Hindlimb Muscles Research Articles

Disparity in the effect of partial gravity simulated using a new apparatus on different rat hindlimb muscles

The days of returning to the Moon and landing on Mars are approaching. These long-duration missions present significant challenges, such as changes in gravity, which pose serious threats to human health. Maintaining muscle function and health is essential for successful spaceflight and exploration of the Moon and Mars. This study aimed to observe the adaptation of rat hindlimb muscles to partial gravity conditions by simulating the gravity of space (microgravity (µG)), Moon (1/6G), and Mars (3/8G) using our recently invented ground-based apparatus. A total of 25 rats were included in this study. The rats were divided into five groups: control (1G), sham (1G), simulated Mars (3/8G), simulated Moon (1/6G), and simulated Space (µG). Muscle mass, fiber proportion, and fiber cross-sectional area (CSA) of four types of hindlimb muscles were measured: gastrocnemius (GA), tibialis anterior (TA), extensor digitorum longus (EDL), soleus (Sol). Sol and GA exhibited the most significant alterations in response to the changes in gravity after 10 days of the experiment. A notable decline in muscle mass was observed in the simulated µG, Moon, and Mars groups, with the µG group exhibiting the most noticeable decline. In Sol, a noteworthy decline in the proportion of slow-twitch type I fibers, CSA of slow-twitch type I fibers, and average CSA of the whole muscle fibers was observed in the simulated groups. The GA red, mixed, and white portions were examined, and the GA mixed portion showed significant differences in fiber proportion and CSA. A notable increase in the proportion of slow-twitch type I fibers was observed in the simulated groups, with a significant decrease in CSA of type IIb. In EDL or TA, no discernible changes in muscle mass, fiber proportion, or fiber CSA were observed in any of the five groups. These findings indicate that weight-bearing muscles, such as Sol and GA, are more sensitive to changes in partial gravity. Furthermore, partial gravity is insufficient to preserve the normal physiological and functional properties of the hindlimb muscles. Therefore, targeted muscle interventions are required to ensure astronauts' health and mission success. Furthermore, these findings demonstrate the viability and durability of our ground-based apparatus for partial gravity simulation.

ROLE OF FORELIMB MORPHOLOGY IN MUSCLE SENSORIMOTOR FUNCTIONS DURING LOCOMOTION IN THE CAT.

Previous studies established strong links between morphological characteristics of mammalian hindlimb muscles and their sensorimotor functions during locomotion. Less is known about the role of forelimb morphology in motor outputs and generation of sensory signals. Here, we measured morphological characteristics of 46 forelimb muscles from 6 cats. These characteristics included muscle attachments, physiological cross-sectional area (PCSA), fascicle length, etc. We also recorded full-body mechanics and EMG activity of forelimb muscles during level overground and treadmill locomotion in 7 and 16 adult cats of either sex, respectively. We computed forelimb muscle forces along with force- and length-dependent sensory signals mapped onto corresponding cervical spinal segments. We found that patterns of computed muscle forces and afferent activities were strongly affected by the muscle's moment arm, PCSA, and fascicle length. Morphology of the shoulder muscles suggests distinct roles of the forelimbs in lateral force production and movements. Patterns of length-dependent sensory activity of muscles with long fibers (brachioradialis, extensor carpi radialis) closely matched patterns of overall forelimb length, whereas the activity pattern of biceps brachii matched forelimb orientation. We conclude that cat forelimb muscle morphology contributes substantially to locomotor function, particularly to control lateral stability and turning, rather than propulsion.

Exercise training induces mild skeletal muscle adaptations without altering disease progression in a TDP-43 mouse model.

Exercise training is considered a nonpharmacological therapeutic approach for many diseases. Mild-to-moderate endurance exercise training is suggested to improve the mental and physical state of people with amyotrophic lateral sclerosis (ALS). The aim of the present study was to determine the capacity of symptomatic rNLS8 mice, which develop ALS-reminiscent TAR DNA-binding protein 43 (TDP-43) pathology and motor dysfunction, to perform mild-to-moderate intensity treadmill exercise training and to evaluate the effects of this training on skeletal muscle health and disease progression. Symptomatic rNLS8 mice were able to complete 4 wk of mild-to-moderate treadmill running (30 min at 6-13 m/min, 3 days a week). Exercise training induced an increase in the percentage of type IIA fibers in the tibialis anterior muscle as well as minor adaptations in molecular markers of myogenic, mitochondrial, and neuromuscular junction health in some forelimb and hindlimb muscles. However, this exercise training protocol did not attenuate the loss in motor function or delay disease progression. Alternative exercise regimens need to be investigated to better understand the role exercise training may play in alleviating symptoms of ALS.NEW & NOTEWORTHY This is the first study to investigate the capacity of symptomatic rNLS8 mice, which develop ALS-reminiscent TDP-43 pathology and motor dysfunction, to perform exercise training. We demonstrate that despite the ALS-reminiscent aggressive disease progression characterizing the rNLS8 mouse model, rNLS8 mice are capable of performing mild-to-moderate endurance treadmill training for at least 3-4 wk. We demonstrate that exercise training induces several minor skeletal muscle adaptations without delaying disease progression in rNLS8 mice.

