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PubMed1998

Different behaviour of the non-sarcomeric cytoskeleton in neonatal and adult rat cardiomyocytes.

Neonatal and adult rat cardiomyocytes display differences when isolated and cultured in vitro. Whereas cells obtained from juvenile hearts adapt quite rapidly as judged by their beating, cells from adult animals undergo a complex degeneration-regeneration process of their myofibrillar apparatus. These differences are also reflected by a distinct sensitivity to drugs that affect the non-sarcomeric cytoskeleton. After long-term treatment with nocodazole, which disassembles microtubules, neonatal rat cardiomyocytes (NRC) remain relatively unaffected, whereas adult rat cardiomyocytes (ARC) are unable to spread on the substrate or to undergo the remodelling process of their myofibrils. If microfilaments are destroyed by cytochalasin D, neither NRC nor ARC spread, and they lose the capacity to assemble new myofibrils. The effects of drug treatment with both cytochalasin and nocodazole, respectively, were reversible, since normal myofibrillogenesis took place after the cells had been washed and cultivated in standard medium for 4 days. This study demonstrates that microfilaments are essential for assembly of new sarcomeres in vitro, and underlines intrinsic differences between NRC and ARC in their requirement for intact microtubules. Adult cardiomyocytes have lost a certain degree of flexibility due to their longer adaptation to the specific situation in the heart, whereas cardiomyocytes isolated from neonatal animals can maintain and assemble myofibrils in vitro even after their microtubules were destroyed.

Actin Cytoskeleton↗
PubMed1987

Chronic cardiac reactions. IV. Effect of drugs and altered functional loads on cardiac energetics as inferred from myofibrillar ATPase and the myosin isoenzyme population.

A major determinant of myocardial energetics is the ATPase activity of myofibrils. In order to account for chronic changes in myofibrillar ATPase, the state equation of the intertropomyosin-interaction model of Tawada et al. was extended by introducing the rates of cross-bridge cycling of myofibrils composed of V-1 or V-3 and the concentration of the myosin isoenzymes. Cross-bridge cycling rates of 1.0 or 0.7 were derived for myofibrils composed of V-1 or V-3, respectively. Ca2+ responsiveness and positive co-operativity were not significantly affected by the myosin isoenzymes. Redistribution of the myosin isoenzyme population and thus altered myocardial energetics was observed following administration of various drugs and as a result of different functional loads. Besides thyroid hormones, catecholamines had a marked influence on myosin. Reducing the adrenergic drive by administration of atenolol, guanethidine or reserpine led to a shift in the direction of V-3. Since serum T3 levels were not significantly reduced by these interventions, the drugs act most probably at the organ level. The functional states responsible for the increase in the proportion of V-3 (pressure load, intermittent feeding, schedule-induced stress) also did not affect circulating T3 in a manner that could entirely explain the redistribution. Hypertrophy-induced dilution of sympathetic nerve fibres or reduced adrenergic responsiveness most likely play a role in the redistribution. An increase in the proportion of V-1 was observed following swimming exercise but not, however, after spontaneous or enforced running. In the swim-exercised rats, T3 was markedly reduced. Thus, the trigger reactions linked most probably to the high adrenergic drive during swimming have to overcome the lower T3 level. It is concluded that myocardial energetics can be decisively altered by a variety of drugs and functional loads, whereby the trigger reactions leading to an altered gene expression of myosin cannot be accounted for entirely by altered circulating T3 but most probably involve the adrenergic system.

Adenosine Triphosphatases↗
PubMed1993

3-D observation of N-cadherin expression during cardiac myofibrillogenesis of the chick embryo using a confocal laser scanning microscope.

