Fiber Myosin Heavy Chain Research Articles

Skeletal muscle myosin heavy chain expression and 3D capillary network changes in streptozotocin-induced diabetic female mice.

It is not well-understood how type 1 diabetes (T1DM) affects skeletal muscle histological phenotype, particularly capillarisation. This study aimed to analyze skeletal muscle myosin heavy chain (MyHC) fibre type changes and 3D capillary network characteristics in experimental T1DM mice. Female C57BL/6J-OlaHsd mice were categorized into streptozotocin (STZ)-induced diabetic (n = 12) and age-matched non-diabetic controls (n =12). The muscle fibre phenotype of the soleus, gluteus maximus, and gastrocnemius muscles were characterized based on the expression of MyHC isoforms, while capillaries of the gluteus maximus were assessed with immunofluorescence staining, confocal laser microscopy and 3D image analysis. STZ-induced diabetic mice exhibited elevated glucose levels, reduced body weight, and prolonged thermal latency, verifying the T1DM phenotype. In both T1DM and non-diabetic mice, the gluteus maximus and gastrocnemius muscles predominantly expressed fast-twitch type 2b fibers, with no significant differences noted. However, the soleus muscle in non-diabetic mice had a greater proportion of type 2a fibers and comparable type 1 fiber densities (26.2 ± 14.6% vs 21.9 ± 13.5%) relative to diabetic mice. T1DM mice showed reduced fiber diameters (P = 0.026), and the 3D capillary network analysis indicated a higher capillary length per muscle volume in the gluteus maximus of diabetic mice compared to controls (P < 0.05). Overall, T1DM induced significant changes in the skeletal muscle, including shifts in MyHC fibre types, decreased fibre diameters, and increased relative capillarisation, possibly due to muscle fibre atrophy. Our findings emphasize the superior detail provided by the 3D analytical method for characterizing skeletal muscle capillary architecture, highlighting caution in interpreting 2D data for capillary changes in T1DM.

Single-Nuclei RNA-Sequencing Uncovers Alterations in the Transcriptional Profile of Skeletal Muscle in Peripheral Artery Disease

BACKGROUND: Lower extremity peripheral artery disease (PAD) is caused by atherosclerotic occlusion of lower limb blood vessels, impacting &gt;200 million people worldwide. PAD causes life-altering symptomatology such as claudication, ischemic rest pain, and gangrene, with limited effective treatment options. Herein, we performed single-nuclei RNA-sequencing (snRNAseq) to facilitate a better understanding of the complex pathobiological environment within the PAD limb and intercellular communication. METHODS: Gastrocnemius muscle specimens were collected from 20 patients with PAD and 12 non-PAD controls. Patients with PAD were classified as Rutherford category 3 (severe claudication). Non-PAD participants were of similar age, body mass index and medication usage as patients with PAD. Nuclei were isolated and snRNAseq was performed. Bioinformatic analysis was conducted to determine differences in gene expression between PAD and non-PAD nuclei, as well as analysis of intercellular signaling networks. Histological analysis of muscle specimens assessed the following: myosin heavy chain fiber distributions, myofiber cross-sectional area and myofiber denervation. RESULTS: 57,364 high-quality nuclei were captured (21,417 PAD; 35,947 non-PAD) and unsupervised clustering revealed 12 distinct nuclei populations based on their transcriptional profile. Quantification of the relative abundance of nuclei populations identified several differences, including PAD muscles having a lower percentage of type I myonuclei, muscle stem cells, and endothelial cells but a higher proportion of type II and regenerating myonuclei. Differentially expressed gene analysis of myonuclei populations uncovered upregulation of genes involved in stress response, autophagy, and atrophy in patients with PAD. Ligand-receptor interactions were used to determine how PAD alters the communication between identified nuclei populations. The inferred strength of interactions was greater in non-PAD compared to PAD patients. Further, intercellular communication network analysis revealed fibro-adipogenic progenitors as a primary signaling hub in PAD muscle. CONCLUSION: snRNAseq uncovered novel transcriptional differences and intercellular communication pathways unique to PAD muscle. This study was supported by National Institutes of Health (NIH) grant R01-HL149704 (T. Ryan). K. Kim supported by the American Heart Association grant POST903198. T. Thome supported by NIH grant F31-DK128920. 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.