Appendicular myology of Skorpiovenator bustingorryi: A first attempt to reconstruct pelvic and hindlimb musculature in an abelisaurid theropod.

We present the pelvic and hindlimb musculature of the abelisaurid Skorpiovenator bustingorryi, constituting the most comprehensive muscle reconstruction to date in ceratosaur theropods. Using extant phylogenetic bracket method, we reconstructed 39 muscles that can commonly found in extant archosaurs. Through the identification of bone correlates, we recognized thigh and hindlimb muscles including knee extensors, m. iliofibularis, m. flexor tibialis externus, mm. caudofemorales, mm. puboischiofemorales, and crus muscles important in foot extension and flexion (e.g., m. tibialis anterior, mm. gastrocnemii). Also, autopodial intrinsic muscles were reconstructed whose function involve extension (m. extensor digiti 2-4), flexion (mm. flexor digitorum brevis superficialis), interdigital adduction (m. interosseus dorsalis) and abduction (m. interosseous plantaris, m. abductor 4). Abelisaurids like Skorpiovenator show a deep pre- and postacetabular blade of the ilia and enlarged cnemial crests, which would have helped increasing the moment arm of muscles related to hip flexion and hindlimb extension. Also, pedal muscles related to pronation were probably present but reduced (e.g., m. pronator profundus). Despite some gross differences in the autopodial morphology in extant outgroups (e.g., crocodilian metatarsus and avian tarsometatarsus), the present study allows us to hypothesize several pedal muscles in Skorpiovenator. These muscles would not be arranged in tendinous bundles as in Neornithes, but rather the condition would be similar to that of crocodilians with several layers formed by fleshy bellies on the plantar and dorsal aspects of the metatarsus. The musculature of Skorpiovenator is key for future studies concerning abelisaurid biomechanics, including the integration of functional morphology and ichnological data.

Evaluation of infrared thermography, arterial Doppler ultrasound, and Doppler echocardiography in healthy adult dogs exposed to a single session of Whole-body vibration at different frequencies

This study aimed to evaluate the infrared thermography, arterial Doppler ultrasound, and Doppler echocardiography in healthy adult dogs exposed to a single Whole-body vibration (WBV) at different frequencies. Sixteen (16) healthy crossbreed dogs males of ages 1 to 5 years, weighing 16.3 to 24.5 kg were enrolled in the study. The dogs were exposed to a single WBV session at frequencies of 30 Hz (5 min), 40 Hz (5 min), and 50 Hz (5 min) with intervals between each frequency exposure of 10 min. The cutaneous temperature, arterial Doppler ultrasound, and Doppler echocardiography were evaluated 10 min before the WBV session, between each frequency, and 1 min after the last frequency. The cutaneous temperature of the regions of the superficial gluteal muscle and biceps femoris muscle of both hind limbs was obtained with an infrared camera. Resistive indexes of carotid and femoral arteries were determined using Doppler ultrasound, and Doppler echocardiography was used to assess the end-systolic volume and end-diastolic volume, heart rate, aortic blood flow velocity, and pulmonary artery flow velocity. Clinical parameters, complete blood count, and serum biochemical (alanine aminotransferase, creatinine, and creatine phosphokinase) were evaluated 10 min before and 60 min after the end of the WBV session. Statistically significant differences were not found in any of the variables among the time points. In conclusion, the protocol of increasing vibration frequencies (30, 40, and 50 Hz) at short-period WBV can be considered appropriate since no change occurred in the parameters evaluated.

Skeletal Muscle UCHL1 Negatively Regulates Muscle Development and Recovery after Muscle Injury.

