Isometric Force Generation Research Articles

Increased CAMKII Activity Impairs Contractile Function in Human HCM Myocardium

Background and Rationale: we have previously shown that the myocardium of patients with Hypertrophic Cardiomyopathy (HCM) shows increased CaMKII activity with respect to non-HCM non-failing surgical patients (controls). We found higher phosphorylation levels of several tested CaMKII targets, such as L-Type Ca2+ channels, Na+ channels and phospholamban (Coppini et al., Circulation 2013). Such changes are likely to determine significant deleterious effects on the electrical and mechanical function of HCM myocardium.Methods: we assessed the effects of a selective inhibitor of CaMKII, the cell-permeant Autocamtide-related Inhibitory Peptide II (AIP-II), on force and intracellular Ca2+ in intact trabeculae and cardiomyocytes isolated from surgical septal samples of HCM patients who underwent myectomy.Results: HCM trabeculae display an increased diastolic tension and a slower kinetics of isometric force generation when compared to control trabeculae. AIP-II determines a reduction of diastolic tension, with larger effects at fast pacing rates (−13±3 % at 2Hz). Additionally, AIP-II accelerated the kinetics of force generation in HCM muscles (time to peak contraction at 1Hz: 255±17 ms at baseline vs. 232±10 ms with AIP-II, p<0.05). Accordingly, in HCM cardiomyocytes AIP-II lowered intracellular diastolic Ca2+ levels (from 278 ± 40 nM to 159 ± 28 nM, p<0.01) and reduced the frequency-dependent increase of diastolic [Ca2+]. The kinetics of Ca2+ transients, however, were not significantly affected by AIP-II. This suggests that reduced CaMKII phosphorylation of myofilament proteins may underlie the accelerated contractile kinetics observed in HCM trabeculae following AIP-II exposure.Discussion: The results indicate that increased activation of CaMKII plays a significant role in determining the abnormalities of myocardial contraction in HCM. Pharmacological inhibition of CaMKII may represent a viable option to reduce diastolic dysfunction in HCM patients.

Optical Trapping Shows a 3-Fold Increase in Myosin Head Stiffness by the FHC-Mutation R723G in the β-Cardiac Myosin Heavy Chain

Missense mutations in the β-cardiac myosin heavy chain (β-MyHC) are the most frequent cause of Familial Hypertrophic Cardiomyopathy (FHC). For these mutations mutant and wildtype β-MyHC are co-incorporated in the contractile apparatus, and direct functional effects of such mutations presumably trigger development of the FHC-phenotype. In M. soleus biopsies of affected patients we previously found increased isometric force generation for the converter mutation R723G, most likely due to increased stiffness of the myosin head domain. Quantification of the increase in head stiffness from fiber studies, however, was complicated by filament compliance, as well as non-equal and variable co-expression of mutant and wildtype protein in the tissue samples.In the present work we directly determined stiffness of individual myosin heads isolated from M. soleus tissue samples of affected patients using the three-bead optical trapping assay. Since each assay contained mutant and wildtype head domains, stiffness of individual heads with and without the R723G mutation could directly be compared. We determined head stiffness by imposing triangular stage displacements and recording bead displacements during binding events, taking into account compliance of the actin-bead link determined by the variance-Hidden-Markov method (Smith et al., BJ 2001). We found two populations of head domains, one with a head stiffness of 0.39 ± 0.24 pN/nm the other with an about 3-fold higher stiffness. The low value is close to our previous optical trapping with β-MyHC (0.38 ± 0.06 pN/nm; Brenner et al., JMRCM 2012), the about 3-fold increase in head stiffness agrees well with our previous estimate from rigor stiffness in fibers with the same mutation (Seebohm et al., BJ 2009) while the absolute value of head stiffness from fibers was about 25% lower than determined here.