383 Effect of Feeding Ractopamine to Finisher Lard-Type Mangalica Pigs on Longissimus Muscle Fiber Myosin-Heavy Chain Isoform Expression and Cross-Sectional Area

Abstract Heritage breeds such as Mangalica pigs are gaining popularity in US niche markets and currently sold as high-quality meat due to greater meat quality characteristics. Research has demonstrated that feeding the β-adrenergic agonist ractopamine hydrochloride (RAC), stimulates hypertrophic growth of type-2, fast-twitch glycolytic muscle fibers via mTOR signaling resulting in greater myofibrillar protein synthesis. Therefore, the current study evaluated the effect of feeding RAC to Mangalica pigs during the finisher phase on longissimus muscle fiber myosin heavy-chain (MHC) isoform expression and cross-sectional area. Mangalica pigs were randomly assigned finisher diets for 21 d with 20 ppm (RAC; n = 8) or without (CTL; n = 6). Longissimus muscle samples were collected during carcass fabrication, flash-frozen in liquid nitrogen and stored at -80 °C. Immunofluorescence staining of 5-µm-thick cryosections to identify Type-1 (Developmental Studies Hybridoma Bank (DSHB) antibody Ba-D5), Type-2a (DSHB SC-71), and 2x (DSHB BF-35) MHC positive fibers. Type 2b fibers were determined by subtraction. Fibers in each image (≥ 150) were assessed for fiber type and their cross-sectional area (CSA) at 200X magnification. Data were analyzed as a one-way ANOVA using the GLIMMIX procedure of SAS V9.4. Means were separated with the PDIFF option and declared different when P ≤ 0.05. Tendencies were declared when 0.0501 ≤ P ≤ 0.1000. Feeding RAC decreased the density of Type 2a (P = 0.0136) and Type 2b (P = 0.0186) fibers in the longissimus muscle of Mangalica pigs. A tendency for RAC to reduce the density of Type 1, slow-twitch (6.61 vs. 16.14 fiber per mm2; P = 0.0505) fibers and increase the density of primarily oxidative fibers expressing a combination of Type 1 + Type 2a MHC isoforms (10.23 vs. 2.88%; P = 0.0524) was also observed. When analyzed as a proportion of total fibers, RAC reduced the proportion of fibers expressing only Type 2a (11.8 vs. 6.70%; P = 0.0468) and Type 2b (0.19 vs. 1.32%; P = 0.0101) MHC isoforms but increased the percentage of fibers expressing both Type 1 + Type 2a MHC isoforms (4.66 vs. 1.13%; P = 0.0280). Feeding RAC did not impact the density or proportion of fibers expressing Type 2x MHC (P ≥ 0.386). Mean longissimus muscle fiber CSA was not impacted by feeding Mangalica pigs RAC for 21 d (P = 0.4815). The CSA of the smallest and largest fibers nor fiber distribution were influenced by RAC inclusion in the diet of Mangalica pigs (P ≥ 0.484). In conclusion, feeding RAC Mangalica pigs impacted fiber MHC isoform expression in the longissimus muscle without altering fiber CSA. Future studies will explore the impact of RAC on the CSA of the various fiber types as well as the impact on other muscles in the Mangalica pig.

Skeletal Muscle Function Is Altered in Male Mice on Low-Dose Androgen Receptor Antagonist or Estrogen Receptor Agonist.

In males, skeletal muscle function may be altered by shifts in either circulating testosterone or estrogen. We examined the effect of acute (2-week) exposures to 17α-ethinyl estradiol (EE2), an estrogen receptor (ER) agonist, or flutamide, an androgen receptor (AR) antagonist, on the contractile function of individual skeletal muscle fibers from slow-contracting soleus and fast-contracting extensor digitorum longus muscles from adult male mice. Single fiber specific tension (force divided by cross-sectional area) was decreased with flutamide treatment in all myosin heavy chain (MHC) fiber types examined (I, IIA, and IIB); similar effects were observed with EE2 treatment but only in the fastest-contracting MHC IIB fibers. The decreases in maximally Ca2+-activated specific tension were primarily a result of fewer strongly bound myosin-actin cross-bridges, with flutamide treatment also showing lower myofilament lattice stiffness. Myosin-actin cross-bridge kinetics were slower in MHC IIA fibers in flutamide-treated mice, but faster in EE2-treated mice, indicating that contractile velocity may be affected differently in this fiber type, which is commonly expressed in human skeletal muscle. Importantly, these effects were observed in the absence of outcomes previously used to evaluate ER agonists or AR antagonists in rodents including weight of reproductive organs or mammary gland morphology. Our findings indicate that substantial shifts in skeletal muscle function occur in male mice following acute exposures to low doses of a pharmacological ER agonist and an AR antagonist. These results suggest that countermeasures to maintain physical function may be needed early in situations that induce similar ER agonist and AR antagonist conditions.

Reversible Effects of Functional Mandibular Lateral Shift on Masticatory Muscles in Growing Rats.

