Myofibrillar protein oxidation and contractile dysfunction in hyperthyroid rat diaphragm.

The purpose of the present study was to test the hypothesis that administration of thyroid hormone [3,5,3'-triiodo-L-thyronine (T(3))] could result in oxidation of myofibrillar proteins and, in turn, induce alterations in respiratory muscle function. Daily injection of T(3) for 21 days depressed isometric forces of diaphragm fiber bundles across a range of stimulus frequencies (1, 10, 20, 40, 75, and 100 Hz) (P < 0.05). These reductions in force production were accompanied by a remarkable increment (104%; P < 0.05) in carbonyl groups of myofibrillar proteins. In contrast, T(3) treatment has no effects on the carbonyl content in myosin heavy chain. In additional experiments, we have also tested the efficacy of carvedilol, a nonselective beta(1)- beta(2)-blocker that possesses antioxidative properties. Treatment with carvedilol dramatically improved isometric tetanic force production at stimulus frequencies from 40 to 100 Hz (P < 0.05). Carvedilol also prevented T(3)-induced contractile protein oxidation (P < 0.05). These data suggest that the oxidative modification of myofibrillar proteins may account, at least in part, for an impairment of diaphragm in hyperthyroidism.     

Myosin and actin expression and oxidation in aging muscle.

While the age-related loss in muscle mass partially explains the decline in strength, other yet undefined mechanisms contribute. This study investigates whether changes in myosin-actin stoichiometry and oxidative modification could help explain the decrement in muscle strength with aging. Protein expression and oxidation were evaluated in myosin and actin isolated from the soleus and semimembranosus muscles from young adult, old, and very old Fischer 344 rats. In the soleus muscle, actin and myosin content did not change with aging. In the semimembranosus, actin content was stable, but myosin exhibited decreased content in muscles from very old rats, resulting in a decrease in the myosin-to-actin ratio. 3-Nitrotyrosine and 4-hydroxy-2-nonenal were used as markers of protein oxidative damage. Although myosin and actin are modified with 3-nitrotyrosine and 4-hydroxy-2-nonenal, the extent of chemical modification does not increase with age. The results suggest that the decline in force production with age is not due to the accumulation of these two specific markers of protein oxidation on the myofibrillar proteins. Additionally, age-dependent changes in myofibrillar stoichiometry do not contribute to the decline in force production in the soleus, but may play a role in the semimembranosus with advanced age.     

Oxidative stress of myosin contributes to skeletal muscle dysfunction in rats with chronic heart failure

Intrinsic muscle abnormalities affecting skeletal muscle are often reported during chronic heart failure (CHF). Because myosin is the molecular motor of force generation, we sought to determine whether its dysfunction contributes to skeletal muscle weakness in CHF and, if so, to identify the underlying causative factors. Severe CHF was induced in rats by aortic stenosis. In diaphragm and soleus muscles, we investigated in vitro mechanical performance, myosin-based actin filament motility, myosin heavy (MHC) and light (MLC) chain isoform compositions, MLC integrity, caspase-3 activation, and oxidative damage. Diaphragm and soleus muscles from CHF exhibited depressed mechanical performance. Myosin sliding velocities were 16 and 20% slower in CHF than in sham in diaphragm (1.9 +/- 0.1 vs. 1.6 +/- 0.1 microm/s) and soleus (0.6 +/- 0.1 vs. 0.5 +/- 0.1 microm/s), respectively (each P < 0.05). The ratio of slow-to-fast myosin isoform did not differ between sham and CHF. Immunoblots with anti-MLC antibodies did not detect the presence of protein fragments, and no activation of caspase-3 was evidenced. Immunolabeling revealed oxidative damage in CHF muscles, and MHC was the main oxidized protein. Lipid peroxidation and expression of oxidized MHC were significantly higher in CHF than in shams. In vitro myosin exposure to increasing ONOO(-) concentrations was associated with an increasing amount of oxidized MHC and a reduced myosin velocity. These data provide experimental evidence that intrinsic myosin dysfunction occurs in CHF and may be related to oxidative damage to myosin.     

