Contraction of striated muscle is initiated by an increase in the cytosolic Ca[2+] concentration, which is regulated by tropomyosin and troponin on thin filaments in sarcomeres; viz., the binding of Ca[2+] to troponin C shifts the "on-off" equilibrium of the thin filament state toward the "on" state, allowing myosin-binding to actin. Also, accumulating evidence shows that in both skeletal and cardiac muscles of mammals, increasing ambient temperature to within the body temperature range not only accelerates the actomyosin ATPase activity but also shifts the "on-off" equilibrium of the thin filament state toward the "on" state, even in the absence of Ca[2+]. Especially noteworthy is that in an in vitro motility assay, the skeletal contractile system, compared with the cardiac contractile system, shows greater temperature dependence of activation, despite a higher threshold temperature for the onset of sliding (by ∼2°C). In this short review, we discuss the differential effects of heating on the skeletal vs. cardiac contractile system, focusing especially on the temperature-dependent shift of the "on-off" equilibrium of the thin filament state in sarcomeres.

Inherited hypertrophic cardiomyopathy (HCM) is considered a disease of the cardiac sarcomere and caused by pathogenic variants present in genes that encode sarcomeric proteins. The human TNNC1 gene is a designated HCM-susceptibility gene encoding the troponin C (TnC) protein, which is expressed in both cardiac and type I slow skeletal muscles and abbreviated as cTnC and ssTnC respectively. HCM patients have been reported to exhibit skeletal muscle weakness and reduced exercise tolerance. Patients bearing TNNC1 cardiac pathogenic variants also express it in type I fibers of their slow skeletal muscles. We hypothesized that the presence of TNNC1 HCM variants in type I fibers may decrease force generating capabilities and alter fatigue resistance of slow skeletal muscles. To address this, we examined the impact of HCM Tnnc1 variants in the soleus muscles of two HCM knock-in mouse models A8V+/-, A8V-/-, and C84Y+/- and their respective wild type (WT) controls. At high stimulation frequencies, we found that A8V-/- soleus muscles had lower tetanic force production than WT, however no differences in fatigue when comparing fatigue indices for either of the variants. The muscles were evaluated by comparing % specific force (N/cm[2])-variant and both A8V-/- and C84+/- had increased responsiveness at low stimulation frequencies. Fiber type analysis uncovered an increase in abundance of type I fibers in A8V-/- soleus, however no other differences were observed in fiber-type percentage of either mouse model. Comparison of the cross-sectional areas (CSA) of type I fibers in soleus muscles revealed lower average values for A8V+/+ and A8V-/-, however an increase in C84Y+/- relative to controls. Examination of the CSA of type IIa fibers uncovered decreased values for both A8V+/- and A8V-/- with no changes in C84Y+/- soleus. Histological evaluation of A8V+/- and A8V-/- soleus muscles revealed central nucleation and the detection of embryonic myosin heavy chain by immunofluorescence suggested the presence of mild regeneration. In contrast, no histopathological changes were detected in the C84Y+/- soleus muscle. Serum myokines were also measured to assess systemic impacts of the pathogenic variants and alterations in the physical activity of the mice but no significant changes were found. Taken together our results suggest that the HCM A8V variant alters force production in soleus muscle that may be attributed to fiber atrophy and heightened pathophysiology.

Resistance training in rodent models is widely used to investigate muscular and cardiovascular adaptations. Traditional models often attach loads to the tail. The present study aimed to develop and validate a resistance training model for rats using a pulley system combined with a load-bearing vest, allowing progressive overload without tail attachment. Spontaneously hypertensive rats were assigned to control (CTR, n = 4) or resistance training (RT, n = 6) groups for a 10-week ladder climbing protocol. Functional and morphometric adaptations were evaluated in the soleus and flexor hallucis longus (FHL) muscles. Resistance training resulted in a significant increase in relative maximum load carried (p = 0.0007), corresponding to a 79.5% improvement in strength capacity. The RT group also showed increased relative mass of the FHL muscle (p = 0.002). Gene expression analyses did not show statistically significant differences between groups (p > 0.05). The pulley-vest system proved to be a viable and effective resistance training model for rats, allowing controlled overload while avoiding tail load attachment and potential vascular interference. This model represents a refined experimental approach for resistance training studies in rodent models, particularly in conditions where tail circulation must be preserved.