Ubiquitin C-terminal hydrolase L1 (UCHL1) is a deubiquitinating enzyme originally found in the brain. Our previous work revealed that UCHL1 was also expressed in skeletal muscle and affected myoblast differentiation and metabolism. In this study, we further tested the role of UCHL1 in myogenesis and muscle regeneration following muscle ischemia-reperfusion (IR) injury. In the C2C12 myoblast, UCHL1 knockdown upregulated MyoD and myogenin and promoted myotube formation. The skeletal muscle-specific knockout (smKO) of UCHL1 increased muscle fiber sizes in young mice (1 to 2 months old) but not in adult mice (3 months old). In IR-injured hindlimb muscle, UCHL1 was upregulated. UCHL1 smKO ameliorated tissue damage and injury-induced inflammation. UCHL1 smKO also upregulated myogenic factors and promoted functional recovery in IR injury muscle. Moreover, UCHL1 smKO increased Akt and Pink1/Parkin activities. The overall results suggest that skeletal muscle UCHL1 is a negative factor in skeletal muscle development and recovery following IR injury and therefore is a potential therapeutic target to improve muscle regeneration and functional recovery following injuries.

The Whole Genome DNA Methylation Signatures of Hindlimb Muscles in Chinese Alligators during Hibernation and Active Periods.

Many ectotherms hibernate to increase their chances of survival during harsh winter conditions. The role of DNA methylation in regulating gene expression related to hibernation in ectotherms remains unclear. Here, we employed whole-genome bisulfite sequencing (WGBS) technology to construct a comprehensive genome-wide DNA methylation landscape of the hindlimb muscles in the Chinese alligator during hibernation and active periods. The results indicated that methylation modifications were most abundant at CG sites, identifying 9447 differentially methylated regions (DMRs) and 2329 differentially methylated genes (DMGs). KEGG pathway enrichment analysis of the DMGs revealed significant enrichment in major pathways such as the neurotrophin signaling pathway, the MAPK signaling pathway, the GnRH signaling pathway, the biosynthesis of amino acids, and the regulation of the actin cytoskeleton, which are closely related to lipid metabolism, energy metabolism, and amino acid metabolism. Among these, 412 differentially methylated genes were located in promoter regions, including genes related to energy metabolism such as ATP5F1C, ATP5MD, PDK3, ANGPTL1, and ANGPTL2, and genes related to ubiquitin-proteasome degradation such as FBXO28, FBXO43, KLHL40, and PSMD5. These findings suggest that methylation in promoter regions may play a significant role in regulating the adaptive hibernation mechanisms in the Chinese alligator. This study contributes to a further understanding of the epigenetic mechanisms behind the hibernation of the Chinese alligator.

Mitochondria Transplantation: Rescuing Innate Muscle Bioenergetic Impairment in a Model of Aging and Exercise Intolerance.

Arroum, T, Hish, GA, Burghardt, KJ, Ghamloush, M, Bazzi, B, Mrech, A, Morse, PT, Britton, SL, Koch, LG, McCully, JD, Hüttemann, M, and Malek, MH. Mitochondria transplantation: Rescuing innate muscle bioenergetic impairment in a model of aging and exercise intolerance. J Strength Cond Res 38(7): 1189-1199, 2024-Mitochondria, through oxidative phosphorylation, are crucial for energy production. Disease, genetic impairment, or deconditioning can harm muscle mitochondria, affecting energy production. Endurance training enhances mitochondrial function but assumes mobility. Individuals with limited mobility lack effective treatments for mitochondrial dysfunction because of disease or aging. Mitochondrial transplantation replaces native mitochondria that have been damaged with viable, respiration-competent mitochondria. Here, we used a rodent model selectively bred for low-capacity running (LCR), which exhibits innate mitochondrial dysfunction in the hind limb muscles. Hence, the purpose of this study was to use a distinct breed of rats (i.e., LCR) that display hereditary skeletal muscle mitochondrial dysfunction to evaluate the consequences of mitochondrial transplantation. We hypothesized that the transplantation of mitochondria would effectively alleviate mitochondrial dysfunction in the hind limb muscles of rats when compared with placebo injections. In addition, we hypothesized that rats receiving the mitochondrial transplantation would experience an improvement in their functional capacity, as evaluated through incremental treadmill testing. Twelve aged LCR male rats (18 months old) were randomized into 2 groups (placebo or mitochondrial transplantation). One LCR rat of the same age and sex was used as the donor to isolate mitochondria from the hindlimb muscles. Isolated mitochondria were injected into both hindlimb muscles (quadriceps femoris, tibialis anterior (TA), and gastrocnemius complex) of a subset LCR (n = 6; LCR-M) rats. The remaining LCR (n = 5; LCR-P) subset received a placebo injection containing only the vehicle without the isolated mitochondria. Four weeks after mitochondrial transplantation, rodents were euthanized and hindlimb muscles harvested. The results indicated a significant (p < 0.05) increase in mitochondrial markers for glycolytic (plantaris and TA) and mixed (quadricep femoris) muscles, but not oxidative muscle (soleus). Moreover, we found significant (p < 0.05) epigenetic changes (i.e., hypomethylation) at the global and site-specific levels for a key mitochondrial regulator (transcription factor A mitochondrial) between the placebo and mitochondrial transplantation groups. To our knowledge, this is the first study to examine the efficacy of mitochondrial transplantation in a rodent model of aging with congenital skeletal muscle dysfunction.