The cost of muscle power production: muscle oxygen consumption per unit work increases at low temperatures inXenopus laevisDaudin

Metabolic energy (ATP) supply to muscle is essential to support activity and behaviour. It is expected, therefore, that there is strong selection to maximise muscle power output for a given rate of ATP use. However, the viscosity and stiffness of muscle increases with a decrease in temperature, which means that more ATP may be required to achieve a given work output. Here, we tested the hypothesis that ATP use increases at lower temperatures for a given power output in Xenopus laevis. To account for temperature variation at different time scales, we considered the interaction between acclimation for 4 weeks (to 15 or 25°C) and acute exposure to these temperatures. Cold-acclimated frogs had greater sprint speed at 15°C than warm-acclimated animals. However, acclimation temperature did not affect isolated gastrocnemius muscle biomechanics. Isolated muscle produced greater tetanus force, and faster isometric force generation and relaxation, and generated more work loop power at 25°C than at 15°C acute test temperature. Oxygen consumption of isolated muscle at rest did not change with test temperature, but oxygen consumption while muscle was performing work was significantly higher at 15°C than at 25°C, regardless of acclimation conditions. Muscle therefore consumed significantly more oxygen at 15°C for a given work output than at 25°C, and plastic responses did not modify this thermodynamic effect. The metabolic cost of muscle performance and activity therefore increased with a decrease in temperature. To maintain activity across a range of temperature, animals must increase ATP production or face an allocation trade-off at lower temperatures. Our data demonstrate the potential energetic benefits of warming up muscle before activity, which is seen in diverse groups of animals such as bees, which warm flight muscle before take-off, and humans performing warm ups before exercise.

Normal isometric strength of rotator cuff muscles in adults

ObjectivesThe most prevalent disorders of the shoulder are related to the muscles of rotator cuff. In order to develop a mechanical method for the evaluation of the rotator cuff muscles, we created a database of isometric force generation by the rotator cuff muscles in normal adult population. We hypothesised the existence of variations according to age, gender and dominancy of limb.MethodsA total of 400 healthy adult volunteers were tested, classified into groups of 50 men and women for each decade of life. Maximal isometric force was measured at standardised positions for supraspinatus, infraspinatus and subscapularis muscles in both shoulders in every person. Torque of the force was calculated and normalised to lean body mass. The profiles of mean torque-time curves for each age and gender group were compared.ResultsOur data showed that men gradually gained maximal strength in the fifth decade, and showed decreased strength in the sixth. In women the maximal strength was gained in the fourth decade with gradual decline to the sixth decade of life. The dominant arm was stronger in most of the tested groups. The torque profiles of the rotator cuff muscles in men at all ages were significantly higher than that in women.ConclusionsWe found previously unrecognised variations of rotator cuff muscles’ isometric strength according to age, gender and dominancy in a normal population. The presented data may serve as a basis for the future studies for identification of the abnormal patterns of muscle isometric strength in patients with pathology of the rotator cuff muscles.Cite this article: Bone Joint Res 2013;2:214–19.

Enhancement of Force Generated by Individual Myosin Heads in Skinned Rabbit Psoas Muscle Fibers at Low Ionic Strength

Although evidence has been presented that, at low ionic strength, myosin heads in relaxed skeletal muscle fibers form linkages with actin filaments, the effect of low ionic strength on contraction characteristics of Ca2+-activated muscle fibers has not yet been studied in detail. To give information about the mechanism of muscle contraction, we have examined the effect of low ionic strength on the mechanical properties and the contraction characteristics of skinned rabbit psoas muscle fibers in both relaxed and maximally Ca2+-activated states. By progressively decreasing KCl concentration from 125 mM to 0 mM (corresponding to a decrease in ionic strength μ from 170 mM to 50 mM), relaxed fibers showed changes in mechanical response to sinusoidal length changes and ramp stretches, which are consistent with the idea of actin-myosin linkage formation at low ionic strength. In maximally Ca2+-activated fibers, on the other hand, the maximum isometric force increased about twofold by reducing KCl concentration from 125 to 0 mM. Unexpectedly, determination of the force-velocity curves indicated that, the maximum unloaded shortening velocity Vmax, remained unchanged at low ionic strength. This finding indicates that the actin-myosin linkages, which has been detected in relaxed fibers at low ionic strength, are broken quickly on Ca2+ activation, so that the linkages in relaxed fibers no longer provide any internal resistance against fiber shortening. The force-velocity curves, obtained at various levels of steady Ca2+-activated isometric force, were found to be identical if they are normalized with respect to the maximum isometric force. The MgATPase activity of muscle fibers during isometric force generation was found not to change appreciably at low ionic strength despite the two-fold increase in Ca2+-activated isometric force. These results can be explained in terms of enhancement of force generated by individual myosin heads, but not by any changes in kinetic properties of cyclic actin-myosin interaction.

