The Effect of Temperature Manipulation on Skeletal Muscle Function and Thermal Stress Responses in Young Males

Influence of temperature in thermal and oxidative stress responses in estuarine fish

Obesity develops when energy intake exceeds energy expenditure. Defect in the function of brown fat and skeletal muscle, two of the major tissues that contribute towards energy expenditure, lead to the development of obesity and metabolic syndrome. Our previous findings suggest that Tyk2 deficient mice become obese and develop the metabolic syndrome. Tyk2, which is a tyrosine kinase of the JAK-STAT signaling family, is important for optimal brown development and function. Since brown fat and skeletal muscle, both are derived from the Myf5+ lineage of mesenchymal stem cells, we also characterized the role of Tyk2 in the development and function of skeletal muscle. We found that Tyk2 deficient mice do not display a structural defect in skeletal muscle development; however, the function of skeletal muscle is severely impaired in these mice. Expression of troponins, which regulate the muscle contraction and muscle creatine kinase, which regulates the levels of phosphocreatine, a major fuel for skeletal muscle, is downregulated in Tyk2 deficient mice. Skeletal muscle mitochondria also display an abnormal morphology along with decreased respiration capacity, which is a function of decreased activity of complex IV of the electron transport chain. Interestingly, Tyk2 deficient mice also exhibit an increased proportion of fast, glycolytic, Type II fibers in the skeletal muscle. Using an in-vitro system for skeletal muscle differentiation, we found that Tyk2 levels increase during differentiation, suggesting a role for Tyk2 in proper development and function of the skeletal muscle. Our previous studies suggested that a kinase-inactive (Tyk2KD) form of Tyk2 is also efficient in restoring the function of Tyk2 deficient brown fat preadipocytes. We generated transgenic mice that expressed a wild type (Tyk2WT) and kinase inactive (Tyk2 KD) form of tyk2 in brown fat and skeletal muscle under Myf5 cre and in skeletal muscle using MCK cre mice. Expression of Tyk2 using the Myf5 cre (E8.0) reverts the obese and the metabolic phenotype observed in the Tyk2 deficient mice. Interestingly, expressing Tyk2 under MCK cre (E13.0) also reverts the obese phenotype, suggesting that the temporal and spatial expression of Tyk2 is critical in regulating energy expenditure. Our studies also highlight the role of Tyk2, not as a kinase, but as a component of the transcriptional assembly regulating the expression of genes involved brown fat and skeletal muscle differentiation and function.

Skeletal muscle is critical for maintaining mobility, independence, and metabolic health in older adults. However, a common feature of aging is the progressive loss of skeletal muscle mass and function, which is often accompanied by mitochondrial impairments, oxidative stress, and insulin resistance. Exercise improves muscle strength, mitochondrial health, and cardiorespiratory fitness, but older adults often exhibit attenuated anabolic responses to acute exercise. Chronic inflammation associated with aging may contribute to this “anabolic resistance” and therapeutic interventions that target inflammation may improve exercise responsiveness. To this end, we conducted a randomized controlled trial to determine the effect of 6 months of dietary omega-3 polyunsaturated fatty acids (n3-PUFA) supplementation on skeletal muscle function (mass, strength), mitochondrial physiology (respiration, ATP production, ROS generation), and acute exercise responsiveness at the level of the muscle (fractional synthesis rate) and the whole-body (amino acid kinetics) in healthy older adults. When compared with a corn oil placebo (n = 33; 71.5 ± 4.8 years), older adults treated with 4 g/day n3-PUFA (n = 30; 71.4 ± 4.5 years) exhibited modest but significant increases in muscle strength (3.1 ± 14.7% increase in placebo vs. 7.5 ± 14.1% increase in n3-PUFA; p = 0.039). These improvements in muscle strength with n3-PUFA supplementation occurred in the absence of any effects on mitochondrial function and a minor attenuation of the acute response to exercise compared to placebo. Together, these data suggest modest benefits of dietary n3-PUFAs to muscle function in healthy older adults. Future studies may elucidate whether n3-PUFA supplementation improves the exercise response in elderly individuals with co-morbidities, such as chronic inflammatory disease or sarcopenia.

