Cheryl A. Smith

Cellular responses in exertion-induced skeletal muscle injury

Muscle injury is a common result of muscle exertion caused by overload and over-activity. In this presentation, an attempt was made to discuss models of muscle injury which involve exertion but not excessive strain, although most functional activities of the extremities require some eccentric muscle actions. Muscle injury is characterized by cellular and extracellular matrix responses which appear to be common to all types of muscle trauma - even in the absence of bleeding. Using tenotomy and functional over-load of the rat hindlimb muscles as examples, illustrations of several of these responses are presented and discussed.

Recovery from 6 weeks of repeated strain injury to rat soleus muscles.

Recovery from chronic strain injury (50 strains daily, five times weekly for 6 weeks to hyperactive soleus muscles) was followed for 3 months in female rats after cessation of chronic hyperactivity induced by pretreatment of the plantar flexor muscles with tetanus toxin. After 6 weeks of repeated strains, muscle mass decreased by 62%, myofiber areas were reduced by 87%, and noncontractile tissue expanded dramatically by 222%. Collagen content increased by almost ninefold (control 40 ± 3 μg/mg, chronic injury 392 ± 53 μg/mg), whereas the molar ratio of collagen (pyridinoline) crosslinks to collagen remained the same (control 0.20 ± 0.01, chronic injury 0.16 ± 0.01). After 3 months of ambulation, muscle mass returned to normal but myofiber areas remained smaller by 21%, noncontractile tissue was still markedly elevated by 18% with increased collagen content (107 ± 15 μg/mg), and the molar ratio of crosslinks to collagen increased by 75% during recovery. Thus, rat soleus muscles recovered very slowly and incompletely from chronic strain injuries that produced muscle fibrosis, highlighting the necessity of devising preventative strategies for repeated strain injuries.

Effects of exercise and creatine on myosin heavy chain isoform composition in patients with Charcot–Marie–Tooth disease

It is not known whether myosin heavy chain (MHC) content changes in response to exercise training or creatine supplementation in subjects with Charcot–Marie–Tooth disease (CMT). Based on previous data, we hypothesized that resistance exercise and creatine would increase the percentage of type I MHC composition in the vastus lateralis muscle and that myosin isoform changes would correlate with improved chair rise‐time in CMT subjects. To test this hypothesis, 18 CMT subjects were randomly assigned to either a placebo or creatine group. All subjects performed a 12‐week, home‐based, moderate‐intensity resistance training program. Chair rise‐time was measured before and after the training program. Muscle biopsies were obtained from the vastus lateralis before and after the 12‐week program. Gel electrophoresis showed a significant decrease (∼30%) in MHC type I in CMT subjects given creatine supplementation when compared with placebo. There was a nonsignificant increase in both MHC type IIa (∼23%) and MHC type IIx (∼7%) in CMT subjects given creatine. Reduced MHC type I content and increased MHC type IIa content correlated with faster chair rise‐times (i.e., improved muscle performance). The training‐induced change in MHC IIa content was inversely correlated with chair rise‐time in CMT subjects given creatine. When the two subject groups were combined, there was a linear, negative relationship between the change in MHC type IIa content and chair rise‐time after training and a positive relationship between the training‐induced change in MHC type I content and chair rise‐time. These data suggest that improved function (chair rise‐time) was associated with a lower level of MHC type I and increased MHC type IIa composition. Furthermore, the data are consistent with the hypothesis that creatine supplementation alters MHC composition in CMT patients undergoing resistance training and that MHC changes associated with creatine supplementation can improve muscle function. Muscle Nerve, 2006

Renal nitric oxide production in rat pregnancy: role of constitutive nitric oxide synthases