In this study, we aimed to determine the effects of functional mandibular lateral shift (FMLS) on the muscle mass, fiber size, myosin heavy chain fiber type, and related gene expression in masticatory muscles (masseter and temporalis), as well as whether the baseline levels could be recovered after FMLS correction in growing rats. The FMLS appliance was placed to shift the mandible leftward by approximately 2 mm. After FMLS placement for 2 and 4 weeks, the muscles on the left side had significantly lower wet weight, mean cross-sectional area, and proportion of type IIa fibers than those on the right side or in the control groups (p < 0.05), with downregulation and upregulation of IGF-1 and GDF-8 gene expression, respectively (p < 0.05). Following 2 weeks devoted to recovery from FMLS, the muscle parameters in the recovery group were not significantly different to those of the control group, and IGF-1 expression in the left-side muscles was enhanced and GDF-8 expression was simultaneously suppressed. These findings indicate that the masticatory muscle changes induced via FMLS tend to revert to normal conditions if the intervention is eliminated at an early stage. Therefore, appropriate orthodontic treatment for FMLS during the growth period is advisable to prevent asymmetric alterations in masticatory muscles.

Revisiting specific force loss in human permeabilised single skeletal muscle fibres obtained from older individuals.

Specific force (SF) has been shown to be reduced in some but not all studies of human ageing using chemically skinned single muscle fibres. This may be due in part to the health status/physical activity levels of different older cohorts, but also from methodological differences in studying skinned fibres. The aim of the present study was to compare SF in fibres from older hip fracture patients (HFP), healthy master cyclists (MC) and healthy non-trained young adults (YA) using two different activating solutions. Quadriceps muscle samples and 316 fibres were obtained from HFPs (74.6±4 years, n=5), MCs (74.8±1, n=5) and YA (25.5±2, n=6). Fibres were activated (pCa 4.5, 15oC) in solutions containing either 60 mM TES or 20 mM imidazole. SF was determined by normalising force to fibre CSA assuming either an elliptical or circular shape and to fibre myosin heavy chain content. Activation in TES resulted in significantly higher MHC-I SF in all groups and YA MHC-IIA fibres, irrespective of normalisation method. Whilst there were no differences in SF between the participant groups, the ratio of SF between the TES and imidazole solutions was lower in HFPs compared to YAs (MHC-I p<0.05; MHC-IIA p=0.055). Activating solution composition, as opposed to donor characteristics, had a more notable effect on single fibre SF. However, this two-solution approach revealed an age-related difference in sensitivity in HFPs which was not shown in MCs. This suggests further novel approaches may be required to probe age/activity-related differences in muscle contractile quality.

A chronic high-fat diet does not exacerbate muscle atrophy in fast-twitch skeletal muscle of aged mice.

What is the central question of this study? Ageing leads to a loss of mass in skeletal muscle, but the effect of obesity on ageing-related muscle wasting is unclear. In this study, we aimed to demonstrate the specific effect of obesity on fast-twitch skeletal muscle in ageing. What is the main finding and its importance? Our findings show that the obesity induced by long-term ingestion of a high-fat diet does not aggravate muscle wasting in fast-twitch skeletal muscle of aged mice, indicating that the present study provides morphological characteristics for skeletal muscle of sarcopenic obesity. Obesity and ageing reduce muscle mass and lead to deficits in muscle maintenance, but it is not known whether obesity accelerates muscle wasting additively in the setting of ageing. We investigated morphological characteristics in fast-twitch extensor digitorum longus (EDL) muscle of mice fed a low-fat diet (LFD) or a high-fat diet (HFD) for 4 or 20 months. The fast-twitch EDL muscle was harvested, and the muscle fibre-type composition, individual muscle cross-sectional area and myotube diameter were measured. We found an increase in the percentage of type IIa and IIx myosin heavy chain fibres in the whole EDL muscle, but a decrease in type IIB myosin heavy chain in both HFD protocols. The cross-sectional area and myofibre diameter were lower in both groups of aged mice (after 20 months of LFD or HFD) compared with young mice (after 4 months of the diets), but there were no differences between mice fed LFD or HFD for 20 months. These data suggest that long-term feeding of HFD does not aggravate muscle wasting in fast-twitch EDL muscle of male mice.

Young bone marrow transplantation prevents aging-related muscle atrophy in a senescence-accelerated mouse prone 10 model.