Chapter 1 - Skeletal muscle structure, physiology, and function

In response to stressors and pathophysiologic conditions, sympathetic neuronal outflows can change heterogeneously among body organs and tissues. This study examined the validity of microdialysis and measurements of microdialysate concentrations of catechols, to assess local sympathetic function in skeletal muscle and adipose tissue in humans. Based on preliminary experiments, a microdialysate perfusion rate of 3 microl/min and collection duration of 30 minutes were chosen. To assess responses to a stimulus that increases sympathetic outflow to skeletal muscle, microdialysate norepinephrine and dihydroxyphenylglycol concentrations in quadriceps muscle, abdominal subcutaneous adipose tissue, and plasma were measured during orthostasis in 8 healthy normal volunteers. To assess responses to decreased postganglionic sympathetic nerve traffic, norepinephrine and dihydroxyphenylglycol concentrations were measured during i. v. infusion of trimethaphan in 5 volunteers. All subjects had detectable norepinephrine and dihydroxyphenylglycol in microdialysate from both skeletal muscle and adipose tissue. Orthostasis significantly increased microdialysate norepinephrine in skeletal muscle (0.38 +/- (SEM) 0.07 nmol/L supine to 1.48+/-0.24 nmol/L standing, p < 0.01) and in adipose tissue (0.31+/-0.02 nmol/L supine to 0.68+/-0.11 nmol/L standing, p < 0.01). Orthostasis also increased microdialysate dihydroxyphenylglycol in both tissues (1.76+/-0.30 nmol/L to 3.08+/-0.43 nmol/L, p < 0.01; 1.37+/-0.15 nmol/L supine to 1.99+/-0.34 nmol/L standing, p < 0.01). Trimethaphan decreased norepinephrine concentrations in skeletal muscle microdialysate by 50%, adipose tissue by 70%, and antecubital venous plasma 50%, with non-significant decreases in dihydroxyphenylglycol concentrations at each site. Microdialysate concentrations of norepinephrine and dihydroxyphenylglycol can be detected reliably and respond appropriately during manipulations that increase or decrease the sympathetically mediated release and turnover of norepinephrine. This approach may provide a means to assess sympathetic neuronal function in skeletal muscle and adipose tissue in humans with known or suspected dysautonomias.

EditorialEmerging roles for estrogen in regulating skeletal muscle physiologyBridget Coyle-Asbil, Leslie M. Ogilvie, and Jeremy A. SimpsonBridget Coyle-AsbilDepartment of Human Health and Nutritional Sciences, University of Guelph, Guelph, Ontario, CanadaIMPART Investigator Team Canada, Saint John, New Brunswick, Canada, Leslie M. OgilvieDepartment of Human Health and Nutritional Sciences, University of Guelph, Guelph, Ontario, CanadaIMPART Investigator Team Canada, Saint John, New Brunswick, Canada, and Jeremy A. SimpsonDepartment of Human Health and Nutritional Sciences, University of Guelph, Guelph, Ontario, CanadaIMPART Investigator Team Canada, Saint John, New Brunswick, CanadaPublished Online:13 Feb 2023https://doi.org/10.1152/physiolgenomics.00158.2022This is the final version - click for previous versionMoreSectionsPDF (682 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations Skeletal muscle mass and strength decline with age, which contributes to impaired balance and reduced mobility, leading to falls. In fact, measures of muscle mass and strength are independent predictors of overall health in men and women (1). Throughout aging, there are sex-specific differences in skeletal muscle function, where declines in muscle strength occur at an earlier age in females than in males (2). Interestingly, this decline in muscle function in females is associated with reductions in circulating estrogen levels—a result of menopause (3–5). This observation is compelling and introduces sex hormones as a mechanism that directly regulates skeletal muscle physiology.Posttranslational modifications (PTMs) are chemical modifications to proteins that change the structure and subsequent function of proteins. There are over 200 different PTMs, which directly influence protein-protein interactions that subsequently drive the phenotype of the organ. One of the most common PTMs of muscle is phosphorylation, the addition of a phosphate group to either serine, or threonine, or tyrosine residues. Protein phosphorylation leads to the activation or inactivation of a protein and impacts homeostatic cell signaling. Modifications to the skeletal muscle phosphoproteome are involved in coordinating muscle growth, metabolism, repair, and contraction to generate force (6, 7). Throughout aging, there are modifications to the skeletal muscle phosphoproteome, indicating a connection between the muscle phosphoproteome and