Increase In Sarcomere Length Research Articles

Exercise training alters length dependence of contractile properties in rat myocardium.

Myocardial function is enhanced by endurance exercise training, but the cellular mechanisms underlying this improved function remain unclear. Exercise training increases the sensitivity of rat cardiac myocytes to activation by Ca(2+), and this Ca(2+) sensitivity has been shown to be highly dependent on sarcomere length. We tested the hypothesis that exercise training increases this length dependence in cardiac myocytes. Female Sprague-Dawley rats were divided into sedentary control (C) and exercise-trained (T) groups. The T rats underwent 11 wk of progressive treadmill exercise. Heart weight increased by 14% in T compared with C rats, and plantaris muscle citrate synthase activity showed a 39% increase with training. Steady-state tension was determined in permeabilized myocytes by using solutions of various Ca(2+) concentration (pCa), and tension-pCa curves were generated at two different sarcomere lengths for each myocyte (1.9 and 2.3 microm). We found an increased sarcomere length dependence of both maximal tension and pCa(50) (the Ca(2+) concentration giving 50% of maximal tension) in T compared with C myocytes. The DeltapCa(50) between the long and short sarcomere length was 0.084 +/- 0.023 (mean +/- SD) in myocytes from C hearts compared with 0.132 +/- 0.014 in myocytes from T hearts (n = 50 myocytes per group). The Deltamaximal tension was 5.11 +/- 1.42 kN/m(2) in C myocytes and 9.01 +/- 1.28 in T myocytes. We conclude that exercise training increases the length dependence of maximal and submaximal tension in cardiac myocytes, and this change may underlie, at least in part, training-induced enhancement of myocardial function.

Myofilament calcium sensitivity in skinned rat cardiac trabeculae: role of interfilament spacing.

The increase in myofilament Ca(2+) responsiveness on an increase in sarcomere length (SL) is, in part, the cellular basis for Frank-Starling's law of the heart. It has been suggested that a decrease in myofilament lattice spacing (LS) in response to an increase in SL underlies this phenomenon. This hypothesis is supported by previous studies in which reduced muscle width induced by osmotic compression was associated with an increase in Ca(2+) sensitivity, mimicking those changes observed with an increase in SL. To evaluate this hypothesis, we directly measured LS by synchrotron x-ray diffraction as function of SL in skinned rat cardiac trabeculae bathed in 0% to 6% dextran solutions (MW 413 000). We found that EC(50), [Ca(2+)] at which force is half-maximal, at SL between 1.95 and 2.25 microm did not vary in proportion to LS when 3% or 6% dextran solutions were applied. We also found that moderate compression (1% dextran) of skinned trabeculae at SL=2.02 microm reduced LS (LS=42.29+/-0.14 nm) to match that of uncompressed fibers at a long SL (SL=2.19 microm; LS=42.28+/-0.15 nm). Whereas increasing SL from 2.02 to 2.19 microm significantly increased Ca(2+) sensitivity as indexed by the EC(50) parameter (2.87+/-0.11 micromol/L to 2.52+/-0.12 micromol/L), similar reduction in myofilament lattice spacing achieved by compression with 1% dextran did not alter Ca(2+) sensitivity (2.87+/-0.10 micromol/L) at the short SL. We conclude that alterations in myofilament lattice spacing may not be the mechanism that underlies the sarcomere length-induced alteration of calcium sensitivity in skinned myocardium.

Increasing tenderness of beef round and sirloin muscles through prerigor skeletal separations.

