Rapid Stretch Research Articles

Bladder Symptoms Provoked by Short, Rapid-Acting Loop Diuretics: A Frequent but Often Overlooked Problem.

Bladder dysfunction entails overactive bladder (OAB) defined as symptoms of urinary urgency, frequency, and/or nocturia with or without incontinence if there is no obvious pathology or infection or lower urinary tract symptoms that includes recognized causes of bladder dysfunction. Literature search. Symptoms of OAB are reported in about 15% of the adult US population. This is increased 2- to 3-fold in patients with congestive heart failure (CHF), hypertension, cardiovascular disease (CVD), chronic kidney disease (CKD), or the elderly where it often accompanies prescription for short, rapid-acting loop diuretics. However, less than 2% of patients seeking care for OAB receive treatment. The fear of urinary incontinence from short, rapid-acting loop diuretics may contribute to medication nonadherence and less well-controlled, apparently resistant hypertension. The bladder contracts to rapid stretch. Thus, less rapid-acting diuretics such as thiazides or extended-release formulations of loop diuretics may be preferable for those with bladder dysfunction. Alternatively, the use of a mineralocorticosteroid receptor antagonist, angiotensin receptor antagonist/neprilysin inhibitor, or sodium glucose-linked transport type 2 inhibitor may allow a reduction in the dose of a short, rapid-acting loop diuretic for those with bladder dysfunction. A worsening of symptoms from bladder dysfunction by short, rapid-acting loop diuretics occurs frequently in patients with CVD, CHF, hypertension, and CKD where it can contribute to impaired quality of life and poor adherence and thereby to worsening outcomes.

The NCX1 calcium exchanger is implicated in delayed axotomy after peripheral nerve stretch injury.

After peripheral nerve stretch injury, most degenerating axons are thought to become disconnected at the time of injury, referred to as primary axotomy. The possibility of secondary axotomy-a delayed and potentially reversible form of disconnection-has not been evaluated. Here, we investigated secondary axotomy in a rat model of sciatic nerve stretch injury. We also evaluated whether axon sparing and functional improvement results from pharmacological blockade of the sodium-calcium exchanger 1 (NCX1), which is widely believed to contribute to traumatic axon degeneration but was previously only investigated in vitro. We studied peripheral nerve secondary axotomy in a clinically relevant rat model of sciatic nerve rapid stretch injury with immunolabeling and fluorescence microscopy. The role of NCX1 in secondary axotomy was studied with pharmacological inhibition with SEA0400 and immunolabeling, immunoblot, and behavioral assays. We found that early after injury, many axons remained in-continuity and that degeneration of axons was delayed, consistent with the occurrence of secondary axotomy. βAPP, a marker of secondary axotomy, accumulated at regions of axon swelling and disconnection, and NCX1 was upregulated and co-localized to βAPP axonal swellings. Pharmacological blockade of NCX1 after injury reduced calpain activation, proteolytic degradation of neurofilaments, βAPP accumulation, distal axon degeneration, and improved hindlimb function. Our data demonstrate a major role for secondary axotomy in peripheral nerve stretch injury and identify NCX1 as a promising therapeutic target to reduce secondary axotomy and improve functional outcome after nerve injury.

Assessing the Post-Activation Performance Enhancement of Upper Limbs in Basketball Athletes: A Sensor-Based Study of Rapid Stretch Compound and Blood Flow Restriction Training.

