Shortening Deactivation Research Articles

Intrinsic mechanisms of right ventricular autoregulation

To elucidate the adaptation of the right ventricle to acute and intermittently sustained afterload elevation, targeted preload reductions and afterload increases were implemented in a porcine model involving 12 pigs. Preload reduction was achieved via balloon occlusion of the inferior vena cava before, immediately and 5 min after acute afterload elevation induced by pulmonary artery occlusion or thromboxane A2 analog (U46619) infusion. Ventricular response was monitored by registration of pressure–volume (PV) loops using a conductance catheter. The end-systolic pressure–volume relationship (ESPVR) during pure preload reduction was adequately described by linear regression (mean and SEM slope of ESPVR (Ees) 0.414 ± 0.064 mmHg/ml), reflecting the classical Frank-Starling mechanism (FSM). The ESPVR during acute afterload elevation exhibited a biphasic trajectory with significantly distinct slopes (mean and SEM Ees bilin1: 1.256 ± 0.066 mmHg ml; Ees bilin2: 0.733 ± 0.063 mmHg ml, p < 0.001). The higher slope during the first phase in the absence of ventricular dilation could be explained by a reduced amount of shortening deactivation (SDA). The changes in PV-loops during the second phase were similar to those observed with a preload intervention. The persistent increase in afterload resulted in an increase in the slopes of ESPVR and preload recruitable stroke work (PRSW) with a slight decrease in filling state, indicating a relevant Anrep effect. This effect became more pronounced after 5 min or TXA infusion. This study demonstrates, for the first time, the relevance of intrinsic mechanisms of cardiac autoregulation in the right ventricle during the adaptation to load. The SDA, FSM, and Anrep effect could be differentiated and occurred successively, potentially with some overlap. Notably, the Anrep effect serves to prevent ventricular dilation.

Exendin-4 affects calcium signalling predominantly during activation and activity of beta cell networks in acute mouse pancreas tissue slices.

Tight control of beta cell stimulus-secretion coupling is crucial for maintaining homeostasis of energy-rich nutrients. While glucose serves as a primary regulator of this process, incretins augment beta cell function, partly by enhancing cytosolic [Ca2+] dynamics. However, the details of how precisely they affect beta cell recruitment during activation, their active time, and functional connectivity during plateau activity, and how they influence beta cell deactivation remain to be described. Performing functional multicellular Ca2+ imaging in acute mouse pancreas tissue slices enabled us to systematically assess the effects of the GLP-1 receptor agonist exendin-4 (Ex-4) simultaneously in many coupled beta cells with high resolution. In otherwise substimulatory glucose, Ex-4 was able to recruit approximately a quarter of beta cells into an active state. Costimulation with Ex-4 and stimulatory glucose shortened the activation delays and accelerated beta cell activation dynamics. More specifically, active time increased faster, and the time required to reach half-maximal activation was effectively halved in the presence of Ex-4. Moreover, the active time and regularity of [Ca2+]IC oscillations increased, especially during the first part of beta cell response. In contrast, subsequent addition of Ex-4 to already active cells did not significantly enhance beta cell activity. Network analyses further confirmed increased connectivity during activation and activity in the presence of Ex-4, with hub cell roles remaining rather stable in both control experiments and experiments with Ex-4. Interestingly, Ex-4 demonstrated a biphasic effect on deactivation, slightly prolonging beta cell activity at physiological concentrations and shortening deactivation delays at supraphysiological concentrations. In sum, costimulation by Ex-4 and glucose increases [Ca2+]IC during beta cell activation and activity, indicating that the effect of incretins may, to an important extent, be explained by enhanced [Ca2+]IC signals. During deactivation, previous incretin stimulation does not critically prolong cellular activity, which corroborates their low risk of hypoglycemia.