Young bone marrow transplantation (YBMT) has been shown to stimulate vascular regeneration in pathological conditions, including ageing. Here, we investigated the benefits and mechanisms of the preventive effects of YBMT on loss of muscle mass and function in a senescence-associated mouse prone 10 (SAMP10) model, with a special focus on the role of growth differentiation factor 11 (GDF-11). Nine-week-old male SAMP10 mice were randomly assigned to a non-YBMT group (n=6) and a YBMT group (n=7) that received the bone marrow of 8-week-old C57BL/6 mice. Compared to the non-YBMT mice, the YBMT mice showed the following significant increases (all P < 0.05 in 6-7 mice): endurance capacity (>61.3%); grip strength (>37.9%), percentage of slow myosin heavy chain fibres (>14.9-15.9%). The YBMT also increased the amounts of proteins or mRNAs for insulin receptor substrate 1, p-Akt, p-extracellular signal-regulated protein kinase1/2, p-mammalian target of rapamycin, Bcl-2, peroxisom proliferator-activated receptor-γ coactivator (PGC-1α), plus cytochrome c oxidase IV and the numbers of proliferating cells (n = 5-7, P < 0.05) and CD34+/integrin-α7+ muscle stem cells (n = 5-6, P < 0.05). The YMBT significantly decreased the levels of gp91phox, caspase-9 proteins and apoptotic cells (n = 5-7, P < 0.05) in both muscles; these beneficial changes were diminished by the blocking of GDF-11 (n = 5-6, P < 0.05). An administration of mouse recombinant GDF-11 improved the YBMT-mediated muscle benefits (n = 5-6, P < 0.05). Cell therapy with young bone marrow from green fluorescent protein (GFP) transgenic mice exhibited GFP+ myofibres in aged muscle tissues. These findings suggest that YBMT can prevent muscle wasting and dysfunction by mitigating apoptosis and proliferation via a modulation of GDF-11 signalling and mitochondrial dysfunction in SAMP10 mice.

A case report of deglutition triggered atrial fibrillation in a patient with Laing distal myopathy

BackgroundDeglutition-induced atrial fibrillation is a rare clinical entity with a reported prevalence of 0.6%. Laing distal myopathy is a rare autosomal dominant muscular dystrophy that is the result of mutations within the slow skeletal muscle fibre myosin heavy chain gene (MYH7). Atrial fibrillation has not been previously reported in patients with Laing distal myopathy. We describe the first reported case of deglutition triggered atrial fibrillation in a female with a history of Laing distal myopathy.Case summaryA 44-year-old female with a history of Laing distal myopathy diagnosed at age 32, began experiencing intermittent episodes of pre-syncope and palpitations which occurred after deglutition with food. An ambulatory 30-day patient triggered event monitor recorded episodes of atrial fibrillation with rapid ventricular response. Family history was significant for Laing distal myopathy, atrial fibrillation, as well as sudden cardiac death. Laboratory data, transthoracic echocardiogram, cardiac magnetic resonance imaging, and an exercise treadmill SPECT Imaging stress test were normal. An oesophagram revealed a mild oesophageal dysmotility with no other abnormalities. She was started on flecainide 50 mg p.o. every 8 h and verapamil 40 mg p.o. every 8 h with no further episodes of atrial fibrillation.DiscussionGiven the strong genetic component of this myopathy, one could postulate as to a possible genetic component in the development of atrial fibrillation in our patient. Although we cannot make definite correlation between deglutition-induced atrial fibrillation and Laing myopathy, it is important to report this unusual association which has not been described before.

Insight into type 2 diabetes impaired exercising mitochondrial oxidative flux: is it blood flow, mitochondria, or neither?

Insight into type 2 diabetes impaired exercising mitochondrial oxidative flux: is it blood flow, mitochondria, or neither?

Skeletal muscle specific mitochondrial dysfunction and altered energy metabolism in a murine model (oim/oim) of severe osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heritable connective tissue disorder with patients exhibiting bone fragility and muscle weakness. The synergistic biochemical and biomechanical relationship between bone and muscle is a critical potential therapeutic target, such that muscle weakness should not be ignored. Previous studies demonstrated mitochondrial dysfunction in the skeletal muscle of oim/oim mice, which model a severe human type III OI. Here, we further characterize this mitochondrial dysfunction and evaluate several parameters of whole body and skeletal muscle metabolism. We demonstrate reduced mitochondrial respiration in female gastrocnemius muscle, but not in liver or heart mitochondria, suggesting that mitochondrial dysfunction is not global in the oim/oim mouse. Myosin heavy chain fiber type distributions were altered in the oim/oim soleus muscle with a decrease (−33 to 50%) in type I myofibers and an increase (+31%) in type IIa myofibers relative to their wildtype (WT) littermates. Additionally, altered body composition and increased energy expenditure were observed oim/oim mice relative to WT littermates. These results suggest that skeletal muscle mitochondrial dysfunction is linked to whole body metabolic alterations and to skeletal muscle weakness in the oim/oim mouse.