This study introduces a novel methodology combining rapid stretch compound training with blood flow restriction (BFR) to investigate post activation performance enhancement (PAPE) in basketball players, a field that has been predominantly explored for lower limbs. We aimed to assess the efficacy of this combined approach on upper limb muscle performance in athletes. We employed a randomized, self-controlled crossover trial with ten male basketball players. The bench press throw (BPT) served as the primary metric, with players undergoing four interventions post-baseline: (1) STR-plyometric training; (2) BFR-blood flow restriction; (3) COMB-STR integrated with BFR; and (4) CON-control. Innovatively, we utilized an intelligent tracking sensor to precisely measure peak power (PP), peak velocity (PV), mean power (MP), and mean velocity (MV) at 4, 8, and 12 min post-intervention, providing a detailed temporal analysis of PAPE. The COMB intervention demonstrated superior PAPE effects at 4 min, significantly outperforming STR and BFR alone and the control group across all measured indices (p < 0.05). Notably, the COMB group maintained these improvements for PV, PP, and H up to 12 min post-intervention, suggesting a prolonged effect. (1) The COMB stimulation has been shown to successfully induce PAPE more effectively than STR and BFR modality alone. (2) It appears that the optimal effects of PAPE are achieved within 4 min of exercising under this COMB. By the 12 min mark, only the COMB group continued to show significant improvements in PV, PP, and H compared to both the baseline and the CON group, while the effects in the STR and BFR groups further diminished. This suggests that although the PAPE effect is maintained over time, its optimal performance may peak at the 4 min mark and then gradually weaken as time progresses.

Predicting mechanical properties of mitotic spindles with a minimal constitutive model

The mitotic spindle, crucial for precise chromosome segregation and cytoplasmic partitioning during cell division, demands stability against forces arising from chromosomal movements and thermal fluctuations. Despite its central role, the mechanical properties of spindles remain largely elusive. In this study, we delve into the mechanical properties of spindles through a comprehensive model encompassing interactions among centrosomes, microtubules, chromosomes, and molecular motors. Our model successfully reproduces the 3D self–assembly of spindles and their responses to mechanical forces. We find that the spindle exhibits viscoelastic properties, responding distinctively to stretch and compression. Rapid stretch induces transient softening of the spindle, while compression leads to temporary hardening. Based on the viscoelastic responses of spindles under constant–force and constant–displacement loadings, we propose a minimal constitutive model for the spindle structure. This constitutive model can not only accurately recapture the viscoelastic responses of spindles under stretch and compression but also predict the mechanical behaviors of spindles under constant–rate loadings and cyclic loadings, which are further verified by simulations. Therefore, our validated constitutive model can replace complex simulations, providing more interesting predictions and guidance for future experiments.

3D Bioprinting Highly Elastic PEG-PCL-DA Hydrogel for Soft Tissue Fabrication and Biomechanical Stimulation.

3-D bioprinting is a promising technology to fabricate custom geometries for tissue engineering. However, most bioprintable hydrogels are weak and fragile, difficult to handle and cannot mimetic the mechanical behaviors of the native soft elastic tissues. We have developed a visible light crosslinked, single-network, elastic and biocompatible hydrogel system based on an acrylated triblock copolymer of poly(ethylene glycol) PEG and polycaprolactone (PCL) (PEG-PCL-DA). To enable its application in bioprinting of soft tissues, we have modified the hydrogel system on its printability and biodegradability. Furthermore, we hypothesize that this elastic material can better transmit pulsatile forces to cells, leading to enhanced cellular response under mechanical stimulation. This central hypothesis was tested using vascular conduits with smooth muscle cells (SMCs) cultured under pulsatile forces in a custom-made bioreactor. The results showed that vascular conduits made of PEG-PCL-DA hydrogel faithfully recapitulate the rapid stretch and recoil under the pulsatile pressure from 1 to 3 Hz frequency, which induced a contractile SMC phenotype, consistently upregulated the core contractile transcription factors. In summary, our work demonstrates the potential of elastic hydrogel for 3D bioprinting of soft tissues by fine tuning the printability, biodegradability, while possess robust elastic property suitable for manual handling and biomechanical stimulation.

Effect of the Novel Myotrope Danicamtiv on Cross-Bridge Behavior in Human Myocardium.