It is not yet understood whether cell adhesion molecules play an active role in early cardiac morphogenesis or not. We present here the spatial and temporal expressions of N-cadherin and its relationships to actin filaments during looping (7- to 13-somite stages) of the chick embryonic heart tube observed by means of a confocal laser scanning microscope. Serial optical tomograms were obtained from the whole-mounted heart tubes stained with antibody to N-cadherin (fluorescein-conjugated) and phalloidin (rhodamine-conjugated). Three patterns of N-cadherin expression were observed during looping; a belt-like pattern, speckled pattern, and clumped pattern, corresponding to adhesion belt, nonjunctional cell contact and early intercalated disks, respectively. At the 7-somite stage, myocytes expressed N-cadherin as adhesion belt and nonjunctional cell contact. At the 8- to 10-somite stages, the clumped pattern of N-cadherin was detected before striated myofibrils appeared. Myofibrils began to develop across the clumps to form transcellular networks in the outer layer, and to form circumferential alignments in the inner layer. These results suggest that N-cadherin is responsible for the connection of myofibrils between the neighboring myocytes, and the alignment of the two layers in the developing heart tube.

Animals↗
PubMed1982

A morphometric analysis of regional differences in myotomal muscle ultrastructure in the juvenile eel (Anguilla anguilla L.).

Subpopulations of fast and slow fibres within the trunk musculature of elvers were examined using morphometric analysis of electron micrographs. Fibre regions were characterised by their histochemical staining characteristics, and individual fibres located using a coordinate mapping system utilising morphological features as reference points. Percentages of fibre volume occupied by mitochondria, myofibrils, sarcoplasmic reticulum (S.R.), and T-system were determined in each of the fibre groups, along a transect from the skin to the vertebral column (fibres 1-14, respectively). The fine structure of slow ("red") fibres (1-2 fibres deep) is relatively homogeneous throughout its range, giving mean values for mitochondria, 21.4%; myofibrils, 61.0%; S.R., 2.10%; T-system, 0.31%. The fibres are relatively small (204 micrometer2) and the mitochondrial cristae poorly developed. In contrast, there is a marked heterogeneity in the ultrastructure of fast ("white") fibres, dependent on both position and size. The moderately small (333 micrometer2) superficial fast fibres (3-4 fibres deep) have a significantly higher mitochondrial content (7.6%) than the larger deep fibres (1.2%) (6-12 fibres deep, 775 micrometer2). The mean fractional volumes occupied by myofibrils, S.R., and T-system in the deep fibres are: 80.4%, 5.95%, and 0.38%, respectively. Fibres less than 100 micrometer2 constitute up to 5% of the fast muscle and have a significantly higher mitochondrial volume (4.3%), more glycogen granules, and a slightly lower volume of S.R. (5.57%) than larger fibres. It is suggested that metabolic subpopulations of fast fibres correspond to different stages of fibre growth. The relatively poorly developed S.R. of eel fast muscle is though to be correlated with the low frequency, high amplitude nature of the propagated waveform found in anguilliform locomotion.

Anguilla↗
PubMed1982

Myocardial lysosomes in pressure-overload hypertrophy.

Combined electron microscopic and cytochemical studies were used to investigate the effects of chronic-pressure overload hypertrophy on myocardial lysosomes, mitochondria, and myofibrils in the left ventricle of the cat. Myocardial hypertrophy was induced by an 84% banding constriction of the ascending aorta. After one month of aortic constriction the experimental animals demonstrated a 51% increase in left ventricular mass. No qualitative ultrastructural differences were noted between the myocardial tissues of the hypertrophy and normal group. However, the cytochemical reaction product to acid phosphatase appeared more frequently in the myocardium of the hypertrophy group compared to that of the normal group. By use of quantitative morphometry the percentage of mitochondria, myofibrils and lysosomes per myocardial cell was determined in both hypertrophy and normal groups of animals. Despite significant increases in the left ventricular mass of hypertrophy animals, a normal balance of mitochondria and myofibrils was maintained within the myocardium. Further analysis indicated an enhanced lysosomal population in the hypertrophy group compared to the normal group.

