Lesion level-dependent systemic muscle wasting after spinal cord injury is mediated by glucocorticoid signaling in mice

An incomplete mechanistic understanding of skeletal muscle wasting early after spinal cord injury (SCI) precludes targeted molecular interventions. Here, we demonstrated systemic wasting that also affected innervated nonparalyzed (supralesional) muscles and emerged within 1 week after experimental SCI in mice. Systemic muscle wasting caused muscle weakness, affected fast type 2 myofibers preferentially, and became exacerbated after high (T3) compared with low (T9) thoracic paraplegia, indicating lesion level-dependent ("neurogenic") mechanisms. The wasting of nonparalyzed muscle and its rapid onset and severity beyond what can be explained by disuse implied unknown systemic drivers. Muscle transcriptome and biochemical analysis revealed a glucocorticoid-mediated catabolic signature early after T3 SCI. SCI-induced systemic muscle wasting was mitigated by (i) endogenous glucocorticoid ablation (adrenalectomy) and (ii) pharmacological glucocorticoid receptor (GR) blockade and was (iii) completely prevented after T3 relative to T9 SCI by genetic muscle-specific GR deletion. These results suggest that neurogenic hypercortisolism contributes to a rapid systemic and functionally relevant muscle wasting syndrome early after paraplegic SCI in mice.

Targeting Troponin C with Small Molecules Containing Diphenyl Moieties: Calcium Sensitivity Effects on Striated Muscles and Structure-Activity Relationship

Despite large investments from academia and industry, heart failure, which results from a disruption of the contractile apparatus, remains a leading cause of death. Cardiac muscle contraction is a calcium-dependent mechanism, which is regulated by the troponin protein complex (cTn) and specifically by the N-terminal domain of its calcium-binding subunit (cNTnC). There is an increasing need for the development of small molecules that increase calcium sensitivity without altering the systolic calcium concentration, thereby strengthening the cardiac function. Here, we examined the effect of our previously identified calcium-sensitizing small molecule, ChemBridge compound 7930079, in the context of several homologous muscle systems. The effect of this molecule on force generation in isolated cardiac trabeculae and slow skeletal muscle fibers was measured. Furthermore, we explored the use of Gaussian accelerated molecular dynamics in sampling highly predictive receptor conformations based on NMR-derived starting structures. Additionally, we took a rational computational approach for lead optimization based on lipophilic diphenyl moieties. This integrated structural-biochemical-physiological approach led to the identification of three novel low-affinity binders, which had similar binding affinities to the known positive inotrope trifluoperazine. The most potent identified calcium sensitizer was compound 16 with an apparent affinity of 117 ± 17 μM.

Association between skeletal muscle mitochondrial dysfunction and insulin resistance in patients with rheumatoid arthritis: a case-control study

BACKGROUND

Insulin resistance affects a substantial proportion of patients with rheumatoid arthritis (RA). Skeletal muscle mitochondrial dysfunction results in the accumulation of lipid intermediates that interfere with insulin signaling. We therefore sought to determine if lower oxidative phosphorylation and muscle mitochondrial content are associated with insulin resistance in patients with RA.

METHODS

This was a cross-sectional prospective study of RA patients. Matsuda index from the glucose tolerance test was used to estimate insulin sensitivity. Mitochondrial content was measured by citrate synthase (CS) activity in snap-frozen muscle samples. Mitochondrial function was measured by using high-resolution respirometry of permeabilized muscle fibers and electron transport chain complex IV enzyme kinetics in isolated mitochondrial subpopulations.

RESULTS

RA participants demonstrated lower insulin sensitivity as measured by the Matsuda index compared to controls [median 3.95 IQR (2.33, 5.64) vs. 7.17 (5.83, 7.75), p = 0.02]. There was lower muscle mitochondrial content among RA vs. controls [median 60 mU/mg IQR (45, 80) vs. 79 mU/mg (65, 97), p = 0.03]. Notably, OxPhos normalized to mitochondrial content was higher among RA vs. controls [mean difference (95% CI) = 0.14 (0.02, 0.26), p = 0.03], indicating a possible compensatory mechanism for lower mitochondrial content or lipid overload. Among RA participants, the activity of muscle CS activity was not correlated with the Matsuda index (ρ =  - 0.05, p = 0.84), but it was positively correlated with self-reported (IPAQ) total MET-minutes/week (ρ = 0.44, p = 0.03) and Actigraph-measured time on physical activity (MET rate) (ρ = 0.47, p = 0.03).

CONCLUSIONS

Mitochondrial content and function were not associated with insulin sensitivity among participants with RA. However, our study demonstrates a significant association between muscle mitochondrial content and physical activity level, highlighting the potential for future exercise interventions that enhance mitochondrial efficiency in RA patients.

Targeting Troponin C with Small Molecules Containing Diphenyl Moieties: Calcium Sensitivity Effects on Striated Muscle and Structure Activity Relationship

Despite large investments from academia and industry, heart failure, which results from a disruption of the contractile apparatus, remains a leading cause of death. Cardiac muscle contraction is a calcium-dependent mechanism, which is regulated by the troponin protein complex (cTn) and specifically by the N-terminal domain of its calcium binding subunit (cNTnC). There is an increasing need for the development of small molecules that increase calcium sensitivity without altering systolic calcium concentration, thereby strengthening cardiac function. Here, we examined the effect of our previously identified calcium sensitizing small molecule, ChemBridge compound 7930079, in the context of several homologous muscle systems. The effect of this molecule on force generation in isolated cardiac trabeculae and slow skeletal muscle fibers was measured. Furthermore, we explored the use of Gaussian accelerated molecular dynamics in sampling highly predictive receptor conformations based on NMR derived starting structures. Additionally, we took a rational computational approach for lead optimization based on lipophilic diphenyl moieties. This led to the identification of three novel low affinity binders, which had similar binding affinities to known positive inotrope trifluoperazine. The most potent identified calcium sensitizer was compound 16 with an apparent affinity of 117 ± 17 μM .

Small Molecule RPI-194 Stabilizes Activated Troponin to Increase the Calcium Sensitivity of Striated Muscle Contraction

Small molecule cardiac troponin activators could potentially enhance cardiac muscle contraction in the treatment of systolic heart failure. We designed a small molecule, RPI-194, to bind cardiac/slow skeletal muscle troponin (Cardiac muscle and slow skeletal muscle share a common isoform of the troponin C subunit.) Using solution NMR and stopped flow fluorescence spectroscopy, we determined that RPI-194 binds to cardiac troponin with a dissociation constant K_D of 6-24 μM, stabilizing the activated complex between troponin C and the switch region of troponin I. The interaction between RPI-194 and troponin C is weak (K_D 311 μM) in the absence of the switch region. RPI-194 acts as a calcium sensitizer, shifting the pCa₅₀ of isometric contraction from 6.28 to 6.99 in mouse slow skeletal muscle fibers and from 5.68 to 5.96 in skinned cardiac trabeculae at 100 μM concentration. There is also some cross-reactivity with fast skeletal muscle fibers (pCa₅₀ increases from 6.27 to 6.52). In the slack test performed on the same skinned skeletal muscle fibers, RPI-194 slowed the velocity of unloaded shortening at saturating calcium concentrations, suggesting that it slows the rate of actin-myosin cross-bridge cycling under these conditions. However, RPI-194 had no effect on the ATPase activity of purified actin-myosin. In isolated unloaded mouse cardiomyocytes, RPI-194 markedly decreased the velocity and amplitude of contractions. In contrast, cardiac function was preserved in mouse isolated perfused working hearts. In summary, the novel troponin activator RPI-194 acts as a calcium sensitizer in all striated muscle types. Surprisingly, it also slows the velocity of unloaded contraction, but the cause and significance of this is uncertain at this time. RPI-194 represents a new class of non-specific troponin activator that could potentially be used either to enhance cardiac muscle contractility in the setting of systolic heart failure or to enhance skeletal muscle contraction in neuromuscular disorders.