Skeletal muscle myofibrillar protein metabolism in heart failure: relationship to immune activation and functional capacity

Chronic heart failure is characterized by changes in skeletal muscle that contribute to physical disability. Most studies to date have investigated defects in skeletal muscle oxidative capacity. In contrast, less is known about how heart failure affects myofibrillar protein metabolism. Thus we examined the effect of heart failure on skeletal muscle myofibrillar protein metabolism, with a specific emphasis on changes in myosin heavy chain (MHC) protein content, synthesis, and isoform distribution in 10 patients with heart failure (63 +/- 3 yr) and 11 controls (70 +/- 3 yr). In addition, we examined the relationship of MHC protein metabolism to inflammatory markers and physical function. Although MHC and actin protein content did not differ between groups, MHC protein content decreased with increasing disease severity in heart failure patients (r = -0.748, P < 0.02), whereas actin protein content was not related to disease severity. No difference in MHC protein synthesis was found between groups, and MHC protein synthesis rates were not related to disease severity. There were, however, relationships between C-reactive protein and both MHC protein synthesis (r = -0.442, P = 0.05) and the ratio of MHC to mixed muscle protein synthesis (r = -0.493, P < 0.03). Heart failure patients showed reduced relative amounts of MHC I (P < 0.05) and a trend toward increased MHC IIx (P = 0.06). In regression analyses, decreased MHC protein content was related to decreased exercise capacity and muscle strength in heart failure patients. Our results demonstrate that heart failure affects both the quantity and isoform distribution of skeletal muscle MHC protein. The fact that MHC protein content was related to both exercise capacity and muscle strength further suggests that quantitative alterations in MHC protein may have functional significance.     

Human soleus single muscle fiber function with exercise or nutrition countermeasures during 60 days of bed rest

The soleus muscle has been consistently shown to atrophy more than other leg muscles during unloading and is difficult to protect using various exercise countermeasure paradigms. However, the efficacy of aerobic exercise, a known stimulus for oxidative adaptations, has not been tested in combination with resistance exercise (RE), a known hypertrophic stimulus. We hypothesized that a concurrent exercise program (AE + RE) would preserve soleus fiber myosin heavy chain (MHC) I size and function during 60 days of bed rest. A secondary objective was to test the hypothesis that a leucine-enriched high protein diet would partially protect soleus single fiber characteristics. Soleus muscle biopsies were obtained before and after bed rest from a control (BR; n = 7), nutrition (BRN; n = 8), and exercise (BRE; n = 6) group. Single muscle fiber diameter (Dia), peak force (Po), contractile velocity, and power were studied. BR decreased (P < 0.05) MHC I Dia (-14%), Po (-38%), and power (-39%) with no change in contractile velocity. Changes in MHC I size (-13%) and contractile function (approximately 30%) from BRN were similar to BR. BRE decreased (P < 0.05) MHC I Dia (-13%) and Po (-23%), while contractile velocity increased (P < 0.05) 26% and maintained power. These soleus muscle data show 1) the AE + RE exercise program maintained MHC I power but not size and strength, and 2) the nutrition countermeasure did not benefit single fiber size and contractile function. The divergent response in size and functional MHC I soleus properties with the concurrent exercise program was a unique finding further highlighting the challenges of protecting the unloaded soleus.     

Altered expression of skeletal muscle myosin isoforms in cancer cachexia

Cachexia is commonly seen in cancer and ischaracterized by severe muscle wasting, but little is known about theeffect of cancer cachexia on expression of contractile protein isoforms such as myosin. Other causes of muscle atrophy shift expression ofmyosin isoforms toward increased fast (type II) isoform expression. Weinjected mice with murine C-26 adenocarcinoma cells, a tumor cell linethat has been shown to cause muscle wasting. Mice were killed 21 daysafter tumor injection, and hindlimb muscles were removed. Myosin heavychain (MHC) and myosin light chain (MLC) content was determined inmuscle homogenates by SDS-PAGE. Body weight was significantly lower intumor-bearing (T) mice. There was a significant decrease in muscle massin all three muscles tested compared with control, with the largestdecrease occurring in the soleus. Although no type IIb MHC was detectedin the soleus samples from control mice, type IIb comprised 19% of thetotal MHC in T soleus. Type I MHC was significantly decreased in T vs. control soleus muscle. MHC isoform content was not significantly different from control in plantaris and gastrocnemius muscles. Thesedata are the first to show a change in myosin isoform expression accompanying muscle atrophy during cancer cachexia.