Animals↗
PubMed1977

Ultrastructure of juvenile hormone-induced degenerating flight muscles in a bark beetle, Ips paraconfusus.

Topical application of 5 microgram of a juvenile hormone analogue (JHA), ZR-615, to female callow adults of Ips paraconfusus induced degeneration of the dorsoventral flight muscles. Within 24 h after JHA-treatment the diameter of the myofibrils was reduced to almost half due to the lysis of the peripheral myofilaments. Mitochondria showed conspicuous degenerative changes like swelling, dissolution of the matrix or presence in the matrix of dense filamentous material or myelin-like figures. Degeneration of the mitochondria seemed to take place inside isolation membranes derived from sarcoplasmic reticulum. A number of granular osmiophilic bodies appeared in the sarcoplasm. Three days after JHA-treatment the muscles were very thin and sheath-like. Most of the mitochondria had already degenerated. The dense sarcoplasm contained numerous crystalline bodies. The granular dense bodies were also more frequent. The myofibrils were comprised of only occasional small bundles of myofilaments. The tubules of the T system enclosed an amorphous material. The nuclei and the tracheal system remained intact but they were crowded due to the decreased volume of the muscle. In some specimens degeneration of the myofibrils and mitochondria was completed by the third day. No sign of degeneration was observed in the flight muscles of acetone treated control insects.

Animals↗
PubMed1976

Ultrastructure and differentiation of ascidian muscle. I. Caudal musculature of the larva of Diplosoma macdonaldi.

The larval caudal musculature of the compound ascidian Diplosoma macdonaldi consists of two longitudinal bands of somatic striated muscle. Approximately 800 mononucleate cells, lying in rows between the epidermis and the notochord, constitute each muscle band. Unlike the caudal muscle cells of most other ascidian larvae, the myofibrils and apposed sarcoplasmic reticulum occupy both the cortical and the medullary sarcoplasm. The cross-striated myofibrils converge near the tapered ends of the caudal muscle cell and integrate into a field of myofilaments. The field originates and terminates at intermediate junctions at the transverse cellular boundaries. Close junctions and longitudinal and transverse segments of nonjunctional sarcolemmata flank the intermediate junctions, creating a transverse myomuscular (TMM) complex which superficially resembles the intercalated disk of the vertebrate heart. A perforated sheet of sarcoplasmic reticulum (SR) invests each myofibril. The sheet of SR spans between sarcomeres and is locally undifferentiated in relation to the cross-striations. Two to four saccular cisternae of SR near each sarcomeric Z-line establish interior (dyadic) couplings with an axial analogue of the vertebrate transverse tubular system. The axial tubules are invaginations of the sarcolemma within and adjacent to the intermediate junctions of the TMM complex. The caudal muscle cells of larval ascidians and the somatic striated muscle fibers of lower vertebrates bear similar relationships to the skeletal organs and share similar locomotor functions. At the cellular level, however, the larval ascidian caudal musculature more closely resembles the vertebrate myocardium.

Animals↗
PubMed1975

Ultrastructure of muscle cells in Siboglinum fiordicum (Pogonophora).

Two different muscle types are found in the body of Siboglinum fiordicum: body wall muscle and blood vessel muscle. Both are of a myomesothelial type. The myofibrils of the body wall muscle are non-striated and consist of thick and thin myofilaments. Scattered dense bodies and attachment plaques are described. The sarcoplasmic reticulum forms a three-dimensional network in the myofibrils and only peripheral couplings are observed. The thick filaments are of a paramyosin type and have a diameter ranging from 400-1500 A. The blood vessels muscle is non-striated, but sometimes a sarcomere-like organization has been observed. Both thick and thin filaments are present. The thick filaments have a diameter of 250-400 A and lack transverse striations. Dense bodies and attachment of plaques are few. The sparse sarcoplasmic reticulum is restricted to the myofibril periphery where it makes peripheral couplings with sarcolemma. The luminal surface of the vessels is lined by a basal lamina with collagen-like inclusions. No endothelium is found. The body wall muscle and the blood vessel muscle are compared with other muscle types described in invertebrates.