Modeling Human Cancer-induced Cachexia

Cachexia is a wasting syndrome characterized by pronounced skeletal muscle loss. In cancer, cachexia is associated with increased morbidity and mortality and decreased treatment tolerance. Although advances have been made in understanding the mechanisms of cachexia, translating these advances to the clinic has been challenging. One reason for this shortcoming may be the current animal models, which fail to fully recapitulate the etiology of human cancer-induced tissue wasting. Because pancreatic ductal adenocarcinoma (PDA) presents with a high incidence of cachexia, we engineered a mouse model of PDA that we named KPP. KPP mice, similar to PDA patients, progressively lose skeletal and adipose mass as a consequence of their tumors. In addition, KPP muscles exhibit a similar gene ontology as cachectic patients. We envision that the KPP model will be a useful resource for advancing our mechanistic understanding and ability to treat cancer cachexia.

Current understanding of conventional and novel co-expression patterns of mammalian sarcomeric myosin heavy chains and light chains

A central tenet of muscle physiology that has accrued from several decades of intense investigations is that myosin, and the vast set of isoforms that constitute its six subunits, is a major regulator of contractile properties of smooth, cardiac and skeletal muscle. Two frequent questions are (1) how many myosin heavy chain (MyHC) isoforms and myosin light chain (MLC) isoforms are expressed in mammalian striated muscles and (2) which isoforms of MyHC and MLC are expressed, at the protein level, with each other - that is, what patterns of co-expression exist in single striated muscle fibers? The answer to the former question is straightforward: eleven MyHC isoforms and nine MLC isoforms, are expressed in a developmentally-regulated and muscle-specific manner. The answer to the latter question, on the other hand, is not clear-cut. The observed number of MyHC and MLC isoform combinations among single fibers is far less than the total number of potential permutations, indicating strict regulation of expression in individual muscle cells. This article provides a review of the current and still evolving understanding of the complexity of muscle fiber types defined on the basis of expression patterns of MyHC and MLC isoforms that constitute an intact functioning molecule.

3-Chlorodiphenylamine activates cardiac troponin by a mechanism distinct from bepridil or TFP

Cardiac troponin activators could be beneficial in systolic heart failure. Tikunova et al. demonstrate that, unlike previously known calcium sensitizers, the small molecule 3-chlorodiphenylamine does not activate isolated cardiac troponin C but instead activates the intact troponin complex.

Despite extensive efforts spanning multiple decades, the development of highly effective Ca²⁺ sensitizers for the heart remains an elusive goal. Existing Ca²⁺ sensitizers have other targets in addition to cardiac troponin (cTn), which can lead to adverse side effects, such as hypotension or arrhythmias. Thus, there is a need to design Ca²⁺-sensitizing drugs with higher affinity and selectivity for cTn. Previously, we determined that many compounds based on diphenylamine (DPA) were able to bind to a cTnC–cTnI chimera with moderate affinity (K_d ∼10–120 µM). Of these compounds, 3-chlorodiphenylamine (3-Cl-DPA) bound most tightly (K_d of 10 µM). Here, we investigate 3-Cl-DPA further and find that it increases the Ca²⁺ sensitivity of force development in skinned cardiac muscle. Using NMR, we show that, like the known Ca²⁺ sensitizers, trifluoperazine (TFP) and bepridil, 3-Cl-DPA is able to bind to the isolated N-terminal domain (N-domain) of cTnC (K_d of 6 µM). However, while the bulky molecules of TFP and bepridil stabilize the open state of the N-domain of cTnC, the small and flexible 3-Cl-DPA molecule is able to bind without stabilizing this open state. Thus, unlike TFP, which drastically slows the rate of Ca²⁺ dissociation from the N-domain of isolated cTnC in a dose-dependent manner, 3-Cl-DPA has no effect on the rate of Ca²⁺ dissociation. On the other hand, the affinity of 3-Cl-DPA for a cTnC–TnI chimera is at least an order of magnitude higher than that of TFP or bepridil, likely because 3-Cl-DPA is less disruptive of cTnI binding to cTnC. Therefore, 3-Cl-DPA has a bigger effect on the rate of Ca²⁺ dissociation from the entire cTn complex than TFP and bepridil. Our data suggest that 3-Cl-DPA activates the cTn complex via a unique mechanism and could be a suitable scaffold for the development of novel treatments for systolic heart failure.

Desensitizing mouse cardiac troponin C to calcium converts slow muscle towards a fast muscle phenotype

KEY POINTS

The Ca2+ -desensitizing D73N mutation in slow skeletal/cardiac troponin C caused dilatated cardiomyopathy in mice, but the consequences of this mutation in skeletal muscle were not known. The D73N mutation led to a rightward shift in the force versus pCa (-log [Ca]) relationship in slow-twitch mouse fibres. The D73N mutation led to a rightward shift in the force-stimulation frequency relationship and reduced fatigue resistance of mouse soleus muscle. The D73N mutation led to reduced cross-sectional area of slow-twitch fibres in mouse soleus muscle without affecting fibre type composition of the muscle. The D73N mutation resulted in significantly shorter times to peak force and to relaxation during isometric twitches and tetani in mouse soleus muscle. The D73N mutation led to major changes in physiological properties of mouse soleus muscle, converting slow muscle toward a fast muscle phenotype.

ABSTRACT

The missense mutation, D73N, in mouse cardiac troponin C has a profound impact on cardiac function, mediated by a decreased myofilament Ca2+ sensitivity. Mammalian cardiac muscle and slow skeletal muscle normally share expression of the same troponin C isoform. Therefore, the objective of this study was to determine the consequences of the D73N mutation in skeletal muscle, as a potential mechanism that contributes to the morbidity associated with heart failure or other conditions in which Ca2+ sensitivity might be altered. Effects of the D73N mutation on physiological properties of mouse soleus muscle, in which slow-twitch fibres are prevalent, were examined. The mutation resulted in a rightward shift of the force-stimulation frequency relationship, and significantly faster kinetics of isometric twitches and tetani in isolated soleus muscle. Furthermore, soleus muscles from D73N mice underwent a significantly greater reduction in force during a fatigue test. The mutation significantly reduced slow fibre mean cross-sectional area without affecting soleus fibre type composition. The effects of the mutation on Ca2+ sensitivity of force development in soleus skinned slow and fast fibres were also examined. As expected, the D73N mutation did not affect the Ca2+ sensitivity of force development in fast fibres but resulted in substantially decreased Ca2+ sensitivity in slow fibres. The results demonstrate that a point mutation in a single constituent of myofilaments (slow/cardiac troponin C) led to major changes in physiological properties of skeletal muscle and converted slow muscle toward a fast muscle phenotype with reduced fatigue resistance and Ca2+ sensitivity of force generation.