Background Omecamtiv mecarbil (OM) and danicamtiv both increase myocardial force output by selectively activating myosin within the cardiac sarcomere. Enhanced force generation is presumably due to an increase in the total number of myosin heads bound to the actin filament; however, detailed comparisons of the molecular mechanisms of OM and danicamtiv are lacking. Methods and Results The effect of OM and danicamtiv on Ca2+ sensitivity of force generation was analyzed by exposing chemically skinned myocardial samples to a series of increasing Ca2+ solutions. The results showed that OM significantly increased Ca2+ sensitivity of force generation, whereas danicamtiv showed similar Ca2+ sensitivity of force generation to untreated preparations. A direct comparison of OM and danicamtiv on dynamic cross-bridge behavior was performed at a concentration that produced a similar force increase when normalized to predrug levels at submaximal force (pCa 6.1). Both OM and danicamtiv-treated groups slowed the rates of cross-bridge detachment from the strongly bound state and cross-bridge recruitment into the force-producing state. Notably, the significant OM-induced prolongation in the time to reach force relaxation and subsequent commencement of force generation following rapid stretch was dramatically reduced in danicamtiv-treated myocardium. Conclusions This is the first study to directly compare the effects of OM and danicamtiv on cross-bridge kinetics. At a similar level of force enhancement, danicamtiv had a less pronounced effect on the slowing of cross-bridge kinetics and, therefore, may provide a similar improvement in systolic function as OM without excessively prolonging systolic ejection time and slowing cardiac relaxation facilitating diastolic filling at the whole-organ level.

Dorsal root ganglion neurons recapitulate the traumatic axonal injury of CNS neurons in response to a rapid stretch in vitro.

Introduction: In vitro models of traumatic brain injury (TBI) commonly use neurons isolated from the central nervous system. Limitations with primary cortical cultures, however, can pose challenges to replicating some aspects of neuronal injury associated with closed head TBI. The known mechanisms of axonal degeneration from mechanical injury in TBI are in many ways similar to degenerative disease, ischemia, and spinal cord injury. It is therefore possible that the mechanisms that result in axonal degeneration in isolated cortical axons after in vitro stretch injury are shared with injured axons from different neuronal types. Dorsal root ganglia neurons (DRGN) are another neuronal source that may overcome some current limitations including remaining healthy in culture for long periods of time, ability to be isolated from adult sources, and myelinated in vitro. Methods: The current study sought to characterize the differential responses between cortical and DRGN axons to mechanical stretch injury associated with TBI. Using an in vitro model of traumatic axonal stretch injury, cortical and DRGN neurons were injured at a moderate (40% strain) and severe stretch (60% strain) and acute alterations in axonal morphology and calcium homeostasis were measured. Results: DRGN and cortical axons immediately form undulations in response to severe injury, experience similar elongation and recovery within 20 min after the initial injury, and had a similar pattern of degeneration over the first 24 h after injury. Additionally, both types of axons experienced comparable degrees of calcium influx after both moderate and severe injury that was prevented through pre-treatment with tetrodotoxin in cortical neurons and lidocaine in DRGNs. Similar to cortical axons, stretch injury also causes calcium activated proteolysis of sodium channel in DRGN axons that is prevented by treatment with lidocaine or protease inhibitors. Discussion: These findings suggest that DRGN axons share the early response of cortical neurons to a rapid stretch injury and the associated secondary injury mechanisms. The utility of a DRGN in vitro TBI model may allow future studies to explore TBI injury progression in myelinated and adult neurons.

Stretch activated force production increases during fatigue in fast-contracting skeletal muscle fibers.

