Sarcomere Length-tension Relationship Research Articles

Ontogenetic changes in bite force and gape in tufted capuchins.

Bite force and gape are two important performance metrics of the feeding system, and these metrics are inversely related for a given muscle size because of fundamental constraints in sarcomere length-tension relationships. How these competing performance metrics change in developing primates is largely unknown. Here we quantify in vivo bite forces and gapes across ontogeny and examine these data in relation to body mass and cranial measurements in captive tufted capuchins, Sapajus spp. Bite forces and gapes were also compared across geometric and mechanical properties of mechanically challenging foods to investigate relationships between bite forces, gapes, and food accessibility (defined here as the ability to breach shelled nuts). Bite forces at a range of gapes and feeding behavioral data were collected from a cross-sectional ontogenetic series of 20 captive and semi-wild tufted capuchins at the Núcleo de Procriação de Macacos-Prego Research Center in Araçatuba, Brazil. These data were paired with body mass, photogrammetric measures of jaw length and facial width, and food geometric and material properties. Tufted capuchins with larger body masses had absolutely higher in vivo bite forces and gapes, and animals with wider faces had absolutely higher bite forces. Bite forces and gapes were significantly smaller in juveniles compared to subadults and adults. These are the first primate data to empirically demonstrate the gapes at which maximum active bite force is generated and to demonstrate relationships to food accessibility. These data advance our understanding of how primates meet the changing performance demands of the feeding system during development.

Muscle in Variable Gravity: "I Do Not Know Where I Am, But I Know What to Do".

Purpose: Fascicle and sarcomere lengths are important predictors of muscle mechanical performance. However, their regulation during stretch-shortening cycle (SSC) activities in usual and challenging conditions is poorly understood. In this study, we aimed to investigate muscle fascicle and sarcomere behavior during drop jumps (a common SSC activity) in conditions of variable gravity.Methods: Fifteen volunteers performed repeated drop jumps in 1 g, hypo-gravity (0 to 1 g), and hyper-gravity (1 to 2 g) during a parabolic flight. Gastrocnemius medialis (GM) electromyographic activity and fascicle length (Lf) were measured at drop-off, ground contact (GC), minimum ankle joint angle (MAJ), and push-off. GM sarcomere number was estimated by dividing Lf, measured by ultrasound at rest, by published data on GM sarcomere length, and measured in vivo at the same joint angle. Changes in sarcomere length were estimated by dividing GM Lf in each jump phase by sarcomere number calculated individually. The sarcomere force-generating capacity in each jump phase was estimated from the sarcomere length-tension relationship previously reported in the literature.Results: The results showed that, regardless of the gravity level, GM sarcomeres operated in the ascending portion of their length-tension relationship in all the jump phases. Interestingly, although in hypo-gravity and hyper-gravity during the braking phase (GC-MAJ) GM fascicles and sarcomeres experienced a stretch (as opposed to the quasi-isometric behavior in 1 g), at MAJ they reached similar lengths as in 1 g, allowing sarcomeres to develop about the 70% of their maximum force.Conclusion: The observed fascicle behavior during drop jumping seems useful for anchoring the tendon, enabling storage of elastic energy and its release in the subsequent push-off phase for effectively re-bouncing in all gravity levels, suggesting that an innate neuromuscular wisdom enables to perform SSC movements also in challenging conditions.

High efficiency preparation of skinned mouse cardiac muscle strips from cryosections for contractility studies.

