Thin Muscle Research Articles

Insulin potentiates the response to capsaicin in dorsal root ganglion neurons in vitro and muscle afferents ex vivo in normal healthy rodents.

Systemic insulin administration evokes sympathoexcitatory actions, but the mechanisms underlying these observations are unknown. We reported that insulin sensitizes the response of thin-fibre primary afferents, as well as the dorsal root ganglion (DRG) that subserves them, to mechanical stimuli. However, little is known about the effects of insulin on primary neuronal responses to chemical stimuli. TRPV1, whose agonist is capsaicin (CAP), is widely expressed on chemically sensitive metaboreceptors and/or nociceptors. The aim of this investigation was to determine the effects of insulin on CAP-activated currents in small DRG neurons and CAP-induced action potentials in thin-fibre muscle afferents of normal healthy rodents. Additionally, we investigated whether insulin potentiates sympathetic nerve activity (SNA) responses to CAP. In whole-cell patch-clamp recordings from cultured mice DRG neurons in vitro, the fold change in CAP-activated current from pre- to post-application of insulin (n=13) was significantly (P<0.05) higher than with a vehicle control (n=14). Similar results were observed in single-fibre recording experiments ex vivo as insulin potentiated CAP-induced action potentials compared to vehicle controls (n=9 per group, P<0.05). Furthermore, insulin receptor blockade with GSK1838705 significantly suppressed the insulin-induced augmentation in CAP-activated currents (n=13) as well as the response magnitude of CAP-induced action potentials (n=9). Likewise, the renal SNA response to CAP after intramuscular injection of insulin (n=8) was significantly (P<0.05) greater compared to vehicle (n=9). The findings suggest that insulin potentiates TRPV1 responsiveness to CAP at the DRG and muscle tissue levels, possibly contributing to the augmentation in sympathoexcitation during activities such as physical exercise. KEY POINTS: Evidence suggests insulin centrally activates the sympathetic nervous system, and a chemical stimulus to tissues activates the sympathetic nervous system via thin fibre muscle afferents. Insulin is reported to modulate putative chemical-sensitive channels in the dorsal root ganglion neurons of these afferents. In the present study, it is demonstrated that insulin potentiates the responsiveness of thin fibre afferents to capsaicin at muscle tissue levels as well as at the level of dorsal root ganglion neurons. In addition, it is demonstrated that insulin augments the sympathetic nerve activity response to capsaicin in vivo. These data suggest that sympathoexcitation is peripherally mediated via insulin-induced chemical sensitization. The present study proposes a possible physiological role of insulin in the regulation of chemical sensitivity in somatosensory thin fibre muscle afferents.

Simple Neural Network Compact Form Model-Free Adaptive Controller for Thin McKibben Muscle System

This paper proposes a simple neural network compact form model-free adaptive controller (NNCFMFAC) for a single thin McKibben muscle (TMM) system. The main contribution of this work is the simplification of the current neural network (NN) based compact form model-free adaptive controller (CFMFAC), which requires only two adaptive weights. This is achieved by designing a NN topology specifically to enhance the CFMFAC response. The prominent control parameters of the CFMFAC are combined and one adaptive weight is used for self-tuning, while the second adaptive weight is used to minimize the offset at each operating point. Hence the issues of redundant adaptive weights in complex neuro based CFMFACs and slow response of the CFMFAC are significantly addressed. The idea is proven in three ways: analytically, simulation on a nonlinear system and experiments on a TMM platform. Experimental results demonstrate the superiority of the proposed method over the conventional CFMFAC is confirmed by a 76% improvement in the convergence speed and a 60% reduction in root mean square error (RMSE). It is envisaged that the proposed controller can be very useful for TMM driven applications as it is model independent, has fast response, high tracking accuracy, and minimal complexity.