Stretch activation (FSA) is the delayed increase in fiber tension (force per cross-sectional area) following a rapid stretch and can improve muscle performance during repetitive cyclical contractions. Historically considered minimal in skeletal muscle, our recent work showed the ratio of stretch activation to calcium-activated tension (FSA/F0) increased from 10 to 40% with greater inorganic phosphate (Pi) levels in soleus muscle fibers (Straight et al., 2019). Given Pi increases with muscle fatigue, we hypothesize that FSA helps maintain force generation during fatigue. To test this, FSA, induced by a stretch of 0.5% fiber length, was examined during control (pCa 4.5, pH 7.0, Pi 5 mM), high calcium fatigue (pCa 4.5, pH 6.2, Pi 30 mM) and low calcium fatigue (pCa 5.1, pH 6.2, Pi 30 mM) in fibers expressing myosin heavy chain (MHC) I, IIA, IIX and IIB isoforms from soleus and extensor digitorum longus muscles of C57BL/6NJ mice. F0 of all MHC isoforms decreased from control to high calcium fatigue to low calcium fatigue, as expected. In MHC IIX and IIB fibers, FSA occurred under all conditions and FSA/F0 increased from control (17-20%) to high calcium fatigue (32-35%) to low calcium fatigue (42-44%). In MHC IIA fibers, FSA/F0 was similar to MHC IIX and IIB fibers in control and high calcium fatigue, however MHC I fiber values were much less (5% and 18%). For MHC I and IIA fibers, no discernable FSA was apparent in low calcium fatigue. These results show that FSA is a significant modulator of force production under fatiguing conditions in fast-contracting muscle fibers. This mechanism could play an important physiological role during cyclical contractions, when the antagonistic muscle rapidly stretches the agonist muscle, by reducing the effect of fatigue on force production.

Prediction of Mechanical Responses of a Uniaxially Stretched Neural Fiber Bundle: Theoretical Approach for a Traumatic Loading Condition

Abstract Viscoelasticity of the spinal nerve roots may play a significant role in predicting nerve root damage caused by overall spinal motion. However, only a few studies have investigated the complex mechanical behavior of this tissue. The current study presents a theoretical protocol for predicting mechanical responses of soft biological materials, and this method was used to a uniaxially stretched neural fiber bundle isolated from porcine spinal nerve roots with various loading configurations. Stress relaxation tests were performed to systematically determine a set of parameters dictating the stress decaying process, i.e., a set of relaxation moduli and the corresponding time constants. Based on the obtained experimental and numerical test data, it was confirmed that the proposed method is effective even for the prediction of mechanical response to a cyclic stretch immediately after the ramp-hold test. In addition, an elastic response, i.e., a stress–strain relationship under a high-rate loading regime, was determined analytically. The results demonstrated that instantaneous mechanical responses of neural fiber bundles can be stiffened against very rapid stretch (&amp;gt;10 s−1); however, the fibers are relatively insensitive to moderate loading rates (&amp;lt;1 s−1). The ultimate tensile strength was estimated to be approximately 8 MPa at the structural failure strain (15%). This information will enable the computational assessment of traumatic nerve root injuries sustained during traffic accidents and contact sports.

Molecular regulation of stretch activation.

Stretch activation is defined as a delayed increase in force after rapid stretches. Although there is considerable evidence for stretch activation in isolated cardiac myofibrillar preparations, few studies have measured mechanisms of stretch activation in mammalian skeletal muscle fibers. We measured stretch activation following rapid step stretches [∼1%-4% sarcomere length (SL)] during submaximal Ca2+ activations of rat permeabilized slow-twitch skeletal muscle fibers before and after protein kinase A (PKA), which phosphorylates slow myosin binding protein-C. PKA significantly increased stretch activation during low (∼25%) Ca2+ activation and accelerated rates of delayed force development (kef) during both low and half-maximal Ca2+ activation. Following the step stretches and subsequent force development, fibers were rapidly shortened to original sarcomere length, which often elicited a shortening-induced transient force overshoot. After PKA, step shortening-induced transient force overshoot increased ∼10-fold following an ∼4% SL shortening during low Ca2+ activation levels. kdf following step shortening also increased after PKA during low and half-maximal Ca2+ activations. We next investigated thin filament regulation of stretch activation. We tested the interplay between cardiac troponin I (cTnI) phosphorylation at the canonical PKA and novel tyrosine kinase sites on stretch activation. Native slow-skeletal Tn complexes were exchanged with recombinant human cTn complex with different human cTnI N-terminal pseudo-phosphorylation molecules: 1) nonphosphorylated wild type (WT), 2) the canonical S22/23D PKA sites, 3) the tyrosine kinase Y26E site, and 4) the combinatorial S22/23D + Y26E cTnI. All three pseudo-phosphorylated cTnIs elicited greater stretch activation than WT. Following stretch activation, a new, elevated stretch-induced steady-state force was reached with pseudo-phosphorylated cTnI. Combinatorial S22/23D + Y26E pseudo-phosphorylated cTnI increased kdf. These results suggest that slow-skeletal myosin binding protein-C (sMyBP-C) phosphorylation modulates stretch activation by a combination of cross-bridge recruitment and faster cycling kinetics, whereas cTnI phosphorylation regulates stretch activation by both redundant and synergistic mechanisms; and, taken together, these sarcomere phosphoproteins offer precision targets for enhanced contractility.