     

Biochemical adaptation of cardiac and skeletal muscle to physical activity.

1. Female Wistar rats were randomly assigned to control (C) or exercising (T) groups and subsequently portioned into 1, 3, 5 and 10 day T and C groups. The T groups completed a progressive endurance running program. Biochemical indices of adaptation were measured in cardiac muscle and in plantaris and soleus muscles of C and T animals after their last exercise bout. 2. In cardiac muscle, myofibrillar ATPase activity was significantly elevated in the 3T (0.241 +/- 0.031) and 5T (0.242 +/- 0.013) groups (P less than or equal to 0.05) compared to their respective controls (3C = 0.187 +/- 0.015 and 5C = 0.190 +/- 0.007). 3. After 10 days of training cardiac myofibrillar ATPase activity was elevated by 17% but this was not significant (P greater than or equal to 0.05). 4. No changes in myofibrillar ATPase activity were seen in skeletal muscle (P greater than or equal to 0.05), however, hexokinase activity progressively increased and was significantly elevated in the 3T, 5T and 10T soleus and plantaris muscles of rats over controls (P less than or equal to 0.05). 5. Minimal nonsignificant changes were noted in the hexokinase activity of the hearts of all T groups (P greater than or equal to 0.05). 6. These results indicate that metabolic adaptation of the heart and skeletal muscles takes place after as little as three training sessions. 7. Although the adaptation of the skeletal muscles continually progresses, the adaptation of the heart appears to be transitory.(ABSTRACT TRUNCATED AT 250 WORDS)     

Slow myosin heavy chain expression in the absence of muscle activity

Innervation has been generally accepted to be a major factor involved in both triggering and maintaining the expression of slow myosin heavy chain (MHC-1) in skeletal muscle. However, previous findings from our laboratory have suggested that, in the mouse, this is not always the case (30). Based on these results, we hypothesized that neurotomy would not markedly reduced the expression of MHC-1 protein in the mouse soleus muscles. In addition, other cellular, biochemical, and functional parameters were also studied in these denervated soleus muscles to complete our study. Our results show that denervation reduced neither the relative amount of MHC-1 protein, nor the percentage of muscle fibers expressing MHC-1 protein (P > 0.05). The fact that MHC-1 protein did not respond to muscle inactivity was confirmed in three different mouse strains (129/SV, C57BL/6, and CD1). In contrast, all of the other histological, biochemical, and functional muscle parameters were markedly altered by denervation. Cross-sectional area (CSA) of muscle fibers, maximal tetanic isometric force, maximal velocity of shortening, maximal power, and citrate synthase activity were all reduced in denervated muscles compared with innervated muscles (P < 0.05). Contraction and one-half relaxation times of the twitch were also increased by denervation (P < 0.05). Addition of tenotomy to denervation had no further effect on the relative expression of MHC-1 protein (P > 0.05), despite a greater reduction in CSA and citrate synthase activity (P < 0.05). In conclusion, a deficit in neural input leads to marked atrophy and reduction in performance in mouse soleus muscles. However, the maintenance of the relative expression of slow MHC protein is independent of neuromuscular activity in mice.     

Adaptations in metabolic capacity of rat soleus after paralysis.