muscle function (8, 9). For example, phosphorylation levels of myosin light chain 2 and tropomyosin α, proteins in the contractile machinery, were increased in skeletal muscle of aged rats (8). Although several studies have investigated how protein phosphorylation impacts muscle function (10–13), these have been carried out largely in male subjects. Despite this, sex hormones are emerging as a novel mechanism regulating skeletal muscle function, which opens up new therapeutic avenues to target age-associated muscle dysfunction, enhance performance, and improve injury recovery. However, what remains to be discovered is the target protein and the specific amino acids that are directly phosphorylated by sex hormones (Fig. 1).Figure 1.Role of sex hormones in regulating skeletal muscle physiology in mice. Sex hormones (e.g., estrogen, testosterone, and progesterone) alter protein signaling through posttranslational modifications, isoform shifting, and de novo synthesis, which impacts skeletal muscle molecular signaling of sarcomeric proteins, transcription factors, calcium signaling, and metabolic proteins. However, the specific impact of each individual sex hormone on protein alterations and their functional impacts remains unknown. [Image created with BioRender.com and published with permission.]Download figureDownload PowerPointIn a recent issue of Physiological Genomics, Peyton and colleagues (14) performed a global phosphoproteomic analysis of tibialis anterior muscle in ovarian-hormone deficient mice to determine whether the decline of estrogen in females alters the skeletal muscle phosphoproteome. Using a mass spectrometric approach, they identified 22 proteins that are differentially phosphorylated in ovariectomized (OVX) versus sham mice, establishing that ovarian-derived sex hormones are involved in regulating the skeletal muscle phosphoproteome under resting conditions (i.e., noncontracting muscle). Pathway enrichment analysis, performed to gain further insight into the functional impact of the differentially regulated phosphopeptides in estrogen-deficient mice, showed an overrepresentation in proteins associated with calcium (e.g., calcium ion channel activity, myofilament calcium sensitivity, intracellular calcium release, and reuptake), metabolic signaling (e.g., insulin signaling, glycolysis, and gluconeogenesis), and cellular functions related to muscle maintenance and cytoskeletal integrity (e.g., sarcomeric organization and contractile function). As these signaling pathways are critical to maintaining normal muscle function, remodeling of the muscle phosphoproteome will have consequences on the protein’s function. These findings lay the molecular foundation linking sex hormones to muscle physiology and outline critical next steps to examining the functional impact of sex hormone depletion on skeletal muscle in females.Our current understanding of how sex hormones impact basic biological functions continues to expand. Sex hormones play important physiological roles that extend beyond their well-established reproductive functions. In fact, estradiol, the ovarian-derived form of estrogen, alters signaling pathways in the musculoskeletal, cardiovascular, and central nervous systems in both females and males (15–17). For example, in male rats, estrogen decreased eccentric cardiac hypertrophy following chronic volume overload, a model of cardiac stress (18). Thus, as estrogen levels are altered throughout aging, this has functional consequences on these biological systems (17, 19–22). As such, further investigation into how estrogen regulates pathways involved in skeletal muscle function may be of benefit to both sexes.The current issue follows their previous work, where Lai et al. (23) showed that OVX decreases phosphorylation of skeletal myosin regulatory light chain (RLC), which impairs muscle contractility in female mice. Furthermore, they demonstrated that muscle contractility and RLC protein phosphorylation are restored with the administration of exogenous estradiol, confirming that the effects of OVX on muscle function are estrogen-mediated rather than other ovarian-derived hormones (i.e., progesterone and testosterone). Although their previous work establishes that estrogen regulates muscle physiology at a sarcomeric level, a broader and unbiased investigation into the effects of estrogen on muscle proteins was lacking until now. Here, Lai et al. (23) present a comprehensive phosphoproteomic profiling of skeletal muscle and provide compelling evidence that sex hormones are involved in regulating signaling pathways such as energy metabolism, calcium signaling, and protein trafficking, which are critical for