Crossbred steers (n = 30) were used to explore and compare tenderness improvements in beef round and sirloin muscles resulting from various methods of prerigor skeletal separations. Animals were slaughtered according to industry procedures, and at 60 min postmortem one of six treatments was applied to each side: A) control, B) saw pelvis at the sirloin-round junction, C) separate the pelvic-femur joint, D) saw femur at mid-point, E) combination of B and C, and F) combination of B and D. After 48 h, the following muscles were excised from each side: semimembranosus, biceps femoris, semitendinosus, and adductor from the round; vastus lateralis and rectus femoris from the knuckle; and gluteus medius, biceps femoris and psoas major from the sirloin. Following a 10-d aging period, samples were removed from each muscle to determine the effect of treatment on sarcomere length and Warner-Bratzler shear force. Most skeletal separation treatments resulted in longer sarcomeres than controls for semimembranosus, adductor, semitendinosus, and gluteus medius muscles. All skeletal separation treatments yielded shorter sarcomeres for the psoas major as compared with controls. Warner-Bratzler shear force differed among treatments for rectus femoris, semitendinosus, and psoas major. For rectus femoris, treatments C, D, E, and F resulted in lower (P < 0.05) shear values than for controls. Treatments B, D, and F increased shear force of the semitendinosus relative to controls (P < 0.05) within muscle. Treatment F resulted in higher shear force values for the PM than controls (P < 0.05). Correlations between sarcomere length and shear force were found to be low and quite variable among muscles. In general, treatments increased sarcomere length of several muscles from the sirloin/round region, but had mixed effects on shear force values.

Sarcomere Length-Induced Alterations of Capillary Hemodynamics in Rat Spinotrapezius Muscle: Vasoactive vs Passive Control

Skeletal muscle blood flow is reduced as fibers are stretched longitudinally. Neither the underlying cause(s) of this decrement in blood flow nor the consequences in terms of capillary red blood cell (rbc) hemodynamics has been established clearly within the physiological range of muscle sarcomere length. Using intravital microscopy, this investigation determined arteriolar diameter and capillary rbc velocity (Vrbc), flux (Frbc), and hematocrit (Hctt) in the rat spinotrapezius muscle at shortened/resting (2.6 μm) and physiological extended (3.2 μm) sarcomere lengths under control (c) and local maximally vasodilated (v, phentolamine, 1 μmol/L; prazosin, 0.1 μmol/L; nitroprusside, 10 μmol/L) conditions. The hypothesis tested was that muscle stretch would reduce Vrbc and Frbc proportionally such that Hctt would remain unchanged and that these reductions in Vrbc and Frbc would be attenuated following maximal vasodilation. Vrbc and Frbc were increased significantly following maximal vasodilation at 2.6-μm (59 and 84%) and 3.2-μm (64 and 104%) sarcomere lengths, respectively. Irrespective of sarcomere length, Hctt was elevated significantly following vasodilation (c, 0.20 ± 0.01; v, 0.27 ± 0.01). At 3.2 μm compared with the 2.6-μm sarcomere length, Vrbc and Frbc were both reduced significantly under control and vasodilated conditions as expected. However, the percent reduction in either Vrbc (c, 27%, and v, 29%) or Frbc (c, 26%, and v, 33%) was not significantly different between the 2.6- and 3.2-μm sarcomere lengths. In addition, arteriolar diameter was not altered discernably as sarcomere length was increased from 2.7 μm (c, 29.0 ± 4.5; v, 37.9 ± 6.7 μm) to 3.2 μm (c, 29.4 ± 4.5; v, 37.3 ± 6.2 μm). These data suggest that increasing sarcomere length from resting to the upper extreme of the physiological range in the rat spinotrapezius muscle reduces Vrbc and Frbc (at constant hematocrit) by a mechanism that is independent of stretch-activated arteriolar vasoconstriction.

Myofilament lattice spacing as a function of sarcomere length in isolated rat myocardium.

The Frank-Starling relationship of the heart has, as its molecular basis, an increase in the activation of myofibrils by calcium as the sarcomere length increases. It has been suggested that this phenomenon may be due to myofilaments moving closer together at longer lengths, thereby enhancing the probability of favorable acto-myosin interaction, resulting in increased calcium sensitivity. Accordingly, we have developed an apparatus so as to obtain accurate measurements of myocardial interfilament spacing (by synchrotron X-ray diffraction) as a function of sarcomere length (by video microscopy) over the working range of the heart, using skinned as well as intact rat trabeculas as model systems. In both these systems, lattice spacing decreased significantly as sarcomere length was increased. Furthermore, lattice spacing in the intact muscle was significantly smaller than that in the skinned muscle at all sarcomere lengths studied. These observations are consistent with the hypothesis that lattice spacing underlies length-dependent activation in the myocardium.