What is the central question of this study? Can frozen cardiac papillary muscles and cryosectioning be used to reliably obtain uniform cardiac muscle strips with high yields? What is the main finding and its importance? A new method was developed using frozen cardiac papillary muscles and cryosectioning to reliably obtain uniform cardiac muscle strips with high yields. Experimental results demonstrate that this new methodology significantly increases the efficiency and application of quantitative biomechanical studies using skinned muscle fibres with an additional advantage of no need for transferring live animals. Skinned cardiac muscle preparations are widely used to study contractile function of myofilament proteins and pathophysiological changes. The current methods applied in these biomechanical studies include detergent permeabilization of freshly isolated papillary muscle, ventricular trabeculae, surgically dissected ventricular muscle strips, mechanically blended cardiac muscle bundles or myocytes, and enzymatically isolated single cardiomyocytes. To facilitate and expand the skinned cardiac muscle approach, we have developed an efficient and readily practical method for mechanical studies of skinned mouse cardiac papillary muscle strips prepared from cryosections. Longitudinal papillary muscle strips of 120-150µm width cut from 35-70µm-thick cryosections are mounted to a force transducer and chemically skinned for the studies of force-pCa and sarcomere length-tension relationship and rate of tension redevelopment. In addition to more effective skinning and perfusion than with whole papillary muscle and much higher yield of useful preparations than that from trabeculae, this new methodology has two more major advantages. One is to allow for the use of frozen cardiac muscle in storage to maximize the value of muscle samples, facilitating resource sharing among research institutions without the need of transferring live animals or fresh biopsies. The other is that the integrity of the muscle strips is well preserved during the preparation and mechanical studies, allowing coupled characterization of myofilament proteins. The combined power of biomechanics and protein biochemistry can provide novel insights into integrative physiological and pathophysiological mechanisms of cardiac muscle contraction while the high yield of high-quality muscle strips also provides an efficient platform for development of therapeutic reagents.

Effects of medial gastrocnemius relative fascicle excursion on stance phase plantarflexion work in children with cerebral palsy

A structural model was developed to explain sarcomere shortening at the expense of tendon lengthening in the frog semitendinosis (ST) muscle-tendon system. The model was based on the data of Lieber et al. [Am. J. Physiol.261, C86–C92 (1991)], who determined the relationship between the sarcomere length, tendon load (as a fraction of maximum isometric tension) and tendon, bone-tendon junction (BTJ), and aponeurosis strain. The model was generated assuming a finite time-course of cross-bridge attachment [Huxley, Prog. Biophys.7, 255–318 (1957)], an ideal sarcomere length-tension relationship [Gordon et al., J. Physiol.184, 170–192 (1966)] and an ideal force-velocity relationship [Katz, J. Physiol.96, 45–64 (1939); Edman, J. Physiol.291, 143–159 (1979)]. Functionally, sarcomeres operated on three distinct regions of the length-tension curve: (1) regions where the muscle force decreased as sarcomeres shortened (the shallow and steep ascending limbs); (2) regions where the muscle force increased as sarcomeres shortened and there was little passive tension (descending limb, where sarcomere length ≤3.0 μm); and (3) regions where the muscle force increased as sarcomeres shortened and there was a significant passive tension (descending limb where sarcomere length >3.0 μm). Using such a physiological model, it was found that the effect of tendon compliance was to ‘skew’ the sarcomere length-tension curve to the right and to increase the operating range of the muscle-tendon unit. Thus, maximum tension in the muscle occurred at an active sarcomere length of 2.0–2.2 μm, whereas in the muscle-tendon system, the maximum tension occurred at a longer resting sarcomere length of about 2.5 μm. The degree to which the tendon affected the muscle system depended on its material properties and dimensions. These data suggest that tendons are not merely rigid links connecting muscles to bones, but impart distinct properties to the muscular system.

Muscle architectural changes after massive human rotator cuff tear.

Rotator cuff (RC) tendon tears lead to negative structural and functional changes in the associated musculature. The structural features of muscle that predict function are termed "muscle architecture." Although the architectural features of "normal" rotator cuff muscles are known, they are poorly understood in the context of cuff pathology. The purpose of this study was to investigate the effects of tear and repair on RC muscle architecture. To this end thirty cadaveric shoulders were grouped into one of four categories based on tear magnitude: Intact, Full-thickness tear (FTT), Massive tear (MT), or Intervention if sutures or hardware were present, and key parameters of muscle architecture were measured. We found that muscle mass and fiber length decreased proportionally with tear size, with significant differences between all groups. Conversely, sarcomere number was reduced in both FTT and MT with no significant difference between these two groups, in large part because sarcomere length was significantly reduced in MT but not FTT. The loss of muscle mass in FTT is due, in part, to subtraction of serial sarcomeres, which may help preserve sarcomere length. This indicates that function in FTT may be impaired, but there is some remaining mechanical loading to maintain "normal" sarcomere length-tension relationships. However, the changes resulting from MT suggest more severe limitations in force-generating capacity because sarcomere length-tension relationships are no longer normal. The architectural deficits observed in MT muscles may indicate deeper deficiencies in muscle adaptability to length change, which could negatively impact RC function despite successful anatomical repair. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:2089-2095, 2016.