To determine whether long-term reductions in neuromuscular activity result in alterations in metabolic capacity, the activities of oxidative, i.e., succinate dehydrogenase (SDH) and citrate synthase (CS), and glycolytic, i.e., alpha-glycerophosphate dehydrogenase (GPD), enzyme markers were quantified in rat soleus muscles 1, 3, and 6 mo after a complete spinal cord transection (ST). In addition, the proportional content of lactate dehydrogenase (LDH) isozymes was used as a marker for oxidative and glycolytic capacities. The myosin heavy chain (MHC) isoform content of a fiber served as a marker of phenotype. In general, MHC isoforms shifted from MHC1 toward MHC2, particularly MHC2x, after ST. Mean SDH and CS activities were higher in ST than control at all time points. The elevated SDH and CS activities were indicative of an enhanced oxidative capacity. GPD activities were higher in ST than control rats at all time points. The increase in activity of SDH was larger than GPD. Thus the GPD-to-SDH (glycolytic-to-oxidative) ratio was decreased after ST. Compared with controls, total LDH activity increased transiently, and the LDH isozyme profile shifted from LDH-1 toward LDH-5, indicative of an enhanced glycolytic capacity. Combined, these results indicate that 1) the metabolic capacities of soleus fibers were not compromised, but the interrelationships among oxidative and glycolytic capacity and MHC content were apparently dissociated after ST; 2) enhancements in oxidative and glycolytic enzyme activities are not mutually exclusive; and 3) chronic reductions in skeletal muscle activity do not necessarily result in a reduced oxidative capacity.     

Changes in muscle proteins and spermidine content in response to unloading and clenbuterol treatment

Anabolic agents such clenbuterol (Cb) are useful tools for probing the mechanisms by which muscles respond to disuse. Cb was examined under different loading conditions with respect to its effects on muscle mass, protein (myofibrillar and cytosolic), and spermidine content in mature male rats. Compared with control treatment, Cb significantly increased loaded and unloaded soleus, plantaris, and extensor digitorum longus (EDL) mass. Likewise, Cb significantly increased loaded and unloaded soleus (24.8 and 21.6%, respectively), plantaris (12.1 and 22.9%, respectively), and EDL (22.4 and 13.3%, respectively) myofibrillar protein content. After unloading, cytosolic proteins significantly increased in the EDL but decreased in the soleus and plantaris. Cb significantly increased cytosolic protein levels in all loaded muscles, while only causing increases in unloaded soleus. When compared with controls, unloading caused significant reductions in spermidine levels in the soleus (40.4%) and plantaris (35.9%) but caused increases in the EDL (54.8%). In contrast, Cb increased spermidine levels in unloaded soleus (42.9%), plantaris (102.8%), and EDL (287%). In loaded muscles, Cb increased spermidine levels in all three muscles, but to a lesser degree than under unloading conditions. Nonlinear regression analyses indicated that the plantaris behaves like a slow-twitch muscle under unloading conditions and like a fast-twitch muscle when loaded. This suggests that the responses of these muscles to unloading and (or) Cb treatment might be influenced by factors beyond fiber type alone.     

Antibody-directed myostatin inhibition enhances muscle mass and function in tumor-bearing mice

Cancer cachexia describes the progressive skeletal muscle wasting and weakness in many cancer patients and accounts for >20% of cancer-related deaths. We tested the hypothesis that antibody-directed myostatin inhibition would attenuate the atrophy and loss of function in muscles of tumor-bearing mice. Twelve-week-old C57BL/6 mice received a subcutaneous injection of saline (control) or Lewis lung carcinoma (LLC) tumor cells. One week later, mice received either once weekly injections of saline (control, n = 12; LLC, n = 9) or a mouse chimera of anti-human myostatin antibody (PF-354, 10 mg·kg?1·wk?1, LLC+PF-354, n = 11) for 5 wk. Injection of LLC cells reduced muscle mass and maximum force of tibialis anterior (TA) muscles by 8-10% (P < 0.05), but the muscle atrophy and weakness were prevented with PF-354 treatment (P > 0.05). Maximum specific (normalized) force of diaphragm muscle strips was reduced with LLC injection (P < 0.05) but was not improved with PF-354 treatment (P > 0.05). PF-354 enhanced activity of oxidative enzymes in TA and diaphragm muscles of tumor-bearing mice by 118% and 89%, respectively (P < 0.05). Compared with controls, apoptosis that was not of myofibrillar or satellite cell origin was 140% higher in TA muscle cross sections from saline-treated LLC tumor-bearing mice (P < 0.05) but was not different in PF-354-treated tumor-bearing mice (P > 0.05). Antibody-directed myostatin inhibition attenuated the skeletal muscle atrophy and loss of muscle force-producing capacity in a murine model of cancer cachexia, in part by reducing apoptosis. The improvements in limb muscle mass and function highlight the therapeutic potential of antibody-directed myostatin inhibition for cancer cachexia.     