Influence of beef carcass stretching and chilling rate on tenderness of m. longissimus dorsi

Sides of 31 non-stimulated carcasses of young bulls were subjected to the muscle stretching methods Tenderstretch (TS) by pelvic bone suspension or Tendercut (TC) with two skeletal cuts or served as controls by traditional Achilles tendon suspension. The sides were chilled at fast and medium rates, resulting in temperatures of 4–5 and 9°C in the m. longissimus dorsi (LD) at 10 h post mortem. The LDs were examined for sarcomere length, Warner–Bratzler peak shear force and sensory properties after 8 days of ageing at 4°C. At the fast chilling rate, TS and TC increased sarcomere lengths, reduced shear force and improved sensory tenderness of the LDs compared to the controls ( P<0.05). At the medium chilling rate, sarcomere lengths increased ( P<0.05), but no significant differences were found in shear force or sensory tenderness ( P>0.05) of the muscles due to stretching. However, the medium chilling rate was efficient in producing tender LDs without applying muscle stretching methods. TS and TC are feasible alternatives for improving overall tenderness and reducing variation in tenderness of beef LD at cold shortening chilling conditions.

Interference fine structure and sarcomere length dependence of the axial x-ray pattern from active single muscle fibers.

Axial x-ray diffraction patterns from single intact fibers of frog skeletal muscle were recorded by using a highly collimated x-ray beam at the European Synchrotron Radiation Facility. During isometric contraction at sarcomere lengths 2.2-3.2 microm, the M3 x-ray reflection, associated with the repeat of myosin heads along the filaments, was resolved into two peaks. The total M3 intensity decreased linearly with increasing sarcomere length and was directly proportional to the degree of overlap between myosin and actin filaments, showing that it comes from myosin heads in the overlap region. The separation between the M3 peaks was smaller at longer sarcomere length and was quantitatively explained by x-ray interference between myosin heads in the two overlap regions of each sarcomere. The relative intensity of the M3 peaks was independent of sarcomere length, showing that the axial periodicities of the nonoverlap and overlap regions of the myosin filament have the same value, 14.57 nm, during active contraction. In resting fibers the periodicity is 14.34 nm, so muscle activation produces a change in myosin filament structure in the nonoverlap as well as the overlap part of the filament. The results establish x-ray interferometry as a new tool for studying the motions of myosin heads during muscle contraction with unprecedented spatial resolution.

Length-dependence of Cross-bridge Mediated Activation of the Cardiac Thin Filament

Length-dependent Ca2+activation of the thin filament plays a critical role in the steep force–length relationship of cardiac muscle (Frank-Starling relation). Recent evidence indicates that the increase in myofilament Ca2+sensitivity and Ca2+-troponin C affinity that occurs with increase in sarcomere length results from a cooperative activation of the thin filament by attached cross-bridges. At short sarcomere length the Ca2+sensitivity is lower because the access of cross-bridges for actin is reduced. The aim of this study was to determine the length-dependence of myosin-mediated thin filament activation in skinned bovine ventricular muscle, as assayed by the generation of force with progressive reduction of MgATP concentration in the absence of Ca2+. If the interaction between myosin and actin is weaker at short sarcomere length there should be a lower MgATP concentration needed to maintain the relaxed state. Contrary to expectation, the force–pMgATP relationship was not significantly influenced by change in sarcomere length. However this relationship became length-sensitive in the presence of phosphate analogs which stabilize weak-binding cross-bridges. We suggest that sarcomere length modulates Ca2+sensitivity by controlling the size of the population of thin filament regulatory units in the weakly-bound state.