A New Method for Characterizing Muscle Shortening Deactivation

A muscle undergoing a cyclical lengthening and shortening contraction pattern requires fast and efficient muscle activation and relaxation. To boost activation rate and increase force levels during shortening, some muscles have evolved stretch activation (SA), a delayed increased in force following rapid muscle lengthening. Equally important is a fast relaxation rate and low force during muscle lengthening. If a muscle incompletely relaxes, its antagonist muscle expends more energy lengthening it, causing decreased power generation and efficiency. Thus, increasing relaxation rate can increase work and power. However, increasing relaxation rate is costly because it typically occurs via ATPases pumping calcium out of the myoplasm. Some flying insect species have greatly reduced this cost in their indirect flight muscle (IFM) by evolving a property known as shortening deactivation (SD), which decreases force levels during a contraction cycle without having to change calcium concentration. SD is a delayed decrease in force following rapid shortening of an active muscle. In IFM, the timing of this force decrease allows the antagonist muscle (DVM) to be lengthened with much less resistance from the agonist muscle (DLM). While there is general appreciation for SD in IFM, there have been few investigations into whether SD is present in other muscle types and if so, how much it benefits these muscles. Even less is known about SD’s mechanism as there have been no investigations into the SD mechanism for any muscle type. To enable mechanistic investigations into SD and quantitatively compare it to SA, we developed a protocol to elicit SA and SD from Drosophila and Lethocerus indirect flight muscles (IFM) and Drosophila jump muscle. All three muscle types exhibited a 4‐phase tension transient in response to a 1% length decrease, with phase 3 defined as SD. Lethocerus IFM presented the greatest SD tension drop, 20 mN/mm2, which was ~1.5‐fold greater than Drosophila jump muscle and ~10‐fold greater than DrosophilaIFM. However, isometric tension in Drosophilajump muscle, 34.3 mN/mm2, was higher than Lethocerus, 18.4 mN/mm2, and DrosophilaIFM, 1.6 mN/mm2. Thus, when normalized to isometric tension, Drosophila IFM exhibited a 118% SD tension decrease. Similarly, SD tension in Lethocerus IFM dropped by 97%, while Drosophilajump muscle decreased by 37%. The same order occurred for normalized phase 3 SA tension: Drosophila IFM displayed the largest gain, 233%, Lethocerus IFM increased by 76%, and Drosophila jump muscle showed only a 11% increase. SD phase 3 occurred a bit earlier than SA phase 3, relative to the length change, for both IFMs; but SD was exceedingly earlier than SA for jump muscle. We suggest that SA and SD evolved together to enable fast and efficient IFM cyclical power generation and may be caused by the same molecular mechanism. However, the SA and SD molecular mechanisms in jump muscle are likely different and might have evolved for an alternate role besides increasing the power output of cyclical muscle contractions.

The importance of shortening deactivation to different muscle types

Generating high power from a muscle undergoing a cyclical lengthening and shortening contraction pattern requires fast and efficient muscle activation and relaxation. To enhance activation and increase force levels during shortening, some muscle types have evolved stretch activation (SA), a delayed increased in force following rapid muscle lengthening. SA’s complementary phenomenon is shortening deactivation (SD), a delayed decrease in force following muscle shortening, which increases muscle relaxation and results in less resistive force during subsequent muscle lengthening.

Shortening deactivation: quantifying a critical component of cyclical muscle contraction.

A muscle undergoing cyclical contractions requires fast and efficient muscle activation and relaxation to generate high power with relatively low energetic cost. To enhance activation and increase force levels during shortening, some muscle types have evolved stretch activation (SA), a delayed increased in force following rapid muscle lengthening. SA's complementary phenomenon is shortening deactivation (SD), a delayed decrease in force following muscle shortening. SD increases muscle relaxation, which decreases resistance to subsequent muscle lengthening. Although it might be just as important to cyclical power output, SD has received less investigation than SA. To enable mechanistic investigations into SD and quantitatively compare it to SA, we developed a protocol to elicit SA and SD from Drosophila and Lethocerus indirect flight muscles (IFM) and Drosophila jump muscle. When normalized to isometric tension, Drosophila IFM exhibited a 118% SD tension decrease, Lethocerus IFM dropped by 97%, and Drosophila jump muscle decreased by 37%. The same order was found for normalized SA tension: Drosophila IFM increased by 233%, Lethocerus IFM by 76%, and Drosophila jump muscle by only 11%. SD occurred slightly earlier than SA, relative to the respective length change, for both IFMs; but SD was exceedingly earlier than SA for jump muscle. Our results suggest SA and SD evolved to enable highly efficient IFM cyclical power generation and may be caused by the same mechanism. However, jump muscle SA and SD mechanisms are likely different, and may have evolved for a role other than to increase the power output of cyclical contractions.