Chimpanzee super strength and human skeletal muscle evolution

Since at least the 1920s, it has been reported that common chimpanzees (Pan troglodytes) differ from humans in being capable of exceptional feats of "super strength," both in the wild and in captive environments. A mix of anecdotal and more controlled studies provides some support for this view; however, a critical review of available data suggests that chimpanzee mass-specific muscular performance is a more modest 1.5 times greater than humans on average. Hypotheses for the muscular basis of this performance differential have included greater isometric force-generating capabilities, faster maximum shortening velocities, and/or a difference in myosin heavy chain (MHC) isoform content in chimpanzee relative to human skeletal muscle. Here, we show that chimpanzee muscle is similar to human muscle in its single-fiber contractile properties, but exhibits a much higher fraction of MHC II isoforms. Unlike humans, chimpanzee muscle is composed of ∼67% fast-twitch fibers (MHC IIa+IId). Computer simulations of species-specific whole-muscle models indicate that maximum dynamic force and power output is 1.35 times higher in a chimpanzee muscle than a human muscle of similar size. Thus, the superior mass-specific muscular performance of chimpanzees does not stem from differences in isometric force-generating capabilities or maximum shortening velocities-as has long been suggested-but rather is due in part to differences in MHC isoform content and fiber length. We propose that the hominin lineage experienced a decline in maximum dynamic force and power output during the past 7-8 million years in response to selection for repetitive, low-cost contractile behavior.

Increased hypoxia-inducible factor-1α in striated muscle of tumor-bearing mice

Cancer cachexia is a progressive wasting disease resulting in significant effects on the quality of life and high mortality. Most studies on cancer cachexia have focused on skeletal muscle; however, the heart is now recognized as a major site of cachexia-related effects. To elucidate possible mechanisms, a proteomic study was performed on the left ventricles of colon-26 (C26) adenocarcinoma tumor-bearing mice. The results revealed several changes in proteins involved in metabolism. An integrated pathway analysis of the results revealed a common mediator in hypoxia-inducible factor-1α (HIF-1α). Work by other laboratories has shown that extensive metabolic restructuring in the C26 mouse model causes changes in gene expression that may be affected directly by HIF-1α, such as glucose metabolic genes. M-mode echocardiography showed progressive decline in heart function by day 19, exhibited by significantly decreased ejection fraction and fractional shortening, along with posterior wall thickness. Using Western blot analysis, we confirmed that HIF-1α is significantly upregulated in the heart, whereas there were no changes in its regulatory proteins, prolyl hydroxylase domain-containing protein 2 (PHD2) and von Hippel-Lindau protein (VHL). PHD2 requires both oxygen and iron as cofactors for the hydroxylation of HIF-1α, marking it for ubiquination via VHL and subsequent destruction by the proteasome complex. We examined venous blood gas values in the tumor-bearing mice and found significantly lower oxygen concentration compared with control animals in the third week after tumor inoculation. We also examined select skeletal muscles to determine whether they are similarly affected. In the diaphragm, extensor digitorum longus, and soleus, we found significantly increased HIF-1α in tumor-bearing mice, indicating a hypoxic response, not only in the heart, but also in skeletal muscle. These results indicate that HIF-1α may contribute, in part, to the metabolic changes that occur during cancer cachexia.NEW & NOTEWORTHY We used proteomics and metadata analysis software to identify contributors to metabolic changes in striated muscle during cancer cachexia. We found increased expression of hypoxia-inducible factor-1α in the heart and skeletal muscle, suggesting a potential target for the therapeutic treatment of cancer cachexia.

Functional Segregation within the Muscles of Aquatic Propulsion in the Asiatic Water Monitor (Varanus salvator)

Water monitor lizards (Varanus salvator) swim using sinusoidal oscillations generated at the base of their long (50% of total body length) tail. In an effort to determine which level of the structural/organizational hierarchy of muscle is associated with functional segregation between the muscles of the tail base, an array of muscle features-myosin heavy chain profiles, enzymatic fiber types, twitch and tetanic force production, rates of fatigue, muscle compliance, and electrical activity patterns-were quantitated. The two examined axial muscles, longissimus, and iliocaudalis, were generally similar at the molecular, biochemical, and physiological levels, but differed at the biomechanics level and in their activation pattern. The appendicular muscle examined, caudofemoralis, differed from the axial muscles particularly at the molecular and physiological levels, and it exhibited a unique compliance profile and pattern of electrical activation. There were some apparent contradictions between the different structural/organizational levels examined. These contradictions, coupled with a unique myosin heavy chain profile, lead to the hypothesis that there are previously un-described molecular/biochemical specializations within varanid skeletal muscles.

Nucleotide and protein sequences for dog masticatory tropomyosin identify a novel Tpm4 gene product

Jaw-closing muscles of several vertebrate species, including members of Carnivora, express a unique, "masticatory", isoform of myosin heavy chain, along with isoforms of other myofibrillar proteins that are not expressed in most other muscles. It is generally believed that the complement of myofibrillar isoforms in these muscles serves high force generation for capturing live prey, breaking down tough plant material and defensive biting. A unique isoform of tropomyosin (Tpm) was reported to be expressed in cat jaw-closing muscle, based upon two-dimensional gel mobility, peptide mapping, and immunohistochemistry. The objective of this study was to obtain protein and gene sequence information for this unique Tpm isoform. Samples of masseter (a jaw-closing muscle), tibialis (predominantly fast-twitch fibers), and the deep lateral gastrocnemius (predominantly slow-twitch fibers) were obtained from adult dogs. Expressed Tpm isoforms were cloned and sequencing yielded cDNAs that were identical to genomic predicted striated muscle Tpm1.1St(a,b,b,a) (historically referred to as αTpm), Tpm2.2St(a,b,b,a) (βTpm) and Tpm3.12St(a,b,b,a) (γTpm) isoforms (nomenclature reflects predominant tissue expression ("St"-striated muscle) and exon splicing pattern), as well as a novel 284 amino acid isoform observed in jaw-closing muscle that is identical to a genomic predicted product of the Tpm4 gene (δTpm) family. The novel isoform is designated as Tpm4.3St(a,b,b,a). The myofibrillar Tpm isoform expressed in dog masseter exhibits a unique electrophoretic mobility on gels containing 6 M urea, compared to other skeletal Tpm isoforms. To validate that the cloned Tpm4.3 isoform is the Tpm expressed in dog masseter, E. coli-expressed Tpm4.3 was electrophoresed in the presence of urea. Results demonstrate that Tpm4.3 has identical electrophoretic mobility to the unique dog masseter Tpm isoform and is of different mobility from that of muscle Tpm1.1, Tpm2.2 and Tpm3.12 isoforms. We conclude that the unique Tpm isoform in dog masseter is a product of the Tpm4 gene and that the 284 amino acid protein product of this gene represents a novel myofibrillar Tpm isoform never before observed to be expressed in striated muscle.