Improved capillary rheometry for viscous Newtonian filaments

The efficacy of the kinematic differential analysis of McCarroll et al. [“Differential analysis of capillary breakup rheometry for Newtonian liquids,” J. Fluid Mech. 804, 116 (2016)], expanding on McKinley and Tripathi [“How to extract the Newtonian viscosity from capillary breakup measurements in a filament rheometer,” J. Rheol. 44, 653 (2000)], to evaluate the surface tension to viscosity ratio for Newtonian filaments is examined. The analysis is valid during and after stretch, while the latter is traditionally applied after cessation of stretch as the midfilament radius approaches zero. Through numerical simulations, the evaluation of viscosity is investigated for two common stretch histories: (a) the ramp function and (b) the modified step strain. The challenges with stretch are twofold: rapid stretch (large capillary number) results in a nearly cylindrical filament with a rapid change near the plate that challenges the one-dimensional (1D) approximation, while slow stretch results in a nearly static solution with limited viscous information. We examine the capillary number-aspect ratio parameter space and find the ramp function with small stretching speeds optimal. Hence, the most accurate measurements are taken while the filament is being stretched.

The role of expansion angle and speed on shish-kebab formation in expansion pipe extruder head explored by high temperature rapid stretch

Morphology and microstructures play a significant role on properties of polymer products. Self-reinforced shish-kebab formed inside the products by elongational flow induced orientation can improve the tensile strength and pressure rating properties, and the density of polymers consisting of shish-kebabs is fairly below those filled with glass fibers, basalt and talc. The threshold parameters to form shish-kebab structure in a novel annular expansion pipe extruder head were determined by high temperature rapid stretch (HTRS) in this study. Different morphologies of HDPE specimens made under the conditions in expansion head by HTRS were studied by SEM, 2D-WAXD and 2D-SAXS, specimen 7 with 15° and 6 mm/s base speed has the optimal shish-kebab structure and specimen 10 with 30° and 5 mm/s has the best orientation parameter and the best shish structure. The DSC results agree well with the morphology results shown in the SEM images. The specimen 7 displays the highest tensile strength, increases about 187% compared with the reference specimen. These outstanding properties were attribute to the perfect and compact shish-kebab structures inside it. The results of HTRS can prove a direction guide for extrusion processing.

Gli1+ Cells Residing in Bone Sutures Respond to Mechanical Force via IP3R to Mediate Osteogenesis.

Early orthodontic correction of skeletal malocclusion takes advantage of mechanical force to stimulate unclosed suture remodeling and to promote bone reconstruction; however, the underlying mechanisms remain largely unclear. Gli1+ cells in maxillofacial sutures have been shown to participate in maxillofacial bone development and damage repair. Nevertheless, it remains to be investigated whether these cells participate in mechanical force-induced bone remodeling during orthodontic treatment of skeletal malocclusion. In this study, rapid maxillary expansion (RME) mouse models and mechanical stretch loading cell models were established using two types of transgenic mice which are able to label Gli1+ cells, and we found that Gli1+ cells participated in mechanical force-induced osteogenesis both in vivo and in vitro. Besides, we found mechanical force-induced osteogenesis through inositol 1,4,5-trisphosphate receptor (IP3R), and we observed for the first time that inhibition of Gli1 suppressed an increase in mechanical force-induced IP3R overexpression, suggesting that Gli1+ cells participate in mechanical force-induced osteogenesis through IP3R. Taken together, this study is the first to demonstrate that Gli1+ cells in maxillofacial sutures are involved in mechanical force-induced bone formation through IP3R during orthodontic treatment of skeletal malocclusion. Furthermore, our results provide novel insights regarding the mechanism of orthodontic treatments of skeletal malocclusion.