Effect of Troponin I Phosphorylation by Protein Kinase A on Length-Dependence of Tension Activation in Skinned Cardiac Muscle Fibers

We examined the effect of troponin I (TnI) phosphorylation by cAMP-dependent protein kinase (PKA) on the length-dependent tension activation in skinned rat cardiac trabeculae. Increasing sarcomere length shifted the pCa (−log[Ca2+])–tension relation to the left. Treatment with PKA decreased the Ca2+ sensitivity of the myofilament and also decreased the length-dependent shift of the pCa–tension relation. Replacement of endogenous TnI with phosphorylated TnI directly demonstrated that TnI phosphorylation is responsible for the decreased length-dependence. When MgATP concentration was lowered in the absence of Ca2+, tension was elicited through rigorous cross-bridge-induced thin filament activation. Increasing sarcomere length shifted the pMgATP (−log[MgATP])–tension relation to the right, and either TnI phosphorylation or partial extraction of troponin C (TnC) abolished this length-dependent shift. We conclude that TnI phosphorylation by PKA attenuates the length-dependence of tension activation in cardiac muscle by decreasing the cross-bridge-dependent thin filament activation through a reduction of the interaction between TnI and TnC.

Analysis of the Mechanisms of Mitochondrial NADH Regulation in Cardiac Trabeculae

We have previously shown that increased cardiac work initially caused a rapid Ca 2+-independent fall of mitochondrial [NADH] ([NADH] m) to a minimum level, and this was followed by a slow Ca 2+-dependent recovery toward control level (Brandes and Bers, Biophys. J. 71:1024–1035, 1996; Brandes and Bers, Circ. Res. 80:82–87, 1997). The purpose of this study is to improve our understanding of the factors that control [NADH] m during increased work. [NADH] m was monitored using fluorescence spectroscopy in intact rat trabeculae isolated from the right ventricular wall. Work was increased by increasing sarcomere length, pacing frequency, external [Ca 2+], or by decreased temperature. The results were: 1) The initial fall of [NADH] m during increased pacing frequency depends independently on increased myofilament work and on increased Ca 2+-transport ATPase activity. 2) The [NADH] m recovery process depends on average cytosolic [Ca 2+] (Av[Ca 2+] c), but not on absolute work level. 3) The initial fall of [NADH] m and the [NADH] m recovery are similar whether increased work is associated with low frequency and high Ca 2+-transient amplitude or vice versa (at the same myofilament work level and Av[Ca 2+] c). 4) The mechanisms associated with the smaller fall and recovery of [NADH] m at 37°C versus 27°C, may be explained by lowered Av[Ca 2+] c and myofilament work. The NADH control mechanisms that operate at lower temperature are thus qualitatively similar at more physiological temperatures.

Non-Elastic Deformation of Myocardium in Low-Flow Ischemia and Reperfusion: Ultrastructure–Function Relations

This study tested the hypothesis that regional low-flow ischemia and reperfusion alter myocardial material properties by causing non-elastic deformation. Twenty-two anesthetized, open-chest pigs were studied. Pigs underwent 90 min of regional low-flow ischemia (anterior LV subendocardial blood flow 29±7% of baseline) followed by 90 min reperfusion. LV pressure and regional subendocardial segment length were recorded to derive end-diastolic pressurevssegment length (EDPvsEDL) and preload-recruitable stroke work (PRSW) relations.In vivo, non-elastic myocardial deformation was inferred from increases in minimally loaded myocardial dimensions: the EDL at zero EDP (L0) and the EDL at which no regional external work was performed (Lw, the PRSW intercept). In 15 pigs, ultrastructural confirmation of non-elastic deformation was obtained from sarcomere dimensions measured by transmission electron microscopy afterin situperfusion fixation under non-ischemic conditions, after 90 min ischemia, or after 90 min ischemia plus 90 min reperfusion. Ischemia increased L0and Lwto 1.17±0.05 and 1.13±0.04 times baseline, respectively. After reperfusion, L0and Lwremained increased to 1.09±0.03 and 1.15±0.02 times baseline (allP<0.05). After reperfusion, PRSW slope was not different from baseline, but regional external work remained depressed (0.38±0.03 times baseline) due to the persistent increase in Lw. Neither L0nor Lwchanged in the posterior (non-ischemic) region. In hearts fixed after ischemia or after ischemia plus reperfusion, sarcomere length was significantly greater and transverse distance between thick myofilaments was significantly smaller in the anterior (ischemic) subendocardium than in the posterior (non-ischemic) subendocardium (P<0.01). We conclude that regional low-flow ischemia and reperfusion cause non-elastic deformation of myocardium, manifestin vivoby increased minimally loaded myocardial dimensions (L0and Lw) and ultrastructurally by increased sarcomere length and decreased transverse interfilament distance. Non-elastic deformation of myocardium may contribute to contractile dysfunction in low-flow ischemia and reperfusion.