Soft Tensegrity Robot Driven by Thin Artificial Muscles

Soft Tensegrity Robot Driven by Thin Artificial Muscles

New data on nudibranchs rhinophore morphology and their spicule complex in Onchidoris muricata (Doridina, Gastropoda)

Nudibranchia is a group of shell-less heterobranch gastropods characterized by highly specialized chemosensory cephalic organs known as rhinophores. Whereas the morphology of the rhinophore is an important taxonomic character, as the external structures are used when describing species. However, the internal morphology of the rhinophore has only been studied in terms of neurophysiology. In this study, for the first time, the external and internal structure of the rhinophore in Onchidoris muricata (O.F. Müller, 1776) is described in detail using methods of light, scanning electron-, transmission electron- and confocal laser scanning microscopy, as well as computer microtomography (micro-CT).Data obtained showed that spicules of the rhinophore significantly differ from the body spicules in both general morphology and localization. The thinner and longer spicules of the rhinophore form the spiral square mesh network, allowing mobility which permits contraction of the rhinophore while maintaining a strong framework. Interestingly, the ultrathin structure of the spicules and scleroblasts resemble those of the body spicules. The lymphatic canal and the large nerve and associated longitudinal muscles are situated in the central part of the rhinophore. Stemming from the large nerve are thin transverse muscles and nerve branches associated with the rhinophore lamellae. Morphological analyses suggest a lymphatically driven mechanism in O. muricata, which is capable of quick muscular contraction and slow retraction of the rhinophore.

Development of soft actuators for stroke rehabilitation using deep learning

Automation has created a mind-blowing impact in diversified fields all over the world. Not only in business but also in various domains like health care sectors, manufacturing, etc. a faultless execution is a prime concern. Robotic Process Automation has paved the way for research in the mechanical and mechatronics field. Software robots are trained well to complete repetitive tasks in an efficient manner. A design of such a soft robot can be greatly helpful in the arena of healing. Automation of Rehabilitation therapy has gained attention in recent years. The main aspiration towards the conduct of this research work is to accomplish a soft exoskeleton robot using a thin McKibben actuator applying Deep Learning approaches to aid automatic therapy to the paralyzed patients and help them carry out the hand movement-based exercises. Convolutional Neural Network (CNN) algorithm will be used to support the training of the AI-enabled automated device. The proposed methodology will support stroke survivors to perform exercises independently to enhance their hand motor recovery. For this purpose, it involves pneumatic soft actuator technology using thin McKibben artificial muscles to create a cognitive potential to induce rehabilitation. A soft actuator is proposed so as to confirm the safety purposes of stroke patients.

Shape Recognition of a Tensegrity With Soft Sensor Threads and Artificial Muscles Using a Recurrent Neural Network

This letter proposes a novel method to accomplish shape recognition by utilizing a tensegrity structure with a soft sensor via a recurrent neural network (RNN). The combination of soft tensegrity and soft sensors make it capable of recognizing the deformation to reflect the shape of its surroundings. As the first step to this goal, we build a three-bar tensegrity prism with nine separate soft sensors in which the resistance value of the sensors changes with length variation. The prism is actuated by thin McKibben muscles and deforms when the pressure inside a muscle. The positions of the six nodes in the prism are obtained using a motion-capture system. The measured resistance and position data are used as training data for the RNN to build a prediction model that can reflect the shape variation during the period of deformation of the tensegrity. If several prisms are connected, they can be used with this approach to recognize the shape of a three-dimensional environment that is difficult to observe directly.