Knock-in mice harboring a Ca(2+) desensitizing mutation in cardiac troponin C develop early onset dilated cardiomyopathy

The physiological consequences of aberrant Ca²⁺ binding and exchange with cardiac myofilaments are not clearly understood. In order to examine the effect of decreasing Ca²⁺ sensitivity of cTnC on cardiac function, we generated knock-in mice carrying a D73N mutation (not known to be associated with heart disease in human patients) in cTnC. The D73N mutation was engineered into the regulatory N-domain of cTnC in order to reduce Ca²⁺ sensitivity of reconstituted thin filaments by increasing the rate of Ca²⁺ dissociation. In addition, the D73N mutation drastically blunted the extent of Ca²⁺ desensitization of reconstituted thin filaments induced by cTnI pseudo-phosphorylation. Compared to wild-type mice, heterozygous knock-in mice carrying the D73N mutation exhibited a substantially decreased Ca²⁺ sensitivity of force development in skinned ventricular trabeculae. Kaplan-Meier survival analysis revealed that median survival time for knock-in mice was 12 weeks. Echocardiographic analysis revealed that knock-in mice exhibited increased left ventricular dimensions with thinner walls. Echocardiographic analysis also revealed that measures of systolic function, such as ejection fraction (EF) and fractional shortening (FS), were dramatically reduced in knock-in mice. In addition, knock-in mice displayed electrophysiological abnormalities, namely prolonged QRS and QT intervals. Furthermore, ventricular myocytes isolated from knock-in mice did not respond to β-adrenergic stimulation. Thus, knock-in mice developed pathological features similar to those observed in human patients with dilated cardiomyopathy (DCM). In conclusion, our results suggest that decreasing Ca²⁺ sensitivity of the regulatory N-domain of cTnC is sufficient to trigger the development of DCM.

Metalloproteinase expression is altered in cardiac and skeletal muscle in cancer cachexia

Cardiac and skeletal muscle dysfunction is a recognized effect of cancer-induced cachexia, with alterations in heart function leading to heart failure and negatively impacting patient morbidity. Cachexia is a complex and multifaceted disease state with several potential contributors to cardiac and skeletal muscle dysfunction. Matrix metalloproteinases (MMPs) are a family of enzymes capable of degrading components of the extracellular matrix (ECM). Changes to the ECM cause disruption both in the connections between cells at the basement membrane and in cell-to-cell interactions. In the present study, we used a murine model of C26 adenocarcinoma-induced cancer cachexia to determine changes in MMP gene and protein expression in cardiac and skeletal muscle. We analyzed MMP-2, MMP-3, MMP-9, and MMP-14 as they have been shown to contribute to both cardiac and skeletal muscle ECM changes and, thereby, to pathology in models of heart failure and muscular dystrophy. In our model, cardiac and skeletal muscles showed a significant increase in RNA and protein levels of several MMPs and tissue inhibitors of metalloproteinases. Cardiac muscle showed significant protein increases in MMP-2, MMP-3, MMP-9, and MMP-14, whereas skeletal muscles showed increases in MMP-2, MMP-3, and MMP-14. Furthermore, collagen deposition was increased after C26 adenocarcinoma-induced cancer cachexia as indicated by an increased left ventricular picrosirius red-positive-stained area. Increases in serum hydroxyproline suggest increased collagen turnover, implicating skeletal muscle remodeling. Our findings demonstrate that cancer cachexia-associated matrix remodeling results in cardiac fibrosis and possible skeletal muscle remodeling. With these findings, MMPs represent a possible therapeutic target for the treatment of cancer-induced cachexia.

Fluoxetine prevents the development of depressive-like behavior in a mouse model of cancer related fatigue

Cancer patients frequently suffer from fatigue, a complex syndrome associated with tiredness and depressed mood. Cancer-related fatigue (CRF) can be present at the time of diagnosis, escalates during treatment, and can persist for years after treatment. CRF negatively influences quality of life, limits functional independence, and is associated with decreased survival in patients with incurable disease. We have previously shown that increased pro-inflammatory cytokine expression in the brain contributes to depressive- and fatigue-like behaviors in a mouse model of CRF. Inflammatory cytokines increase the activity of indoleamine 2,3-dioxygenase (IDO) and kynurenine 3-monooxygenase (KMO), which competitively reduce serotonin synthesis. Reduced serotonin availability in the brain and increased production of alternative neuroactive metabolites of tryptophan are thought to contribute to the development of depression and fatigue. The purpose of this study was to determine the effects of fluoxetine, a selective serotonin reuptake inhibitor (SSRI), on brain cytokines and behavioral measures of fatigue and depression in tumor-bearing mice. Here we show that tumor growth increased brain expression of pro-inflammatory cytokines and KMO. Treatment with fluoxetine had no effect on tumor growth, muscle wasting, fatigue behavior, or cytokine expression in the brain. Fluoxetine, however, reduced depressive-like behaviors in tumor bearing mice. In conclusion, our data confirm that increased brain expression of pro-inflammatory cytokines is associated with tumor-induced fatigue- and depressive-like behaviors. However, it is possible to separate the effects of tumor growth on mood and fatigue-like behaviors using SSRIs such as fluoxetine.

Sex differences in myosin heavy chain isoforms of human failing and nonfailing atria

Mammalian hearts express two myosin heavy chain (MHC) isoforms, which drive contractions with different kinetics and power-generating ability. The expression of the isoform that is associated with more rapid contraction kinetics and greater power output, MHC-α, is downregulated, with a concurrent increase in the relative amount of the slower isoform, MHC-β, during the progression to experimentally induced or disease-related heart failure. This change in protein expression has been well studied in right and left ventricles in heart failure models and in humans with failure. Relatively little quantitative data exists regarding MHC isoform expression shifts in human failing atria. We previously reported significant increases in the relative amount of MHC-β in the human failing left atrium. The results of that study suggested that there might be a sex-related difference in the level of MHC-β in the left atrium, but the number of female subjects was insufficient for statistical analysis. The objective of this study was to test whether there is, in fact, a sex-related difference in the level of MHC-β in the right and left atria of humans with cardiomyopathy. The results indicate that significant differences exist in atrial MHC isoform expression between men and women who are in failure. The results also revealed an unexpected twofold greater amount of MHC-β in the nonfailing left atrium of women, compared with men. The observed sex-related differences in MHC isoform expression could impact ventricular diastolic filling during normal daily activities, as well as during physiologically stressful events.