Older Age Increases the Amplitude of Muscle Stretch-Induced Cortical Beta-Band Suppression But Does not Affect Rebound Strength.

Healthy aging is associated with deterioration of the sensorimotor system, which impairs balance and somatosensation. However, the exact age-related changes in the cortical processing of sensorimotor integration are unclear. This study investigated primary sensorimotor cortex (SM1) oscillations in the 15–30 Hz beta band at rest and following (involuntary) rapid stretches to the triceps surae muscles (i.e., proprioceptive stimulation) of young and older adults. A custom-built, magnetoencephalography (MEG)-compatible device was used to deliver rapid (190°·s−1) ankle rotations as subjects sat passively in a magnetically-shielded room while MEG recorded their cortical signals. Eleven young (age 25 ± 3 years) and 12 older (age 70 ± 3 years) adults matched for physical activity level demonstrated clear 15–30 Hz beta band suppression and rebound in response to the stretches. A sub-sample (10 young and nine older) were tested for dynamic balance control on a sliding platform. Older adults had greater cortical beta power pre-stretch (e.g., right leg: 4.0 ± 1.6 fT vs. 5.6 ± 1.7 fT, P = 0.044) and, subsequently, greater normalized movement-related cortical beta suppression post-proprioceptive stimulation (e.g., right leg: −5.8 ± 1.3 vs. −7.6 ± 1.7, P = 0.01) than young adults. Furthermore, poorer balance was associated with stronger cortical beta suppression following proprioceptive stimulation (r = −0.478, P = 0.038, n = 19). These results provide further support that cortical processing of proprioception is hindered in older adults, potentially (adversely) influencing sensorimotor integration. This was demonstrated by the impairment of prompt motor action control, i.e., regaining perturbed balance. Finally, SM1 cortex beta suppression to a proprioceptive stimulus seems to indicate poorer sensorimotor functioning in older adults.

Rapid-Stretch Injury to Peripheral Nerves: Histologic Results.

Although most severe peripheral nerve injuries result from high-speed mechanisms, there is no laboratory model to replicate this clinical condition. To qualitatively and quantitatively describe microanatomical injury of rapid stretch. The sciatic nerves of 36 Sprague-Dawley rats were subjected to rapid-stretch nerve injury, using fixed-direction strain produced via constrained weight drop applied to an intact nerve. Nerve injury severity was categorized by biomechanical parameters. Injury to nerve microarchitecture was quantified with serial longitudinal sectioning, with specific focus on the endoneurium, perineurium, and epineurium. Four grades of stretch injury severity were determined by mathematical cluster analysis: sham, elastic stretch, inelastic stretch, and stretch rupture. Two patterns of injury to endoneurial architecture were quantified: loss of fiber undulation (straightened fibers) and rupturing of individual fibers ("microruptures"). Straightening of nerve fibers was the earliest accommodation to stretch injury and accounted for elongation during elastic stretch. Microruptures were distributed along the length of the nerve and were more severe and involved greater volume of the nerve at higher biomechanical severity. Epineurium and perineurium disruption increased in frequency with progressive injury severity, yet did not predict transition from one injury grade to another (P=.3), nor was it a hallmark of severe injury. Conversely, accumulation of microruptures provided strong correlation to nerve injury severity (Pearson's R=.9897) and progression to mechanical failure. Nerve architecture is injured in a graded fashion during stretch injury, which likely reflects tissue biomechanics. This study suggests new considerations in the theoretical framework of nerve stretch trauma.