Lipopolysaccharide increases agonist‐induced contractile responses in Sprague Dawley rat corpus cavernosum

Chronic inflammatory conditions are frequent co‐morbidities of erectile dysfunction (ED) and elevated lipopolysaccharide (LPS) levels may increase inflammation via Toll‐like receptor 4 (TLR4) activation. An imbalance between vascular dilatation and contraction underlies ED. We hypothesized that LPS increases the agonist‐induced contractile response in corpus cavernosum (CC). The CC tissues were isolated, incubated for 6 hours in tissue culture media with 1 ug/mL LPS or vehicle, before being mounted in a muscle strip myograph for studies of isometric force generation. Contraction to 120 mM KCl did not differ between LPS and vehicle treated CC. KCl‐induced contraction data were used to normalize contraction data from the serotonin concentration response curve (30 nM‐100 uM). The CC tissue exposed to LPS exhibited enhanced contraction to serotonin (as % of KCl contraction) compared to control values. At 30 uM concentration: 67.34±5.15 vs. 43.34±6.67% (p&lt;0.05), and at 100 uM: 77.61±5.4 vs. 56.61±6.98% (p&lt;0.05). These results show that treatment with the TLR4 ligand LPS increased the contractile response to serotonin in rat corpus cavernosum. Thus, TLR4 activation may play a role in the etiology of ED which is a condition that has been shown to predict cardiovascular disease.Support: NIH and SMSNA.

Modulation of local field potential power of the subthalamic nucleus during isometric force generation in patients with Parkinson’s disease

Investigations of local field potentials of the subthalamic nucleus of patients with Parkinson’s disease have provided evidence for pathologically exaggerated oscillatory beta-band activity (13–30Hz) which is amenable to physiological modulation by, e.g., voluntary movement. Previous functional magnetic resonance imaging studies in healthy controls have provided evidence for an increase of subthalamic nucleus blood-oxygenation-level-dependant signal in incremental force generation tasks.However, the modulation of neuronal activity by force generation and its relationship to peripheral feedback remain to be elucidated. We hypothesised that beta-band activity in the subthalamic nucleus is modulated by incremental force generation. Subthalamic nucleus local field potentials were recorded intraoperatively in 13 patients with Parkinson’s disease (37 recording sites) during rest and five incremental isometric force generation conditions of the arm with applied loads of 0–400g (in 100-g increments).Repeated measures analysis of variance (ANOVA) revealed a modulation of local field potential (LFP) power in the upper beta-band (in 24–30Hz; F(3.042)=4.693, p=0.036) and the gamma-band (in 70–76Hz; F(4)=4.116, p=0.036). Granger-causality was computed with the squared partial directed coherence and showed no significant modulation during incremental isometric force generation.Our findings indicate that the upper beta- and gamma-band power of subthalamic nucleus local field potentials are modulated by the physiological task of force generation in patients with Parkinson’s disease. This modulation seems to be not an effect of a modulation of peripheral feedback.