Electrophoretic separation of reptilian skeletal and cardiac muscle myosin heavy chain isoforms: dependence on gel format

This report provides a comparison of multiple gel formats to study myosin heavy chain (MHC) isoforms that are expressed in reptilian skeletal and cardiac muscles of five turtle species, water monitor, and prehensile tailed skink. Three gel formats were tested. The results identify one format that is superior, for the overall extent of electrophoretic separation and for the assessment of the number of MHC isoforms in reptilian striated muscles. The same format was shown previously to separate MHC isoforms that are expressed in American alligator. The results also show that another gel format reveals the distinct electrophoretic mobility of MHC isoforms in atrial, ventricular, and jaw adductor samples, compared to those expressed in skeletal muscles in the limbs and elsewhere in the body. In addition, the results reveal that the electrophoretic mobility of specific MHC isoforms, relative to other isoforms, depends on the gel format, as shown previously for mammalian and avian species. The discovery of the expression of masticatory MHC, which is abundantly expressed in jaw adductors of members of Carnivora and several other vertebrate orders, in the homologous muscles of prehensile tailed skink, an herbivore, and the carnivorous water monitor, was made during the course of this study.

Very low force-generating ability and unusually high temperature-dependency in hummingbird flight muscle fibers

Hummingbird flight muscle is estimated to have among the highest mass-specific power output among vertebrates, based on aerodynamic models. However, little is known about fundamental contractile properties of their remarkable flight muscles. We hypothesized that hummingbird pectoralis fibers generate relatively low force when activated in a tradeoff for high shortening speeds associated with the characteristic high wing beat frequencies that are required for sustained hovering. Our objective was to measure maximal force-generating ability (maximal force/cross-sectional area, Po/CSA) in single, skinned fibers from the pectoralis and supracoracoideus muscles, which power the wing downstroke and upstroke, respectively, in hummingbirds (Calypte anna) and in another similarly-sized species, zebra finch (Taeniopygia guttata), which also has a very high wingbeat frequency during flight but does not perform a sustained hover. Mean Po/CSA in hummingbird pectoralis fibers was very low - 1.6, 6.1 and 12.2 kN/m2, at 10, 15 and 20oC, respectively. Po/CSA in finch pectoralis fibers was also very low (for both species, ~5% of the reported Po/CSA of chicken pectoralis fast fibers at 15oC). Force generated at 20oC/force generated at 10oC ('Q10-force' value) was very high for hummingbird and finch pectoralis fibers (mean = 15.3 and 11.5, respectively), compared to rat slow and fast fibers (1.8 and 1.9, respectively). Po/CSA in hummingbird leg fibers was much higher than in pectoralis fibers, at each temperature, and the mean Q10-force was much lower. Thus, hummingbird and finch pectoralis fibers have an extremely low force-generating ability, compared to other bird and mammalian limb fibers, and an extremely high temperature-dependence of force generation. The extrapolated maximum force-generating ability of hummingbird pectoralis fibers in vivo (~48 kN/m2) is, however, substantially higher than the estimated requirements for hovering flight of C. anna. The unusually low Po/CSA of hummingbird and zebra finch pectoralis fibers may reflect a constraint imposed by a need for extremely high contraction frequencies, especially during hummingbird hovering.

Patterns of tropomyosin and troponin-T isoform expression in jaw-closing muscles of mammals and reptiles that express masticatory myosin

We recently reported that masticatory ('superfast') myosin is expressed in jaw-closing muscles of some rodent species. Most mammalian limb muscle fibers express tropomyosin-β (Tm-β), along with fast-type or slow-type tropomyosin-β (Tm-β), but jaw-closing muscle fibers in members of Carnivora express a unique isoform of Tm [Tm-masticatory (Tm-M)] and little or no Tm-β. The goal of this study was to determine patterns of Tm and troponin-T (TnT) isoform expression in the jaw-closing muscles of rodents and other vertebrate species that express masticatory myosin, and compare the results to those from members of Carnivora. Comparisons of electrophoretic mobility, immunoblotting and mass spectrometry were used to probe the Tm and fast-type TnT isoform composition of jaw-closing and limb muscles of six species of Carnivora, eight species of Rodentia, five species of Marsupialia, big brown bat, long-tailed macaque and six species of Reptilia. Extensive heterogeneity exists in Tm and TnT isoform expression in jaw-closing muscles between phylogenetic groups, but there are fairly consistent patterns within each group. We propose that the differences in Tm and TnT isoform expression patterns between phylogenetic groups, which share the expression of masticatory myosin, may impart fundamental differences in thin-filament-mediated muscle activation to accommodate markedly different feeding styles that may require high force generation in some species (e.g. many members of Carnivora) and high speed in others (e.g. Rodentia).

Cardiac alterations in cancer-induced cachexia in mice

Cachexia is a common syndrome in advanced cancer patients and causes up to 22% of cancer-related deaths. It remains elusive whether cancer cachexia causes heart failure. We investigated the effect of cancer cachexia on heart function and cardiac muscle structure in a mouse model. Male CD2F1 mice were inoculated with either colon-26 adenocarcinoma cells (Tumor group) or vehicle (PBS) (No Tumor group and Pair-fed group). Heart function as measured by fractional shortening in vivo using transthoracic echocardiography was performed on day 14 after tumor or PBS inoculation. At necropsy (day 17), hearts were collected for histology, transmission electron microscopy, RT-PCR and SDS-PAGE analysis. Mice from the Tumor group displayed a significantly reduced fractional shortening compared to mice in the No Tumor and Pair-fed groups. In hearts of the Tumor mice compared to the other groups, there was marked fibrosis and transmission electron microscopy revealed disrupted myocardial ultrastructure. Gene expression of troponin I, a regulator of cardiac muscle contraction, was reduced. Moreover, both mRNA and protein levels of myosin heavy chain (MHC) were altered whereby MHCalpha (adult isoform) was decreased and MHCbeta (fetal isoform) was increased indicating reactivation of the fetal gene expression pattern. In conclusion, heart function was diminished in mice with tumor-induced cachexia, and this impaired function was associated with increased fibrosis, disrupted myocardial structure and altered composition of contractile proteins of cardiac muscle.

The myosin light chain 1 isoform associated with masticatory myosin heavy chain in mammals and reptiles is embryonic/atrial MLC1

We recently reported that masticatory myosin heavy chain (MHC-M) is expressed as the exclusive or predominant MHC isoform in masseter and temporalis muscles of several rodent species, contrary to the prevailing dogma that rodents express almost exclusively MHC isoforms that are typically found in fast limb muscles and not masticatory myosin. We also reported that the same rodent species express the embryonic/atrial isoform of myosin light chain 1 (MLC1E/A) in jaw-closing muscles and not a unique masticatory MLC1 isoform that others have reported as being expressed in jaw-closing muscles of carnivores that express MHC-M. The objective of this study was to test the hypothesis that MLC1E/A is consistently expressed in jaw-closing muscles whenever MHC-M is expressed as the predominant or exclusive MHC isoform. Jaw-closing muscles, fast and slow limb muscles, and cardiac atria and ventricles of 19 species (six Carnivora species, one Primates species, one Chiroptera species, five marsupial species, an alligator and five turtle species) were analyzed using protein gel electrophoresis, immunoblotting, mass spectrometry and RNA sequencing. Gel electrophoresis and immunoblotting indicate that MHC-M is the exclusive or predominant MHC isoform in the jaw-closing muscles of each of the studied species. The results from all of the approaches collectively show that MLC1E/A is exclusively or predominantly expressed in jaw-closing muscles of the same species. We conclude that MLC1E/A is the exclusive or predominant MLC1 isoform that is expressed in jaw-closing muscles of vertebrates that express MHC-M, and that a unique masticatory isoform of MLC1 probably does not exist.