Alterations in thin filament length during postnatal skeletal muscle development and aging in mice.

The lengths of the sarcomeric thin filaments vary in a skeletal muscle-specific manner and help specify the physiological properties of skeletal muscle. Since the extent of overlap between the thin and thick filaments determines the amount of contractile force that a sarcomere can actively produce, thin filament lengths are accurate predictors of muscle-specific sarcomere length-tension relationships and sarcomere operating length ranges. However, the striking uniformity of thin filament lengths within sarcomeres, specified during myofibril assembly, has led to the widely held assumption that thin filament lengths remain constant throughout an organism's lifespan. Here, we rigorously tested this assumption by using computational super-resolution image analysis of confocal fluorescence images to explore the effects of postnatal development and aging on thin filament length in mice. We found that thin filaments shorten in postnatal tibialis anterior (TA) and gastrocnemius muscles between postnatal days 7 and 21, consistent with the developmental program of myosin heavy chain (MHC) gene expression in this interval. By contrast, thin filament lengths in TA and extensor digitorum longus (EDL) muscles remained constant between 2 mo and 2 yr of age, while thin filament lengths in soleus muscle became shorter, suggestive of a slow-muscle-specific mechanism of thin filament destabilization associated with aging. Collectively, these data are the first to show that thin filament lengths change as part of normal skeletal muscle development and aging, motivating future investigations into the cellular and molecular mechanisms underlying thin filament adaptation across the lifespan.

Length Dependence of Force Generation is Controlled by Phosphorylation of cTnI at Serines 23/24

The steepness of the Frank-Starling relationship is modulated by numerous physiological factors including beta-adrenergic stimulation, which steepens the relationship. This arises in part from increased myofibrillar length dependence of force and power by PKA, a downstream signaling molecule of the beta-adrenergic system. Since PKA has multiple myofibrillar substrates including titin, myosin binding protein-C (MyBP-C), and cardiac troponin I (cTnI), we sought to define if phosphorylation of one of these molecules was sufficient to control length-tension relationships. We focused on cTnI since (i) we previously observed a relationship between cTnI phosphorylation and the steepness of ventricular function curves in rat working hearts, (ii) 2D-DIGE indicated a distribution of cTnI phosphorylation states consistent with our previous observation of two populations of length-tension relationships (one shallow the other steep) in rat skinned cardiac myocytes, and (iii) troponin can be more readily exchanged in permeabilized striated muscle cell preparations. To test if PKA-mediated phosphorylation of cTnI is sufficient to mediate length dependence of force we used skinned rat slow-twitch skeletal muscle fibers, which exhibit shallow length-tension relationships that are unaltered by PKA. A sarcomere length-tension relationship was measured during submaximal Ca2+ activation, then the fiber was incubated in a troponin (Tn) exchange solution, which replaced ∼80% of the endogenous slow-skeletal Tn with cTn. After cTn incorporation the length-tension relationship remained shallow as predicted since the purified cTnI lacks phosphate incorporation. Next, the fiber was incubated in PKA, which caused the length-tension relationship to shift from shallow to steep. In addition, exchange with cTn having cTnI serines 23/24 modified to aspartic acids to mimic phosphorylation steepened the length-tension relationship. These results indicate that phosphorylation of cTnI serines 23/24 is sufficient to control length dependence of force generation in striated muscle.