Ca 2+ and Cross-Bridge-Induced Changes in Troponin C in Skinned Skeletal Muscle Fibers: Effects of Force Inhibition

Changes in skeletal troponin C (sTnC) structure during thin filament activation by Ca 2+ and strongly bound cross-bridge states were monitored by measuring the linear dichroism of the 5′ isomer of iodoacetamidotetramethylrhodamine (5′IATR), attached to Cys 98 (sTnC-5′ATR), in sTnC-5′ATR reconstituted single skinned fibers from rabbit psoas muscle. To isolate the effects of Ca 2+ and cross-bridge binding on sTnC structure, maximum Ca 2+-activated force was inhibited with 0.5 mM AlF 4 − or with 30 mM 2,3 butanedione-monoxime (BDM) during measurements of the Ca 2+ dependence of force and dichroism. Dichroism was 0.08 ± 0.01 (± SEM, n = 9) in relaxing solution (pCa 9.2) and decreased to 0.004 ± 0.002 (± SEM, n = 9) at pCa 4.0. Force and dichroism had similar Ca 2+ sensitivities. Force inhibition with BDM caused no change in the amplitude and Ca 2+ sensitivity of dichroism. Similarly, inhibition of force at pCa 4.0 with 0.5 mM AlF 4 − decreased force to 0.04 ± 0.01 of maximum (± SEM, n = 3), and dichroism was 0.04 ± 0.03 (± SEM, n = 3) of the value at pCa 9.2 and unchanged relative to the corresponding normalized value at pCa 4.0 (0.11 ± 0.05, ± SEM; n = 3). Inhibition of force with AlF 4 − also had no effect when sTnC structure was monitored by labeling with either 5-dimethylamino-1-napthalenylsulfonylaziridine (DANZ) or 4-( N-(iodoacetoxy)ethyl- N-methyl)amino-7-nitrobenz-2-oxa-1,3-diazole (NBD). Increasing sarcomere length from 2.5 to 3.6 μm caused force (pCa 4.0) to decrease, but had no effect on dichroism. In contrast, rigor cross-bridge attachment caused dichroism at pCa 9.2 to decrease to 0.56 ± 0.03 (± SEM, n = 5) of the value at pCa 9.2, and force was 0.51 ± 0.04 (± SEM, n = 6) of pCa 4.0 control. At pCa 4.0 in rigor, dichroism decreased further to 0.19 ± 0.03 (± SEM, n = 6), slightly above the pCa 4.0 control level; force was 0.66 ± 0.04 of pCa 4.0 control. These results indicate that cross-bridge binding in the rigor state alters sTnC structure, whereas cycling cross-bridges have little influence at either submaximum or maximum activating [Ca 2+].

A cross-bridge mechanism can explain the thixotropic short-range elastic component of relaxed frog skeletal muscle.

1. The passive tension and sarcomere length of relaxed frog skeletal muscle fibres were measured in response to imposed length stretches. The tension response to a constant-velocity stretch exhibited a clear discontinuity. Tension rose more rapidly during the initial approximately 0.4 % L0 of the stretch than during the latter stages (where L0 is the resting length of the fibre). This initial tension response is attributed to the short-range elastic component (SREC). 2. The use of paired triangular stretches revealed that the maximum tension produced during the SREC response of the second stretch was significantly reduced by the first stretch. This history-dependent behaviour of the SREC reflects thixotropic stiffness changes that have been previously described in relaxed muscle. 3. The biphasic nature of the SREC tension response to movement was most apparent during the first imposed length change after a period at a fixed length, irrespective of the direction of movement. 4. If a relaxed muscle was subjected to an imposed triangular length change so that the muscle was initially stretched and subsequently shortened back to its original fibre length, the resting tension at the end of the stretch was reduced relative to its initial pre-stretch value. Following the end of the stretch, tension slowly increased towards its initial value but the tension recovery was not accompanied by a contemporaneous increase in sarcomere length. This finding suggests that the resting tension of a relaxed muscle fibre is not entirely due to passive elasticity. The results are compatible with the suggestion that a portion of the resting tension - the filamentary resting tension (FRT) - is produced by a low level of active force generation. 5. If a second identical stretch was imposed on the muscle at a time when the resting tension was reduced by the previous stretch, the maximal tension produced during the second stretch was the same as that produced during the first, despite the second stretch commencing from a lower initial resting tension. 6. In experiments using paired triangular length changes, an inter-stretch interval of zero did not produce a substantially greater thixotropic reduction in the second stretch elastic limit force than an inter-stretch interval in the range 0.5-1 s. 7. A theoretical model was developed in which the SREC and FRT arise as manifestations of a small number of slowly cycling cross-bridges linking the actin and myosin filaments of a relaxed skeletal muscle. The predictions of the model are compatible with many of the experimental observations. If the SREC and FRT are indeed due to cross-bridge activity, the model suggests that the cross-bridge attachment rate must increase during interfilamentary movement. A mechanism (based on misregistration between the actin binding sites and the myosin cross-bridges) by which this might arise is presented.