Distal Intra Articular Tibial Fracture Treated by Ilizarov with Ligamentotaxis

Abstract Background The thin soft tissue and muscles envelope that surrounds the distal tibia makes treatment of these fractures difficult. These fractures are often referred to as ―pilon‖ fractures or ―plafond‖ fractures‖. If the articular surface of the tibia is involved; in such cases an anatomic realignment of the involved articular fracture in conjunction with a stable fixation is crucial. Objectives The aim of this paper is to conduct a systematic review of literature about functional and radiological outcomes in patients with distal intra articular tibial fracture treated by external fixator with ligamentotaxis. Materials and Methods We performed this systematic review and meta-analysis in accordance to the recommendations of the Meta-analysis of Observational Studies in Epidemiology (MOOSE) statement. MOOSE is a reporting checklist for Authors, Editors, and Reviewers of Meta-analyses of interventional and observational studies. According to International committee of medical journal association (ICJME), reviewers must report their findings according to each of the items listed in those checklists Results Over all, five studies reported the time to union. The overall effect estimates showed that the time to union after Ilizarov external fixator was 12.83 weeks. Three studies reported the rate of union. The overall effect estimates showed that the rate of union after Ilizarov external fixator was 88.4%. Over all, eight studies reported the good-excellent AOFS score. The overall effect estimates showed that the rate of good-excellent AOFS score after Ilizarov external fixator was 69.4%. Nine studies reported the rate of malunion. The overall effect estimates showed that the rate of malunion after Ilizarov external fixator was 10.2%. Over all, seven studies reported the rate of delayed union. The overall effect estimates showed that the rate of delayed union after Ilizarov external fixator was 6.3%. All studies reported the rate of pin infection. The overall effect estimates showed that the rate of pin infection after Ilizarov external fixator was 33.7%. Ten studies reported the rate of major infection. The overall effect estimates showed that the rate of major infection after Ilizarov external fixator was 3.4. Conclusion External fixation and ligamentotaxis by either ilizarov or any external fixator is a good and easy method for fixation of distal intraarticulat tibial fracture with few serious complications. due to easy to apply it, less rate of infection, no risk of bleeding since no opening the fracture site, good aligment of the joint, no streaping of the periosteomy that lead to later on good and rapid healing of the fracture.

Sensory neuron inositol 1,4,5-trisphosphate receptors contribute to chronic mechanoreflex sensitization in rats with simulated peripheral artery disease.

The mechanoreflex is exaggerated in patients with peripheral artery disease (PAD) and in a rat model of simulated PAD in which a femoral artery is chronically (∼72 h) ligated. We found recently that, in rats with a ligated femoral artery, blockade of thromboxane A2 (TxA2) receptors on the sensory endings of thin fiber muscle afferents reduced the pressor response to 1 Hz repetitive/dynamic hindlimb skeletal muscle stretch (a model of mechanoreflex activation isolated from contraction-induced metabolite production). Conversely, we found no effect of TxA2 receptor blockade in rats with freely perfused femoral arteries. Here, we extended the isolated mechanoreflex findings in "ligated" rats to experiments evoking dynamic hindlimb skeletal muscle contractions. We also investigated the role played by inositol 1,4,5-trisphosphate (IP3) receptors, receptors associated with intracellular signaling linked to TxA2 receptors, in the exaggerated response to dynamic mechanoreflex and exercise pressor reflex activation in ligated rats. Injection of the TxA2 receptor antagonist daltroban into the arterial supply of the hindlimb reduced the pressor response to 1 Hz dynamic contraction in ligated but not "freely perfused" rats. Moreover, injection of the IP3 receptor antagonist xestospongin C into the arterial supply of the hindlimb reduced the pressor response to 1 Hz dynamic stretch and contraction in ligated but not freely perfused rats. These findings demonstrate that, in rats with a ligated femoral artery, sensory neuron TxA2 receptor and IP3 receptor-mediated signaling contributes to a chronic sensitization of the mechanically activated channels associated with the mechanoreflex and the exercise pressor reflex.

Thromboxane A2 receptors contribute to the exaggerated exercise pressor reflex in male rats with heart failure.