Length Dependence of Cardiac Myocyte Force Generation is Mediated by CTnI Phosphorylation

According to the Frank-Starling relationship, greater ventricular volume increases ventricular output. The Frank-Starling relationship is based partly on the length-tension relationship in cardiac myocytes. Recently, we identified two populations of length-tension relationships in mammalian cardiac myocytes, which was dependent on PKA-induced myofibrillar phosphorylation. We also observed a relationship between cardiac troponin I (cTnI) phosphorylation and steepness of ventricular function curves in rat working hearts. Thus, we tested the hypothesis that phosphorylation of cTnI is sufficient and/or necessary to control the length dependence of force generation. Our approach took advantage of the fact that troponin can be readily exchanged in permeabilized striated muscle cell preparations. We first used permeabilized rat slow-twitch skeletal muscle fibers, which exhibit shallow length-tension relationships that are unaltered by PKA. A sarcomere length-tension relationship was measured during submaximal Ca2+ activation then the fiber was incubated in a troponin (Tn) exchange solution, which replaced nearly all the endogenous slow Tn with cTn. After cTn incorporation the length-tension relationship remained shallow as predicted since the purified cTnI lacks phosphate incorporation. Next, the fiber was incubated in PKA and a final sarcomere length-tension relationship measured. Contrary to our prediction, the length-tension relationship remained shallow after PKA phosphorylation of exogeneous cTnI. We next tested whether cTnI phosphorylation is necessary to steepen length-tension relationships in a cardiac myofibrillar background. For these experiments, we exchanged unphosphorylated cTn into a PKA treated myocyte that displayed a steep length-tension relationship. In this case cTn exchange shifted the length-tension relationship from a steep to a shallow relation. These results indicate that cTnI phosphorylation is not sufficient to alter length dependence of force generation in slow-twitch skeletal muscle fibers but appears necessary to convert the length-tension relationship from steep to shallow in cardiac myocytes.

Sarcomere overextension reduces stretch-induced tension loss in myofibrils of rabbit psoas

Stretch-induced damage to skeletal muscles results in loss of isometric tension. Although there is no direct evidence, loss of tension has been implicitly assumed to be the consequence of permanent loss of myofilament overlap in some sarcomeres (‘sarcomere overextension’). Using isolated myofibrils of rabbit psoas muscle (n=38; 6 control and 32 test specimens) at 12–15°C, we directly tested the idea that loss of tension following stretch is caused by sarcomere overextension. Experimental myofibrils were maximally activated at the edge of the descending limb (sarcomere length ∼2.9μm) of the sarcomere length-tension relationship and then stretched by 1μmsarcomere–1 at a constant speed of 0.1μms–1sarcomere–1 to result in an average strain of 33.6±0.9% (mean±1SE). Myofibrils were immediately returned to the original lengths and relaxed. Isometric tension measured in a subsequent re-activation 3–5min later was reduced by 24.6±1.5% from its original value. In 22 out of the 32 test specimens, all sarcomeres maintained myofilament overlap, while in 10 myofibrils one or two sarcomeres were stretched permanently beyond myofilament overlap (>4.0μm), and thus exhibited overextended sarcomeres. Loss of tension following stretch was significantly smaller in myofibrils with overextended sarcomeres compared to myofibrils with no overextended sarcomeres (19.5±2.3% and 27.1±1.8%, respectively; p=0.017). Combined, these results suggest that the loss of tension associated with stretch-induced damage can occur in the absence of sarcomere overextension and that sarcomere overextension limits rather than causes stretch-induced tension loss.

The interdependence of Ca2+ activation, sarcomere length, and power output in the heart.

Myocardium generates power to perform external work on the circulation; yet, many questions regarding intermolecular mechanisms regulating power output remain unresolved. Power output equals force × shortening velocity, and some interesting new observations regarding control of these two factors have arisen. While it is well established that sarcomere length tightly controls myocyte force, sarcomere length-tension relationships also appear to be markedly modulated by PKA-mediated phosphorylation of myofibrillar proteins. Concerning loaded shortening, historical models predict independent cross-bridge mechanics; however, it seems that the mechanical state of one population of cross-bridges affects the activity of other cross-bridges by, for example, recruitment of cross-bridges from the non-cycling pool to the cycling force-generating pool during submaximal Ca(2+) activation. This is supported by the findings that Ca(2+) activation levels, myofilament phosphorylation, and sarcomere length are all modulators of loaded shortening and power output independent of their effects on force. This fine tuning of power output probably helps optimize myocardial energetics and to match ventricular supply with peripheral demand; yet, the discernment of the chemo-mechanical signals that modulate loaded shortening needs further clarification since power output may be a key convergent point and feedback regulator of cytoskeleton and cellular signals that control myocyte growth and survival.