Non-linearity of end-systolic pressure\u2013volume relation in afterload increases is caused by an overlay of shortening deactivation and the Frank\u2013Starling mechanism

The linearity and load insensitivity of the end-systolic pressure–volume-relationship (ESPVR), a parameter that describes the ventricular contractile state, are controversial. We hypothesize that linearity is influenced by a variable overlay of the intrinsic mechanism of autoregulation to afterload (shortening deactivation) and preload (Frank-Starling mechanism). To study the effect of different short-term loading alterations on the shape of the ESPVR, experiments on twenty-four healthy pigs were executed. Preload reductions, afterload increases and preload reductions while the afterload level was increased were performed. The ESPVR was described either by a linear or a bilinear regression through the end-systolic pressure volume (ES-PV) points. Increases in afterload caused a biphasic course of the ES-PV points, which led to a better fit of the bilinear ESPVRs (r2 0.929 linear ESPVR vs. r2 0.96 and 0.943 bilinear ESPVR). ES-PV points of a preload reduction on a normal and augmented afterload level could be well described by a linear regression (r2 0.974 linear ESPVR vs. r2 0.976 and 0.975 bilinear ESPVR). The intercept of the second ESPVR (V0) but not the slope demonstrated a significant linear correlation with the reached afterload level (effective arterial elastance Ea). Thus, the early response to load could be described by the fixed slope of the ESPVR and variable V0, which was determined by the actual afterload. The ESPVR is only apparently nonlinear, as its course over several heartbeats was affected by an overlay of SDA and FSM. These findings could be easily transferred to cardiovascular simulation models to improve their accuracy.

Is the Myosin Head Conformation Coupled to the Thick Filament Backbone Structure?

Some striated muscles thick filaments show evidence of structural changes in response to activation or tension. Relaxed thick filaments from Lethocerus flight muscle change their helical angle by −0.15°/crown (145Å repeat) in response to externally applied tension. The change in helical angle, which is a structural parameter defined by the myosin rods within the filament backbone, is accompanied by intensity changes in the 145Å layer line, which is sensitive to the arrangement and order of the myosin heads. The intensity change could indicate that putting the relaxed muscle under tension alters the order or arrangement of at least some of the myosin heads. Vertebrate striated muscle thick filaments, which are not helical, change their axial repeat from 143Å to 145Å, the latter being the axial spacing in Lethocerus thick filaments. We reasoned that if externally applied tension could change the helical angle and the myosin head arrangement, that relaxed Lethocerus thick filaments with partially disordered heads might display an altered helical angle coupled to changes in myosin head arrangement. The 3-D images obtained by cryoEM, provide the first structural evidence that the arrangement of myosin heads within the interacting heads motif is coupled to the structure of the thick filament backbone. We find that a −0.16° change in helical angle disorders the blocked head preferentially, which suggests a mechanism for how tension affects the dynamics of the myosin heads and their contribution to stretch activation and shortening deactivation. We suggest the blocked head preferentially binds the thin filament followed by the free head when the blocked head produces force. The shift in blocked head dynamics from ordered to disordered is achieved by tension applied by the antagonist muscle. Supported by NIH and AHA.

Coupling between myosin head conformation and the thick filament backbone structure.

The recent high-resolution structure of the thick filament from Lethocerus asynchronous flight muscle shows aspects of thick filament structure never before revealed that may shed some light on how striated muscles function. The phenomenon of stretch activation underlies the function of asynchronous flight muscle. It is most highly developed in flight muscle, but is also observed in other striated muscles such as cardiac muscle. Although stretch activation is likely to be complex, involving more than a single structural aspect of striated muscle, the thick filament itself, would be a prime site for regulatory function because it must bear all of the tension produced by both its associated myosin motors and any externally applied force. Here we show the first structural evidence that the arrangement of myosin heads within the interacting heads motif is coupled to the structure of the thick filament backbone. We find that a change in helical angle of 0.16° disorders the blocked head preferentially within the Lethocerus interacting heads motif. This observation suggests a mechanism for how tension affects the dynamics of the myosin heads leading to a detailed hypothesis for stretch activation and shortening deactivation, in which the blocked head preferentially binds the thin filament followed by the free head when force production occurs.