Mechanical and metabolic signals associated with skeletal muscle contraction stimulate the sensory endings of thin fiber muscle afferents and produce reflex increases in sympathetic nerve activity and blood pressure during exercise (i.e., the exercise pressor reflex; EPR). The EPR is exaggerated in patients and animals with heart failure with reduced ejection fraction (HF‐rEF) and its activation contributes to reduced exercise capacity within this patient population. Accumulating evidence suggests that the exaggerated EPR in HF‐rEF is partially attributable to a sensitization of mechanically activated channels produced by thromboxane A2 receptors (TxA2‐Rs) on those sensory endings; however, this has not been investigated. Accordingly, the purpose of this investigation was to determine the role played by TxA2‐Rs on the sensory endings of thin fiber muscle afferents in the exaggerated EPR in rats with HF‐rEF induced by coronary artery ligation. In decerebrate, unanesthetized rats, we found that injection of the TxA2‐R antagonist daltroban (80 μg) into the arterial supply of the hindlimb reduced the pressor response to 30 s of electrically induced 1 Hz dynamic hindlimb muscle contraction in HF‐rEF (n = 8, peak ∆MAP pre: 22 ± 3; post: 14 ± 2 mmHg; p = 0.01) but not sham (n = 10, peak ∆MAP pre: 13 ± 3; post: 11 ± 2 mmHg; p = 0.68) rats. In a separate group of HF‐rEF rats (n = 4), we found that the systemic (intravenous) injection of daltroban had no effect on the EPR (peak ΔMAP pre: 26 ± 7; post: 25 ± 7 mmHg; p = 0.50). Our data suggest that TxA2‐Rs on thin fiber muscle afferents contribute to the exaggerated EPR evoked in response to dynamic muscle contraction in HF‐rEF.

Study of the relationship between acupuncture dose and effect

Study of the relationship between acupuncture dose and effect

Human orbital muscle in adult cadavers and near-term fetuses: its bony attachments and individual variation identified by immunohistochemistry.

To compare fetal and adult morphologies of the orbital muscle (OM) and to describe the detailed topographical anatomy in adults. Using unilateral orbits from 15 near-term fetuses and 21 elderly cadavers, semiserial horizontal or sagittal paraffin sections were prepared at intervals of 20-100µm. In addition to routine histology, we performed immunohistochemistry for smooth muscle actin. At near term, the OM consistently extended widely from the zygomatic bone or the greater wing of the sphenoid to the maxilla or ethmoid. Thus, it was a large sheet covering the future inferior orbital fissure. In contrast, the adult OM was a thin and small muscle bundle connecting (1) the greater wing of the sphenoid to the maxilla (11/19 cadavers), (2) the lesser wing of the sphenoid to the maxilla (5/19) or the greater wing (3/19). The small OM was likely to be restricted within the greater wing (5/19 cadavers) or the maxilla (3/19). Two of these five types of OM coexisted in eight orbits. OM attachment to the lesser wing was not seen in fetuses, whereas ethmoid attachment was absent in adults. The lesser wing attachment of the OM seemed to establish after birth. A growing common origin of the three recti was likely involved in "stealing" the near-term OM attachment from the ethmoid. The strong immunoreactivity of remnant-like OM in the elderly suggests that OM contraction is still likely to occur against the increased flow through a thin vein. However, the contraction might have no clinical significance.

A Thin and Short Muscle Flap Acted as a Savior for A Child’s Calcaneal Defect: A Case Report

Managing, a distal leg defect is reconstructive challenge for any plastic surgeon. With the advent of microsurgery, the work hose flap for this area shifted from cross leg to free and local perforator flaps. Children with such defects pose a bigger problem because of smaller caliber of vessels, difficult immobilization and poor compliance. We, hereby, report a case of a 12 year old child who had an exposed distal half of calcaneum, which warranted a cross leg or free flap, but was managed successfully by distally based peroneus brevis muscle.

Fascial plane mapping for superficial tumor resection in dogs. Part II: Forelimb.