Length dependence of force generation exhibit similarities between rat cardiac myocytes and skeletal muscle fibres

According to the Frank-Starling relationship, increased ventricular volume increases cardiac output, which helps match cardiac output to peripheral circulatory demand. The cellular basis for this relationship is in large part the myofilament length-tension relationship. Length-tension relationships in maximally calcium activated preparations are relatively shallow and similar between cardiac myocytes and skeletal muscle fibres. During twitch activations length-tension relationships become steeper in both cardiac and skeletal muscle; however, it remains unclear whether length dependence of tension differs between striated muscle cell types during submaximal activations. The purpose of this study was to compare sarcomere length-tension relationships and the sarcomere length dependence of force development between rat skinned left ventricular cardiac myocytes and fast-twitch and slow-twitch skeletal muscle fibres. Muscle cell preparations were calcium activated to yield 50% maximal force, after which isometric force and rate constants (k(tr)) of force development were measured over a range of sarcomere lengths. Myofilament length-tension relationships were considerably steeper in fast-twitch fibres compared to slow-twitch fibres. Interestingly, cardiac myocyte preparations exhibited two populations of length-tension relationships, one steeper than fast-twitch fibres and the other similar to slow-twitch fibres. Moreover, myocytes with shallow length-tension relationships were converted to steeper length-tension relationships by protein kinase A (PKA)-induced myofilament phosphorylation. Sarcomere length-k(tr) relationships were distinct between all three cell types and exhibited patterns markedly different from Ca(2+) activation-dependent k(tr) relationships. Overall, these findings indicate cardiac myocytes exhibit varied length-tension relationships and sarcomere length appears a dominant modulator of force development rates. Importantly, cardiac myocyte length-tension relationships appear able to switch between slow-twitch-like and fast-twitch-like by PKA-mediated myofibrillar phosphorylation, which implicates a novel means for controlling Frank-Starling relationships.

Determinants of Loaded Shortening in Cardiac Myocytes

Ventricular performance is dictated by stroke volume, which ultimately depends on the extent of myocyte shortening during loaded contractions. We propose that the extent of loaded shortening is determined by the balance between two processes: (i) Ca2+-cross-bridge-induced cooperative activation of the thin filament and (ii) shortening-induced cooperative deactivation of the thin filament. Accordingly, any modulator that augments contractility (i.e., stroke volume) should favor process (i) and diminish process (ii). Since β-adrenergic stimulation is known to increase contractility, we tested whether PKA (the myofibrillar ligand of β-adrenergic signaling) would increase cooperative activation and diminish shortening-induced deactivation in rat permeabilized cardiac myocytes during submaximal Ca2+ activations. Regarding cooperative activation, PKA increased the slope of tension-pCa relationships (nH = 3.85 ± 0.09 before versus nH = 5.03 ± 0.71 after PKA). PKA also slowed rates of force redevelopment, increased the transient force overshoot after a slack-restretch maneuver, and increased the rate and amplitude of spontaneous oscillatory contractions (SPOCs); all of which are consistent with greater cooperative activation of the thin filament. Regarding cooperative deactivation, PKA increased the curvature of myocyte length traces during lightly loaded shortening (kshortening = 6.41 ± 0.28 before versus kshortening = 9.45 ± 0.53 after PKA) and steepened sarcomere length-tension relationships; both of which implicate enhanced (rather than diminished) shortening-induced cooperative deactivation. Taken together, PKA-induced myofibrillar phosphorylation appears to augment both Ca2+-cross-bridge-induced cooperative activation of the thin filament and (ii) shortening-induced cooperative deactivation. Greater cooperative activation should lead to more cycling cross-bridges, which would speed loaded shortening against a given afterload. On the other hand, greater shortening-induced cooperative deactivation may be necessary to help accelerate relaxation and assist diastolic filling in the face of shorter systolic and diastolic times in the presence of higher heart rates induced by β-adrenergic stimulation.