Isometric and Eccentric Force Generation Assessment of Skeletal Muscles Isolated from Murine Models of Muscular Dystrophies

Critical to the evaluation of potential therapeutics for muscular disease are sensitive and reproducible physiological assessments of muscle function. Because many pre-clinical trials rely on mouse models for these diseases, isolated muscle function has become one of the standards for Go/NoGo decisions in moving drug candidates forward into patients. We will demonstrate the preparation of the extensor digitorum longus (EDL) and diaphragm muscles for functional testing, which are the predominant muscles utilized for these studies. The EDL muscle geometry is ideal for isolated muscle preparations, with two easily accessible tendons, and a small size that can be supported by superfusion in a bath. The diaphragm exhibits profound progressive pathology in dystrophic animals, and can serve as a platform for evaluating many potential therapies countering fibrosis, and promoting myofiber stability. Protocols for routine testing, including isometric and eccentric contractions, will be shown. Isometric force provides assessment of strength, and eccentric contractions help to evaluate sarcolemma stability, which is disrupted in many types of muscular dystrophies. Comparisons of the expected results between muscles from wildtype and dystrophic muscles will also be provided. These measures can complement morphological and biochemical measurements of tissue homeostasis, as well as whole animal assessments of muscle function.

Isometric and Eccentric Force Generation Assessment of Skeletal Muscles Isolated from Murine Models of Muscular Dystrophies

Critical to the evaluation of potential therapeutics for muscular disease are sensitive and reproducible physiological assessments of muscle function. Because many pre-clinical trials rely on mouse models for these diseases, isolated muscle function has become one of the standards for Go/NoGo decisions in moving drug candidates forward into patients. We will demonstrate the preparation of the extensor digitorum longus (EDL) and diaphragm muscles for functional testing, which are the predominant muscles utilized for these studies. The EDL muscle geometry is ideal for isolated muscle preparations, with two easily accessible tendons, and a small size that can be supported by superfusion in a bath. The diaphragm exhibits profound progressive pathology in dystrophic animals, and can serve as a platform for evaluating many potential therapies countering fibrosis, and promoting myofiber stability. Protocols for routine testing, including isometric and eccentric contractions, will be shown. Isometric force provides assessment of strength, and eccentric contractions help to evaluate sarcolemma stability, which is disrupted in many types of muscular dystrophies. Comparisons of the expected results between muscles from wildtype and dystrophic muscles will also be provided. These measures can complement morphological and biochemical measurements of tissue homeostasis, as well as whole animal assessments of muscle function.

Analysis of Increasing and Decreasing Isometric Finger Force Generation and the Possible Role of the Corticospinal System in This Process

This study investigated the possible role of the corticospinal system during force generation and force relaxation. Nine young and healthy subjects were instructed to produce a total force with four fingers within a hand following a preset force generation and relaxation ramp template closely. Excitability of corticospinal (CS) projections was assessed by single- and paired-pulse TMS. Errors introduced by a finger force were partially compensated by other finger forces during force generation, but were amplified during force relaxation. The CS excitability was greater during force generation than maintenance or relaxation. No difference in intracortical inhibition or facilitation was found. Nonnormalized finger extensor EMG responses remained unchanged. The findings suggest that force relaxation is not just a withdrawal from activation, and multifinger interactions are likely controlled beyond the primary motor cortex.

The role of contractile unit reorganization in force generation in airway smooth muscle

Airway smooth muscle (ASM) cells undergo remodelling and reside in a tissue structure that is subject to heterogenous stress distributions that change dynamically during the breathing cycle. In this paper, we develop a structural model of an ASM cell that consists of contractile units (actin and myosin filaments) in series and parallel, anchored to a nonlinearly elastic cytoskeleton. We mimic a typical experimental protocol that involves isometric force generation through triggering of the contractile machinery, followed by oscillatory length fluctuation of the cell. We use the model to predict the effect of a single instance of rearrangement of the contractile machinery, combined with strain-stiffening of the cytoskeleton, on the force generated by the sarcomeres, and the total force generated by the cell. By linking intra-cellular events to whole-cell behaviour, the model reveals mechanistic relationships between structural properties and cell-level force–length loops. We show how contractile force, shortening velocity and sarcomere operating lengths vary as the internal cell architecture is altered. Additionally, we show how interactions between the internal contractile machinery and cytoskeletal structure play a role in the regulation of force generation and hysteresis of the cell.