Human vastus lateralis and soleus muscles display divergent cellular contractile properties

The purpose of this study was to investigate potential differences in single-fiber contractile physiology of fibers with the same myosin heavy chain isoform (MHC I and MHC IIa) originating from different muscles. Vastus lateralis (VL) and soleus biopsies were obtained from 27 recreationally active females (31 +/- 1 yr, 59 +/- 1 kg). A total of 943 single fibers (MHC I = 562; MHC IIa = 301) were isolated and examined for diameter, peak tension (Po), shortening velocity (Vo), and power. The soleus had larger (P < 0.05) fibers (MHC I +18%; MHC IIa +19%), higher MHC I Vo (+13%), and higher MHC I Po (+18%) compared with fibers from the VL. In contrast, fibers from the VL had higher (P < 0.05) specific tension (MHC I +18%; MHC IIa +20%), and MHC I normalized power (+25%) compared with the soleus. There was a trend for MHC IIa soleus fibers to have higher Vo [MHC IIa +13% (P = 0.058)], whereas VL MHC IIa fibers showed a trend for higher normalized power compared with soleus fibers [MHC IIa +33% (P = 0.079)]. No differences in absolute power were detected between muscles. These data highlight muscle-specific differences in single-fiber contractile function that should serve as a scientific basis for consideration when extending observations of skeletal muscle tissue from one muscle of interest to other muscles of origin. This is important when examining skeletal muscle adaptation to physical states such as aging, unloading, and training.     

Exercise training alleviates MCT1 and MCT4 reductions in heart and skeletal muscles of STZ-induced diabetic rats.

We compared the changes in monocarboxylate transporter 1 (MCT1) and 4 (MCT4) proteins in heart and skeletal muscles in sedentary control and streptozotocin (STZ)-induced diabetic rats (3 wk) and in trained (3 wk) control and STZ-induced diabetic animals. In nondiabetic animals, training increased MCT1 in the plantaris (+51%; P < 0.01) but not in the soleus (+9%) or the heart (+14%). MCT4 was increased in the plantaris (+48%; P < 0.01) but not in the soleus muscles of trained nondiabetic animals. In sedentary diabetic animals, MCT1 was reduced in the heart (-30%), and in the plantaris (-31%; P < 0.01) and soleus (-26%) muscles. MCT4 content was also reduced in sedentary diabetic animals in the plantaris (-52%; P < 0.01) and soleus (-25%) muscles. In contrast, in trained diabetic animals, MCT1 and MCT4 in heart and/or muscle were similar to those of sedentary, nondiabetic animals (P > 0.05) but were markedly greater than in the sedentary diabetic animals [MCT1: plantaris +63%, soleus +51%, heart +51% (P > 0.05); MCT4: plantaris +107%, soleus +17% (P > 0.05)]. These studies have shown that 1) with STZ-induced diabetes, MCT1 and MCT4 are reduced in skeletal muscle and/or the heart and 2) exercise training alleviated these diabetes-induced reductions.     

Alteration of contractile force and mass in the senescent diaphragm with beta(2)-agonist treatment.