Selective Activation of the Spinal Cord with Epidural Electrical Stimulation.

Spinal cord epidural electrical stimulation (EES) has been successfully employed to treat chronic pain and to restore lost functions after spinal cord injury. Yet, the efficacy of this approach is largely challenged by the suboptimal spatial distribution of the electrode contacts across anatomical targets, limiting the spatial selectivity of stimulation. In this study, we exploited different ESS paradigms, designed as either Spatial-Selective Stimulation (SSES) or Orientation-Selective Epidural Stimulation (OSES), and compared them to Conventional Monopolar Epidural Stimulation (CMES). SSES, OSES, and CMES were delivered with a 3- or 4-contact electrode array. Amplitudes and latencies of the Spinally Evoked Motor Potentials (SEMPs) were evaluated with different EES modalities. The results demonstrate that the amplitudes of SEMPs in hindlimb muscles depend on the orientation of the electrical field and vary between stimulation modalities. These findings show that the electric field applied with SSES or OSES provides more selective control of amplitudes of the SEMPs as compared to CMES. We demonstrate that spinal cord epidural stimulation applied with SSES or OSES paradigms in the rodent model could be tailored to the functional spinal cord neuroanatomy and can be tuned to specific target fibers and their orientation, optimizing the effect of neuromodulation.

Dynamics of Trypanosoma cruzi infection in hamsters and novel association with progressive motor dysfunction.

Chagas disease is a zoonosis caused by the protozoan parasite Trypanosoma cruzi. Clinical outcomes range from long-term asymptomatic carriage to cardiac, digestive, neurological and composite presentations that can be fatal in both acute and chronic stages of the disease. Studies of T. cruzi in animal models, principally mice, have informed our understanding of the biological basis of this variability and its relationship to infection and host response dynamics. Hamsters have higher translational value for many human infectious diseases, but they have not been well developed as models of Chagas disease. We transposed a real-time bioluminescence imaging system for T. cruzi infection from mice into female Syrian hamsters (Mesocricetus auratus). This enabled us to study chronic tissue pathology in the context of spatiotemporal infection dynamics. Acute infections were widely disseminated, whereas chronic infections were almost entirely restricted to the skin and subcutaneous adipose tissue. Neither cardiac nor digestive tract disease were reproducible features of the model. Skeletal muscle had only sporadic parasitism in the chronic phase, but nevertheless displayed significant inflammation and fibrosis, features also seen in mouse models. Whereas mice had normal locomotion, all chronically infected hamsters developed hindlimb muscle hypertonia and a gait dysfunction resembling spastic diplegia. With further development, this model may therefore prove valuable in studies of peripheral nervous system involvement in Chagas disease.

Forelimb movements contribute to hindlimb cutaneous reflexes during locomotion in cats.

During quadrupedal locomotion, interactions between spinal and supraspinal circuits and somatosensory feedback coordinate forelimb and hindlimb movements. How this is achieved is not clear. To determine whether forelimb movements modulate hindlimb cutaneous reflexes involved in responding to an external perturbation, we stimulated the superficial peroneal nerve in six intact cats during quadrupedal locomotion and during hindlimb-only locomotion (with forelimbs standing on stationary platform) and in two cats with a low spinal transection (T12-T13) during hindlimb-only locomotion. We compared cutaneous reflexes evoked in six ipsilateral and four contralateral hindlimb muscles. Results showed similar occurrence and phase-dependent modulation of short-latency inhibitory and excitatory responses during quadrupedal and hindlimb-only locomotion in intact cats. However, the depth of modulation was reduced in the ipsilateral semitendinosus during hindlimb-only locomotion. Additionally, longer-latency responses occurred less frequently in extensor muscles bilaterally during hindlimb-only locomotion, whereas short-latency inhibitory and longer-latency excitatory responses occurred more frequently in the ipsilateral and contralateral sartorius anterior, respectively. After spinal transection, short-latency inhibitory and excitatory responses were similar to both intact conditions, whereas mid- or longer-latency excitatory responses were reduced or abolished. Our results in intact cats and the comparison with spinal-transected cats suggest that the absence of forelimb movements suppresses inputs from supraspinal structures and/or cervical cord that normally contribute to longer-latency reflex responses in hindlimb extensor muscles.NEW & NOTEWORTHY During quadrupedal locomotion, the coordination of forelimb and hindlimb movements involves central circuits and somatosensory feedback. To demonstrate how forelimb movement affects hindlimb cutaneous reflexes during locomotion, we stimulated the superficial peroneal nerve in intact cats during quadrupedal and hindlimb-only locomotion as well as in spinal-transected cats during hindlimb-only locomotion. We show that forelimb movement influences the modulation of hindlimb cutaneous reflexes, particularly the occurrence of long-latency reflex responses.