From cycling between coupled reactions to the cross-bridge cycle: mechanical power output as an integral part of energy metabolism.

ATP delivery and its usage are achieved by cycling of respective intermediates through interconnected coupled reactions. At steady state, cycling between coupled reactions always occurs at zero resistance of the whole cycle without dissipation of free energy. The cross-bridge cycle can also be described by a system of coupled reactions: one energising reaction, which energises myosin heads by coupled ATP splitting, and one de-energising reaction, which transduces free energy from myosin heads to coupled actin movement. The whole cycle of myosin heads via cross-bridge formation and dissociation proceeds at zero resistance. Dissipation of free energy from coupled reactions occurs whenever the input potential overcomes the counteracting output potential. In addition, dissipation is produced by uncoupling. This is brought about by a load dependent shortening of the cross-bridge stroke to zero, which allows isometric force generation without mechanical power output. The occurrence of maximal efficiency is caused by uncoupling. Under coupled conditions, Hill’s equation (velocity as a function of load) is fulfilled. In addition, force and shortening velocity both depend on [Ca2+]. Muscular fatigue is triggered when ATP consumption overcomes ATP delivery. As a result, the substrate of the cycle, [MgATP2−], is reduced. This leads to a switch off of cycling and ATP consumption, so that a recovery of [ATP] is possible. In this way a potentially harmful, persistent low energy state of the cell can be avoided.

Gone in 0.6 Seconds: The Encoding of Motor Memories Depends on Recent Sensorimotor States

Real-world tasks often require movements that depend on a previous action or on changes in the state of the world. Here we investigate whether motor memories encode the current action in a manner that depends on previous sensorimotor states. Human subjects performed trials in which they made movements in a randomly selected clockwise or counterclockwise velocity-dependent curl force field. Movements during this adaptation phase were preceded by a contextual phase that determined which of the two fields would be experienced on any given trial. As expected from previous research, when static visual cues were presented in the contextual phase, strong interference (resulting in an inability to learn either field) was observed. In contrast, when the contextual phase involved subjects making a movement that was continuous with the adaptation-phase movement, a substantial reduction in interference was seen. As the time between the contextual and adaptation movement increased, so did the interference, reaching a level similar to that seen for static visual cues for delays >600 ms. This contextual effect generalized to purely visual motion, active movement without vision, passive movement, and isometric force generation. Our results show that sensorimotor states that differ in their recent temporal history can engage distinct representations in motor memory, but this effect decays progressively over time and is abolished by ∼600 ms. This suggests that motor memories are encoded not simply as a mapping from current state to motor command but are encoded in terms of the recent history of sensorimotor states.

Biomechanical versus robotic indices to assess isometric force generation capabilities

Biomechanical versus robotic indices to assess isometric force generation capabilities

Mechanics of myosin function in white muscle fibres of the dogfish, Scyliorhinus canicula

The contractile properties of muscle fibres have been extensively investigated by fast perturbation in sarcomere length to define the mechanical characteristics of myofilaments and myosin heads that underpin refined models of the acto-myosin cycle. Comparison of published data from intact fast-twitch fibres of frog muscle and demembranated fibres from fast muscle of rabbit shows that stiffness of the rabbit myosin head is only ∼62% of that in frog. To clarify if and how much the mechanical characteristics of the filaments and myosin heads vary in muscles of different animals we apply the same high resolution mechanical methods, in combination with X-ray diffraction, to fast-twitch fibres from the dogfish (Scyliorhinus canicula). The values of equivalent filament compliance (Cf) measured by X-ray diffraction and in mechanical experiments are not significantly different; the best estimate from combining these values is 17.1 ± 1.0 nm MPa−1. This value is larger than Cf in frog, 13.0 ± 0.4 nm MPa−1. The longer thin filaments in dogfish account for only part of this difference. The average isometric force exerted by each attached myosin head at 5°C, 4.5 pN, and the maximum sliding distance accounted for by the myosin working stroke, 11 nm, are similar to those in frog, while the average myosin head stiffness of dogfish (1.98 ± 0.31 pN nm−1) is smaller than that of frog (2.78 ± 0.30 pN nm−1). Taken together these results indicate that the working stroke responsible for the generation of isometric force is a larger fraction of the total myosin head working stroke in the dogfish than in the frog.