Tropomyosin as a Stretch Sensor in the Troponin Bridges of Insect Flight Muscle

The flight muscles of many insects contract at high frequency and nearly constant concentrations of calcium. The fibres are activated by rapid stretches, and bridges between thick and thin filaments have been identified as possible stretch sensors. The bridges are spaced at 38.7 nm at the position of troponin on the thin filament, alternating with force-producing crossbridges at target sites in actin. The tropomyosin-troponin complex (Tm-Tn) in Lethocerus flight muscle has two isoforms of tropomyosin (Tm1 and Tm2). Tm-Tn and Tm1 bind to thick filaments or myosin, whereas Tm2 does not. The Tm isoforms exist as homodimers in situ and could act independently: Tm1 forming part of a Tn bridge and Tm2 acting as conventional Tm. Electron micrographs of Tm-Tn incubated with isolated thick filaments showed the complex following the helical arrangement of myosin molecules, with projections every ∼40 nm. Thick filaments were sometimes crosslinked by Tm-Tn or by Tm1 alone. Atomic force microscopy (AFM) of thick filaments in relaxing buffer showed crossbridge crowns every ∼43 nm that were ∼30 nm in height above the mica surface, and intermediate crowns ∼19 nm above the surface. The higher crowns would be more likely to interact with Tm-Tn on thin filaments. Thick filaments crosslinked by Tm1 were also observed by tapping mode AFM. Direct binding of Tm1 to myosin confirms the suggestion that non-force producing crossbridges could activate the thin filament directly when fibres are stretched, pulling Tm from an inhibitory position on actin every cycle of oscillatory contraction, without changes in calcium concentration. We will describe the differences in biophysical properties of Tm1 and Tm2.

Microtubules Increase Diastolic Stiffness in Failing Human Cardiomyocytes and Myocardium.

Diastolic dysfunction is a prevalent and therapeutically intractable feature of heart failure (HF). Increasing ventricular compliance can improve diastolic performance, but the viscoelastic forces that resist diastolic filling and become elevated in human HF are poorly defined. Having recently identified posttranslationally detyrosinated microtubules as a source of viscoelasticity in cardiomyocytes, we sought to test whether microtubules contribute meaningful viscoelastic resistance to diastolic stretch in human myocardium. Experiments were conducted in isolated human cardiomyocytes and trabeculae. First, slow and rapid (diastolic) stretch was applied to intact cardiomyocytes from nonfailing and HF hearts and viscoelasticity was characterized after interventions targeting microtubules. Next, intact left ventricular trabeculae from HF patient hearts were incubated with colchicine or vehicle and subject to pre- and posttreatment mechanical testing, which consisted of a staircase protocol and rapid stretches from slack length to increasing strains. Viscoelasticity was increased during diastolic stretch of HF cardiomyocytes compared with nonfailing counterparts. Reducing either microtubule density or detyrosination reduced myocyte stiffness, particularly at diastolic strain rates, indicating reduced viscous forces. In myocardial tissue, we found microtubule depolymerization reduced myocardial viscoelasticity, with an effect that decreased with increasing strain. Colchicine reduced viscoelasticity at strains below, but not above, 15%, with a 2-fold reduction in energy dissipation upon microtubule depolymerization. Post hoc subgroup analysis revealed that myocardium from patients with HF with reduced ejection fraction were more fibrotic and elastic than myocardium from patients with HF with preserved ejection fraction, which were relatively more viscous. Colchicine reduced viscoelasticity in both HF with preserved ejection fraction and HF with reduced ejection fraction myocardium. Failing cardiomyocytes exhibit elevated viscosity and reducing microtubule density or detyrosination lowers viscoelastic resistance to diastolic stretch in human myocytes and myocardium. In failing myocardium, microtubules elevate stiffness over the typical working range of strains and strain rates, but exhibited diminishing effects with increasing length, consistent with an increasing contribution of the extracellular matrix or myofilament proteins at larger excursions. These studies indicate that a stabilized microtubule network provides a viscous impediment to diastolic stretch, particularly in HF.

A myosin-based mechanism for stretch activation and its possible role revealed by varying phosphate concentration in fast and slow mouse skeletal muscle fibers.