To detail the qualitative fascial categories and fascial intersections of the forelimb of the dog to facilitate preoperative planning for superficial cancers. Qualitative anatomical study. Three male and four female mixed breed canine cadavers weighing approximately 20-35 kg. The skin and subcutaneous fat were excised. Fascial planes were incised and elevated to allow exploration of their quality and borders. Fascia was categorized as type I (discrete sheets), type II (tightly adhered to thin muscles), type III (tightly adhered to thick muscles), or type IV (associated with periosteum). Photographs of specimens were digitally modified with overlays to map tissue types. Differences between the cadavers used were largely based on muscle mass and sex, with minimal other subjective differences affecting fascial mapping. The fasciae of the forelimb were largely type II or type III, with type I fascia at the antebrachium and type IV fascia at the olecranon, scapular spine, and accessory carpal bone. Fascial integrity was often questionable or lacking distal to the distal quarter of the antebrachium. The fascial types and integrity of the forelimb varied with anatomic location with thin or absent fascia for surgical use at the elbow, carpus, and manus. This study provides information for preoperative planning and excision of superficial tumors of the forelimb. Knowledge of the potential limitations of fascia to provide a deep margin may influence selection of treatment modalities.

New aspects of the esophageal histology of the domestic goat (Capra hircus) and European roe deer (Capreolus capreolus).

The present study examines the esophageal wall of animals from two distinct families of the Ruminantia: domestic goats and European roe deer. Five fragments were collected from the entire length of the esophageal wall in five goats and four roe deer and subjected to microscopic and morphometric analyses. All layers of the esophageal wall except the tela submucosa were found to be thicker in the goats. In both species, the esophagus was lined by parakeratinized stratified squamous epithelium, and the tela submucosa was deprived of glands along its entire length. However, the structure of the lamina muscularis mucosae was better developed in goats: it was found to be discontinuous in the proximal part, and then became fused in the cervical part, that is around the most proximal quarter of its length. In contrast, in roe deer, the lamina muscularis mucosae began as sparse, thin muscle bundles at the pharyngeal‐esophageal junction, which thickened and clustered further down the esophagus, but did not fuse. Our findings regarding the microscopic structure of the ruminant esophagus are not fully consistent with the widely‐accepted view and suggest that the histological structure of the esophagus demonstrates interspecies variation within this large suborder. More precisely, species‐specific differences can be seen regarding the presence of esophageal glands and parakeratinized epithelium, and in the organization of the lamina muscularis mucosae.

How to Perform a High-Quality Endoscopic Submucosal Dissection

How to Perform a High-Quality Endoscopic Submucosal Dissection

The free serratus carpaccio flap: indications and technique

Soft tissue reconstructions of the hand require a thin and resistant flap palmarly as well as sliding abilities between the extensor tendons and the flap on the dorsum of the hand. Elasticity is required to maintain the normal wrist range of motion. One option in these cases is the free serratus fascia flap that sometimes shows limitations regarding resistance as well as reliability. Here, we describe an easy modification including a thin muscle cuff in the serratus fascia flap - the serratus carpaccio flap - that improves the ease of flap harvest and morbidity rates. The indications for the serratus carpaccio flap, the technique of flap harvest and contouring as well as flap inset are described in detail. Clinical examples are given. The main advantage of the serratus carpaccio flap is the ability of the surgeon to adapt the flap thickness to the recipient site requirements. This allows excellent results regarding contour and function. Split thickness skin grafting fromthe instep region of the foot additionally allows optimal results for palmar as well as plantar defect reconstruction. Eighteen flaps for soft tissue defects of the hand (n = 5), foot (n = 10), and lower leg (n = 3) were performed. Complications included one flap loss, one venous re-anastomosis, two partial wound dehiscences and one postoperative hematoma at the donor site. Seventeen flaps survived completely. Secondary thinning procedures were not required. The serratus carpaccio flap is an excellent option for the reconstruction of medium-sized skin and soft tissue defects of the dorsum of the hand or foot, the palm, and the distal forearm.