maintaining normal muscle function. Their results provide fundamental knowledge of female physiology and the impact of female sex hormones on skeletal muscle at a molecular level. These findings present a critical first step in identifying the molecular intermediates involved in driving specific muscle phenotypes. It is intriguing to consider the consequences of estrogen deficiency in older females and the impact this has on their strength, mobility, and quality of life. It will be of great interest to investigate the effects of sex hormones on other tissues such as cardiac and respiratory muscles.As estrogen is involved in regulating a variety of physiological mechanisms, it is imperative to investigate how the loss of ovarian-derived estrogen alters the regulation of normal physiology. Currently, there are two primary animal models used to evaluate the effects of estrogen loss in females. OVX, the surgical removal of one or both ovaries, has been the gold standard menopause model used to evaluate the effects of ovarian hormone loss, including estrogen, on various biological systems (24). However, when the ovaries are removed, the levels of other sex hormones (e.g., progesterone, testosterone, follicle-stimulating hormone, and luteinizing hormone) are also altered, which may influence the interpretation of how estrogen loss affects physiology. An emerging rodent model of menopause is produced by administering the chemical, 4-vinylcyclohexene diepoxide (VCD). VCD selectively targets and depletes the ovarian follicles, resulting in a hormone profile that is more similar to the natural menopause transition in women. However, VCD is toxic at high doses, which may present confounding effects on other organs if the appropriate administration doses are exceeded. Overall, the OVX and VCD models of menopause both provide the opportunity to study how ovarian-derived hormones influence biological systems. In both cases, it is important to consider the advantages of each model when designing research experiments and to understand the limitations to ensure that findings are interpreted in the appropriate biological context. Furthermore, how male sex hormones affect the phosphoproteome is an intriguing comparison that requires further consideration where orchiectomy, the surgical removal of the gonads and spermatic cord, may be used as a valuable model to examine changes in testosterone in humans with age.Throughout their manuscript, Peyton et al. (14) highlight insightful similarities between cardiac and skeletal muscle physiology and discuss the implications of how alterations in the phosphoproteome are the basis for various pathologies in both muscle types. Indeed, phosphoproteomic modifications in cardiac muscle are key features of several cardiomyopathies. For example, cardiac troponin I (cTnI) and RLC are sarcomeric proteins that play important roles in calcium regulation and mediating actin-myosin interactions to generate force. In heart failure, both of these proteins are dephosphorylated, leading to severe impairments in cardiac muscle contraction and relaxation (25–27). This demonstrates the importance of regulating protein phosphorylation to maintain normal muscle function. Furthermore, estrogen acts directly on the heart through various estrogen receptors localized in different cardiac cells (e.g., cardiomyocytes, endothelial cells, and cardiac fibroblasts) suggesting that sex hormones are also involved in regulating cardiac function. Estrogen has a protective effect on the heart contributing to a lower incidence of heart disease in females premenopause compared with aged-matched males and postmenopausal females when estrogen levels decline (28). The identification of altered protein phosphorylation patterns and the role of sex hormones in muscle physiology may reveal targets for therapeutic intervention to improve muscle weakness and dysfunction in both cardiac and skeletal muscle pathologies.Here, Peyton and colleagues (14) have presented the first global phosphoproteomic analysis in skeletal muscle examining phosphorylation modifications following OVX in female mice. Their results show that with OVX, the phosphorylation of proteins associated with calcium signaling, metabolic regulation, and sarcomere organization is altered, all of which are important for regulating normal muscle function. With the identification of these phosphoproteins that are regulated by sex hormones, research into the receptors, signaling pathways, and kinases involved is required along with which sex hormones are driving each specific change. We are eager to learn the functional impact of these phosphoproteomic changes on skeletal muscle. These data provide