Stretch activation (SA) is a delayed increase in force following a rapid muscle length increase. SA is best known for its role in asynchronous insect flight muscle, where it has replaced calcium's typical role of modulating muscle force levels during a contraction cycle. SA also occurs in mammalian skeletal muscle but has previously been thought to be too low in magnitude, relative to calcium-activated (CA) force, to be a significant contributor to force generation during locomotion. To test this supposition, we compared SA and CA force at different Pi concentrations (0-16 mM) in skinned mouse soleus (slow-twitch) and extensor digitorum longus (EDL; fast-twitch) muscle fibers. CA isometric force decreased similarly in both muscles with increasing Pi, as expected. SA force decreased with Pi in EDL (40%), leaving the SA to CA force ratio relatively constant across Pi concentrations (17-25%). In contrast, SA force increased in soleus (42%), causing a quadrupling of the SA to CA force ratio, from 11% at 0 mM Pi to 43% at 16 mM Pi, showing that SA is a significant force modulator in slow-twitch mammalian fibers. This modulation would be most prominent during prolonged muscle use, which increases Pi concentration and impairs calcium cycling. Based upon our previous Drosophila myosin isoform studies and this work, we propose that in slow-twitch fibers a rapid stretch in the presence of Pi reverses myosin's power stroke, enabling quick rebinding to actin and enhanced force production, while in fast-twitch fibers, stretch and Pi cause myosin to detach from actin.

Rapid Stretch Injury to Peripheral Nerves: Biomechanical Results

Although most adult brachial plexus injuries result from high-speed mechanisms, no laboratory model has been created to mimic rapid-stretch nerve injuries. Understanding the biomechanical response of nerves to rapid stretch is essential to understanding clinical injury patterns and developing models that mimic the clinical scenario. To assess the influence of rate, loading direction, and excursion of stretch injuries on the biomechanical properties of peripheral nerves. The sciatic nerves of 138 Sprague-Dawley rats were dissected and subjected to rapid- and slow-stretch methods. Maximal nerve strain, persistent deformation, regional strain variation, and location of nerve failure were recorded. Nerve rupture was primarily determined by weight-drop momentum>1 N/sec (odds ratio=27.8, P<.0001), suggesting a threshold condition. Loading direction strongly determined maximal strain at rupture (P=.028); pull along the nerve axis resulted in nerve rupture at lower strain than orthogonal loading. Regional variations in nerve compliance and rupture location correlated with anatomic zones. Nerve branch anatomy was the largest contributing factor on maximum strain and rupture location. Rapidly stretched nerves are characterized by a zone of elastic recovery, followed by inelastic response at increasing strain, and finally rupture. The large variation in previous results for nerve strain at rupture can be attributed to different testing conditions and is largely due to loading direction or segment of nerve tested, which has significant clinical implications. Nerve stretch injuries do not reflect a continuous variability to applied force but instead fall into biomechanical patterns of elastic, inelastic, and rupture injuries.

High-speed mechano-active multielectrode array for investigating rapid stretch effects on cardiac tissue

Systematic investigations of the effects of mechano-electric coupling (MEC) on cellular cardiac electrophysiology lack experimental systems suitable to subject tissues to in-vivo like strain patterns while simultaneously reporting changes in electrical activation. Here, we describe a self-contained motor-less device (mechano-active multielectrode-array, MaMEA) that permits the assessment of impulse conduction along bioengineered strands of cardiac tissue in response to dynamic strain cycles. The device is based on polydimethylsiloxane (PDMS) cell culture substrates patterned with dielectric actuators (DEAs) and compliant gold ion-implanted extracellular electrodes. The DEAs induce uniaxial stretch and compression in defined regions of the PDMS substrate at selectable amplitudes and with rates up to 18 s−1. Conduction along cardiomyocyte strands was found to depend linearly on static strain according to cable theory while, unexpectedly, being completely independent on strain rates. Parallel operation of multiple MaMEAs provides for systematic high-throughput investigations of MEC during spatially patterned mechanical perturbations mimicking in-vivo conditions.