Mechanical properties of intact single fibres from wild-type and MLC/mIgf-1 transgenic mouse muscle

The effects of overexpression of the local form of insulin like growth factor-1 (mIgf-1) on skeletal muscle were investigated by comparing the mechanical properties of single intact fibres from the flexor digitorum brevis of wild-type (WT) and (MLC/mIgf-1) transgenic mice (TG)at 21-24 degrees C. Isolated single fibres were clean enough to measure accurately the sarcomere length. The parameters investigated were: tetanic absolute and specific force, the force-velocity relationship, and the sarcomere length-tension relationship. In addition, we investigated the properties of the "static stiffness", a non-crossbridge Ca(2+)-dependent increase of fibre stiffness previously found in frog muscle. Both average cross-sectional area and tetanic force almost doubled in TG fibres, so that specific force was the same in both preparation: 312 +/- 20 and 344 +/- 34 kN m(-2) in WT and TG fibres, respectively. None of the relative force-velocity parameters was altered by Igf-1 overexpression, however, V(max) (8-10 l(0) s(-1)) was greater than previously reported in whole muscles. The sarcomere length-tension relationship was the same in TG and WT fibres showing the classical shape with a plateau region between 2.28 and 2.52 microm and a linear descending limb. The static stiffness was present in both WT and TG fibres and showed similar characteristics to that of frog skeletal muscle. In contrast to the other parameters, static stiffness in TG fibres was about 24% smaller than in WT fibres suggesting a possible effect of Igf-1 overexpression on its mechanism.

Length-Tension Relationships Are Similar Between Cardiac Myocytes And Skeletal Muscle Fibers

According to the Frank-Starling relationship increased ventricular volume increases ventricular output, which helps match peripheral energy supply with demand. The cellular basis for this relationship is, in large part, the myofilament length-tension relationship. The length-tension relationship in maximally activated skinned preparations is relatively shallow and similar between cardiac myocytes and skeletal muscle fibers. During twitch activations the length tension relationship becomes steeper in both cardiac and skeletal muscle; however, it remains unclear whether length dependence of tension differs between striated muscle cell types during submaximal activations. The purpose of this study was to compare myofilament length-tension relationships between cardiac myocytes, fast-twitch and slow twitch skeletal muscle fibers during half-maximal calcium activations. For these experiments, passive and active sarcomere length-tension relationships were characterized in skinned rat left ventricular cardiac myocytes, fast-twitch and slow-twitch skeletal muscle fibers. For active length-tension relationships, cells were mounted between a force transducer and motor and activated to yield 50-65% maximal force after which isometric force was measured over a range of ascending limb sarcomere lengths monitored by IonOptix SarLen system. Passive tension was considerably greater in cardiac myocytes (3.5 ± 2.5 kN/m2 at 2.30 ± 0.04μm) compared to both fast-twitch (1.1 ± 0.8 kN/m2 at 2.70 ± 0.07μm) and slow-twitch (1.4 ± 0.5 kN/m2 at 2.70 ± 0.07μm) muscle fibers. Active myofilament length-tension relationships were considerably shallower in slow-twitch fibers compared to fast-twitch fibers. Interestingly, cardiac myocytes exhibited two distinct populations of length-tension relationships, one nearly identical to fast-twitch fibers and the other similar to slow-twitch fibers. These results indicate that cardiac myocytes exhibit length-tension relationships very similar to skeletal muscle fibers. The finding of two populations of cardiac myocyte length-tension relationships suggests a mode for adjusting Frank-Starling relationships following acute and/or chronic ventricular adaptations.