fundamental insight into how female sex hormones regulate pathways important for maintaining normal muscle function. This information will ultimately provide novel therapeutic pathways for targeting muscle weakness and improving muscle function. Overall, given the importance of integrating sex and gender into research, a specific focus on improving our understanding of female physiology provides new insight into how estrogen deficiency impacts the skeletal muscle phosphoproteome and the consequences for muscle strength and function in aging females.GRANTSThis work was supported by the Canadian Institutes of Health Research (CIHR), Natural Sciences and Engineering Research Council of Canada (NSERC), and Heart and Stroke Foundation of Canada grants to J. A. Simpson. B. Coyle-Asbil was supported by a Canada Graduate Scholarship-Master’s NSERC and L. M. Ogilvie was supported by an Alexander Graham Bell Canada Graduate Scholarship-Doctoral NSERC.DISCLOSURESNo conflicts of interest, financial or otherwise, are declared by the authors.AUTHOR CONTRIBUTIONSB.C.-A., L.M.O., and J.A.S. prepared figures; drafted manuscript; edited and revised manuscript; and approved final version of manuscript.REFERENCES1. Santilli V, Bernetti A, Mangone M, Paoloni M. Clinical definition of sarcopenia. Clin Cases Miner Bone Metab 11: 177–180, 2014. PubMed | Google Scholar2. Haynes EMK, Neubauer NA, Cornett KMD, O'Connor BP, Jones GR, Jakobi JM. Age and sex-related decline of muscle strength across the adult lifespan: a scoping review of aggregated data. Appl Physiol Nutr Metab 45: 1185–1196, 2020. doi:10.1139/apnm-2020-0081. Crossref | PubMed | ISI | Google Scholar3. Greising SM, Baltgalvis KA, Lowe DA, Warren GL. Hormone therapy and skeletal muscle strength: a meta-analysis. J Gerontol A Biol Sci Med Sci 64: 1071–1081, 2009. doi:10.1093/gerona/glp082. 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Download PDF Back to Top Next FiguresReferencesRelatedInformation Related ArticlesGlobal phosphoproteomic profiling of skeletal muscle in ovarian hormone-deficient mice 24 Oct 2022Physiological Genomics More from this issue > Volume 55Issue 2February 2023Pages 75-78 Crossmark Copyright & PermissionsCopyright © 2023 the American Physiological Society.https://doi.org/10.1152/physiolgenomics.00158.2022PubMed36622080History Received 8 November 2022 Accepted 5 January 2023 Published online 13 February 2023 Published in print 1 February 2023 Keywordsestrogenphosphoproteomesex hormonesskeletal muscle Metrics

The impact of COVID-19 has been largely described after symptom development. Although the SARS-CoV-2 virus elevates heart rate (HR) prior to symptom onset, whether this virus evokes other presymptomatic alterations is unknown. This case study details the presymptomatic impact of COVID-19 on vascular and skeletal muscle function in a young woman [24 yr, 173.5 cm, 89 kg, body mass index (BMI): 29.6 kg·m–2]. Vascular and skeletal muscle function were assessed as part of a separate study with the first and second visits separated by 2 wk. On the evening following the second visit, the participant developed a fever and a rapid antigen test confirmed a positive COVID-19 diagnosis. Compared with the first visit, the participant presented with a markedly elevated HR (∼30 beats/min) and a lower mean blood pressure (∼8 mmHg) at the second visit. Vascular function measured by brachial artery flow-mediated dilation, reactive hyperemia, and passive leg movement were all noticeably attenuated (25%–65%) as was leg blood flow during knee extension exercise. Muscle strength was diminished as was ADP-stimulated respiration (30%), assessed in vitro, whereas there was a 25% increase in the apparent Km. Lastly, an elevation in IL-10 was observed prior to symptom onset. Notably, 2.5 mo after diagnosis symptoms of fatigue and cough were still present. Together, these findings provide unique insight into the physiological responses immediately prior to onset of COVID-19 symptoms; they suggest that SARS-CoV-2 negatively impacts vascular and skeletal muscle function prior to the onset of common symptoms and may set the stage for the widespread sequelae observed following COVID-19 diagnosis.NEW & NOTEWORTHY This unique case study details the impact of SARS-CoV-2 infection on vascular and skeletal muscle function in a young predominantly presymptomatic woman. Prior to COVID-19 diagnosis, substantial reductions in vascular, skeletal muscle, and mitochondrial function were observed along with an elevation in IL-10. This integrative case study indicates that the presymptomatic impact of COVID-19 is widespread and may help elucidate the acute and long-term sequelae of this disease.