Proteomic reference map for sarcopenia research: mass spectrometric identification of key muscle proteins located in the sarcomere, cytoskeleton and the extracellular matrix.

Sarcopenia of old age is characterized by the progressive loss of skeletal muscle mass and concomitant decrease in contractile strength. Age-related skeletal muscle dysfunctions play a key pathophysiological role in the frailty syndrome and can result in a drastically diminished quality of life in the elderly. Here we have used mass spectrometric analysis of the mouse hindlimb musculature to establish the muscle protein constellation at advanced age of a widely used sarcopenic animal model. Proteomic results were further analyzed by systems bioinformatics of voluntary muscles. In this report, the proteomic survey of aged muscles has focused on the expression patterns of proteins involved in the contraction-relaxation cycle, membrane cytoskeletal maintenance and the formation of the extracellular matrix. This includes proteomic markers of the fast versus slow phenotypes of myosin-containing thick filaments and actin-containing thin filaments, as well as proteins that are associated with the non-sarcomeric cytoskeleton and various matrisomal layers. The bioanalytical usefulness of the newly established reference map was demonstrated by the comparative screening of normal versus dystrophic muscles of old age, and findings were verified by immunoblot analysis.

Defects in integrin complex formation promote CHKB-mediated muscular dystrophy.

Phosphatidylcholine (PC) is the major membrane phospholipid in most eukaryotic cells. Bi-allelic loss of function variants in CHKB, encoding the first step in the synthesis of PC, is the cause of a rostrocaudal muscular dystrophy in both humans and mice. Loss of sarcolemma integrity is a hallmark of muscular dystrophies; however, how this occurs in the absence of choline kinase function is not known. We determine that in Chkb -/- mice there is a failure of the α7β1 integrin complex that is specific to affected muscle. We observed that in Chkb -/- hindlimb muscles there is a decrease in sarcolemma association/abundance of the PI(4,5)P2 binding integrin complex proteins vinculin, and α-actinin, and a decrease in actin association with the sarcolemma. In cells, pharmacological inhibition of choline kinase activity results in internalization of a fluorescent PI(4,5)P2 reporter from discrete plasma membrane clusters at the cell surface membrane to cytosol, this corresponds with a decreased vinculin localization at plasma membrane focal adhesions that was rescued by overexpression of CHKB.

Histological analysis and etiology of a pathological iguanodontian femur from England.

Derived ornithopods, such as hadrosaurids, show a high occurrence of fossilized lesions and diseases. However, paleopathologies in iguanodontians seem to be less common, considering the rich fossil record of these taxa in Europe, in particular in Belgium, Britain and Spain. Here, we describe an iguanodontian femur discovered in England that exhibits a large overgrowth of its lateral aspect, not previously recognized in any other similar remains. The specimen was scanned with micro-computed tomography (microCT) and later sectioned in three sites of the overgrowth for histological analysis. The femur belongs to an early adult Iguanodontia indet., based on the presence of a woven parallel fibered complex in the outer cortex and three to four lines of arrested growth. Internal analysis of the dome-like overgrowth suggests it can be diagnosed as a fracture callus. The injury might have negatively impacted upon the animal's locomotion as the trauma had occurred in the region above the knee, a crucial spot for hindlimb musculature. Finally, a cancellous medullary bone-like tissue was recognized in the medullary cavity next to the pathological overgrowth. An attempt was made to determine the precise nature of this tissue, as medullary bone is linked with the ovulation period in (avian) dinosaurs, whereas other types of endosteal, medullary bone-like tissue have previously been recognized in pathological bones.

A surgical technique for individual control of the muscles of the rabbit lower hindlimb.

