Changes in myosin heavy chain mRNA and protein isoforms in single fibers of unloaded rat soleus muscle.
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Changes in myosin heavy chain (MHC) mRNA and protein isoforms were investigated in single fibers from rat soleus muscle unloaded by hindlimb suspension for 4 and 7 days. Dramatic changes were seen after 4 days, when all fibers co-expressed slow and fast MHC mRNAs. Most fibers contained mRNAs for MHCIbeta, MHCIIa, MHCIId(x), and MHCIIb. The up-regulation of the fast isoforms was only partially transmitted to the protein level. Atypical combinations of MHC mRNA isoforms, which deviated from the 'next-neighbor rule', were frequent in fibers from unloaded soleus. These atypical combinations increased with time and were not observed in the controls. The results suggest that hindlimb suspension elicits in soleus muscle pronounced perturbations in the expression of MHC isoforms by disrupting transcriptional and translational activities. (+info)
Time of contact and step length: the effect of limb length, running speed, load carrying and incline.
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Using published values for twelve species of birds and mammals, we investigated the effects of limb length and running speed on time of contact and step length. In addition, we measured the time of contact in horses trotting up a 10 % incline and when carrying a load averaging 19 % of their body mass. From these values, we calculated stride period and step length. Our analysis of the interspecific data yielded the following relationship between time of contact (t(c) in s) and leg length (L in m) and running speed (v in m s(-)(1)): t(c)=0.80L(0.84)/v(0.87) (r(2)=0.97). Both exponents in this relationship are significantly different from 1.0, indicating that step length increases with speed and that small species use a step length that, relative to their leg length, is longer than the relative step length used by larger species. Time of contact increased when a horse carried a load but not when it trotted up an incline. (+info)
An electromyographic study of the hip abductor muscles as subjects with a hip prosthesis walked with different methods of using a cane and carrying a load.
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BACKGROUND AND PURPOSE: Certain methods of carrying handheld loads or using a cane can reduce the demands placed on the hip abductor (HA) muscles and the loads on the underlying prosthetic hip. In certain conditions, unusually large forces from the HA muscles may contribute to premature loosening of a prosthetic hip. The purpose of this study was to examine HA use by measuring the amplitude of the electromyographic (EMG) signal from the HA muscles as subjects carried a load and simultaneously used a cane. SUBJECTS: Twenty-four active subjects (mean age = 63.3 years, SD = 10.7, range = 40-86) with a unilateral prosthetic hip were tested. METHODS: The HA muscle surface EMG activity was analyzed as subjects carried loads weighing 5%, 10%, or 15% of body weight held by either their contralateral or ipsilateral arm relative to their prosthetic hip. They simultaneously used a cane with their free hand. RESULTS: The contralateral cane and ipsilateral load conditions produced HA muscle EMG activity that was approximately 40% less than the EMG activity produced while walking without carrying a load or using a cane. CONCLUSION AND DISCUSSION: People who are in danger of premature loosening of their prosthetic hip should, if possible, avoid carrying loads. If a load must be carried, however, then the contralateral cane and ipsilateral load condition appears to minimize the loads placed on the prosthetic hip due to HA muscle activity. (+info)
Unloading of juvenile muscle results in a reduced muscle size 9 wk after reloading.
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The role of satellite cells and DNA unit size in determining muscle size was examined by inhibiting postnatal skeletal muscle development by using hindlimb suspension. Satellite cell mitotic activity and DNA unit size were determined in the soleus muscles from hindlimb-suspended and age-matched weight-bearing rats before the initiation of hindlimb suspension, at the conclusion of a 28-day hindlimb-suspension period, 2 wk after reloading, and 9 wk after reloading. The body weights of hindlimb-suspended rats were significantly (P < 0.05) less than those of weight-bearing rats at the conclusion of hindlimb suspension, but they were the same (P > 0. 05) as those of weight-bearing rats 9 wk after reloading. The soleus muscle weight, soleus muscle weight-to-body weight ratio, myofiber diameter, nuclei per millimeter, and DNA unit size for the hindlimb-suspended rats were significantly (P < 0.05) smaller than for the weight-bearing rats at all recovery times. Satellite cell mitotic activity was significantly (P < 0.05) higher in the soleus muscles from hindlimb-suspended rats 2 wk after reloading, but it was the same (P > 0.05) as in weight-bearing rats 9 wk after reloading. Juvenile soleus muscles failed to achieve normal muscle size 9 wk after reloading because there was incomplete compensation for the hindlimb-suspension-induced interruptions in myonuclear accretion and DNA unit size expansion. (+info)
Integrin signaling's potential for mediating gene expression in hypertrophying skeletal muscle.
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Overloaded skeletal muscle undergoes dramatic shifts in gene expression, which alter both the phenotype and mass. Molecular biology techniques employing both in vivo and in vitro hypertrophy models have demonstrated that mechanical forces can alter skeletal muscle gene regulation. This review's purpose is to support integrin-mediated signaling as a candidate for mechanical load-induced hypertrophy. Research quantifying components of the integrin-signaling pathway in overloaded skeletal muscle have been integrated with knowledge regarding integrins role during development and cardiac hypertrophy, with the hope of demonstrating the pathway's importance. The role of integrin signaling as an integrator of mechanical forces and growth factor signaling during hypertrophy is discussed. Specific components of integrin signaling, including focal adhesion kinase and low-molecular-weight GTPase Rho are mentioned as downstream targets of this signaling pathway. There is a need for additional mechanistic studies capable of providing a stronger linkage between integrin-mediated signaling and skeletal muscle hypertrophy; however, there appears to be abundant justification for this type of research. (+info)
Skeletal muscle Ca(2+)-independent kinase activity increases during either hypertrophy or running.