Reduced First-Phase Ejection Fraction and Sustained Myocardial Wall Stress in Hypertensive Patients With Diastolic Dysfunction: A Manifestation of Impaired Shortening Deactivation That Links Systolic to Diastolic Dysfunction and Preserves Systolic Ejection Fraction.

Impaired shortening deactivation of cardiac myocytes could sustain myocardial contraction, preserving ejection fraction at the expense of diastolic dysfunction. We examined the relationship between first-phase ejection fraction (EF1), the fraction of left ventricular volume ejected from the start of systole to the time of the first peak in left ventricular pressure (corresponding to the time of maximal ventricular shortening) to the duration of myocardial contraction and diastolic function in patients with hypertension (n=163), and varying degrees of diastolic dysfunction. Left ventricular systolic pressure was estimated by carotid tonometry; time-resolved left ventricular cavity and wall volume were obtained by echocardiography with speckle wall tracking. Measurements were repeated after nitroglycerin, a drug known to influence ventricular dynamics, in a subsample (n=18) of patients. EF1 and time of onset of ventricular relaxation (as determined from the temporal pattern of myocardial wall stress) were independently correlated with diastolic relaxation as measured by tissue Doppler early diastolic mitral annular velocity (E', standardized regression coefficients 0.48 and -0.34 for EF1 and time of onset of ventricular relaxation, respectively, each P<0.001, irrespective of adjustment for age, sex, antihypertensive treatment, measures of afterload, and ventricular geometry) and with diastolic function measured by the ratio of transmitral Doppler early filling velocity (E) to E' (E/E', regression coefficients -0.34 and 0.34, respectively, each P<0.001). Nitroglycerin increased EF1, decreased time of onset of ventricular relaxation, and improved diastolic function (each P<0.05). Hypertensive patients with diastolic dysfunction exhibit reduced EF1 which may sustain myocardial contraction, preserving systolic ejection fraction at the expense of impaired diastolic function.

The Structure of the Relaxed Thick Filaments from Lethocerus Asynchronous Flight Muscle

The structure of relaxed muscle thick filaments is a key element for understanding muscle physiology and the effect of mutations on muscle function. Previous thick filament structures by 3D-EM resolved individual myosin heads and revealed an interacting head motif (IHM) first identified in smooth muscle myosin. A later model based on X-ray fiber diffraction of Lethocerus flight muscle showed a different, intermolecular, interaction between myosin heads. Almost nothing is known about the structure of the long α-helical coiled-coil myosin rod in thick filaments. Here we show the relaxed structure of Lethocerus thick filaments by cryoEM at sufficient resolution to reveal the relative placement of individual myosin heads at ∼20 A resolution, as well as the myosin rod α-helices which can be traced from the head-rod junction all the way to their C-termini at a resolution of 5.5A. In contrast to the model suggested by X-ray diffraction, we find an IHM somewhat different from IHMs of other relaxed thick filaments. The myosin rod follows a tortuous path from the outside of the thick filament to the inside across a region we call the myosin rod annulus. The pitch of the rod coiled-coil is variable with long pitch regions in the approximate location of skip residues. Surprisingly, threading their way among the forest of myosin rods are four copies, matching the 4-fold symmetry of the filament, of four polypeptide chains. Their presence in this location could modulate the mechanical properties of the myosin backbone. Our results demonstrate the ubiquity of the IHM in relaxed myosin filaments but with a variation that might shed light on the mechanism of stretch activation and shortening deactivation. Supported by NIH and AHA.

