Behavior of contractile proteins of rat muscle fibers under microgravity conditions

Microgravity effects were studied on three muscles: gastrocnemius lateralis, plantaris and diaphragm, after the biocosmos 1514 and 1667 space flights. Results showed a decrease of maximal mechanical activity on both gastrocnemius and plantaris while no modification was observed on diaphragm. Cross-bridge cycling speed was reduced on both gastrocnemius and plantaris. Moreover, a reduced calcium binding affinity appeared in gastrocnemius.     

Effect of microgravity on metabolic enzymes of individual muscle fibers

Eleven enzymes were measured in individual fibers of soleus and tibialis anterior (TA) muscles from two flight and two control (synchronous) animals. There were five enzymes of glycogenolytic metabolism: phosphorylase, glucose-6-phosphate isomerase, glycerol-3-phosphate dehydrogenase, pyruvate kinase, and lactate dehydrogenase (group GLY); five of oxidative metabolism: citrate synthase, malate dehydrogenase, beta-hydroxyacyl-CoA dehydrogenase, 3-ketoacid CoA-transferase, and mitochondrial thiolase (group OX); and hexokinase, subserving both groups. Fiber size (dry weight per unit length) was reduced about 35% in both muscles. On a dry weight basis, hexokinase levels were increased 100% or more in flight fibers from both soleus and TA. Group OX enzymes increased 56-193% in TA without significant change in soleus. Group GLY enzymes increased an average of 28% in soleus fibers but underwent, if anything, a modest decrease (20%) in TA fibers. These changes in composition of TA fibers were those anticipated for a conversion of about half of the originally predominant fast glycolytic fibers into fast oxidative glycolytic fibers. Calculation on the basis of fiber length, rather than dry weight, gave an estimate of absolute enzyme changes: hexokinase was still calculated to have increased in both soleus and TA fibers, but only by 50 and 25%, respectively. Three of the OX enzymes were, on this basis, unchanged in TA fibers, but 3-ketoacid CoA-transferase and thiolase had still nearly doubled, whereas TA GLY enzymes had fallen about 40%. In soleus fibers, absolute levels of OX enzymes had decreased an average of 25% and GLY enzymes were marginally decreased.     

Plasticity of skeletal muscle: regenerating fibers adapt more rapidly than surviving fibers

The properties of mammalian skeletal muscle demonstrate a high degree of structural and functional plasticity as evidenced by their adaptability to an atypical site after cross-transplantation and to atypical innervation after cross-innervation. We tested the hypothesis that, regardless of fiber type, skeletal muscles composed of regenerating fibers adapt more readily than muscles composed of surviving fibers when placed in an atypical site with atypical innervation. Fast muscles of rats were autografted into the site of slow muscles or vice versa with the donor muscle innervated by the motor nerve to the recipient site. Surviving fibers in donor muscles were obtained by grafting with vasculature intact (vascularized muscle graft), and regenerating fibers were obtained by grafting with vasculature severed (free muscle graft). Our hypothesis was supported because 60 days after grafting, transposed muscles with surviving fibers demonstrated only a slight change from the contractile properties and fiber typing of donor muscles, whereas transplanted muscles with regenerating fibers demonstrated almost complete change to those of the muscle formerly in the atypical site.     

Energy metabolism pathways in rat muscle under conditions of simulated microgravity

Evidence from rats flown in space suggests that there is a decrease in the ability of the soleus muscle to oxidize long chain fatty acids during space flight. The observation suggests that a shift in the pathways involved in muscle fuel utilization in the absence of load on the muscle has occurred. It is also possible that the reduction is part of a general down-sizing of metabolic capacity since energy needs of inactive muscle are necessarily less. The rodent hind limb suspension model has proved to be a useful ground based model for studying the musculo-skeletal systems changes that occur with space flight. Microarray technology permits the screening of a large number of the enzymes of the relevant pathways thereby permitting a distinction to be made between a shift fuel utilization pattern or a general decrease in metabolic activity. The soleus muscle was isolated from 5 control and 5 hindlimb suspended rats (21 days) and the Affymetrix system for assessing gene expression used to determine the impact of hindlimb unloading on fuel pathways within the muscle of each animal. RESULTS: Suspended rats failed to gain weight at the same rate as the controls (337 +/- 5 g vs 318 +/- 6 g, p < 0.05) and muscle mass from the soleus was reduced (135 +/- 3 mg vs 48 +/- 4 mg, p < 0.05). There was a consistent decrease (p < 0.05) in gene expression of proteins involved in fatty acid oxidation in the suspended group whereas glycolytic activity was increased (p < 0.05). Gene expressions of individual key regulatory enzymes reflected these changes. Carnitine palmitoyltransferase I and II were decreased (p < 0.05) whereas expression of hexokinase, phosphofructokinase and pyruvate kinase were increased (p < 0.05). CONCLUSION: Disuse atrophy is associated with a change in mRNA levels of enzymes involved in fuel metabolism indicative of a shift in substrate utilization away from fat towards glucose.     