Duchenne muscular dystrophy (DMD) is a severe genetic disorder characterized by progressive muscle degeneration, and the mdx mouse has been widely used to investigate disease mechanisms and therapeutic strategies, including the impact of different exercise modalities and sex-related responses on dystrophic muscle. This study compared moderate/low-intensity swimming and loaded ladder-climbing in male and female mdx, including sedentary control groups for each sex. Animals were subjected to a 4-week swimming (30 min/day, 4 days/week) or a 15-day ladder-climbing resistance protocol (6 sessions, 3 sets of 10 climbs, every 48 h), starting at 30 and 45 days of age, respectively. All groups (male and female sedentary controls, swimming, and resistance-trained mice) were evaluated at 60 days of age. In male mdx mice, swimming reduced serum creatine kinase levels and improved limb functional performance compared to sedentary controls. In contrast, resistance exercise increased diaphragmatic fibrosis and reduced centrally nucleated fibers in biceps brachii in male mdx mice. Resistance-trained males also exhibited lower holding impulse compared to swimming-trained males. Females exhibited a milder response, with superior fatigue resistance, greater mechanical work during climbing, and superior final holding impulse compared to males. These findings demonstrate that exercise outcomes in mdx mice depend strongly on modality, intensity, and sex. Swimming provided functional benefits, while ladder-climbing induced measurable but controlled muscle adaptations, without consistent evidence of widespread damage.

UNC-89 is a giant modular protein located at the sarcomeric M-line of C. elegans striated muscle and is required for sarcomere organization and function. UNC-89 contains two protein kinase domains, PK1 and PK2, separated by 850 residues, that includes a 645-residue long intrinsically disordered sequence that acts like an elastic spring. Bioinformatic analysis suggests that PK2 is an active kinase whereas PK1 is a pseudokinase. We recently reported that a genome-edited worm, unc-89(sf22), that expresses UNC-89 carrying a kinase-inactivating point mutation in PK2 has an unusual phenotype with normally organized sarcomeres and SR, normal muscle function and yet fragmented mitochondria, increased ATP levels, increased glycolysis and alterations in electron transport chain complexes and respiration. Here, we show that a genome-edited worm unc-89(sf23), that expresses UNC-89 with an in-frame deletion of the C-lobe of PK1 has approximately the same phenotype as the PK2 catalytically dead mutant. The fact that mutations in two different regions of UNC-89 result in a mitochondrial phenotype is further evidence of communication between the sarcomere and mitochondria. We further demonstrate that in vitro PK2 interacts with full length PK1 and the C-lobe of PK1. The protein kinase domains of giant sarcomeric proteins are autoinhibited by parts of their own sequence, and this is also likely for PK2, but the mechanism by which PK2 would be activated is unknown. Our data is compatible with a model in which PK1 interacts with PK2 and thereby stimulates PK2 kinase activity.