Little is known regarding the precise muscle, bone and joint actions resulting from individual and simultaneous muscle activation(s) of the lower limb. An in situ experimental approach is described herein to control the muscles of the rabbit lower hindlimb, including the medial and lateral gastrocnemius, soleus, plantaris and tibialis anterior. The muscles were stimulated using nerve-cuff electrodes placed around the innervating nerves of each muscle. Animals were fixed in a stereotactic frame with the ankle angle set at 90deg. To demonstrate the efficacy of the experimental technique, isometric plantarflexion torque was measured at the 90deg ankle joint angle at a stimulation frequency of 100, 60 and 30 Hz. Individual muscle torque and the torque produced during simultaneous activation of all plantarflexor muscles are presented for four animals. These results demonstrate that the experimental approach was reliable, with insignificant variation in torque between repeated contractions. The experimental approach described herein provides the potential for measuring a diverse array of muscle properties, which is important to improve our understanding of musculoskeletal biomechanics.

Impact of Hindlimb Disuse on Sepsis-Induced Myopathy in Older Adult Mice

BACKGROUND: Sepsis, a life-threatening condition due to an overactive immune response, adversely affects multiple organ systems. Despite its significance of accounting for 40% of total body mass, the skeletal muscle system is considered the forgotten organ system affected by sepsis. In addition to the inflammatory aspects of sepsis, patients in the intensive care unit (ICU) frequently experience prolonged periods of bed rest, which can lead to muscle disuse-induced myopathy characterized by muscle atrophy, weakness, and impaired regenerative capacity. Despite the higher incidence of sepsis in bedridden older adults, most research on sepsis-induced myopathy primarily focuses on younger adults under conditions that do not mimic bed rest. In this study, we simulated the clinical conditions of older adult ICU patients by combining sepsis with hindlimb disuse. Our hypotheses were that the combination of sepsis, disuse, and aging would exacerbate muscle atrophy and weakness, while also reducing the number of regenerative stem cells in muscle tissue. Methods: To investigate the functional and morphological responses of skeletal muscle to sepsis, we conducted experiments on 78-week-old male wild-type C57Bl6 mice. They underwent either cecal ligation and puncture (CLP, n = 17) or Sham (n = 14) surgeries. Four days post-surgery, the animals were allocated to hindlimb suspension (HLS — CLP, n=9; Sham, n=10) or normal ambulation (NA — CLP, n=8; Sham, n=4) for seven days. Terminal experiments were conducted after hindlimb suspension or normal ambulation treatments. We assessed peak force production ex vivo in solei muscles, examined the cross-sectional area (CSA) of relevant hindlimb muscles using immunohistochemistry, and quantified the abundance of stem cells through immunohistochemistry (e.g., PAX7+ cells) in muscle sections. Additionally, we evaluated markers of senescence by measuring the protein abundance of p16 and p53 in hindlimb muscle homogenates. Results: CLP surgery resulted in 50% mortality rate (p&lt;0.05). Percentage body mass loss was as follows: 5.3 ± 4.2 for Sham/NA group, 13.3 ± 5.2 for Sham/HLS group, 7.3 ± 4.4 for CLP/NA group, and 17.7 ± 5.5 for CLP/HLS group. Peak absolute force was significantly lower in Sham/HLS and CLP/HLS compared to Sham/NA (Sham/NA = 277±2.4, Sham/HLS = 189±28.4, CLP/NA = 264±23.0, CLP/HLS = 180±48.9 mN, p&lt;0.05). Mice exposed to CLP/HLS displayed a higher cumulative fiber area at smaller fiber sizes (800 to 1000 μm2) (p&lt;0.05), indicating muscle atrophy. CLP/NA caused a 9.8% reduction in solei muscle mass (p &lt; 0.05), while CLP/HLS led to an additional 30% loss in solei mass (p&lt;0.05). Furthermore, CLP/HLS significantly decreased the number of PAX7+ stem cells by 75% (p&lt;0.05). Notably, p16 was significantly increased in response to CLP/HLS (p&lt;0.05), while no changes were observed in p53 abundance. CONCLUSION: This study demonstrates the role of muscle disuse in exacerbating skeletal muscle functional and morphological abnormalities in older adult septic mice, emphasizing the critical contribution of disuse to the pathophysiology of sepsis-induced myopathy. NIH R21AG072011 to OL. 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.