Timing and magnitude of prestretch strongly affect cardiac muscle tension development (545.2)

Ectopic ventricular pacing leads to regional myocardial prestretch. In‐vivo studies suggest that the timing of prestretch determines the effect on pump function and can lead to regional wall remodeling. Here we measure the direct effects of prestretch timing on the magnitude of tension development in isolated cardiac muscle. The effects of prestretch were simulated using excised murine right ventricular papillary muscles. A servo motor was used to impose precisely timed stretches, while a force transducer measured force output and strain was monitored using a CCD camera and topical markers. A critical stretch timing interval was observed to statistically increase peak tension by as much as 500%, while shortening during this interval inhibited peak tension significantly by as much as 40%. A simple model consisting of experimentally obtained time varying elastance and the force‐velocity relation could not completely explain the results. An estimate of shortening deactivation was measured by performing quick stretches to dissociate crossbridges. The results showed that the timing of shortening deactivation could explain the remaining differences between the model and the experimental results. These evaluations have led us to the conclusion that prestretches performed at slow physiologic velocities cause alterations in twitch kinetics that cannot be explained solely by time varying elastance.

The Influence of Troponin C, Isoform 4 on Drosophila Development, Stretch Activation, and Power Generation

Drosophila indirect flight muscle (IFM) is one of the most rapidly contracting muscle types known. To increase IFM efficiently and power generation many insect species have evolved high degrees of stretch activation and shortening deactivation in their IFMs. In addition to expressing TnC1, a typical calcium binding troponin C, these IFMs express an unusual unique, additional isoform, TnC4. TnC1 has two calcium binding sites, a high-affinity structural site and a lower-affinity regulatory site. TnC4 has only the high-affinity structural binding site. It is hypothesized to respond to stretch rather than calcium concentration to further activate the thin filament. We are investigating the roles of TnC4 and TnC1 in Drosophila using RNAi and by creating a TnC4 null mutant. Using RNAi, we eliminated TnC1 expression, which resulted in upregulation of TnC4. Power generation and stretch activation of IFMs without TnC1 were statistically identical to wild-type. IFMs with undetectable levels of TnC4 expression produced by RNAi do not generate power or display stretch-activation, suggesting that only TnC4 is necessary for normal muscle function. We have also created a TnC4 null, which will allow us to mechanically evaluate transgenically expressed TnC isoforms and mutants in IFM. Preliminary characterization of the TnC4 null indicates that it is homozygous lethal, but viable as a heterozygote. Flight ability of the heterozygotes is reduced, displaying a 10% reduction in wing-beat frequency than wild-type. The homozygous nulls die at the first molt suggesting that TnC4 is also expressed in some larval muscle types and that it plays a vital role in fly development.

The Influence of Troponin C Isoforms on Drosophils Stretch Activation and Power Generation

Drosophila indirect flight muscle (IFM) is one of the most rapidly contracting muscle types known. To efficiently generate power for flight at such high speeds, insects have evolved stretch activation and shortening deactivation. In IFM of some insects, in addition to a typical calcium binding troponin C, TnC1, there is a unique, additional isoform, TnC4. TnC1 has 2 calcium binding sites, a high-affinity structural site and a lower-affinity regulatory site. TnC4 has only the high-affinity structural binding site. It is hypothesized to respond to stretch rather than Ca2+ concentration to further activate the thin filament. We are investigating TnC4 in Drosophila via two different routes. The first is to chemically remove TnC from IFM using either o-vanadate or trifluoperazine which eliminates power generation and stretch activation. Bacterially-expressed TnC1 and TnC4 are reincorporated into the fiber in physiological proportions, which restores power-generating ability and stretch activation to the IFM. We are currently optimizing the extraction and reintroduction protocol to determine the effects on stretch activation of varying the ratio of TnC1 and TnC4. The second method is to breed a TnC4 null using p-element mediated male homologous recombination. We are performing this by crossing flies containing identical p-elements in positions which bracket the TnC4 gene on chromosome 2R. When the p-elements are mobilized, some of the offspring will have the region between the p-elements excised. We are currently screening offspring of the final cross for null TnC4 alleles. These two methods of investigation into TnC4 function will allow modified TnC4 to either be introduced or expressed in IFM to help elucidate TnC4's structural mechanism.