Aging is associated with a decrease in diaphragmatic maximal tetanic force production (P(o)) in senescent rats. Treatment with the beta(2)-agonist clenbuterol (CB) has been shown to increase skeletal muscle mass and P(o) in weak locomotor skeletal muscles from dystrophic rodents. It is unknown whether CB can increase diaphragmatic mass and P(o) in senescent rats. Therefore, we tested the hypothesis that CB treatment will increase specific P(o) (i.e., force per cross-sectional area) and mass in the diaphragm of old rats. Young (5 mo) and old (23 mo) male Fischer 344 rats were randomly assigned to one of the following groups (n = 10/group): 1) young CB treated; 2) young control; 3) old CB treated; and 4) old control. Animals were injected daily with either CB (2 mg/kg) or saline for 28 days. CB increased (P < 0.05) the mass of the costal diaphragm in both young and old animals. CB treatment increased diaphragmatic-specific P(o) in old animals (approximately 15%; P < 0.05) but did not alter (P > 0.05) diaphragmatic-specific P(o) in young animals. Biochemical analysis indicated that the improved maximal specific P(o) in the diaphragm of CB-treated old animals was not due to increased myofibrillar protein concentration. Analysis of the myosin heavy chain (MHC) content of the costal diaphragm revealed a CB-induced increase (P < 0.05) in type IIb MHC and a decrease in type I, IIa, and IIx MHC in both young and old animals. These data support the hypothesis that CB treatment can restore the age-associated decline in both diaphragmatic-specific P(o) and muscle mass.     

Quantitative histochemistry of three mouse hind-limb muscles: the relationship between calcium-stimulated myofibrillar ATPase and succinate dehydrogenase activities

Summary A quantitative modification of Meijer's calcium-lead capture method, for the demonstration of calcium-stimulated myofibrillar ATPase activity at physiological pH, is described. A range of myofibrillar ATPase activities has been found among fast muscle fibres in two mouse hind-limb muscles. The myofibrillar ATPase activity of fast muscle fibres is 1.5–3 times higher than the myofibrillar ATPase activity of slow muscle fibres.Myofibrillar ATPase activities and succinate dehydrogenase activities of individual muscle fibres have been determined in serial sections. Activities of the two enzymes are correlated positively in soleus (fast and slow fibres), and negatively in plantaris (almost all fast) and extensor digitorum longus muscle (all fast). However, this correlation is not significant among the oxidative fibres in the extensor digitorum longus. The fibres of the latter muscle cannot be classified satisfactorily into two sub-types.     

Effects of thyroid hormone receptor gene disruption on myosin isoform expression in mouse skeletal muscles

Skeletal muscle is known to be a target for the active metabolite of thyroid hormone, i.e., 3,5,3'-triiodothyronine (T(3)). T(3) acts by repressing or activating genes coding for different myosin heavy chain (MHC) isoforms via T(3) receptors (TRs). The diverse function of T(3) is presumed to be mediated by TR-alpha(1) and TR-beta, but the function of specific TRs in regulating MHC isoform expression has remained undefined. In this study, TR-deficient mice were used to expand our knowledge of the mechanisms by which T(3) regulates the expression of specific MHC isoforms via distinct TRs. In fast-twitch extensor digitorum longus (EDL) muscle, TR-alpha(1)-, TR-beta-, or TR-alpha(1)beta-deficient mice showed a small but statistically significant decrease (P < 0.05) of type IIB MHC content and an increased number of type I fibers. In the slow-twitch soleus, the beta/slow MHC (type I) isoform was significantly (P < 0. 001) upregulated in the TR-deficient mice, but this effect was highly dependent on the type of receptor deleted. The lack of TR-beta had no significant effect on the expression of MHC isoforms. An increase (P < 0.05) of type I MHC was observed in the TR-alpha(1)-deficient muscle. A dramatic overexpression (P < 0.001) of the slow type I MHC and a corresponding downregulation of the fast type IIA MHC (P < 0.001) was observed in TR-alpha(1)beta-deficient mice. The muscle- and fiber-specific differences in MHC isoform expression in the TR-alpha(1)beta-deficient mice resembled the MHC isoform transitions reported in hypothyroid animals, i.e., a mild MHC transition in the EDL, a dramatic but not complete upregulation of the beta/slow MHC isoform in the soleus, and a variable response to TR deficiency in different soleus muscle fibers. Thus the consequences on muscle are similar in the absence of thyroid hormone or absence of thyroid hormone receptors, indicating that TR-alpha(1) and TR-beta together mediate the known actions of T(3). However, it remains unknown how thyroid hormone exerts muscle- and muscle fiber-specific effects in its action. Finally, although developmental MHC transitions were not studied specifically in this study, the absence of embryonic and fetal MHC isoforms in the TR-deficient mice indicates that ultimately the transition to the adult MHC isoforms is not solely mediated by TRs.     