Demonstration of ATPase activity in skeletal muscle fibers of the triceps muscle of arm in white rats during development

On the base of modified calcium and method from Zebe and Falk (1964) for demonstration of ATPase activity results at the long head of the triceps muscle of white rats aged 1, 3, 6 weeks and 3 month were shown. This method is proper to make visible alteration of the localisation of the reaction product during the postnatal development. Further investigations are necessary in order to correlate the steps of differentiation of muscle fibres with the strength of ATPase activity.     

Plasticity in mammalian skeletal muscle.

While it has been recognized for many years that different limb muscles belonging to the same mammal may have markedly differing contractile characteristics, it is only comparatively recently that it has been demonstrated that these differences depend upon the motor innervation. By appropriately changing the peripheral nerve innervating a mammalian skeletal muscle, it is possible to change dramatically the contractile behaviour of the reinnervated muscle. The manner by which the motor innervation determines the nature of a muscle fibre's contractile machinery is not completely understood, but it appears that the number and pattern of motor nerve impulses reaching the muscle play an important role. The biochemical changes occurring within muscle fibres whose contractile properties have been modified by altered motor innervation include the synthesis of different contractile proteins.     

Histochemical characteristics of the muscle fibers of the biceps and triceps brachii muscles in human ontogeny

By means of morphometrical and histochemical methods for revealing myosin ATPase and SDG activity development of various types of muscle fibers (MF) has been studied in the postmortem material, using m. biceps and m. triceps brachii in human ontogenesis. The flexors and extensors have features in common in the dynamics of the MF maturation, and some distinctive peculiarities. The appearance of histochemical distinctions between the MF takes place on the 5th-6th months of the intrauterine development. Morphofunctional specialization begins with formation of tonic fibers. During the 1st-2nd years phasic fibers form. A relative amount of fast MF in both muscles increases at the age of 11-12 years. The dynamics of final specialization of the MF is connected with stages of sexual maturation. The first stage of the sexual maturation (about 14 years of age) is connected with decrease in the relative amount of the MF of glycolytic type of energy supply and corresponding increase in the number of oxidative type structures. From 15-17 years of age a final differentiation begins, it is connected with an intensive transversal growth of all the MF and distinguish of thick glycolytic MF. The m. biceps brachii has a relatively greater amount of oxidative fibers, and the m. triceps brachii, glycolytic ones. The transversal section area of the MF in the m. triceps brachii exceeds that of the m. biceps brachii, beginning from the 7th month of the intrauterine development up to 14 years of age. The investigation performed does not reveal any anticipating development either in the flexors or in the extensors. The differentiating processes in the m. biceps and m. triceps brachii occur nearly simultaneously.     

Radial stiffness of frog skinned muscle fibers in relaxed and rigor conditions.

Radial stiffness in various conditions of mechanically skinned fibers of semitendinosus muscle of Rana catesbeiana was determined by compressing the fiber with polyvinylpyrrolidone (PVP K-30, Mr = 40,000) in incubating solution. The change in width (D) of fibers with increasing and decreasing PVP concentrations was highly reproducible at a range 0-6% PVP. Radial stiffness of relaxed fibers was almost independent of the sarcomere length. On the other hand, radial stiffness of rigor fibers showed a linear relation against the sarcomere length. These results indicate that cross-bridge attachment would be a major factor in the increase of the radial stiffness. Radial stiffness of relaxed and rigor fibers was (2.14 +/- 0.52) X 10(4) N/m2 (mean +/- SD) and (8.76 +/- 2.04) X 10(4) N/m2, respectively, at the relative fiber width (D/D0) of 0.92, where D0 denotes the fiber width in the rigor solution at 0% PVP. Radial stiffness of a fiber in a rigor solution containing pyrophosphate (PPi) was between those of relaxed and rigor fibers, i.e., (4.76 +/- 0.86) X 10(4) N/m2 at D/Do of 0.92. In PPi and rigor solutions, radial stiffness reversibly increased to around 150 and 130%, respectively, in the presence of 10(-6) M Ca2+. To explain these results, especially the Ca2+-induced change in the radial stiffness, some factor in addition to the number of attached cross-bridges has to be taken into account. The variation of radial stiffness under various conditions will be discussed in relation to the possible manner of cross-bridge attachment.     

Physiology of a microgravity environment invited review: microgravity and skeletal muscle.

Spaceflight (SF) has been shown to cause skeletal muscle atrophy; a loss in force and power; and, in the first few weeks, a preferential atrophy of extensors over flexors. The atrophy primarily results from a reduced protein synthesis that is likely triggered by the removal of the antigravity load. Contractile proteins are lost out of proportion to other cellular proteins, and the actin thin filament is lost disproportionately to the myosin thick filament. The decline in contractile protein explains the decrease in force per cross-sectional area, whereas the thin-filament loss may explain the observed postflight increase in the maximal velocity of shortening in the type I and IIa fiber types. Importantly, the microgravity-induced decline in peak power is partially offset by the increased fiber velocity. Muscle velocity is further increased by the microgravity-induced expression of fast-type myosin isozymes in slow fibers (hybrid I/II fibers) and by the increased expression of fast type II fiber types. SF increases the susceptibility of skeletal muscle to damage, with the actual damage elicited during postflight reloading. Evidence in rats indicates that SF increases fatigability and reduces the capacity for fat oxidation in skeletal muscles. Future studies will be required to establish the cellular and molecular mechanisms of the SF-induced muscle atrophy and functional loss and to develop effective exercise countermeasures.     