Thromboxane A 2 Receptors Contribute to the Exaggerated Exercise Pressor Reflex in Rats with Heart Failure

Background Mechanical and metabolic signals associated with skeletal muscle contraction stimulate the sensory endings of thin fiber skeletal muscle afferents. During exercise, stimulation of these afferents initiates a reflex, termed the exercise pressor reflex (EPR), which contributes importantly to reflex increases in sympathetic nerve activity (SNA) and mean arterial blood pressure (MAP). In patients and animals with heart failure with reduced ejection fraction (HF-rEF), activation of the EPR contributes to exaggerated increases in SNA and is linked to decreased exercise tolerance and increased mortality. The exaggerated EPR in HF-rEF is attributed, at least in part, to an increased responsiveness of thin fiber skeletal muscle afferents produced by cyclooxygenase (COX) products of arachidonic metabolism. However, the COX metabolite receptors on the sensory endings that mediate the COX-induced afferent sensitization in HF-rEF are unknown. Purpose & Hypothesi The purpose of this investigation was to determine the role played by one of the major receptors for products of COX metabolism present on sensory neurons, namely thromboxane A2 receptors (TxA2-Rs), in the exaggerated EPR in HF-rEF rats. We tested the hypothesis that injection of the TxA2-R antagonist daltroban (80μg) into the arterial supply of the hindlimb would reduce the reflex increase in renal SNA (RSNA) and MAP evoked in response to 30 seconds of 1 Hz dynamic hindlimb muscle contraction in HF-rEF rats, but not control rats. Methods Experiments were performed on male Sprague-Dawley rats subjected to either a coronary artery ligation surgery to produce myocardial infarction and HF-rEF or a sham ligation. At least six weeks after the initial surgery, in decerebrate, unanesthetized rats we compared the EPR evoked by electrically induced hindlimb skeletal muscle contraction before and after injection of daltroban into the arterial supply of the hindlimb. Results Ejection fraction was significantly reduced in HF-rEF (47±3%) compared to sham (84±1%, p<0.01). We found that daltroban reduced the EPR response in HF-rEF (n=8; peak ΔMAP pre: 17±4; post: 12±2mmHg; P=0.04; peak ΔRSNA pre: 135±55; post: 46±13%; P=0.07), but not in SHAM rats (n=10; peak ΔMAP pre: 13±3; post: 11±2mmHg; P=0.48; peak ΔRSNA pre: 64±17; post: 63±15%; P=0.96;) rats. In a separate group of HF-rEF rats (n=4), we determined whether systemic circulation of daltroban accounted for the attenuating effects on EPR activation seen when daltroban was injected into the hindlimb arterial supply. We found that intravenous injection of daltroban had no effect on the EPR (peak ΔMAP pre: 26±7; post: 25±7mmHg; P=0.50). In these same four HF-rEF rats we also found that hindlimb arterial injection of the vehicle for daltroban (0.4mL saline containing 1% DMSO) had no effect on the EPR (peak ΔMAP pre: 23±8; post: 20±6mmHg; P=0.40). Conclusions These data suggest that that TxA2-R receptors on the sensory endings of muscle afferents contribute to the exaggerated EPR in HF-rEF. Our data enhance the understanding of the sympathetic and cardiovascular adjustments to exercise in the ~6 million Americans with HF-rEF.

The panniculus carnosus layer exhibits poor regeneration and deformed geometry following pressure injury: Implications for biomechanical protection