A maternal high-fat/low-fiber diet impairs glucose tolerance and induces the formation of glycolytic muscle fibers in neonatal offspring.

In our previous study, the maternal high-fat/low-fiber (HF-LF) diet was suggested to induce metabolic disorders and placental dysfunction of the dam, but the effects of this diet on glucose metabolism of neonatal offspring remain largely unknown. Here, a neonatal pig model was used to evaluate the effects of maternal HF-LF diet during pregnancy on glucose tolerance, transition of skeletal muscle fiber types, and mitochondrial function in offspring. A total of 66 pregnant gilts (Guangdong Small-ear Spotted pig) at day 60 of gestation were randomly divided into two groups: control group (CON group; 2.86% crude fat, 9.37% crude fiber), and high-fat/low-fiber diet group (HF-LF group; 5.99% crude fat, 4.13% crude fiber). The maternal HF-LF diet was shown to impair the glucose tolerance of neonatal offspring, downregulate the protein level of slow-twitch fiber myosin heavy chain I (MyHC I), and upregulate the protein levels of fast-twitch fiber myosin heavy chain IIb (MyHC IIb) and IIx (MyHC IIx) in soleus muscle. Additionally, compared with the CON group, the HF-LF offspring showed inhibition of insulin signaling pathway and decrease in mitochondrial function in liver and soleus muscle. Maternal HF-LF diet during pregnancy impairs glucose tolerance, induces the formation of glycolytic muscle fibers, and decreases the hepatic and muscular mitochondrial function in neonatal piglets.

Laing Myopathy: Report of 4 New Families With Novel MYH7 Mutations, Double Mutations, and Severe Phenotype.

Laing distal myopathy (LDM) is an autosomal dominant disorder caused by mutations in the slow skeletal muscle fiber myosin heavy chain (MYH7) gene on chromosome 14q11.2. The classic LDM phenotype-including early-onset, initial involvement of foot dorsiflexors and great toe extensors, followed by weakness of neck flexors and finger extensors-is well documented. Since the original report by Laing et al in 1995, the spectrum of MYH7-related myopathies has expanded to include congenital myopathies, late-onset myopathies, myosin storage myopathy, and scapuloperoneal myopathies. Most patients with LDM harbor mutations in the midrod domain of the MYH7 gene, but rare cases document disease-associated mutations in the globular head region. In this report, we add to the medical literature by describing the clinicopathological findings in 8 affected family members from 4 new LDM families-including 2 with novel MYH7 mutations (Y162D and A1438P), one with dual mutations (V39M and K1617del), and one family (E1508del) with severe early-onset weakness associated with contractures, respiratory insufficiency, and dilated cardiomyopathy. Our families highlight the ever-expanding clinical spectrum and genetic variation of the skeletal myopathies related to MYH7 gene mutations.

Myosin Heavy Chain Composition, Creatine Analogues, and the Relationship of Muscle Creatine Content and Fast-Twitch Proportion to Wilks Coefficient in Powerlifters.

Machek, SB, Hwang, PS, Cardaci, TD, Wilburn, DT, Bagley, JR, Blake, DT, Galpin, AJ, and Willoughby, DS. Myosin heavy chain composition, creatine analogues, and the relationship of muscle creatine content and fast-twitch proportion to Wilks coefficient in powerlifters. J Strength Cond Res 34(11): 3022-3030, 2020-Little data exist on powerlifting-specific skeletal muscle adaptations, and none elucidate sex differences in powerlifters. Powerlifters tend to display higher fast-twitch fiber content and phosphagen system dependence. Nevertheless, it is unknown whether fast-twitch fiber or muscle creatine content are predictive of competitive powerlifting performance (via Wilks coefficient). Twelve actively competing powerlifters (PL; n = 6M/6F; age = 21.3 ± 1.0; 3.0 ± 1.8 year competing; 7.3 ± 6.6 meets attended) and 10 sedentary controls (CON; n = 5M/5F; age = 19.4 ± 2.0 year) underwent vastus lateralis muscle biopsies and venipuncture to compare the myosin heavy chain (MHC) fiber type and creatine analogue profiles between groups of both sexes, and determine whether MHC IIa and muscle total creatine (MTC) composition predict powerlifting performance. Samples were analyzed for specific MHC isoform (I, IIa, and IIx) content via mixed homogenate SDS-PAGE, and creatine analogues (MTC, muscle creatine transporter [SLC6A8], serum total creatine [STC], and serum creatinine [CRT]). Furthermore, MHC IIa and MTC content were compared with Wilks coefficient using Pearson correlation coefficients. Male PL MHC content was 50 ± 6% I, 45 ± 6% IIa, and 5 ± 11% IIx, versus 46 ± 6% I, 53 ± 6 IIa, and 0% IIx in female PL. Conversely, male CON MHC content was 33 ± 5% I, 38 ± 7% IIa, and 30 ± 8% IIx, vs. 35 ± 9% I, 44 ± 8% IIa, and 21 ± 17% IIx in female CON. Muscle total creatine, SLC6A8, STC, and CRT did not significantly differ between groups nor sexes. Finally, neither MHC IIa content (r = -0.288; p = 0.364) nor MTC (r = 0.488; p = 0.108) significantly predicted Wilks coefficient, suggesting these characteristics alone do not determine powerlifting skill variation.