Functional studies show that increased renal nitric oxide (NO) mediates the renal vasodilation and increased glomerular filtration rate that occur during normal pregnancy. We investigated whether changes in the constitutive NO synthases (NOS), endothelial (eNOS) and neuronal (nNOS), were associated with the increased renal NO production in normal midterm pregnancy in the rat. In kidneys from midterm pregnant (MP: 11-13 days gestation), late-term pregnant (LP: 18-20 days gestation), and similarly aged virgin (V) rats, transcript and protein abundance for eNOS and the nNOSα and nNOSβ splice variants, as well as the rate of L-arginine-to-L-citrulline conversion, were determined as a measure of NOS activity. At MP, renal cortical abundance of the total eNOS protein and phosphorylated (Ser(1177)) eNOS was reduced, and L-arginine-to-L-citrulline conversion in the cortical membrane fraction was decreased; these declines were also seen in LP. There were no changes in the eNOS transcript. In contrast, L-arginine-to-L-citrulline conversion in the soluble fraction of renal cortex increased at MP and then declined at LP. This MP increase was ablated by S-methylthiocitrulline, a nNOS inhibitor. Using Western blotting, we did not detect a change in the protein abundance or transcript of the 160-kDa nNOSα, but protein abundance and transcript of the nNOSβ were increased at MP in cortex. Collectively, these studies suggest that the soluble nNOSβ is responsible for the increased renal cortical NO production during pregnancy.

Determination of Fibrosis from Cryostat Sections Using High Performance Liquid Chromatography: Skeletal Muscle

Analysis of hydroxyproline (collagen) and pyridinoline (collagen cross-links) in biopsies prepared for routine histological evaluation with OCT compound was performed. Frozen sections (250 µm-thick) were cut from cardiac muscle, diaphragm, liver, and soleus muscle from the rat. After removal of OCT compound by rinsing, the samples were dried, weighed and hydrolyzed in 6 N HCl. A portion of the hydrolysate was analyzed for hydroxyproline using high performance liquid chromatography with collagen type I as the standard. Collagen concentrations ranged from 6.6 µg/mg dry weight (liver) to 74.7 µg/mg dry weight (diaphragm). From the remainder of the hydrolyzate, pyridinoline cross-links of collagen were separated and analyzed similarly by high performance liquid chromatography. The concentration of pyridinoline ranged from 2.6 ng/mg dry weight (liver) to 35.6 ng/mg dry weight (diaphragm). These techniques were adequate to analyze both collagen and pyridinoline (i.e. collagen cross-links) in small biopsy samples (<1 mg dry weight) routinely used in clinical pathology. The method proved useful in the quantitation of focal fibrosis in a partially denervated rat soleus. Denervation was confirmed using fast myosin immunohistochemistry which revealed large areas of small myofibres containing fast myosin. Collagen concentration increased by five-fold and collagen cross-links by more than 7-fold consistent with fibrotic changes known to occur with denervation.

Myotonic Dystrophy Type 1 Management and Therapeutics.

Opinion statementMyotonic dystrophy (DM1) is the most common form of adult muscular dystrophy. It is a multisystem disorder with a complex pathophysiology. Although inheritance is autosomal dominant, disease variability is attributed to anticipation, a maternal expansion bias, variable penetrance, somatic mosaicism, and a multitude of aberrant pre-mRNA splicing events. Patient presentations range from asymptomatic or mild late onset adult to severe congenital forms. Multiple organ systems may be affected. Patients may experience early cataracts, myotonia, muscle weakness/atrophy, fatigue, excessive daytime sleepiness, central/obstructive apnea, respiratory failure, cardiac arrhythmia, insulin resistance, dysphagia, GI dysmotility, cognitive impairment, Cluster C personality traits, and/or mood disorders. At present, there is no curative or disease-modifying treatment, although clinical treatment trials have become more promising. Management focuses on genetic counseling, preserving function and independence, preventing cardiopulmonary complications, and symptomatic treatment (e.g., pain, myotonia, hypersomnolence, etc.). Currently, there is an increasing international consensus on monitoring and treatment options for these patients which necessitates a multidisciplinary team to provide comprehensive, coordinated clinical care.