A novel preclinical model of human lung cancer cachexia

Most patients with lung cancer are afflicted with cancer cachexia (CC), a syndrome characterized by wasting of muscle and fat tissues, which leads to poor treatment response and increased mortality. Pre-clinical models used to study CC often do not mimic the human condition, which may explain why there are no approved treatments for CC. To overcome this barrier, we sought to develop a model of lung CC that better recapitulates the pathoetiological characteristics of human lung CC using mice with a tamoxifen-inducible, lung epithelial cell (club cell)-specific KrasG12D allele ( G12D mice). We hypothesized that this model would mimic human CC in tumor location and burden, rate of progression, and tumor-driven tissue wasting. Following induction by tamoxifen injection, adenocarcinomas developed in the terminal bronchioles as early as 3.5 weeks post-induction and were accompanied by loss of 15% of animal body weight over a 12-week period. This weight loss began at 6 weeks and yielded a slow rate of CC progression (-1.9%/week) relative to currently available lung CC models (-5-10%/week). Body weight loss resulted from reductions in hindlimb muscle mass (18-30%) and profound depletion of inguinal and gonadal adipose tissue depots (60-70%) compared to age-matched wild-type (WT) littermates. At 6 weeks post-induction, fat tissue was reduced at the tissue and cellular levels while reductions in hindlimb muscle weight and fiber size were not observed. This fat loss corresponded with increased serum glycerol levels at 6 and 12 weeks, a metabolic characteristic that was also found in patients with lung cancer receiving treatment. Lung organoids were developed from G12D and WT mice. G12D organoids exhibited similarities in gene expression to human lung tumors ( RNAseq) and their conditioned medium (CM) elicited glycerol release from cultured 3T3L1 adipocytes compared to CM from WT organoids, suggesting that tumor-derived factors may contribute to early adipose tissue atrophy. Collectively, these findings suggest that G12D mice may be a valuable model, well-suited to identify mechanisms leading to CC and test preventative therapies. NIH R01 AR065826(MJT), NCI R21 CA283492(MJT), T32- HL076122-18(DBS/MEP), R01 CA273238 (YJ-H) and the University of Vermont Cancer Center. Funding support for RNAseq provided by Zymo Impact Initiative Grant (DBS). 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.

Safety of mapping the motor networks in the spinal cord using penetrating microelectrodes in Yucatan minipigs.

The goal of this study was to assess the safety of mapping spinal cord locomotor networks using penetrating stimulation microelectrodes in Yucatan minipigs (YMPs) as a clinically translational animal model. Eleven YMPs were trained to walk up and down a straight line. Motion capture was performed, and electromyographic (EMG) activity of hindlimb muscles was recorded during overground walking. The YMPs underwent a laminectomy and durotomy to expose the lumbar spinal cord. Using an ultrasound-guided stereotaxic frame, microelectrodes were inserted into the spinal cord in 8 animals. Pial cuts were made to prevent tissue dimpling before microelectrode insertion. Different locations within the lumbar enlargement were electrically stimulated to map the locomotor networks. The remaining 3 YMPs served as sham controls, receiving the laminectomy, durotomy, and pial cuts but not microelectrode insertion. The Porcine Thoracic Injury Behavioral Scale (PTIBS) and hindlimb reflex assessment results were recorded for 4 weeks postoperatively. Overground gait kinematics and hindlimb EMG activity were recorded again at weeks 3 and 4 postoperatively and compared with preoperative measures. The animals were euthanized at the end of week 4, and the lumbar spinal cords were extracted and preserved for immunohistochemical analysis. All YMPs showed transient deficits in hindlimb function postoperatively. Except for 1 YMP in the experimental group, all animals regained normal ambulation and balance (PTIBS score 10) at the end of weeks 3 and 4. One animal in the experimental group showed gait and balance deficits by week 4 (PTIBS score 4). This animal was excluded from the kinematics and EMG analyses. Overground gait kinematic measures and EMG activity showed no significant (p > 0.05) differences between preoperative and postoperative values, and between the experimental and sham groups. Less than 5% of electrode tracks were visible in the tissue analysis of the animals in the experimental group. There was no statistically significant difference in damage caused by pial cuts between the experimental and sham groups. Tissue damage due to the pial cuts was more frequently observed in immunohistochemical analyses than microelectrode tracks. These findings suggest that mapping spinal locomotor networks in porcine models can be performed safely, without lasting damage to the spinal cord.