Perivascular fat impairs contraction in aorta from obese but not lean adult rats

Aortic perivascular fat limits the ability of the aorta to contract. We hypothesized that constriction would be further impaired in rats fed a high fat (HF) diet. Male Sprague Dawley (SD) rats were fed a HF diet from 3–20 weeks of age, control rats received normal chow (n=4–6 in each group). Aortae were removed and isometric force generation was measured in the presence and absence of fat. The HF rats weighed more than the controls (401±9 vs 465±10g, control vs HF p&lt;0.05) and had more abdominal fat (5.6±0.3 vs 16.5±1.2g, control vs HF p&lt;0.05). Surprisingly, the presence of fat in the control rats had no effect on the contraction mediated by phenylephrine (PE) or angiotensin II (Ang II). Fat did cause a marked impairment in the contraction in aorta from HF rats. Interestingly, the clean aorta from the HF rats also exhibited impaired PE and Ang II mediated contraction when compared to clean aorta from control rats. We have shown that fat impairs contraction in aorta from young control chow fed SD rats. The present study suggests that fats ability to limit contraction is reduced with age in control but not obese rats; the mechanisms responsible for this remain to be elucidated. Maximum contraction (mg increase from baseline) Contractile Agent Control Control + Fat HF HF+Fat PE 1580±210 1595±68 1280±76# 855±59* Ang II 545±90 533±73 367±80# 157±67* different from clean aorta from HF rats, different from clean aorta from control rats

Obesity impairs alpha 1 adrenergic receptor mediated contraction in carotid arteries

Hypertension is generally thought to cause a hyper contractile state in arteries. This does not appear to be the case in aorta from rats with diet‐induced obesity associated hypertension. We tested the hypothesis that contractile responses would also be impaired in carotid arteries from rats fed a high fat (HF) diet. Male Sprague Dawley rats received a HF diet from 3–20 weeks of age; control rats were fed normal chow (n=4). Carotid arteries were removed and isometric force generation was measured in response to phenylephrine (PE) and the thromboxane A2 receptor agonist U46619. The HF rats were heavier than the controls (401±9 vs 465±10g, control vs HF p&lt;0.05) and had more abdominal fat (5.6±0.3 vs 16.5±1.2g, control vs HF p&lt;0.05). The PE mediated contraction was impaired in carotids from the HF rats as evidenced by an increase in the EC50 and a reduction in the maximum contraction. This was not the case for U46619 where the responses were similar in the control and HF rats. These studies suggest that alpha 1 adrenergic receptor mediated contraction is impaired by a HF diet; this may be linked to the increased sympathetic drive observed with obesity. PE U46619 Max contraction (mN) EC50 Max contraction (mN) EC50 Control 8.3±0.4 −7.0±0.1 20.4±0.6 −8.0±0.3 HF 6.8±0.6 * −6.7±0.1* 20.0±0.7 −7.8±0.4 Different from control

Modulation of subthalamic nucleus local field potential power during incremental isometric force generation in Parkinson's disease patients