Masticatory (;superfast') myosin heavy chain and embryonic/atrial myosin light chain 1 in rodent jaw-closing muscles

Masticatory myosin is widely expressed among several vertebrate classes. Generally, the expression of masticatory myosin has been associated with high bite force for a carnivorous feeding style (including capturing/restraining live prey), breaking down tough plant material and defensive biting in different species. Masticatory myosin expression in the largest mammalian order, Rodentia, has not been reported. Several members of Rodentia consume large numbers of tree nuts that are encased in very hard shells, presumably requiring large forces to access the nutmeat. We, therefore, tested whether some rodent species express masticatory myosin in jaw-closing muscles. Myosin isoform expression in six Sciuridae species was examined, using protein gel electrophoresis, immunoblotting, mass spectrometry and RNA analysis. The results indicate that masticatory myosin is expressed in some Sciuridae species but not in other closely related species with similar diets but having different nut-opening strategies. We also discovered that the myosin light chain 1 isoform associated with masticatory myosin heavy chain, in the same four Sciuridae species, is the embryonic/atrial isoform. We conclude that rodent speciation did not completely eliminate masticatory myosin and that its persistent expression in some rodent species might be related to not only diet but also to feeding style.

Redox modulation of global phosphatase activity and protein phosphorylation in intact skeletal muscle

Skeletal muscles produce transient reactive oxygen species (ROS) in response to intense stimulation, disuse atrophy, heat stress, hypoxia, osmotic stress, stretch and cell receptor activation. The physiological significance is not well understood. Protein phosphatases (PPases) are known to be highly sensitive to oxidants and could contribute to many different signalling responses in muscle. We tested whether broad categories of PPases are inhibited by levels of acute oxidant exposure that do not result in loss of contractile function or gross oxidative stress. We also tested if this exposure results in elevated levels of global protein phosphorylation. Rat diaphragm muscles were treated with either 2,3-dimethoxy-1-naphthoquinone (DMNQ; 1, 10, 100 microm; a mitochondrial O(2)(.-)/H2O2 generator) or exogenous H2O2 (5, 50, 500 microm) for 30 min. Supernatants were assayed for serine/threonine PPase (Ser/Thr-PPase) or protein tyrosine PPase (PTP) activities. With the exception of 500 microm H2O2, no other oxidant exposures significantly elevated protein carbonyl formation, nor did they alter the magnitude of twitch force. DMNQ significantly decreased all categories of PPase activity at 10 and 100 microm and reduced PTP at 1 microm. Similar reductions in Ser/Thr-PPase activity were seen in response to 50 and 500 microm H2O2 and PTP at 500 microm H2O2. ROS treatments resulted a dose-dependent increase in the phosphorylation states of many proteins. The data are consistent with the concept that PPases, within intact skeletal muscles, are highly sensitive to acute changes in ROS activity and that localized ROS play a critical role in lowering the barriers for effective phosphorylation events to occur in muscle cells, thus increasing the probability for cell signalling responses to proceed.

Induction of muscle weakness by local inflammation: an experimental animal model

OBJECTIVE AND DESIGN

The objective of this study was to characterize the response of skeletal muscle to a localized inflammation induced by the inflammatory agent casein.

METHODS

An inflammatory agent, casein, was injected into the right hindlimb and saline was injected into the left hindlimb of normal adult mice, once daily for six consecutive days. Inflammatory response was monitored by immunohistochemical labeling of leukocytes. Muscle protein levels were determined by electrophoresis and muscle function was determined by isometric force measurements.

RESULTS

Local inflammation was induced by casein in association with the accumulation of extensive neutrophils and macrophages in the soleus muscle. This local inflammation resulted in a shift in myosin heavy chain (MHC) isoform expression and a significant reduction in total MHC concentration in the soleus. Maximal twitch and tetanic forces were significantly reduced in the inflamed soleus. Contractile function in soleus was fully restored after two weeks of recovery, along with the restoration of protein concentration and the disappearance of inflammatory cells.

CONCLUSION

This study establishes a unique and robust model in which mechanisms of local inflammation induced muscle protein degradation, reduction of contractile force, and subsequent recovery from this condition can be further studied.

Myosin isoform expression in dog rectus muscles: patterns in global and orbital layers and among single fibers

PURPOSE

To quantitate the distribution of myosin heavy chain (MyHC) isoforms along the global and orbital layers of dog rectus muscles and determine MyHC and myosin light chain (MLC) isoform patterns among single fibers from both layers.

METHODS

Serial samples of both layers of rectus muscles were prepared for gel electrophoresis. Relative amounts of each MyHC isoform in each sample were determined with scanning densitometry. Single fibers were isolated from each layer for analyses of MyHC and MLC isoforms.

RESULTS

Nine MyHC isoforms were detected. Four prominent MyHC isoforms, and an additional MyHC isoform at very low levels, are expressed in the global layer. Evidence suggests that all nine MyHC isoforms are expressed in the orbital layer. There are marked gradients in the levels of some MyHC isoforms along the length of both layers. Complex patterns of coexpression of multiple MyHC isoforms exist in single fibers from both layers. Most fibers express conventional slow or fast MLC isoforms, in accordance with the type (slow or fast) of MyHC isoform(s) in a given fiber, with the exception that slow fibers in the orbital layer express the atrial/embryonic isoform of MLC1.

CONCLUSIONS

MyHC isoform expression patterns differ markedly between and along global and orbital layers of dog rectus muscles, with greater complexity in the orbital layer. Heterogeneity in MyHC isoform expression in rectus muscles is much greater than in limb muscles and presumably is the basis for the broad spectrum of extraocular muscle (EOM) contractile properties in driving oculomotor functions.

Increased myosin heavy chain-beta with atrial expression of ventricular light chain-2 in canine cardiomyopathy

BACKGROUND

Dilated cardiomyopathy is a naturally occurring disease in humans and dogs. Human studies have shown increased levels of myosin heavy chain (MHC)-beta in failing ventricles and the left atria (LA) and of ventricular light chain (VLC)-2 in the right atria in dilated cardiomyopathy.

METHODS AND RESULTS

This study evaluates the levels of MHC-beta in all heart chambers in prolonged canine right ventricular pacing. In addition, we determined whether levels of VLC2 were altered in these hearts. Failing hearts demonstrated significantly increased levels of MHC-beta in the right atria, right atrial appendage, LA, left atrial appendage (LAA), and right ventricle compared with controls. Significant levels of VLC2 were detected in the right atria of paced hearts. Differences in MHC-beta expression were observed between the LA and the LAA of paced and control dogs. MHC-beta expression was significantly greater in the LA of paced and control dogs compared with their respective LAA.

