expression in both slow and fast fibres. Based on the possible dependency of GLUT4 expression on volume, we hypothesize that the reduced GLUT4 expression in obesity and type 2 diabetes may partly be compensated for by physical activity.