The role of intrinsic muscle mechanics in the neuromuscular control of stable running in the guinea fowl.

Here we investigate the interplay between intrinsic mechanical and neural factors in muscle contractile performance during running, which has been less studied than during walking. We report in vivo recordings of the gastrocnemius muscle of the guinea fowl (Numida meleagris), during the response and recovery from an unexpected drop in terrain. Previous studies on leg and joint mechanics following this perturbation suggested that distal leg extensor muscles play a key role in stabilisation. Here, we test this through direct recordings of gastrocnemius fascicle length (using sonomicrometry), muscle-tendon force (using buckle transducers), and activity (using indwelling EMG). Muscle recordings were analysed from the stride just before to the second stride following the perturbation. The gastrocnemius exhibits altered force and work output in the perturbed and first recovery strides. Muscle work correlates strongly with leg posture at the time of ground contact. When the leg is more extended in the drop step, net gastrocnemius work decreases (-5.2 J kg(-1) versus control), and when the leg is more flexed in the step back up, it increases (+9.8 J kg(-1) versus control). The muscle's work output is inherently stabilising because it pushes the body back toward its pre-perturbation (level running) speed and leg posture. Gastrocnemius length and force return to level running means by the second stride following the perturbation. EMG intensity differs significantly from level running only in the first recovery stride following the perturbation, not within the perturbed stride. The findings suggest that intrinsic mechanical factors contribute substantially to the initial changes in muscle force and work. The statistical results suggest that a history-dependent effect, shortening deactivation, may be an important factor in the intrinsic mechanical changes, in addition to instantaneous force-velocity and force-length effects. This finding suggests the potential need to incorporate history-dependent muscle properties into neuromechanical simulations of running, particularly if high muscle strains are involved and stability characteristics are important. Future work should test whether a Hill or modified Hill type model provides adequate prediction in such conditions. Interpreted in light of previous studies on walking, the findings support the concept of speed-dependent roles of reflex feedback.

Antihistamine mepyramine directly inhibits KCNQ/M channel and depolarizes rat superior cervical ganglion neurons

The first-generation antihistamines are widely prescribed medications that relieve allergic reactions and urticaria by blocking the peripheral histamine H 1 receptor. Overdose of these drugs often results in serious neuronal toxic effects, including seizures, convulsions and worsening of epileptic symptoms. The KCNQ/M K + channel plays a crucial role in controlling neuron excitability. Here, we demonstrate that mepyramine and diphenhydramine, two structurally related first-generation antihistamines, can act as potent KCNQ/M channel blockers. Extracellular application of these drugs quickly and reversibly reduced KCNQ2/Q3 currents heterologously expressed in HEK293 cells. The current inhibition was concentration and voltage dependent. The estimated IC 50 (12.5 and 48.1 μM, respectively) is within the range of drug concentrations detected in poisoned patients (30–300 μM). Both drugs shifted the I–V curve of KCNQ2/Q3 channel to more depolarized potentials and altered channel gating properties by prolonging activation and shortening deactivation kinetics. Mepyramine also inhibited the individual homomeric KCNQ1–4 and heteromeric KCNQ3/Q5 currents. Moreover, mepyramine inhibited KCNQ2/Q3 current in an outside-out patch excised from HEK293 cells and the inhibitory effect was neither observed when it was applied intracellularly nor affected by blocking phospholipase C (PLC) activity, indicating an extracellular and direct channel blocking mechanism. Finally, in cultured rat superior cervical ganglion (SCG) neurons, mepyramine reduced the M type K + current in a concentration-dependent manner and led to marked membrane potential depolarization. It is likely that these effects may be involved in the adverse neuroexcitatory effects observed in patients experiencing an overdose of antihistamines.