Sarcomere length-tension relationship in failing human myocardium

The Starling relationship is a fundamental component of cardiac muscle physiology. Although many researchers have studied the length-tension relationship in human myocardium, none have done it at the sarcomere level. The present study examined the effects of sarcomere length (SL) on contractile parameters in cardiac trabeculae from ischemic and non-ischemic failing human hearts, as well as those mechanically unloaded by a left ventricular assist device (LVAD).Methods:Using laser diffraction techniques, resting SLs were measured in right ventricular cardiac trabeculae (n 10: Average dimensions (mm): 0.33 0.05 Width, 0.19 0.01 Thickness, 2.94 0.32 Length) from failing human hearts. As muscle length was incremented in a stepwise manner (25.0 um/step), resting SL and isometric contractile tension were assessed. Based on the SL-tension relationship, we determined the SLs that produced a developed tension of 20, 50, and 80% of maximal developed tension (Tmax) for each trabeculae. Results: For all groups, developed tension, rate of maximal tension rise, and rate of maximal tension decline all increased linearly as SL was increased. Diastolic tension increased minimally between SLs of 1.7 um to 2.0 um, and in some cases at SLs beyond 2.0 um, it increased in a nonlinear fashion. The SL associated with a particular percent of Tmax was remarkably consistent across all trabeculae: 1.92 0.02 um 20%(Tmax-To); 2.09 0.02 um 50%(Tmax-To); and 2.25 ± 0.01 um 80%(Tmax-To); (To represents the minimal developed tension). These relationships did not seem to be affected by etiology or prior LVAD support. Conclusion: SL can be reliably measured in very thin cardiac trabeculae from human hearts. Although the shapes of the active SL-tension relationships are quite similar, there is substantial heterogeneity in the shapes of the passive SL-tension relationships. Our study also shows that Tmax, often used as a starting point during in vitro analysis, is achieved at non-physiological SLs ( 2.35 um). Therefore, in studies where SL cannot be measured, using a percent of the tension between Tmax and To one can reliably set the preload at a more physiologic SL.

In vivo measurement of human wrist extensor muscle sarcomere length changes.

1. Human extensor carpi radialis brevis (ECRB) sarcomere length was measured intraoperatively in five subjects using laser diffraction. 2. In a separate cadaveric study, ECRB tendons were loaded to the muscle's predicted maximum tetanic tension, and tendon strain was measured to estimate active sarcomere shortening at the expense of tendon lengthening. 3. As the wrist joint was passively flexed from full extension to full flexion, ECRB sarcomere length increased from 2.6 to 3.4 microns at a rate of 7.6 nm/deg joint angle rotation. Correcting for tendon elongation during muscle activation yielded an active sarcomere length range of 2.44 to 3.33 microns. Maximal predicted sarcomere shortening accompanying muscle activation was dependent on initial sarcomere length and was always < 0.15 microns, suggesting a minimal effect of tendon compliance. 4. Thin filament lengths measured from electron micrographs of muscle biopsies obtained from the same region of the ECRB muscles were 1.30 +/- .027 (SE) microns whereas thick filaments were 1.66 +/- .027 microns long, suggesting an optimal sarcomere length of 2.80 microns and a maximum sarcomere length for active force generation of 4.26 microns. 5. These experiments demonstrate that human skeletal muscles can function on the descending limb of their sarcomere length-tension relationship under physiological conditions. Thus, muscle force changes during joint rotation are an important component of the motor control system.

Myosin and actin filament lengths in diaphragms from emphysematous hamsters

In vitro studies of the diaphragm from emphysematous animals have, in some instances, shown an alteration in its sarcomere length-tension relationship and a decreased maximal specific tension. To our knowledge, it has never been determined whether such functional changes may be indicative of ultrastructural adaptations, e.g., changes in filament lengths and thus cross-bridge number. To address this, we compared filament lengths in diaphragms from hamsters in which emphysema was induced by endotracheal instillation of elastase (E) 5 mo before the hamsters were killed with those from control hamsters (C; saline instillation). Diaphragms were then fixed by vascular perfusion with buffered glutaraldehyde in situ at airway pressures set to approximate the physiological range of lung volumes from residual volume (RV) to total lung capacity (TLC). Ultrathin sections (50-70 nm) were taken parallel to the muscle fiber axis and examined by electron microscopy (x33,000). Sarcomere and filament length measurements were calibrated using an actin periodicity of 39 nm and an M-band width of 86 nm to correct for dimensional changes during preparation. Emphysema increased the change in lung volume from -20 to +25 cmH2O airway pressure (from RV to TLC) by approximately 88%, and the displacement volume of excised lung at 0 cmH2O airway pressure was increased by approximately 138% on average. Neither myosin (C = 1.592 +/- 0.027; E = 1.572 +/- 0.035 micron; P = 0.72) nor actin (C = 1.210 +/- 0.035; E = 1.221 +/- 0.014 micron; P = 0.76) filament lengths were affected by emphysema. Thus, filament length changes do not underlie the diaphragm functional adaptations observed previously in emphysema.