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Spikes in free Ca(2+) initiate contractions in skeletal muscle cells, but whether and how they might signal to transcription factors in skeletal muscles of living animals is unknown. Since previous studies in non-muscle cells have shown that serum response factor (SRF) protein, a transcription factor, is phosphorylated rapidly by Ca(2+)/calmodulin (CaM)-dependent protein kinase after rises in intracellular Ca(2+), we measured enzymatic activity that phosphorylates SRF (designated SRF kinase activity). Homogenates from 7-day-hypertrophied anterior latissimus dorsi muscles of roosters had more Ca(2+)-independent SRF kinase activity than their respective control muscles. However, no differences were noted in Ca(2+)/CaM-dependent SRF kinase activity between control and trained muscles. To determine whether the Ca(2+)-independent and Ca(2+)/CaM-dependent forms of Ca(2+)/CaM-dependent protein kinase II (CaMKII) might contribute to some of the SRF kinase activity, autocamtide-3, a synthetic substrate that is specific for CaMKII, was employed. While the Ca(2+)-independent form of CaMKII was increased, like the Ca(2+)-independent form of SRF kinase, no alteration in CaMKII occurred at 7 days of stretch overload. These observations suggest that some of SRF phosphorylation by skeletal muscle extracts could be due to CaMKII. To determine whether this adaptation was specific to the exercise type (i.e., hypertrophy), similar measurements were made in the white vastus lateralis muscle of rats that had completed 2 wk of voluntary running. Although Ca(2+)-independent SRF kinase was increased, no alteration occurred in Ca(2+)/CaM-dependent SRF kinase activity. Thus any role of Ca(2+)-independent SRF kinase signaling has downstream modulators specific to the exercise phenotype. (+info)
Novel salmon cardiac peptide hormone is released from the ventricle by regulated secretory pathway.
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We used the secretion of the novel salmon cardiac peptide (sCP) as a model to examine the mechanisms of ventricular hormone release. Mechanical load increased dose dependently the secretion of immunoreactive sCP from isolated perfused salmon ventricle, with 3. 3-fold increase when a load of 13 cmH(2)O was applied. Endothelin-1 (5 nmol/l) was also able to rapidly increase the secretion of sCP. The released peptide corresponded to the biologically active sCP-29, whereas the large ventricular storage consisted of pro-sCP-sized material. With the use of immunoelectron microscopy, a large number of granules containing immunoreactive sCP could be detected in salmon ventricle. As judged by RNA blot analysis, there was very active basal expression of the sCP gene in the ventricle, which was not increased by mechanical load of up to 2-h duration. Our results show that the ventricle actively expresses the gene of sCP, stores the prohormone in secretory granules, and releases the peptide in response to mechanical load and endothelin-1. Thus the salmon ventricle uses the regulated pathway to produce and release a hormone structurally related to the mammalian natriuretic peptides. (+info)
Assessment of disc injury in subjects exposed to long-term whole-body vibration.
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Long-term exposure to whole-body vibration is known to increase the risk of low back problems. The chain of events leading from repeated loading of the lumbar spine to back complaints and the exact nature of the vibration-induced damage are, however, obscure. Fluid in- and outflow as well as viscoelastic deformation are important aspects of the physiological function of the lumbar disc. Precision measurement of stature, termed 'stadiometry', has previously been applied in healthy subjects to document changes in disc height in relation to the load on the lumbar spine. The purpose of this study was to explore the relation between spinal loading and stature in a cohort of 20 subjects with long-term exposure to whole-body vibration. If the change of stature (and thus the change of disc height) caused by changes in spinal loading differed between exposed and normal subjects, this would point to vibration-induced changes in structure and material properties of the discs. For this purpose, four hypotheses were tested: (1) the viscoelastic deformation and fluid exchange of intervertebral discs during phases of spinal loading and unloading differs from normal; (2) the water content of lumbar discs of subjects exposed to long-term whole-body vibration deviates from normal; (3) the mean disc height of the lumbar spine depends on the total time of vibration exposure; (4) repeated loading influences trabecular bone density of vertebrae in the lumbar spine. A cohort of 20 operators of heavy earth-moving machinery was enrolled. Back complaints suspected to be due to long-term exposure (mean 17.6 +/- 2.1 years) to whole-body vibration and application for early retirement were the selection criteria used. Change of stature during a regular 8-h shift and change of stature in standing, carrying and sitting activities were measured. The stadiometric investigations were supplemented by magnetic resonance imaging (MRI) of the lumbar spine to assess whether the water content of the discs exhibited deviations from normal. In addition, quantitative computed tomography (QCT) was performed to assess whether the trabecular bone density of the third lumbar vertebra deviated from normal. The results showed no significant difference in change of stature while standing, carrying or sitting between exposed machine operators and non-exposed operators. Likewise, MRI examinations revealed no significant differences in the water content of the discs averaged over the lumbar spine. In addition, QCT examinations revealed no significant difference in the trabecular bone density of the third lumbar vertebra. The study thus revealed no significant difference between a cohort with long-term exposure and non-exposed controls with respect to viscoelastic properties of discs as determined by stadiometry, average water content of lumbar discs and trabecular bone density of L3. (+info)