The Impact Of Age On The Transcriptional And Morphological Profile Of Skeletal Muscle

Aging is associated with many physiological changes that impact physical function. Most notably, older adults experience a progressive loss of skeletal muscle mass and function, termed sarcopenia. A better understanding of the molecular and phenotypical changes associated with advancing age may provide therapeutic targets for interventions to slow the progression of sarcopenia. PURPOSE: To characterize the transcriptional and morphological profile of aging skeletal muscle. METHODS: Resting vastus lateralis muscle biopsies were collected from 9 young (Y; 27±3yr, 179±7cm, 82±10kg, 26±3BMI) and 9 older adults (O; 68±5yr, 172±8cm, 77±19kg, 26±5BMI) following an overnight fast. Whole transcriptome next-generation RNA sequencing was performed on cDNA synthesized from skeletal muscle RNA. Differentially expressed genes (FDR-adjusted P-value ≤ 0.05) were identified through DESeq2 and subjected to bioinformatic analyses using DAVID (v6.8). Skeletal muscle morphology including fiber type, satellite cell (SC) content, and capillarization was assessed through immunofluorescent microscopy RESULTS: In total, 900 differentially expressed genes were identified in the skeletal muscle of O versus Y (+1.5 fold change = 213; -1.5 fold change = 127). DAVID functional analyses indicated that aging was associated with functions related to glycogen metabolism, amino acid metabolism, ubiquitination, and transition between fast and slow fibers. Consistent with the latter, a significant difference (P=0.048) in myosin heavy chain (MyHC) fiber type profile was identified (Y = MyHCI: 29±4%, MyHCII: 71±4%; O = MyHCI: 46±7%, MyHCII: 54±7%). Moreover, aging was associated with a numerical reduction in SC specific to MyHCI (Y = 0.13±0.02, O = 0.07±0.02 SC/MyHCI fiber, P=0.07) but not MyHCII fibers (Y=0.12±0.03, O=0.08±0.02 P=0.373). Independent of fiber type, capillaries per fiber was significantly lower (P=0.015) in O (1.53±0.34) vs. Y (4.59±0.85). CONCLUSION: Advancing age is associated with changes in the transcriptional and morphological profile of skeletal muscle. These findings highlight potential therapeutic targets for the preservation of skeletal muscle mass and function with advancing age. Supported by a JumpStart Grant, CHS, ASU.

Postnatal development of skeletal muscle in pigs with intrauterine growth restriction: morphofunctional phenotype and molecular mechanisms.

Intrauterine growth restriction (IUGR) is a serious condition which impairs the achievement of the fetus' full growth potential and occurs in a natural and severe manner in pigs as a result of placental insufficiency. Reduced skeletal muscle mass in the fetus with IUGR persists into adulthood and may contribute to increased metabolic disease risk. To investigate skeletal muscle postnatal development, histomorphometrical patterns of the semitendinosus muscle, myosin heavy chain (MyHC; embryonic I, IIA, IIB and IIX isoforms) fiber composition and the relative expression of genes related to myogenesis, adipogenesis and growth during three specific periods: postnatal myogenesis (newborn to 100days old), hypertrophy (100-150days old), and postnatal development (newborn to 150days old) were evaluated in female pigs with IUGR and normal birth weight (NW) female littermates. NW females presented higher body weights compared to their IUGR counterparts at all ages evaluated (P<0.05). Moreover, growth restriction in utero affected the semitendinosus muscle weight, muscle fiber diameter, and muscle cross-sectional area, which were smaller in IUGR pigs at birth (P<0.05). Notwithstanding the effects on muscle morphology, IUGR also affected muscle fiber composition, as the percentage of MyHC-I myofibers was higher at birth (P<0.05), and, in 150-day-old gilts, a lower percentage of MyHC-IIX isoform (P<0.05) and the presence of embryonic MyHC isoform were also observed. Regarding the pattern of gene expression in both the postnatal myogenesis and postnatal development periods, IUGR led to the downregulation of myogenic factors, which delayed skeletal muscle myogenesis (PAX7, MYOD, MYOG, MYF5 and DES). Altogether, growth restriction in utero affects muscle fiber number and size at birth and muscle fiber composition through the downregulation of myogenic factors, which determines the individual´s postnatal growth rate. This fact, associated with delayed myofiber development in growth-restricted animals, may affect meat quality characteristics in animal production. Hence, knowledge of the morphofunctional phenotype of the skeletal muscle throughout postnatal development in individuals with IUGR, and the mechanism that governs it, may provide a better understanding of the mechanisms that limit postnatal muscle growth, and help the establishment of potential strategies to improve muscle development and prevent the onset of later-life metabolic diseases.