Background Human pressure injuries (PI) exhibit delayed healing and are classified as chronic wounds, and muscle damage increases the chronicity and severity of PIs. Pressure or load redistribution is typically the gold standard for preventing PIs in bedridden patients or those with limited mobility. The panniculus carnosus (PC) is a thin muscle in the cutaneous layer. Although evidence of the PC layer in multiple anatomical locations such as the heel has been reported, with high inter-individual variability, the PC is traditionally considered to be vestigial in humans. In this work, we investigate the role of the PC layer in biomechanics and load distribution, and ask how the PC regenerates after a pressure-induced injury. Methods To examine the biomechanical function of the PC, a finite element analysis was performed on a three-dimensional model of the human heel, comparing the change in soft tissue deformation during supine weight-bearing, with and without the PC. To assess PC regeneration, a mouse with inducible confetti-colored fluorescent satellite cells (muscle stem cells) was developed by crossing the Brainbow2.1 mouse with a conditional Pax7CreER mouse. The resulting mouse, upon tamoxifen induction, provides fluorescent colors for lineage tracing of muscle regeneration in vivo. Muscle PIs were created in these mice by applying a pair of 12mm magnets to the dorsal skinfold and PC muscle, in two intervals of 12 hours. Results In computational simulations of heel tissue, we find extensive overall displacement of soft tissues around the bony prominence during weight bearing, which was greatly decreased in the presence of the PC layer. Additionally, the presence of PC decreased the volume of tissue experiencing high strain and stress. When the PCs of mice were injured by pressure, tissue histology and fluorescent tracing of endogenous stem cells showed that the PC failed to regenerate across the distances necessary for this two-dimensional sheet-like structure. Even at 90 days following injury, a hole remained in the PC, while the other tissue layers regenerated successfully. Furthermore, multiple split and deformed myofibers were observed in the newly regenerated muscle at the wound edge. Conclusions Our work shows that the PC layer is sufficient to redistribute load around the posterior of the heel bone, reducing the volume of displaced tissue and the volume of soft tissue experiencing high strain and stress. Moreover, upon pressure injury, the PC layer is highly susceptible to poor regeneration and abnormalities in the geometry of the resulting fibers. Split or deformed fibers are weaker and more susceptible to repeated injury and this could be an underlying pathology of recurrent human PIs. We conclude that the PC might play an important protective function in pressure redistribution and that the PC regenerates poorly after PI. This absence or deformation of PC could be a potential risk factor for PI recurrence.

TRPV1 Channel Stimulation Reduces the Mechanoreflex in Decerebrate Rats

Background Mechanically-sensitive channels (MSCs) located on group III and IV thin fiber skeletal muscle afferents are stimulated by skeletal muscle contraction during exercise and contribute to reflex increases in sympathetic nervous system activity and blood pressure. This exercise-induced mechanically activated reflex, termed the mechanoreflex, is dependent on MSC stimulation produced by changes in muscle length and concurrent increases in intramuscular pressure during contraction. A relatively novel class of mechanically activated channel known as “piezo” is thought to contribute importantly to the generation of the mechanoreflex. For example, we found recently that hindlimb arterial injection of GsMTx4, a partially selective piezo channel inhibitor, reduced the pressor response to isolated dynamic mechanoreflex activation in rats. Recent work on the gating mechanics of piezo channels reveal that their activation can be regulated by changes in the phospholipid components of the cell membrane. Stimulation of transient receptor potential vanilloid 1 (TRPV1) has been shown to alter membrane concentrations of phospholipid component phosphatidylinositol 4,5-bisphosphate (PIP2). Specifically, stimulation of TRPV1 by capsaicin has been shown to completely inhibit piezo channel currents by depleting PIP2 in dorsal root ganglia neurons. Whether TRPV1 stimulation with capsaicin can modulate MSCs involved in mechanoreflex activation and reduce the reflex increase in blood pressure during dynamic hindlimb muscle stretch remains unknown. Purpose and Hypothesis Our purpose was to investigate the effect of TRPV1 stimulation produced by the injection of capsaicin into the arterial supply of the hindlimb on the mechanoreflex in healthy rats with patent femoral arteries (“freely perfused”, FP) and in a chronically ligated femoral artery rat model of simulated peripheral artery disease (“ligated”) in which the mechanoreflex is exaggerated. We tested the hypothesis that hindlimb arterial injection of capsaicin would reduce the pressor response to 30 s of 1 Hz dynamic hindlimb muscle stretch in both FP and ligated rats. Methods We compared (paired Student's t-tests) the pressor response (peak Δ mean arterial blood pressure) to muscle stretch before and 30 minutes after the injection of the TRPV1 agonist capsaicin (0.5 µg in 0.25ml) into the arterial supply of the hindlimb circulation of decerebrate, unanesthetized rats. Results Injection of capsaicin into the arterial supply of the hindlimb produced a pressor response in 57±14 mmHg in FP rats and 52±4 mmHg in ligated rats. More importantly, we found that capsaicin significantly reduced the peak pressor response to dynamic stretch in both FP (n=4, pre: 19±5, post: 5±1 mmHg, p=0.04) and ligated (n=7, pre: 27±3, post: 19±4 mmHg, p=0.01) rats. Conclusion We conclude that TRPV1 stimulation with capsaicin reduced the mechanoreflex evoked by dynamic stretch in both FP and ligated rats. These data provide additional, albeit indirect, evidence that piezo channels contribute importantly to the generation of the mechanoreflex and that TRPV1 stimulation-induced desensitization of piezo channels is capable of modulating mechanoreflex activation.