Short communication. Sarcomere length changes during fish swimming.

The interaction between actin and myosin represents the fundamental force-producing mechanism in all skeletal muscles. Interaction between analogous proteins also occurs within cells, resulting in movement of subcellular organelles. Although considerable detail is known about the muscle sarcomere length-tension relationship and how isometric force potential is a function of myofilament overlap (Gordon et al. 1966), there are no direct measurements of myofilament movement during locomotion. We have taken advantage of the transparent morphology of the glass catfish (Kryptopterus bicirri) to investigate noninvasively sarcomere length changes which occur during free swimming.

Model of muscle-tendon interaction during frog semitendinosis fixed-end contractions

A structural model was developed to explain sarcomere shortening at the expense of tendon lengthening in the frog semitendinosis (ST) muscle-tendon system. The model was based on the data of Lieber et al. [ Am. J. Physiol. 261, C86–C92 (1991)], who determined the relationship between the sarcomere length, tendon load (as a fraction of maximum isometric tension) and tendon, bone-tendon junction (BTJ), and aponeurosis strain. The model was generated assuming a finite time-course of cross-bridge attachment [Huxley, Prog. Biophys. 7, 255–318 (1957)], an ideal sarcomere length-tension relationship [Gordon et al., J. Physiol. 184, 170–192 (1966)] and an ideal force-velocity relationship [Katz, J. Physiol. 96, 45–64 (1939); Edman, J. Physiol. 291, 143–159 (1979)]. Functionally, sarcomeres operated on three distinct regions of the length-tension curve: (1) regions where the muscle force decreased as sarcomeres shortened (the shallow and steep ascending limbs); (2) regions where the muscle force increased as sarcomeres shortened and there was little passive tension (descending limb, where sarcomere length ≤3.0 μm); and (3) regions where the muscle force increased as sarcomeres shortened and there was a significant passive tension (descending limb where sarcomere length >3.0 μm). Using such a physiological model, it was found that the effect of tendon compliance was to ‘skew’ the sarcomere length-tension curve to the right and to increase the operating range of the muscle-tendon unit. Thus, maximum tension in the muscle occurred at an active sarcomere length of 2.0–2.2 μm, whereas in the muscle-tendon system, the maximum tension occurred at a longer resting sarcomere length of about 2.5 μm. The degree to which the tendon affected the muscle system depended on its material properties and dimensions. These data suggest that tendons are not merely rigid links connecting muscles to bones, but impart distinct properties to the muscular system.

Stimulation rate, potentiators, and sarcomere length-tension relationship of muscle.

Isometric tetani of single muscle fibers of Rana temporaria were studied as a function of stimulation rate, sarcomere length (1.7-2.3 micron), twitch-to-tetanus ratio, and exposure to twitch potentiators (Zn2+ and NO3-) at 20 degrees C. As the stimulation rate was decreased below a maximal level, tension generation decreased. This depression in tension generation was more pronounced at shorter sarcomere lengths. Therefore the magnitude and shape of the sarcomere length curve was dependent on stimulation rate. Although the depression in tension generation was always accompanied by a noticeable ripple in the tension record in fibers with large twitch-to-tetanus ratios, it could be observed even during well-fused tetani in fibers with low twitch-to-tetanus ratios. In all fibers, however, high stimulation rates or exposure to potentiators resulted in maximum tension generation at each length, and the sarcomere length-tension curve followed that found by Gordon, Huxley, and Julian. This indicates that the fall in tension between sarcomere lengths of 2.0 and 1.7 micron is not due to length-dependent activation but is more likely to be the result of mechanical interference in the force-generating interaction between cross bridges and thin filament sites.