Role of ASIC3 in Evoking the Exercise Pressor Reflex in Type 1 Diabetes Mellitus Rats

Previous studies have suggested that the exercise pressor reflex is exaggerated in individuals with type 1 diabetes mellitus (T1DM); however, the mechanisms responsible for this altered reflex are poorly understood. Acid-sensing ion channel 3 (ASIC3) likely contributes to this exaggeration as previous studies found that peripheral blockade, as well as functional knockout (KO) of the ASIC3 gene, prevented the exaggeration of exercise pressor reflex in rats with cardiovascular-related diseases. However, whether ASIC3 plays a similar role in the exaggerated exercise pressor reflex in T1DM is currently unknown. Therefore, the purpose of this study was to determine whether ASIC3 contributes to the exaggerated exercise pressor reflex in T1DM rats. We hypothesized that the exaggerated exercise pressor reflex would be attenuated in ASIC3 KO T1DM rats compared to their wild-type (WT) counterparts. To test this, we used adult, male and female Wistar-Kyoto ASIC3 KO (n=3; body weight: 348 ± 84 g) and WT counterparts (n=5; body weight: 318 ± 112 g) rats. Streptozotocin (50 mg/kg) was injected intraperitoneally in fasted anesthetized rats to induce T1DM, and a random blood glucose over 300 mg/dl was used to diagnose diabetes. One week later, the exercise pressor reflex was evoked in unanesthetized, decerebrated rats by statically contracting the hindlimb muscles for 30 s. Peak mean arterial pressure (MAP) and heart rate (HR) were calculated as the highest change (Δ) during 30 s contraction baseline. We found that the peak pressor and cardioaccelerator responses to static contraction, although is not statistically significant, appear to be lower in ASIC3 KO (ΔMAP: 17 ± 14 mmHg; ΔHR: 13 ± 12 bpm), compared to WT rats (ΔMAP: 23 ± 11 mmHg; P=0.26 ΔHR:23 ± 9 bpm; P=0.11). Furthermore, we found that within female rats, the peak pressor and heart rate responses were lower in ASIC3 KO (n=1) rat (ΔMAP: 7 mmHg; ΔHR: 15 bpm) compared to WT rats (n=3) (ΔMAP: 27 ± 12 mmHg; ΔHR: 27 ± 11 bpm). Our preliminary findings suggest that ASIC3 may play a role in the exaggerated exercise pressor reflex in T1DM rats and that sex differences may also be present. Further studies involving larger sample sizes will reveal if trends from the current results become significant. This project was supported by NIH R01HL 166323-01A1. 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.

Endomorphin-2 and oxycodone paradoxically potentiate the exercise pressor reflex via acid-sensing ion channel 3 (ASIC3) in decerebrated rat

The exercise pressor reflex is evoked by the stimulation of group III and IV muscle afferents in response to muscle contractions. When the contracting muscles are freely perfused, acid-sensing ion channel 3 (ASIC3) on the endings of group IV afferents appear to play a minor role in evoking the exercise pressor reflex. We recently showed in isolated dorsal root ganglion (DRG) neurons innervating the gastrocnemius muscles that two μ opioid receptor agonists, namely endomorphin-2, and oxycodone, potentiated the sustained inward ASIC3 current evoked by acidic saline (pH 6.0). This in vitro finding prompted us to investigate whether endomorphin-2 and oxycodone, when infused into the arterial supply of freely perfused contracting hindlimb muscles, could potentiate the exercise pressor reflex. We found that infusing endomorphin-2 and naloxone (a μ opioid receptor antagonist) into the superficial epigastric artery of decerebrated unanesthetized rats potentiated the peak and integrated pressor responses to static contraction of the triceps surae muscles. The opioid-induced potentiation of the pressor responses to contraction were prevented by infusion of APETx2, an ASIC3 antagonist. Specifically, the peak pressor response to contraction averaged 19.3 ± 5.6 mmHg for control (n=10), 27.2 ± 8.1 mmHg after naloxone and endomorphin-2 infusion (n=10), and 20 ± 8 mmHg after APETx2 and endomorphin-2 infusion (n=10). Infusion of endomorphin-2 and naloxone did not potentiate the pressor responses to contraction in ASIC3 knockout rats (n=6), averaging 21.5 ± 7.7 mmHg for control (n=6), 20 ± 6.2 mmHg after naloxone and endomorphin-2 infusion (n=6), and 19.8 ± 6.9 mmHg after APETx2 and endomorphin-2 infusion (n=4). Similar findings were observed when oxycodone was substituted for endomorphin-2. Specifically, the peak pressor response to contraction averaged 24.3 ± 8.9 mmHg for control (n=10), 33.9 ± 10.6 mmHg after naloxone and oxycodone were infused (n=10), and 26.8 ± 8.1 mmHg after APETx2 and oxycodone were infused (n=10). Altogether, our data strongly suggest that ASIC3 stimulation by mu opioid agonists can potentiate the exercise pressor reflex, provided that mu opioid receptors on group III and IV afferents are not operative. (Supported by HL156594 and HL 156513). HL156594 and HL 156513. 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.