Thyroid status and endothelium-dependent vasodilation in skeletal muscle

Cardiovascular dysfunction is characteristic of both hypo- and hyperthyroidism. Endothelium-dependent dilation of conductance vessels is impaired in hypothyroidism but augmented in hyperthyroidism. We hypothesized that these alterations in dilation extend into the resistance vasculature of skeletal muscle. To test this hypothesis, rats were made hypothyroid with propylthiouracil (Hypo; n = 13) or hyperthyroid with triiodothyronine (Hyper; n = 9) over 3-4 mo. Compared with euthyroid controls (Eut; n = 14), Hypo rats were characterized by reduced skeletal muscle oxidative capacity and blunted growth; Hyper rats exhibited increased muscle oxidative capacity and left ventricular hypertrophy (P < 0.05 for all effects). Vasodilation to the endothelium-dependent agent acetylcholine ( approximately 2 x 10(-4) M) in skeletal muscle was determined in situ. Conductance in certain muscles increased from control [e.g., soleus: 0.98 +/- 0.15 (Eut), 0.79 +/- 0.14 (Hypo), and 1.06 +/- 0.24 ml.min(-1).100 g(-1).mmHg(-1) (Hyper); not significant among groups] to acetylcholine [1.91 +/- 0.21 (Eut), 2.28 +/- 0.26 (Hypo), and 2.15 +/- 0.33 ml.min(-1).100 g(-1).mmHg(-1) (Hyper); P < 0.05 vs. control values for all groups] but did not differ among groups. Expression of mRNA for the endothelial isoform of nitric oxide synthase in resistance vessels isolated from various muscles was similarly unchanged with alterations in thyroid status [e.g., soleus 1A arterioles: 33.15 +/- 0.58 (Eut), 32.73 +/- 0.27 (Hypo), and 32.80 +/- 0.54 (Hyper) cycles at threshold; not significant]. These data suggest that endothelium-dependent dilation of resistance vasculature in skeletal muscle is unchanged in both hypo- and hyperthyroidism. These data also emphasize the importance of examining resistance vasculature to improve understanding of effects of chronic disease on integrated cardiovascular function.     

Atrophy responses to muscle inactivity. I. Cellular markers of protein deficits.

The goal of this study was to use the model of spinal cord isolation (SI), which blocks nearly all neuromuscular activity while leaving the motoneuron muscle-fiber connections intact, to characterize the cellular processes linked to marked muscle atrophy. Rats randomly assigned to normal control and SI groups were studied at 0, 2, 4, 8, and 15 days after SI surgery. The slow soleus muscle atrophied by approximately 50%, with the greatest degree of loss occurring during the first 8 days. Throughout the SI duration, muscle protein concentration was maintained at the control level, whereas myofibrillar protein concentration steadily decreased between 4 and 15 days of SI, and this was associated with a 50% decrease in myosin heavy chain (MHC) normalized to total protein. Actin relative to the total protein was maintained at the control level. Marked reductions occurred in total RNA and DNA content and in total MHC and actin mRNA expressed relative to 18S ribosomal RNA. These findings suggest that two key factors contributing to the muscle atrophy in the SI model are 1). a reduction in ribosomal RNA that is consistent with a reduction in protein translational capacity, and 2). insufficient mRNA substrate for translating key sarcomeric proteins comprising the myofibril fraction, such as MHC and actin. In addition, the marked selective depletion of MHC protein in the muscles of SI rats suggests that this protein is more vulnerable to inactivity than actin protein. This selective MHC loss could be a major contributor for the previously reported loss in the functional integrity of SI muscles. Collectively, these data are consistent with the involvement of pretranslational and translational processes in muscle atrophy due to SI.