Cellular mechanisms for the slow phase of the frank—starling response

Following a step increase in sarcomere length, isometric cardiac muscle tension increases instantaneously by the Frank—Starling mechanism. In isolated papillary muscle and myocytes, there is an additional significant rise in developed tension over the following 15 min due to an unknown mechanism. This slow change in tension could not be explained by mechanical heterogeneity of the muscle preparations or by an increase in myofilament sensitivity to Ca 2+. The slow change in tension was not dependent on sarcoplasmic reticulum Ca 2+ loading assessed with rapid cooling contractures, and was not significantly altered by sarcoplasmic reticulum Ca 2+ depletion (ryanodine) or inhibition of sarcoplasmic reticulum Ca 2+ reuptake (cyclopiazonic acid). We used the Luo—Rudy ionic model of the ventricular myocyte together with a model of the length-dependent myofilament activation by Ca 2+ to examine the effects of step changes in the parameters of sarcolemmal ion fluxes as possible mechanisms for the slow change in stress. The slow increase in tension was simulated by step changes in the Na +-K + pump or Na + leak currents, suggesting that the slow change in stress may be caused by length induced changes in Na + fluxes. The model also predicted a slow increase in the magnitude of the initial repolarization during phase 1 of the action potential. The combination of experimental and computational models used in this investigation represents a valuable technique in elucidating the cellular mechanisms of fundamental processes in cardiac excitation-contraction coupling.

Changes in Interfilament Spacing Mimic the Effects of Myosin Regulatory Light Chain Phosphorylation in Rabbit Psoas Fibers

The modulatory effect of myosin regulatory light chain phosphorylation in mammalian skeletal muscle, first documented as posttetanic potentiation of twitch tension, was subsequently shown to enhance the expression and development of tension at submaximal levels of activating calcium. Structural analyses demonstrated that thick filaments with phosphorylated myosin regulatory light chains appeared disordered: they lost the near-helical, periodic arrangement of myosin head characteristic of the relaxed state. We suggested that disordered heads may be more mobile than ordered heads and are likely to spend more time close to their binding sites on thin filaments. In this study we determined that the physiological effects of phosphorylation could be mimicked by decreasing the lattice spacing between the thick and the thin filaments, either by osmotic compression with dextran or by increasing the sarcomere length of permeabilized rabbit psoas fibers. Phosphorylation of regulatory light chains by incubation of permeabilized fibers with myosin light chain kinase and calmodulin, followed by low levels of activating calcium, potentiated tension development at resting or lower sarcomere lengths in the absence of dextran but had no additional effect on tension potentiation or development in fibers with decreased lattice spacing due to either osmotic compression or increased sarcomere length.

In vivo microvascular structural and functional consequences of muscle length changes.