Local field potential (LFP) investigations of the subthalamic nucleus (STN) of patients with Parkinson’s disease have provided evidence for pathologically exaggerated oscillatory beta-band-activity (12–30 Hz) which is amenable to physiological modulation by, e.g., voluntary movement1. Previous fMRI studies in healthy controls have provided evidence for an increase of STN blood-oxygenation-level-dependent (BOLD) signal in incremental force generation tasks2. However, how neuronal activity encodes force generation and the topography of its neural correlates within the human STN remain to be elucidated. We hypothesised that incremental force generation is encoded in the sensorimotor part of the STN by modulation of beta-band activity. STN-LFPs were recorded intraoperatively in 13 patients with Parkinson’s disease (37 recording sites) during rest and five incremental force generation conditions of the arm with applied loads of 0g to 400g (in 100g increments). LFP power group data analyses by repeated measures ANOVAs revealed a significant modulation in the high beta- (24 to 30 Hz; p=0.024) and gamma-band (70 to 76 Hz; p=0.036) after Bonferroni α-correction. Supporting regression analyses revealed sites with both negative (low force – high beta-band power, high force – low beta-band power) and positive modulation. The former was found throughout dorsal, lateral and caudal STN regions, whereas the latter was localised in a caudal STN region. Our findings indicate that the modulation of STN-LFP beta- and gamma-band power is involved in encoding physiological force generation in patients with Parkinson’s Disease and that this modulation occurs in STN regions commonly attributed with sensorimotor functions3.

Protein kinase C delta contributes to increase in EP3 agonist-induced contraction in mesenteric arteries from type 2 diabetic Goto-Kakizaki rats

Prostaglandin E(2) (PGE(2)), an important and ubiquitously present vasoactive eicosanoid, may either constrict or dilate systemic vascular beds. However, little is known about the vascular contractile responsiveness to and signaling pathways for PGE(2) at the chronic stage of type 2 diabetes. We hypothesized that PGE(2)-induced arterial contraction is augmented in type 2 diabetic Goto-Kakizaki (GK) rats via the protein kinase Cδ (PKCδ) pathway. Here, we investigated the vasoconstrictor effects of PGE(2) and of sulprostone (EP1-/EP3-receptor agonist) in rings cut from superior mesenteric arteries isolated from GK rats (37-44weeks old). In arteries from GK rats (vs. those from age-matched Wistar rats), examined in the presence of a nitric oxide synthase inhibitor: 1) the PGE(2)- and sulprostone-induced vasocontractions (which were not blocked by the selective EP1 receptor antagonist sc19220) were enhanced, and these enhancements were suppressed by rottlerin (selective PKCδ inhibitor) but not by Gö6976 (selective PKCα/β inhibitor); 2) the sulprostone-stimulated phosphorylation of PKCδ (at Thr(505)), which yields an active form, was increased and 3) sulprostone-stimulated caldesmon phosphorylations, which are related to isometric force generation in smooth muscle, were increased. The protein expression of EP3 receptor in superior mesenteric arteries was similar between the two groups of rats. Our data suggest that the diabetes-related enhancement of EP3 receptor-mediated vasocontraction results from activation of the PKCδ pathway. Alterations in EP3 receptor-mediated vasocontraction may be important factors in the pathophysiological influences over arterial tone that are present in diabetic states.

Robustness of muscle synergies underlying three-dimensional force generation at the hand in healthy humans.

Previous studies using advanced matrix factorization techniques have shown that the coordination of human voluntary limb movements may be accomplished using combinations of a small number of intermuscular coordination patterns, or muscle synergies. However, the potential use of muscle synergies for isometric force generation has been evaluated mostly using correlational methods. The results of such studies suggest that fixed relationships between the activations of pairs of muscles are relatively rare. There is also emerging evidence that the nervous system uses independent strategies to control movement and force generation, which suggests that one cannot conclude a priori that isometric force generation is accomplished by combining muscle synergies, as shown in movement control. In this study, we used non-negative matrix factorization to evaluate the ability of a few muscle synergies to reconstruct the activation patterns of human arm muscles underlying the generation of three-dimensional (3-D) isometric forces at the hand. Surface electromyographic (EMG) data were recorded from eight key elbow and shoulder muscles during 3-D force target-matching protocols performed across a range of load levels and hand positions. Four synergies were sufficient to explain, on average, 95% of the variance in EMG datasets. Furthermore, we found that muscle synergy composition was conserved across biomechanical task conditions, experimental protocols, and subjects. Our findings are consistent with the view that the nervous system can generate isometric forces by assembling a combination of a small number of muscle synergies, differentially weighted according to task constraints.