The transcriptional regulatory protein Rgg coordinates amino acid catabolism and virulence factor expression in Streptococcus pyogenes. We used a proteomic approach to compare cytoplasmic proteins isolated from S. pyogenes wild-type strain NZ131 (serotype M49) to proteins isolated from an rgg mutant strain during the exponential and stationary phases of growth. Proteins were separated by two-dimensional gel electrophoresis, and 125 protein spots of interest were identified by tandem mass spectrometry. Comparative analysis of proteins isolated from the isogenic strains revealed that growth phase-associated regulation of enzymes involved in the metabolism of arginine (ArcABC), histidine (HutI), and serine (SdhA) was abrogated in the rgg mutant strain, which synthesized the proteins in the exponential phase of growth. In contrast, the enzymes were detected only among wild-type proteins isolated from organisms in the stationary phase of growth. The differences in protein composition were correlated with previously described metabolic changes. In addition, proteins associated with thermal and oxidative stress responses, including ClpE and ClpL, were present in samples isolated from the rgg mutant strain but not in samples isolated from the wild-type strain. The rgg mutant strain was more tolerant to elevated temperature and puromycin than the wild-type strain; however, the mutant was less tolerant to paraquat. We concluded that Rgg is a global regulatory factor that contributes to growth phase-dependent synthesis of proteins associated with secondary metabolism and oxidative and thermal stress responses.

The ubiquitously expressed large GTPase Dynamin 2 (DNM2) plays a critical role in the regulation of intracellular membrane trafficking through its crucial function in membrane fission, particularly in endocytosis. Autosomal-dominant mutations in DNM2 cause tissue-specific human disorders. Different sets of DNM2 mutations are linked to dominant intermediate Charcot-Marie-Tooth neuropathy type B, a motor and sensory neuropathy affecting primarily peripheral nerves, or autosomal-dominant centronuclear myopathy (CNM) presenting with primary damage in skeletal muscles. To understand the underlying disease mechanisms, it is imperative to determine to which degree the primary affected cell types require DNM2. Thus, we used cell type-specific gene ablation to examine the consequences of DNM2 loss in skeletal muscle cells, the major relevant cell type involved in CNM. We found that DNM2 function in skeletal muscle is required for proper mouse development. Skeletal muscle-specific loss of DNM2 causes a reduction in muscle mass and in the numbers of muscle fibers, altered muscle fiber size distributions, irregular neuromuscular junctions (NMJs) and isolated degenerating intramuscular peripheral nerve fibers. Intriguingly, a lack of muscle-expressed DNM2 triggers an increase of lipid droplets (LDs) and mitochondrial defects. We conclude that loss of DNM2 function in skeletal muscles initiates a chain of harmful parallel and serial events, involving dysregulation of LDs and mitochondrial defects within altered muscle fibers, defective NMJs and peripheral nerve degeneration. These findings provide the essential basis for further studies on DNM2 function and malfunction in skeletal muscles in health and disease, potentially including metabolic diseases such as diabetes.

Skeletal muscle wasting in the intensive care unit (ICU) has been associated with mortality, but it is unclear whether sarcopenia, defined by skeletal muscle mass and function, is useful for detailed risk stratification after ICU discharge. In this cohort study, 72 critically ill patients with an ICU stay of ≥48 h were identified. Skeletal muscle mass was assessed from the muscle thickness (MT) of the patients’ quadriceps using ultrasound images before ICU discharge. Skeletal muscle function was assessed from the patients’ muscle strength (MS) before ICU discharge according to the Medical Research Council sum score. A diagnosis of sarcopenia in the ICU was made in patients with low MT and low MS. The study endpoint was 1-year mortality. Sarcopenia in the ICU was diagnosed in 26/72 patients (36%). After adjusting for covariates in the Cox regression, sarcopenia in the ICU was significantly associated with 1-year mortality (hazard ratio 3.82; 95% confidence interval, 1.40–10.42). Sarcopenia in the ICU, defined by low skeletal muscle mass and function, was associated with 1-year mortality in survivors of critical illness. Skeletal muscle mass and function assessed at the bedside could be used to identify higher-risk patients in the ICU.