As muscles are stretched, blood flow and oxygen delivery are compromised, and consequently muscle function is impaired. We tested the hypothesis that the structural microvascular sequellae associated with muscle extension in vivo would impair capillary red blood cell hemodynamics. We developed an intravital spinotrapezius preparation that facilitated direct on-line measurement and alteration of sarcomere length simultaneously with determination of capillary geometry and red blood cell flow dynamics. The range of spinotrapezius sarcomere lengths achievable in vivo was 2.17 +/- 0.05 to 3.13 +/- 0.11 microns. Capillary tortuosity decreased systematically with increases of sarcomere length up to 2.6 microns, at which point most capillaries appeared to be highly oriented along the fiber longitudinal axis. Further increases in sarcomere length above this value reduced mean capillary diameter from 5.61 +/- 0.03 microns at 2.4-2.6 microns sarcomere length to 4.12 +/- 0.05 microns at 3.2-3.4 microns sarcomere length. Over the range of physiological sarcomere lengths, bulk blood flow (radioactive microspheres) decreased approximately 40% from 24.3 +/- 7.5 to 14.5 +/- 4.6 ml.100 g-1.min-1. The proportion of continuously perfused capillaries, i.e., those with continuous flow throughout the 60-s observation period, decreased from 95.9 +/- 0.6% at the shortest sarcomere lengths to 56.5 +/- 0.7% at the longest sarcomere lengths and was correlated significantly with the reduced capillary diameter (r = 0.711, P < 0.01; n = 18). We conclude that alterations in capillary geometry and luminal diameter consequent to increased muscle sarcomere length are associated with a reduction in mean capillary red blood cell velocity and a greater proportion of capillaries in which red blood cell flow is stopped or intermittent. Thus not only does muscle stretching reduce bulk blood (and oxygen) delivery, it also alters capillary red blood cell flow dynamics, which may further impair blood-tissue oxygen exchange.

Extent of radial sarcomere coupling revealed in passively stretched cardiac myocytes.

The complex geometry of the heart leads to a time-varying and nonuniform distribution of stress and strain on the myocardium during the cardiac cycle. This study investigated the sarcomere length and striation registration behavior resulting from a gradient of strain imposed on the cytoskeleton of isolated cardiac myocytes. Within a myocyte, sarcomeres are organized into domains separated by strips of mitochondria and nuclei. Sarcomeres are interconnected radially at the Z-disc by the exosarcomeric cytoskeleton, composed primarily of the intermediate filament desmin. Chemically skinned myocytes isolated from adult rat hearts were attached using to concentric double-barreled pipettes in such a way as to induce a gradient of applied strain across the width of the cell. At rest lengths, there was variation in the sarcomere length between domains of attached cells. Upon stretch, most domains exhibited proportional increases in sarcomere length, with the initial differences being maintained. However, some domains did not behave in synchrony with the others at shorter sarcomere lengths. Thus, the coupling between sarcomeric domains can allow for a non-linear relationship between sarcomere length and strain. Sarcomeres were tightly coupled in the radial direction within a single domain, but striation mis-registration of up to 0.20 micron between domains was induced by stretch. This indicates the looser nature of the cytoskeletal coupling between domains compared to that within domains. Thus, cardiac myocytes are not rigid functional units, but composite structures whose components consist of functionally semi-independent domains tethered by the cytoskeletal intermediate filament lattice.

Intracellular Ca2+ increases the mitochondrial NADH concentration during elevated work in intact cardiac muscle.

It is not clear how mitochondrial energy production is regulated in intact tissue when energy consumption suddenly changes. Whereas mitochondrial [NADH] ([NADH]m) may regulate cellular respiration rate and energetic state, it is not clear how [NADH]m itself is controlled during increased work in vivo. We have varied work and [Ca2+] in intact cardiac muscle while assessing [NADH]m using fluorescence spectroscopy. When increased work was accompanied by increasing average [Ca2+]c (by increasing [Ca2+]c or pacing frequency), [NADH]m initially fell and subsequently recovered to a new steady state level. Upon reduction of work, [NADH]m overshot and then returned to control levels. In contrast, when work was increased without increasing average [Ca2+]o (by increasing sarcomere length), [NADH]m fell similarly, but no recovery or overshoot was observed. This Ca(2+)-dependent recovery and overshoot may be attributed to Ca(2+)-dependent stimulation of mitochondrial dehydrogenases. We conclude that the immediate initial increase in respiration rate upon elevation of work is not activated by increased [NADH]m (since [NADH]m rapidly fell) or by [Ca2+]o (since work could also be increased at constant [Ca2+]c). However, during sustained high work, a Ca(2+)-dependent mechanism causes slow recovery of [NADH]m toward control values. This demonstrates a Ca(2+)-dependent feed-forward control mechanism of cellular energetics in cardiac muscle during increased work.