Effects of inhaled fluticasone propionate on extrinsic tongue muscles in rats.

Obstructive sleep apnea (OSA) is more common in patients with asthma, and inhaled corticosteroids may contribute to OSA pathogenesis in these patients. This study tested the effects of orally inhaled fluticasone propionate (FP) on extrinsic tongue muscles. Unanesthetized rats were treated with FP or placebo for 28 days. On day 29, tongue retrusive and protrusive functions were tested via hypoglossal nerve stimulation under a state of anesthesia, followed by genioglossus (GG), styloglossus (SG) and hyoglossus (HG) muscle extraction, after euthanasia, for histology [myosin heavy chain (MHC) fibers and laminin content reflecting extracellular matrix (ECM)]. On protrusive testing, FP increased percent maximum tetanic force at 40 Hz (P = 0.03 vs. placebo) and endurance index (P = 0.029 vs. placebo). On retrusive testing, FP increased maximum twitch (P = 0.026 vs. placebo) and tetanic forces (P = 0.02 vs. placebo) with no effect on endurance index. On histology, FP increased GG cross-sectional area of MHC type IIa (P = 0.036 vs. placebo) and tended to increase type IIb (P = 0.057 vs. placebo) fibers and HG MHC IIx fibers (P = 0.065). The FP group had significantly increased laminin-stained areas, of greatest magnitude in the HG muscle. FP affects tongue protrusive and retrusive functions differently, concurrent with a shift in MHC fibers and increased ECM accumulation. These differential alterations may destabilize the tongue's "muscle hydrostat" during sleep and promote collapse.NEW & NOTEWORTHY The effects of inhaled corticosteroid on upper airway may contribute to OSA pathogenesis in asthma. In this study, we tested the effects of orally inhaled fluticasone propionate on tongue protrusive and retrusive functions and on tongue extrinsic muscle fiber composition and molecular properties. We found that fluticasone treatment: 1) increased protrusive endurance and retrusive maximum twitch and tetanic force; and 2) on histology, increased cross-sectional area of myosin heavy chain (MHC) type IIa fibers and tended to increase cross-sectional area of MHC type IIb fibers in the protrusive muscle and of MHC IIx fibers in the retrusors. It also increased laminin-stained areas, across extrinsic tongue muscles, of greatest magnitude in the retrusors; and 3) reduced protein degradation and activated pathways associated with increased protein synthesis in the protrusor. These differential effects on the protrusors and retrusors may destabilize the tongue's "muscle hydrostat" properties during sleep and promote collapse.