Muscles undergo stretch-shortening cycles (SSCs) and following active stretching, work, and power during shortening are amplified compared with pure shortening contractions (i.e., SSC effect). We investigated the effects of immobilization on SSCs and the contribution of residual force depression (rFD) and enhancement (rFE). Single muscle fibres (n = 120) were chemically permeabilized from the rat soleus and medial gastrocnemius of control and cast (14 days unilateral immobilization). Fibres were maximally activated while mounted between a force transducer and length controller. For SSCs, fibres were actively lengthened from sarcomere lengths of 2.5-to-3.0 µm and immediately shortened back to 2.5 µm (all 0.6Lo/s), work and power during shortening was compared to a shortening contraction not preceded by active lengthening. rFD and rFE were assessed using active shortening (3.0-to-2.5 µm) or lengthening (2.5-to-3.0 µm) contractions, respectively, both 0.6Lo/s, with steady-state isometric force compared to a fixed-end isometric contraction at the same length. Cast fibres had smaller cross-sectional areas compared to control (both P < 0.05). Compared to control fibres, cast soleus showed ~ 6% greater rFD% (P = 0.0027) but no difference in rFE% (P = 0.55), whereas cast medial gastrocnemius showed ~ 5% reduced rFE% (P = 0.001) and unaffected rFD% (P = 0.09). Cast fibres demonstrated SSC performance enhancement (~ 120% and ~ 210%, all P < 0.001), however, the enhancement was less in the cast compared to control medial gastrocnemius (~ 88% vs ~ 120%, P = 0.022), while the soleus was not (both P > 0.86). Across muscles the SSC effect mitigated deficits in absolute work and power in the cast compared to control. Therefore, SSCs can mitigate contractile deficits at the cellular level after immobilization.

Diabetes mellitus can apply its first adverse effects by developing endothelial dysfunction, which can progress to diabetic vasculopathy. This study was assessed the influence of melatonin on aortic reactivity to endothelin-1 (ET-1) in streptozotocin (STZ)-induced diabetic rats and to identify its mechanisms through calcium channels and endothelium-derived relaxing factors (EDRFs). This experimental study was carried out on rats randomly assigned to three groups: control, diabetic (induced by a single intraperitoneal dose of STZ), and diabetic treated with melatonin (30 mg/kg for two weeks). After the rats were anesthetized and aortic rings were prepared, concentration-response curves to ET-1 were recorded alone and following preincubation with amlodipine (L-type calcium channel blocker), L-NAME (NOS inhibitor), and indomethacin (COX inhibitor). Histomorphic changes evaluated in all groups. Melatonin reduced vascular reactivity to ET-1 in the aorta of diabetic rats. Amlodipine did not reduce ET-1–induced contraction in diabetic group, but in the melatonin group, the inhibitory effect of amlodipine was restored (P < 0.05). Pretreatment with L-NAME decreased the induced contraction in diabetic rats, unlike in the control group. Melatonin led to a contraction response close to that of the control group in diabetic rats. Diabetes increased aortic wall thickness and perinuclear space, and melatonin reversed these changes (P < 0.05). The findings indicate that melatonin reduces vascular dysfunction and structural damage in diabetes by restoring L-type calcium channel sensitivity and modulating the balance of EDRFs, such as NO and prostanoids. These results highlight the potential of melatonin as an adjunct therapy for preventing diabetic vascular complications.

Smooth muscle cells are widespread in the human body, found in a range of tissues from the vasculature to the gut, airways, bladder and so on. They can be found as isolated cells or organised into syncytia (groups of cells). The organisation of the contractile cytoskeleton (smooth muscle actin and myosin filaments) is less well understood than that of striated (cardiac and skeletal) muscle. This review aims to explore what is known, the potential effects of mutations in smooth muscle actin, myosin and tropomyosin and how they lead to disease, and to highlight what we have still to learn about the smooth muscle contractile cytoskeleton.

In vitro skeletal muscle culture models provide important insight into the cellular mechanisms which underpin skeletal muscle physiology and metabolism in health and disease. The establishment of a model that can be cultured in physiological concentrations of glucose is an important factor in its translatability to more complex models and systems. Using the human skeletal muscle cell line, LHCN-M2 myoblasts, we aimed to determine the effects of different concentrations of glucose in culture media on cell viability, proliferation, ATP production and differentiation. LHCN-M2 myoblasts were cultured in NORM (1 g· L[- 1]) or HIGH (3.8 g· L[- 1]) glucose growth media, and cell viability, ATP production, and proliferation were measured. Immunofluorescence microscopy was used to determine LHCN-M2 differentiation into multinucleated myotubes with increasing concentrations of human serum (0.5%, 1% and 2% v/v). There were no differences in the viability, proliferation or basal ATP production rates of LHCN-M2 cells grown in NORM compared to HIGH glucose (P > 0.05). Morphological analysis revealed that myotube area was greater when differentiated in 2% compared to 0.5% human serum (P = 0.02), but myotube number and fusion index were unaffected (P > 0.05). These findings demonstrate that LHCN-M2 cells are capable of proliferating and differentiating into multinucleated myotubes under normal glucose concentrations in the culture media. Further work is required to determine the implications of media glucose concentration on the wider metabolic function and phenotype of LHCN-M2 myoblasts cells and myotubes.