368 Telomere shortening and senescence checkpoint deactivation during multistep human hepatocarcinogenesis

368 Telomere shortening and senescence checkpoint deactivation during multistep human hepatocarcinogenesis

Dynamics of crossbridge-mediated activation in the heart

Both intracellular calcium and strongly bound crossbridges contribute to thin filament activation in the heart, but the magnitude and the duration of the effects due to crossbridges are not well characterized. In this study, crossbridge attachment was altered in tetanized ferret papillary muscles and changes in the rate constant for the recovery of force (k (TR)) and unloaded shortening velocity (V (U)) were measured to track thin filament activation. k (TR) decreased as the time the muscles spent at low levels of crossbridge attachment (shortening deactivation) increased (0.02 s=17.9+/-2.3 s(-1), 0.32 s=3.3+/-0.4 s(-1); half-time=0.052 s; P<0.05). Furthermore, the deactivation was reversible and k (TR) recovered when muscles were allowed to regenerate force isometrically during the same tetanus. V (U) also decreased when the preceding load was lower (isometric load, V (U)=1.93+/-0.26 muscle lengths/s (ML/s); zero load, V (U)=0.93+/-0.14 ML/s, P<0.05) and as the length of time the muscle spent unloaded increased (>60% decline after 0.3 s). In addition, V (U) recovered when the muscle was allowed to regenerate force isometrically. These results indicate that crossbridge attachment increases thin filament activation as reflected in measurements of V (U) and k (TR). This 'extra' activation by crossbridges appears to be a dynamic process that decays during unloaded shortening and redevelops during isometric contraction.

A modified Hill muscle model that predicts muscle power output and efficiency during sinusoidal length changes

The power output of a muscle and its efficiency vary widely under different activation conditions. This is partially due to the complex interaction between the contractile component of a muscle and the serial elasticity. We investigated the relationship between power output and efficiency of muscle by developing a model to predict the power output and efficiency of muscles under varying activation conditions during cyclical length changes. A comparison to experimental data from two different muscle types suggests that the model can effectively predict the time course of force and mechanical energetic output of muscle for a wide range of contraction conditions, particularly during activation of the muscle. With fixed activation properties, discrepancies in the work output between the model and the experimental results were greatest at the faster and slower cycle frequencies than that for which the model was optimised. Further optimisation of the activation properties across each individual cycle frequency examined demonstrated that a change in the relationship between the concentration of the activator (Ca2+) and the activation level could account for these discrepancies. The variation in activation properties with speed provides evidence for the phenomenon of shortening deactivation, whereby at higher speeds of contraction the muscle deactivates at a faster rate. The results of this study demonstrate that predictions about the mechanics and energetics of muscle are possible when sufficient information is known about the specific muscle.

Molecular dynamics of cyclically contracting insect flight muscle in vivo

Flight in insects--which constitute the largest group of species in the animal kingdom--is powered by specialized muscles located within the thorax. In most insects each contraction is triggered not by a motor neuron spike but by mechanical stretch imposed by antagonistic muscles. Whereas 'stretch activation' and its reciprocal phenomenon 'shortening deactivation' are observed to varying extents in all striated muscles, both are particularly prominent in the indirect flight muscles of insects. Here we show changes in thick-filament structure and actin-myosin interactions in living, flying Drosophila with the use of synchrotron small-angle X-ray diffraction. To elicit stable flight behaviour and permit the capture of images at specific phases within the 5-ms wingbeat cycle, we tethered flies within a visual flight simulator. We recorded images of 340 micros duration every 625 micros to create an eight-frame diffraction movie, with each frame reflecting the instantaneous structure of the contractile apparatus. These time-resolved measurements of molecular-level structure provide new insight into the unique ability of insect flight muscle to generate elevated power at high frequency.

Asynchronous Muscle: A Primer

The asynchronous muscles of insects are characterized by asynchrony between muscle electrical and mechanical activity, a fibrillar organization with poorly developed sarcoplasmic reticulum, a slow time course of isometric contraction, low isometric force, high passive stiffness and delayed stretch activation and shortening deactivation. These properties are illustrated by comparing an asynchronous muscle, the basalar flight muscle of the beetle Cotinus mutabilis, with synchronous wing muscles from the locust, Schistocerca americana. Because of delayed stretch activation and shortening deactivation, a tetanically stimulated beetle muscle can do work when subjected to repetitive lengthening and shortening. The synchronous locust muscle, subjected to similar stimulation and length change, absorbs rather than produces work.