Role of Piezo channels in group IV muscle afferent fiber mechanosensation in rats

During exercise, skeletal muscle afferentnerves are activated by both mechanical and chemical stimuli within contracting muscle, subsequently contributing to an increase in blood pressure (termed the exercise pressor reflex; EPR). Although metabolically-sensitive receptors mediating the EPR have been described, the mechano-gated channels responsible for mechanosensation in thin muscle afferents (i.e. group III and IV fibers) remain to be elucidated. Piezo channels are known to be mechanically-activated ion channels that play crucial roles in several mechanotransduction processes in various organs. Moreover, Piezo channels are expressed in sensory tissues including dorsal root ganglion associated with skeletal muscle group III and IV afferents. Evidence suggests that antagonism of Piezo channels reduces the pressor response to activation of the EPR in rats. However, whether antagonism of Piezo channels affects muscle afferent discharge during mechanical stimulation remains unknown. PURPOSE The purpose of this study was to determine the impact of GsMTx4, a selective Piezo channel inhibitor, on neuronal responses to mechanical stimulation in group IV muscle afferents. It was hypothesized that GsMTx4 reduces the neural response to mechanical stimulation in sensory afferent fibers innervating skeletal muscle. METHODS Sprague-Dawley rats were used in this experiment (8-12 wks old). Using the ex vivo extensor digitorum longus muscle-nerve preparation, the responsiveness of mechanically activated group IV fibers was assessed by obtaining single-fiber recordings. Action potentials elicited by a mechanical stimulus (0-196 mN ramped stimulus) were recorded before and 20-min after injection of either Krebs-buffer solution (control), gadolinium (a non-specific mechano-gated channel antagonist, 10 mM) or GsMTx4 (250 μM) near the receptive field. RESULTS Conduction velocities of the group IV fibers assessed ranged between 0.32 and 0.77 m/s (average 0.56±0.10 m/s, n=22). The percentage of group IV fibers unable to generate an action potential during mechanical stimulation after injection of Krebs-buffer, Gadolinium and GsMTx4 were 0% (0 out of 6 fibers), 29% (2 out of 7 fibers) and 22% (2 out of 9 fibers), respectively. Fibers whose action potentials were partially attenuated by Krebs-buffer, Gadolinium and GsMTx4 were 50% (3 out of 6 fibers), 29% (2 out of 7 fibers) and 56% (5 out of 9 fibers), respectively. Compared to control, the reduction in neural responsiveness to mechanical stimulation tended to be greater in GsMTx4 (-1±49 vs. -48±38 %, P=0.08) albeit similar to Gadolinium (-71±33%, P=0.32). CONCLUSIONS The data demonstrate, for the first time, that GsMTx4 reduces the neuronal response to mechanical stimulation in group IV muscle afferent fibers of rats. These findings suggest that Piezo channels may significantly contribute to mechanosensation in sensory afferent fibers innervating skeletal muscle.