Cardiac autonomic responses to hyperinsulinemia are associated with skeletal muscle function in healthy human subjects

Skeletal muscle mass and function are essential components of health and homeostasis. In addition to its role in balance and locomotion, skeletal muscle is the body’s largest metabolic reservoir, absorbing significant amounts of glucose and amino acids from the blood stream. The loss of muscle mass is associated with impaired glucose tolerance, threatens vascular health and contributes to cardiovascular disease in obese and aging (sarcopenic) populations, along with driving overall mortality. The myokine myostatin (GDF-8) is a potent negative regulator of skeletal muscle growth that is upregulated in humans and animal models of obesity and aging. Loss of myostatin is known to increase the number of glycolytic muscle fibers and has been shown to correct indices that govern cardiometabolic health in obesity and aging, improving endothelial function and glucose homeostasis, and protecting against renal injury and hypertension. However, these studies have largely been conducted using male mice, leaving the question regarding the effectiveness of myostatin deletion on skeletal muscle function, independent of sex, largely unexplored. As such, our hypothesis is that the deletion of myostatin will improve skeletal muscle function in both male and female mice, regardless of the disease phenotype being obesity or aging. Young (3-5 mo.) lean and obese mice were used, along with aged (18-24 mo.) mice. The obese db/db model was utilized as it is a well characterized model of hyperphagia that results in obesity and subsequent metabolic disease, including significant whole-body adiposity and reduced muscle mass. On this background we have crossed mice harboring constitutive genetic deletion of myostatin to reverse muscle loss. Thus, myostatin deletion was crossed onto the relevant backgrounds in combination with aging or obesity and included both sexes. Muscle function was assessed in all groups using in vivo plantarflexion. Young male mice demonstrated an ~20% increase in force production compared to the females. Results show that aging and obesity inhibited skeletal muscle function in both sexes, with female mice showing a lessened impact of obesity or aging. Myostatin deletion improved muscle function in male mice, returning both obesity-derived weakness and aging-induced sarcopenia back to levels of young control. However, female mice without myostatin experienced a relative gain-of-function, with force production significantly exceeding that of the young control. The implication of these results is that weight loss may be more effective in protecting muscle function in diseases in the male population whereas myostatin inhibition, either pharmaceutically or with prescription exercise, may result in greater improvements in the female sex comparatively. This research gratefully acknowledges support from the NIA (K01 AG064121), OCAST (HR21-045-1) and the Niblack Research Scholars Program. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Recent evidence has shown that mitochondrial respiratory function contributes to exercise performance and metabolic health. Given that lactate is considered a potential signaling molecule that induces mitochondrial adaptations, we tested the hypothesis that lactate would change mitochondrial respiratory function in skeletal muscle. Male ICR mice (8 weeks old) received intraperitoneal injection of PBS or sodium lactate (1 g/kg BW) 5 days a week for 4 weeks. Mitochondria were isolated from freshly excised gastrocnemius muscle using differential centrifugation and were used for all analyses. Lactate administration significantly enhanced pyruvate + malate- and glutamate + malate-induced (complex I-driven) state 3 (maximal/ATP synthesis-coupled) respiration, but not state 2 (basal/proton conductance) respiration. In contrast, lactate administration significantly decreased succinate + rotenone-induced (complex II-driven) state 3 and 2 respiration. No significant differences were observed in malate + octanoyl-l-carnitine-induced state 3 or 2 respiration. The enzymatic activity of complex I was tended to increase and those of complexes I + III and IV were significantly increased after lactate administration. No differences were observed in the activities of complexes II or II + III. Moreover, lactate administration increased the protein content of NDUFS4, a subunit of complex I, but not those of the other components. The present findings suggest that lactate alters mitochondrial respiratory function in skeletal muscle.

Duchenne muscular dystrophy (DMD) is a degenerative disorder affecting skeletal and cardiac muscle for which there is no effective therapy. Angiotension receptor blockade (ARB) has excellent therapeutic potential in DMD based on recent data demonstrating attenuation of skeletal muscle disease progression during 6–9 months of therapy in the mdx mouse model of DMD. Since cardiac-related death is major cause of mortality in DMD, it is important to evaluate the effect of any novel treatment on the heart. Therefore, we evaluated the long-term impact of ARB on both the skeletal muscle and cardiac phenotype of the mdx mouse. Mdx mice received either losartan (0.6 g/L) (n = 8) or standard drinking water (n = 9) for two years, after which echocardiography was performed to assess cardiac function. Skeletal muscle weight, morphology, and function were assessed. Fibrosis was evaluated in the diaphragm and heart by Trichrome stain and by determination of tissue hydroxyproline content. By the study endpoint, 88% of treated mice were alive compared to only 44% of untreated (p = 0.05). No difference in skeletal muscle morphology, function, or fibrosis was noted in losartan-treated animals. Cardiac function was significantly preserved with losartan treatment, with a trend towards reduction in cardiac fibrosis. We saw no impact on the skeletal muscle disease progression, suggesting that other pathways that trigger fibrosis dominate over angiotensin II in skeletal muscle long term, unlike the situation in the heart. Our study suggests that ARB may be an important prophylactic treatment for DMD-associated cardiomyopathy, but will not impact skeletal muscle disease.