CONCLUSIONS

The cardiac myosin isoform shifts in this study were similar to those observed in end-stage human heart failure and more severe than those reported in less prolonged pacing models, supporting the use of this model for further study of end-stage human heart failure. The observation of consistent differences between sampling sites, especially LA versus LAA, indicates the need for rigorous sampling consistency in future studies.

Variations in apparent mass of mammalian fast-type myosin light chains correlate with species body size, from shrew to elephant

A recent study (Bicer S and Reiser PJ. J Muscle Res Cell Motil 25: 623-633, 2004) suggested considerable variation in the apparent molecular mass (M(a)), deduced from electrophoretic mobility, in fast-type myosin light chains (MLCF), especially MLC1F, among mammalian species. Furthermore, there was an indication that MLC1F M(a) generally correlates with species body mass, over an approximately 4,000-fold range in body mass. The results also suggested that M(a) of other low-molecular-weight myofibrillar proteins is less variable and not as strongly correlated with body mass among the same species. The objective of this study was to test the hypotheses that the M(a) of MLCs does, in fact, vary and correlate with species body mass. The electrophoretic mobilities of MLCF isoforms from 19 species, varying in size approximately 500,000-fold, were quantitated. The results confirm that the M(a) of MLC1F and MLC2F vary significantly among mammals, spanning a very broad range in body mass; the MLC1F M(a) varies more than that of other low-molecular-weight myofibrillar proteins; and there is a significant correlation between species body mass and MLC1F M(a). Differences in MLC1F M(a) among five species can be accounted for by differences in the reported amino acid sequence, especially the length of a common polyalanine region near the NH(2)-terminal actin-binding site. The possibility that the differences in MLC1F sequence among mammalian species, in and adjacent to the actin-binding region, are related to differences in modulation of cross-bridge kinetics in species with diverse locomotion kinetics is discussed.

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.

Three-dimensional compartmentalization of myosin heavy chain and myosin light chain isoforms in dog thyroarytenoid muscle

The thyroarytenoid muscle, a vocal fold adductor, has important roles in airway protection (e.g., prevention of aspiration) and phonation. Isoform expression of myosin heavy chain (MHC), a major determinant of muscle-shortening velocity, has been reported to be heterogeneous in this muscle in several mammals, differing markedly between the medial and lateral divisions. The objective was to determine the isoform expression patterns of both MHC and myosin light chain (MLC), with the latter having a modulatory role in determining shortening velocity, to further test whether the expression of both myosin subunits differs in multiple specific sites within the divisions of the dog thyroarytenoid muscle, potentially revealing even greater compartmentalization in this muscle. Our results indicate the existence of large gradients in the relative levels of individual MHC isoforms in the craniocaudal axis along the medial layer (i.e., airflow axis), where levels of MHC-I and MHC-IIA are low at both ends of the axis and high in the middle and MHC-IIB has a reciprocal distribution. The lateral layer is more uniform, with high levels of MHC-IIB throughout. The level of MHC-IID is relatively constant along the axis in both layers. Large differences exist in the distribution of MHC isoforms among single fibers isolated from sites along the craniocaudal axis, especially in the lateral layer. Systematic regional variations are apparent in the MLC isoform composition of single fibers as well, including some MLC isoform combinations that are not observed in dog limb muscles. Variations of MHC and MLC isoform expression in the dog thyroarytenoid muscle are greater than previously recognized and suggest an even broader range of contractile properties within this multifunctional muscle.

Selective androgen receptor modulator treatment improves muscle strength and body composition and prevents bone loss in orchidectomized rats

The partial agonist activity of a selective androgen receptor modulator (SARM) in the prostate was demonstrated in orchidectomized rats. In the current study, we characterized the full agonist activity of S-3-(4-acetylamino-phenoxy)-2-hydroxy-2-methyl-N-(4-nitro-3-trifluoromethyl-phenyl)-propionamide (a structurally related SARM referred to in other publications and hereafter as S-4) in skeletal muscle, bone, and pituitary of castrated male rats. Twelve weeks after castration, animals were treated with S-4 (3 or 10 mg/kg), dihydrotestosterone (DHT) (3 mg/kg), or vehicle for 8 wk. S-4 (3 and 10 mg/kg) restored soleus muscle mass and strength and levator ani muscle mass to that seen in intact animals. Similar changes were also observed in DHT-treated (3 mg/kg) animals. Compared with the anabolic effects observed in muscle, DHT (3 mg/kg) stimulated prostate and seminal vesicle weights more than 2-fold greater than that observed in intact controls, whereas S-4 (3 mg/kg) returned these androgenic organs to only 16 and 17%, respectively, of the control levels. S-4 (3 and 10 mg/kg) and DHT (3 mg/kg) restored castration-induced loss in lean body mass. Furthermore, S-4 treatment caused a significantly larger increase in total body bone mineral density than DHT. S-4 (3 and 10 mg/kg) also demonstrated agonist activity in the pituitary and significantly decreased plasma LH and FSH levels in castrated animals in a dose-dependent manner. In summary, the strong anabolic effects of S-4 in skeletal muscle, bone, and pituitary were achieved with minimal pharmacologic effect in the prostate. The tissue-selective pharmacologic activity of SARMs provides obvious advantages over steroidal androgen therapy and demonstrates the promising therapeutic utility that this new class of drugs may hold.

A cardiac myosin binding protein C mutation in the Maine Coon cat with familial hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is one of the most common causes of sudden cardiac death in young adults and is a familial disease in at least 60% of cases. Causative mutations have been identified in several sarcomeric genes, including the myosin binding protein C (MYBPC3) gene. Although numerous causative mutations have been identified, the pathogenetic process is still poorly understood. A large animal model of familial HCM in the cat has been identified and may be used for additional study. As the first spontaneous large animal model of this familial disease, feline familial HCM provides a valuable model for investigators to evaluate pathophysiologic processes and therapeutic (pharmacologic or genetic) manipulations. The MYBPC3 gene was chosen as a candidate gene in this model after identifying a reduction in the protein in myocardium from affected cats in comparison to control cats (P<0.001). DNA sequencing was performed and sequence alterations were evaluated for evidence that they changed the amino acid produced, that the amino acid was conserved and that the protein structure was altered. We identified a single base pair change (G to C) in the feline MYBPC3 gene in affected cats that computationally alters the protein conformation of this gene and results in sarcomeric disorganization. We have identified a causative mutation in the feline MYBPC3 gene that results in the development of familial HCM. This is the first report of a spontaneous mutation causing HCM in a non-human species. It should provide a valuable model for evaluating pathophysiologic processes and therapeutic manipulations.