Thyroid hormones (THs) are critical regulators of vertebrate embryonic development, governing cell proliferation, lineage commitment, and tissue maturation, including skeletal muscle formation. Disruption of TH homeostasis during early development can therefore have profound consequences for myogenesis and locomotor function. In this study, we investigated the effects of amiodarone, a known disruptor of TH metabolism, on muscle development and motility in zebrafish (Danio rerio) embryos. A dose-range analysis identified 0.01 mM amiodarone as a sublethal concentration suitable for mechanistic investigation, and LC-MS analysis confirmed compound uptake in embryos. Amiodarone exposure resulted in distinct developmental abnormalities, including craniofacial defects at 48 hpf and axial curvature and tail malformations by 72 hpf, accompanied by significantly reduced hatching success and bradycardia. Consistent with impaired neuromuscular function, treated larvae exhibited a markedly attenuated touch-evoked escape response with reduced swimming distance and velocity. Hormonal profiling revealed a significant reduction in triiodothyronine (T3) levels with a modest accumulation of thyroxine (T4), indicating disruption of TH homeostasis. At the molecular level, amiodarone altered the expression of Notch-associated regulators and key myogenic and cell-cycle genes, including downregulation of her6, her9, her12, hey2, sox9, and pcna, alongside upregulation of hey1, heyl, tp53, and casp3. These transcriptional changes were supported by reduced MYOD and PCNA protein levels and increased cleaved CASPASE-3, indicating impaired myogenic progression, reduced proliferation, and enhanced apoptosis. Collectively, these findings demonstrate that disruption of TH homeostasis by amiodarone compromises muscle development and locomotor function during zebrafish embryogenesis, underscoring the central role of endocrine regulation in early myogenic programs.

Skeletal muscle dysfunction (SMD) associated with Chronic obstructive pulmonary disease (COPD), characterized by muscle atrophy and altered fiber type distribution. High-intensity interval training (HIIT) is increasingly used clinically to improve cardiopulmonary and exercise functions in COPD patients, yet robust scientific evidence and exploration of its molecular mechanisms remain limited. In this research, bioinformatics analysis of GSE datasets revealed the differentially expressed genes in the gastrocnemius muscle tissues with COPD preliminarily. COPD rat models were established using cigarette smoke exposure. Cigarette smoke extract (CSE)-induced L6 myoblasts were used as cell models. It was found that BRD4 and PGC-1α were both significantly downregulated in the gastrocnemius muscle tissues with COPD and show a certain correlation. In vitro, BRD4 overexpression could restore PGC-1α levels and attenuate mitochondrial damage of L6 myoblasts caused by CSE treatment. HIIT significantly improved pulmonary function, muscle strength, and oxidative fiber composition in COPD rats by restoring the BRD4/PGC-1α axis and mitochondrial function, but the BET bromodomain inhibitor JQ1 could partially reverse the positive effects of HIIT. These findings not only provides some scientific evidence for the beneficial effects of HIIT on gastrocnemius muscle dysfunction caused by COPD but also suggest that the BRD4/PGC-1α axis plays functional roles in mediating the beneficial effects of HIIT on COPD-induced muscle dysfunction, highlighting its potential as a therapeutic target.