-We examined the relationship between function, water content, and growth rate of skeletal muscles in the European Starling (Sturnus vulgaris). Function was measured as the ability of nestlings to increase their rate of oxygen consumption in response to cold stress. Water content and growth rate of pectoral and leg muscles were determined for tissues dissected from a series of known-age nestlings. The maximum metabolic response to cold stress increased from 0 at 4 days of age to 4.5-5.5 cm3 O2g'-'h-' at 15-16 days. During this period, the mass of the leg muscles increased 5-fold and that of the pectoral muscles, 12.5fold. The water fraction (WF) of the pectoral muscles decreased from about 0.86 to 0.72, while that of the leg muscles decreased from 0.83 to 0.72. Of the variatlon in metabolic response to cold stress per gram of muscle, 89% could be related to the WF of skeletal muscle by a linear relationship. Over the ages surveyed, the metabolic response varied from 0 at WF = 0.85 to an increase above resting metabolism amounting to 60 cm3 02 gram of pectoral and leg muscle-' h-' at WF = 0.72. Also during this period, the growth rate of both muscle masses decreased with age and proportion of water, from a mass-specific growth rate of 0.5/day at WF = 0.85 to near 0 at WF = 0.72. If these relationships represent a balance between growth rate and functional maturity in developing tissues, then even small changes in function, associated with a change in water content of only a few percent, could have large consequences for the growth rate of the individual. Received 30 August 1983, accepted 30 November 1984. IN comparisons among species of birds of the same size, the growth rate of chicks varies over an approximately 10-fold range (Ricklefs 1979b). Much of this variation is associated with the mode of development. Chicks of precocial species, which are relatively independent of their parents from an early age, grow more slowly than do those of altricial species, which depend upon their parents for food and warmth during much of their development. Such comparisons, and the observation that mass-specific growth rates of individuals slow as the individual matures functionally, led Ricklefs (1969, 1973, 1979a) to suggest that growth rate is inversely related to functional maturity at the tissue level. As a tissue differentiates and begins to function at an adult level, its proliferation by cell division and growth by cell enlargement decrease. Skeletal muscle provides a model for this constraint in that as mesenchyme cells (myoblasts) of the embryonic muscles differentiate, they fuse to form muscle fiI Present address: Department of Zoology, University of Washington, Seattle, Washington 98195 USA. bers and cease to proliferate (Holtzer 1970, Cameron and Jeter 1971). Associated with the maturation of tissues is a decrease in content of water (Ricklefs 1967, 1979a; Bilby and Widdowson 1971; O'Connor 1977; Dunn and Brisbin 1980). In comparisons among species and in comparisons among individuals during development, the water content of a tissue has frequently been used as an index to its differentiation and functional maturity. But in spite of the general association between water content, function, and growth rate, these relationships have not been determined quantitatively. Indeed, Marsh and Wickler (1982) have questioned the use of water content as a suitable index of muscle function. We examined the relationship between function, water content, and growth rate of skeletal muscles in the European Starling (Sturnus vulgaris). Skeletal muscle was chosen for two reasons: first, its pattern of development has been suggested to underlie differences in growth rate (1/time) among species of birds (Ricklefs 1979a, b); second, the function of skeletal muscle can be determined directly by simple physiological measurement. During development, increase in 369 The Auk 102: 369-376. April 1985 This content downloaded from 157.55.39.25 on Tue, 02 Aug 2016 06:21:48 UTC All use subject to http://about.jstor.org/terms 370 RICKLEFS AND WEBB [Auk, Vol. 102