Effects of exercise training on contractile function in myocardial trabeculae after ischemia-reperfusion

Potential protective effects of aerobic exercise training on the myocardium, before an ischemic event, are not completely understood. The purpose of the study was to investigate the effects of exercise training on contractile function after ischemia-reperfusion (Langendorff preparation with 15-min global ischemia/30-min reperfusion). Trabeculae were isolated from the left ventricles of both sedentary control and 10- to 12-wk treadmill exercise-trained rats. The maximal normalized isometric force (force/cross-sectional area; Po/CSA) and shortening velocity ( Vo) in isolated, skinned ventricular trabeculae were measured using the slack test. Ischemia-reperfusion induced significant contractile dysfunction in hearts from both sedentary and trained animals; left ventricular developed pressure (LVDP) and maximal rates of pressure development and relaxation (±dP/d tmax) decreased, whereas end-diastolic pressure (EDP) increased. However, this dysfunction (as expressed as percent change from the last 5 min before ischemia) was attenuated in trained myocardium [LVDP: sedentary −60.8 ± 6.4% (32.0 ± 5.5 mmHg) vs. trained −15.6 ± 8.6% (64.9 ± 6.6 mmHg); +dP/d tmax: sedentary −54.1 ± 4.7% (1,058.7 ± 124.2 mmHg/s) vs. trained −16.7 ± 8.4% (1,931.9 ± 188.3 mmHg/s); −dP/d tmax: sedentary −44.4 ± 2.5% (−829.3 ± 52.0 mmHg/s) vs. trained −17.9 ± 7.2% (−1,341.3 ± 142.8 mmHg/s); EDP: sedentary 539.5 ± 147.6%; (41.3 ± 6.0 mmHg) vs. trained 71.6 ± 30.6%; 11.4 ± 1.2 mmHg]. There was an average 26% increase in Po/CSA in trained trabeculae compared with sedentary controls, and this increase was not affected by ischemia-reperfusion. Ischemia-reperfusion reduced V0 by 39% in both control and trained trabeculae. The relative amount of the β-isoform of myosin heavy chain (MHC-β) was twofold greater in trained trabeculae as well as in the ventricular free walls. Despite a possible increase in the economy in the trained heart, presumed from a greater amount of MHC-β, ischemia-reperfusion reduced Vo, to a similar extent in both control and trained animals. Nevertheless, the trained myocardium appears to have a greater maximum force-generating ability that may, at least partially, compensate for reduced contractile function induced by a brief period of ischemia.

Myosin light chain isoform expression among single mammalian skeletal muscle fibers: species variations

Extensive heterogeneity in myosin heavy chain and light chain (MLC) isoform expression in skeletal muscle has been well documented in several mammalian species. The initial objective of this study was to determine the extent of heterogeneity in myosin isoform expression among single fibers in limb muscles of dogs, a species for which relatively little has been reported. Fibers were isolated from muscles that have different functions with respect to limb extension and limb flexion and were analyzed on SDS gels, with respect to myosin isoform composition. The results of this part of the study indicate that there are at least four distinct fiber types in dog limb and diaphragm muscles, on the basis of MLC isoform expression: conventional fast (expressing fast-type isoforms of MLC1 (MLC1F) and MLC2 (MLC2F), plus MLC3), conventional slow (expressing slow-type MLC1 (MLC1S) and MLC2 (MLC2S)), hybrid (expressing MLC1S, MLC1F, MLC2S, MLC2F and MLC3) and a second slow fiber type, designated as S1F. S1F fibers express MLC1F, along with MLC1S and MLC2S and relatively low levels of MLC3. The fraction of slow fibers that are S1F fibers varies among dog limb muscles, being greater in limb extensors than flexors. Furthermore, the mean level of MLC1F in S1F fibers is greater in extensors than flexors (mean levels range from approximately 3% to 50% of total MLC1). The study was, therefore, extended to include six additional species, spanning a broad range in adult body size to more thoroughly characterize heterogeneity in MLC isoform expression among mammals. The results indicate that there are distinct patterns in MLC isoform expression among fast and slow fibers among different species. Specifically, large-size mammals have two distinct types of slow fibers, based upon MLC isoform composition (conventional and S1F fibers), whereas small mammals exhibit variations in MLC isoforms between different types of fast fibers, including a fast fiber type that expresses MLC1S (designated as F1S fibers). S1F fibers were absent in rodent muscles and F1S fibers were not found in large mammals. We conclude that extensive variation exists in MLC isoform expression in mammalian skeletal muscle fibers, yet there are distinct patterns among different species and among muscles within an individual species.

Age-dependent changes in contraction and regional myocardial myosin heavy chain isoform expression in rats

The goals of this study were to measure the relative levels of the alpha- and beta-isoforms of myosin heavy chain (MHC-alpha and MHC-beta, respectively) in multiple, specific regions of the adult rat heart and to determine whether age-dependent changes in isoform levels in different regions are uniform. Relative amounts of MHC-alpha and MHC-beta were determined in right and left atria and left ventricular (LV) Purkinje fibers (PF), papillary muscles, trabeculae, and endo-, mid-, and epicardial regions at 2, 5, 10, 16, and 21 mo. PFs contained substantial amounts of myosin and were striated and capable of generating force and shortening on activation. Levels of MHC-beta increased in all LV compartments with age, especially between 2 and 5 mo. There was more MHC-beta in PFs than other LV sites. There were regional differences in the level of MHC-beta throughout the LV at all ages, and the rates of change within regions differed. Ca(2+)-activated tension in PFs and trabeculae was compared at 2 and 22 mo. PF tension was less than trabecula tension, and this difference may be explained by differences in MHC content. V(max) and tension-generating ability in PFs decreased with age. Maximal tension generated by trabeculae did not change during aging. A large proportion of the increase in the level of MHC-beta that is normally associated with aging occurs at a relatively early age in rat LV. PFs, with their small diameters and short diffusion distance, should be considered for skinned multicellular myocardial studies.

Degradation of rat cardiac troponin I during ischemia independent of reperfusion

Cardiac troponin I (cTnI) degradation has been noted in the stunned myocardium of rodents after ischemia and reperfusion and is one proposed mechanism for the decreased left ventricular (LV) contractility in postischemic hearts. cTnI degradation has been best described after reperfusion of the ischemic myocardium. The effect of ischemia, independent of reperfusion, on cTnI breakdown has not been well characterized. We tested the hypothesis that progressive cTnI degradation occurs with increasing durations of ischemia and that this ischemia-based degradation is, in part, oxidant mediated. Isolated perfused rat hearts underwent global ischemia of 15, 20, or 25 min with and without reperfusion. A second series of hearts was treated with the antioxidants tiron (10 mM) and N-acetylcysteine (4 mM) before 20 min of global ischemia without reperfusion. cTnI degradation was measured using a cTnI-specific antibody and Western blot analyses. A progressive increase in cTnI degradation was seen with increasing duration of ischemia (no reperfusion), which correlated with the return of LV developed pressure during reperfusion. The extent of cTnI degradation was increased in hearts pretreated with antioxidants, although the qualitative degradation pattern was not altered. We conclude that a time-dependent cTnI breakdown occurs during global ischemia that is independent of reperfusion. cTnI breakdown during ischemia is further increased in the presence of antioxidants, suggesting ROS generated during ischemia may play a cTnI protective role.