Characterizing Pork Quality of Carcasses with an Average Weight of 119 Kg

ObjectivesBetween 1995 and 2018, average hot carcass weight of U.S. pork carcasses increased from 82 kg to 96 kg, which is an increase of approximately 17%. At current rates, pork carcasses in the U.S. will weigh on average, 105 kg by the year 2030 and over 118 kg by 2050. Although this represents an increase in throughput efficiency due to increases in economy of scale, projecting continued increases in the future raises some concerns. Therefore, the objective was to characterize pork quality of carcasses ranging from 78 to 145 kg with a mean weight of 119 kg.Materials and MethodsCarcass composition, such as hot carcass weight (HCW), back fat depth and loin depth were measured on 666 carcasses. Additionally, loin quality measurements, such as pH, loin instrumental and visual color, and iodine value of clear plate fat were measured on approximately 90% of the total population. Ham quality, 14 d aged loin and chop quality measurements, and loin chop slice shear force (SSF) were evaluated on approximately 30% of the total population. Finally, myosin heavy chain fiber type determination was completed on approximately 50 carcasses selected from carcasses ranging from 97 to 133 kg. The slope of regression lines and coefficients of determination between hot carcass weights and quality traits were calculated using the REG procedure in SAS and considered significantly different from 0 at P ≤ 0.05.ResultsAs HCW increased loin depth (b1 = 0.2496, P &lt; 0.0001), back fat depth (b1 = 0.1374, P &lt; 0.0001), loin weight (b1 = 0.0345, P &lt; 0.0001), chop weight (b1 = 1.6626, P &lt; 0.0001), and ham weight (b1 = 0.1044, P &lt; 0.0001) increased. There was a decrease in estimated lean (b1 = –0.0751, P &lt; 0.0001) and iodine value (b1 = –0.0923, P &lt; 0.0001) as carcass weight increased, however, HCW only accounted for ≤ 24% (R2 = 0.24) of the variation in estimated lean and iodine value. Additionally, there were no significant differences in gluteus medius pH (b1 = 0.0009, P = 0.30) or instrumental lightness (b1 = 0.0301, P = 0.15), redness (b1 = –0.0036, P = 0.73) or yellowness (b1 = 0.0058, P = 0.57) of the ham as carcass weight increased. As carcass weight increased, 1 d loin instrumental yellowness (b*) increased (b1 = 0.0092 P &lt; 0.01), however HCW only explained 1% of the variation in b*. Heavier carcasses were more tender (decreased SSF of chops cooked to 71°C, (b1 = –0.0674, P &lt; 0.0001), although HCW only explained 9% of the variation in SSF. Total cook loss of chops used for SSF determination decreased as HCW increased (b1 = –0.0512, P &lt; 0.0001), and HCW explained 15% (R2 = 0.15) of the variation in total cook loss. There were no significant differences in fiber type percentage, type 1 (b1 = –0.0170, P = 0.81), 2a (b1 = –0.0786, P = 0.23), 2x (b1 = –0.0201, P = 0.80), or 2b (b1 = 0.1224, P = 0.37), or fiber type area, type 1 (b1 = –26.6331, P = 0.22), 2a (b1 = –40.7257, P = 0.07), 2x (b1 = –46.9459, P = 0.25), or 2b (b1 = –26.2537, P = 0.38) as HCW increased.ConclusionDue to the lack of variation explained by HCW (≤ 15%), pork quality traits are not expected to be compromised as HCW continues to increase. The results suggest that increasing HCW to 119 kg did not have detrimental effects on pork quality attributes.

Fiber typing human skeletal muscle with fluorescent immunohistochemistry.

Skeletal muscle myosin heavy chain (MyHC) fiber type composition is a critical determinant of overall muscle function and health. Various approaches interrogate fiber type at the single cell, but the two most commonly utilized are single-muscle fiber sodium dodecyl sulfate-polyacrylamide gel electrophoresis (smfSDS-PAGE) and fluorescent immunohistochemistry (IHC). Although smfSDS-PAGE is generally considered the "gold standard," IHC is more commonly used because of its time-effectiveness and relative ease. Unfortunately, there is lingering inconsistency on how best to accurately and quickly determine fiber type via IHC and an overall misunderstanding regarding pure fiber type proportions, specifically the abundance of fibers exclusively expressing highly glycolytic MyHC IIX in humans. We therefore 1) present information and data showing the low abundance of pure MyHC IIX muscle fibers in healthy human skeletal muscle and 2) leverage this information to provide straightforward protocols that are informed by human biology and employ inexpensive, easily attainable antibodies for the accurate determination of fiber type.

Skeletal Muscle Fiber Type and Morphology in a Middle-Aged Elite Male Powerlifter Using Anabolic Steroids

Powerlifting regularly exposes athletes to extreme stimuli such as chronic heavy resistance training (HRT), and many powerlifters choose to augment their performance with anabolic–androgenic steroids (AAS). However, little is known about the myocellular adaptations that occur from long-term HRT and AAS use, especially into middle age. We were presented with the unique opportunity to study muscle cells from an elite-level powerlifter (EPL; age 40 years) with ≥ 30 years of HRT experience and ≥ 15 years of AAS use. The purpose of this case study was to identify myocellular characteristics [myosin heavy chain (MHC) fiber type, fiber size, and myonuclear content] in EPL, as well as compare these data to existing literature. The participant underwent a resting vastus lateralis muscle biopsy and single fibers were analyzed for MHC content via SDS-PAGE. A subset of fibers underwent MHC-specific imaging analysis via confocal microscopy to identify cell size (cross-sectional area, CSA) and myonuclear domain (MND) size. MHC fiber type distribution was 9% I, 12% I/IIa, 79% IIa, and 0% other isoforms. This pure MHC IIa (fast-twitch) fiber content was amongst the highest reported in the literature. Imaging analysis of MHC IIa fibers revealed a mean CSA of 4218 ± 933 μm2 and MND of 12,548 ± 3181 μm3. While the fast-twitch fiber CSA was comparable to values in previous literature, mean MND was smaller than has been reported in untrained men, implying greater capacity for growth and repair. These findings showcase the unique muscle cell structure of an elite powerlifter, extending the known physiological limits of human muscle size and strength.