Existence of the Frank-Starling mechanism in the failing human heart. Investigations on the organ, tissue, and sarcomere levels.

The Frank-Starling mechanism is one of the most important physiological principles for regulation of contractile performance. We therefore studied the question of whether this mechanism may be absent or attenuated in end-stage failing human left ventricular myocardium. Different methodological approaches were used to analyze the effects of this mechanism on the organ, tissue, and sarcomere levels: (1) In excised human whole left ventricles (2 donor hearts, 5 failing hearts), diastolic and systolic pressure-volume relationships were obtained. (2) In isolated muscle strip preparations from the left ventricular wall of donor hearts (n = 14) and failing hearts from patients with idiopathic dilated cardiomyopathy (n = 21) and ischemic cardiomyopathy (n = 11), peak developed force was measured at different muscle lengths of the preparation. (3) Skinned fiber preparations were obtained from failing right and left ventricles (n = 12). In all three studies, we clearly observed the existence of the Frank-Starling mechanism: (1) In isolated failing human left ventricles, peak developed isometric pressure is increased when the preload is elevated. (2) Peak developed tension is increased by approximately 50% to 70% (P < .01) in left ventricular preparations of failing and nonfailing ventricles when the muscles are stretched from 90% to 100% optimum length. (3) An increase in sarcomere length leads to a sensitization of contractile proteins of ventricular skinned fiber preparations from failing human hearts. At 1.9-microns sarcomere length, the EC50 value was 5.56 +/- 0.06, and at 2.3 microns it was 5.70 +/- 0.05 (P < .01; n = 7). The Frank-Starling mechanism is maintained in end-stage failing human hearts, whereas significant alterations of diastolic myocardial distensibility are evident in chronic heart failure.

Sarcomere Length changes after flexor carpi ulnaris to extensor digitorum communis tendon transfer

Sarcomere length was measured intraoperatively on five patients undergoing tendon transfer of the flexor carpi ulnaris (FCU) to the extensor digitorum communis (EDC) for radial nerve palsy. The most significant result was that the absolute sarcomere length and sarcomere length operating range of the FCU increased after transfer into the EDC (p < .001). Preoperatively, with the wrist fully extended and fingers flexed, FCU sarcomere length was 4.22 +/- .24 microns and decreased to 3.19 +/- .05 microns as the wrist was fully flexed. This represented an overall sarcomere length range of 1.03 microns. After the tendon transfer using standard recommended techniques, all sarcomere lengths were significantly longer (p < .001). Specifically, sarcomeres were 0.74 +/- .14 microns longer with the muscle in its fully lengthened position (4.96 +/- .43 microns with the wrist and digits flexed) and 0.31 +/- .16 microns longer with the FCU in the fully shortened position (3.50 +/- .06 microns with the wrist and digits extended). At these sarcomere lengths, the FCU muscle was predicted to develop relatively high force only during movement involving synergistic wrist flexion and finger extension. Under the conditions of the procedures performed, the transferred FCU muscle was predicted to produce maximum force over the range of about 30 degrees of wrist flexion and 0 degree of finger flexion to 70 degrees of wrist extension and 90 degrees of finger flexion. While this is acceptable, a more desirable result was predicted to occur if the muscle was transferred at a longer length. In this latter case, greater stretch of the FCU during transfer (increasing sarcomere length to about 5 microns) was predicted to improve the transfer. The more highly stretched FCU was predicted to result in maximum force as the wrist and fingers progressed from about 60 degrees of wrist extension and 0 degree of finger flexion to 80 degrees of wrist extension and 70 degrees of finger flexion. These results quantify the relationship between the passive tension chosen for transfer, sarcomere length, and the estimated active tension that can be generated by the muscle. The results also demonstrate the feasibility of using intraoperative laser diffraction during tendon transfer as a guide for optimal placement of the transferred muscle.