Chronic kidney disease (CKD) and end-stage renal disease (ESRD) are serious health concerns worldwide that require regular dialysis before a kidney transplant for a better quality of life. Due to aging and frailty, dialysis patients are often limited in physical activity, which can lead to multiple health complications and sometimes result in patient death. Therefore, maintaining muscle health is crucial for individuals undergoing dialysis. We recently reported that personalized resistance-based exercise can improve frailty in dialysis patients; however, the molecular mechanisms underlying its effects on muscle health remain unknown. RNA-Seq analysis revealed that exercise upregulates genes associated with muscle stemness, regeneration, and differentiation. Furthermore, our study demonstrated that exercise upregulates mitochondrial proteins involved in the oxidation of organic compounds and mitochondrial function in muscle. Moreover, we observed that exercise increased energy production via glucose uptake, glycogenesis, and fatty acid metabolism. These results suggest that personalized resistance-based muscle therapy in dialysis patients restores the expression of genes and proteins associated with muscle stemness, regeneration, differentiation, and energy generation.

Rheumatoid arthritis is a chronic systemic inflammatory disease associated with joint inflammation, skeletal muscle dysfunction, and reduced physical capacity, reinforcing the need for complementary non-pharmacological strategies aimed at preserving musculoskeletal function. This study investigated the functional and morphological effects of turmeric supplementation associated with stair-climbing resistance exercise in Wistar rats with adjuvant-induced arthritis (AIA), an experimental model representative of rheumatoid arthritis. Thirty-one Wistar rats were randomly allocated into five groups: control (CTRL, n = 6), adjuvant-induced arthritis (AIA, n = 6), AIA treated with turmeric (AIA + T, n = 6), AIA subjected to stair-climbing resistance exercise (AIA + E, n = 6), and AIA treated with turmeric combined with exercise (AIA + T+E, n = 7). Arthritis was induced using Complete Freund’s Adjuvant. Functional outcomes included joint swelling and hindlimb grip strength. On Day 23, animals were euthanized, and analyses of muscle fibers, connective tissue, and neuromuscular junctions were performed. AIA induced joint swelling and reduced muscle strength. Histological analyses revealed muscle fiber alterations characterized by fiber size variability, centrally located nuclei, and mononuclear inflammatory infiltration. These alterations were less pronounced in exercise-treated groups. Stair-climbing resistance exercise improved functional performance and preserved muscle morphology, whereas turmeric supplementation alone produced limited functional effects. Neuromuscular junction morphometry showed an increase in smaller diameter in arthritic animals that was attenuated by exercise. Stair-climbing resistance exercise attenuated functional and morphological impairments associated with adjuvant-induced arthritis, supporting its role as a complementary therapeutic strategy. Turmeric supplementation alone showed limited effects under the conditions tested and warrants further investigation.

Resistance training is currently a widely targeted practice due to its ability to induce vital adaptations resulting from increased muscle strength (MS) and hypertrophy. There are reports of the concomitant use of nonsteroidal anti-inflammatory drugs with this practice for pain relief, which arises from the adaptive signaling of training. Based on this, the present study aimed to investigate whether the use of nimesulide concomitant with training could interfere with MS and hypertrophy. The sample consisted of 18 male Wistar rats, aged 91 days, with a body mass of 331 ± 20 g, divided into three groups: Control (CTRL, n = 4); Trained (TR, n = 7); and Trained and Treated with nimesulide (COMB, n = 7). The training protocol followed the ladder-climbing model. Nimesulide was administered orally at a dose of 2.5 mg/kg every day after training. Tissue collection of the Flexor Hallucis Longus (FHL) muscle was performed two days after the last training session. After eight weeks of the training protocol, a significant increase in MS was observed in TR and COMB compared to CTRL (p < 0.05). A significant increase in FHL muscle volume was observed in COMB compared to CTRL and TR only when muscle mass was normalized to body mass (p < 0.05), whereas no significant differences were detected among groups for absolute FHL mass. Additionally, there were no differences in training load or volume between COMB and TR (p ≥ 0.05). Therefore, based on the results, it is concluded that the administration of nimesulide during eight weeks of training promoted a greater hypertrophic response of the FHL muscle in rats.