失重状态下肌萎缩研究进展

失重条件下人和动物生理状态会发生一系列的变化,其中骨骼肌萎缩和力量下降较为显著,目前其发生的机制仍不明确且缺少特效的干预措施。本文从肌肉湿重及肌纤维横截面积的变化、肌纤维类型的变化、肌纤维超微结构的变化、肌梭的适应性变化四个方面进行简要阐述,探讨肌肉萎缩的可能发生机制。     

Optical diffraction study of muscle fibers. II. Electro-optical properties of muscle fibers.

When an electric field is applied along the fiber axis, the intensities of all observable optical diffraction lines of skeletal muscle fibers increase. This electro-optical effect was extensively studied and it was confirmed that the effect is due to the interaction between electric dipole moments of thin filaments and the applied field. From the present study on the intensity modulation due to applied field in sinusoidal and square forms, we confirmed that (1) the thin filament is a semiflexible rod, (2) the second order mode of the bending motion of thin filaments contributes to the electro-optical effect of muscle fibers at higher frequencies of a sinusodidal field or shorter durations of a square field, (3) the induced moment has no appreciable effect, and (4) the estimated value of the flexural rigidity of thin filaments strongly depends on the concentrations of free calcium ions in the myofibrillar space.     

Catalase in skeletal muscle fibers.

Catalase has been localized immunocytochemically with anti-bovine catalase in long thin filament structures in aerobic type I fibers in the skeletal muscles of normal and genetically dystrophic hamsters. The filaments range in length from 1 to 60 micron, are orientated regularly along the long axis of the fibers, and also seem to surround and project from muscle nuclei. The enzyme thus appears to be more prominent in the sarcoplasmic reticulum than in peroxisomes, and in this situation is suitably placed for destroying toxic hydrogen peroxide which may be continously generated in aerobic fibers.     

Redistribution of bodily fluids under conditions of microgravity and in microgravity models

This review is focused on the redistribution of blood and other bodily fluids along the body axis in the cranial direction under conditions of microgravity or during simulation of the physiological effects of microgravity. This redistribution of bodily fluids in the direction of the thorax or head results in respective physiological responses and induces a whole cascade of secondary adaptation mechanisms. Changes in central venous pressure, heart cavity volume, kidney functioning, and hormonal volume regulation lead to adaptive modifications in bodily fluid sectors. Modification of the hemodynamic in the splanchnic vascular system influences the organs of the abdominal cavity. Pharmacological correction accelerates the adaptation of the human body to unusual living conditions.     

Passive tension of rat skeletal soleus muscle fibers: effects of unloading conditions.

In this work we studied changes in passive elastic properties of rat soleus muscle fibers subjected to 14 days of hindlimb unloading (HU). For this purpose, we investigated the titin isoform expression in soleus muscles, passive tension-fiber strain relationships of single fibers, and the effects of the thick filament depolymerization on passive tension development. The myosin heavy chain composition was also measured for all fibers studied. Despite a slow-to-fast transformation of the soleus muscles on the basis of their myosin heavy chain content, no modification in the titin isoform expression was detected after 14 days of HU. However, the passive tension-fiber strain relationships revealed that passive tension of both slow and fast HU soleus fibers increased less steeply with sarcomere length than that of control fibers. Gel analysis suggested that this result could be explained by a decrease in the amount of titin in soleus muscle after HU. Furthermore, the thick filament depolymerization was found to similarly decrease passive tension in control and HU soleus fibers. Taken together, these results suggested that HU did not change titin isoform expression in the soleus muscle, but rather modified muscle stiffness by decreasing the amount of titin.     

Skeletal muscle adaptations to microgravity exposure in the mouse.

To investigate the effects of microgravity on murine skeletal muscle fiber size, muscle contractile protein, and enzymatic activity, female C57BL/6J mice, aged 64 days, were divided into animal enclosure module (AEM) ground control and spaceflight (SF) treatment groups. SF animals were flown on the space shuttle Endeavour (STS-108/UF-1) and subjected to approximately 11 days and 19 h of microgravity. Immunohistochemical analysis of muscle fiber cross-sectional area revealed that, in each of the muscles analyzed, mean muscle fiber cross-sectional area was significantly reduced (P < 0.0001) for all fiber types for SF vs. AEM control. In the soleus, immunohistochemical analysis of myosin heavy chain (MHC) isoform expression revealed a significant increase in the percentage of muscle fibers expressing MHC IIx and MHC IIb (P < 0.05). For the gastrocnemius and plantaris, no significant changes in MHC isoform expression were observed. For the muscles analyzed, no alterations in MHC I or MHC IIa protein expression were observed. Enzymatic analysis of the gastrocnemius revealed a significant decrease in citrate synthase activity in SF vs. AEM control.