Animals↗
PubMed1987

A comparative study of the rat heart sarcolemmal Ca2+-dependent ATPase and myosin ATPase.

In order to gain some information regarding Ca2+-dependent ATPase, the enzyme was purified from cardiac sarcolemma and its properties were compared with Ca2+-ATPase activity of myosin purified from rat heart. Both Ca2+-dependent ATPase and myosin ATPase were stimulated by Ca2+ but the maximal activation of Ca2+-dependent ATPase required 4 mM Ca2+ whereas that of myosin ATPase required 10 mM Ca2+. These ATPases were also activated by other divalent cations in the order of Ca2+ greater than Mn2+ greater than Sr2+ greater than Br2+ greater than Mg2+; however, there was a marked difference in the pattern of their activation by these cations. Unlike the myosin ATPase, the ATP hydrolysis by Ca2+-dependent ATPase was not activated by actin. The pH optima of Ca2+-dependent ATPase and myosin ATPase were 9.5 and 6.5 respectively. Na+ markedly inhibited Ca2+-dependent ATPase but had no effect on the myosin ATPase activity. N-ethylmaleimide inhibited Ca2+-dependent ATPase more than myosin ATPase whereas the inhibitory effect of vanadate was more on myosin ATPase than Ca2+-dependent ATPase. Both Ca2+-dependent ATPase and myosin ATPase were stimulated by K-EDTA and NH4-EDTA. When myofibrils were treated with trypsin and passed through columns similar to those used for purifying Ca2+-ATPase from sarcolemma, an enzyme with ATPase activity was obtained. This myofibrillar ATPase was maximally activated at 3-4 mM Ca2+ and 3 to 4 mM ATP like sarcolemmal Ca2+-dependent ATPase. K+ stimulated both ATPase activities in the absence of Ca2+ and inhibited in the presence of Ca2+. Both enzymes were inhibited by Na+, Mg2+, La3+, and azide similarly. However, Ca2+ ATPase from myofibrils showed three peptide bands in SDS polyacrylamide gel electrophoresis whereas Ca2+ ATPase from sarcolemma contained only two bands. Sarcolemmal Ca2+-ATPase had two affinity sites for ATP (0.012 mM and 0.23 mM) while myofibrillar Ca2+-ATPase had only one affinity site (0.34 mM). Myofibrillar Ca2+-ATPase was more sensitive to maleic anhydride and iodoacetamide than sarcolemmal Ca2+-ATPase. These observations suggest that Ca2+-dependent ATPase may be a myosin like protein in the heart sarcolemma and is unlikely to be a tryptic fragment of myosin present in the myofibrils.

Adenosine Triphosphatases↗
PubMed1983

A peculiar myofibrillar pattern in the murine muscle-tendon junction.

The muscle-tendon junctions of adult mice show a special arrangement of myofibrils. Essentially, the myofibrils are oriented in an anchor-like manner in the large finger-shaped processes of the muscle fiber, which contain "mitochondrial sacks" (Demmel et al. 1979) in a subsarcolemmal position. Furthermore, the single myofibril frequently branches out into smaller fibrils. It is suggested that due to the three-dimensional arrangement, these processes gain a high rigidity during contraction, thus maintaining their shape and preserving the "mitochondrial sacks".

Animals↗
PubMed1983

Ultrastructural morphometry of the myocardium of Thunnus alalunga.

The common ventricle in the heart of the Thunnus alalunga was studied. The ventricular myocardium consists of an outer compact layer and a thick inner spongy layer. The compact layer has slightly larger cells (4-6 microns diameter) than the spongy layer (2.5-5 microns diameter). Ultrastructurally the myocardium displays normal arrangements of myofibrils and mitochondria. The sarcoplasmic reticulum is poorly developed. The intercalated discs are simple with the fascia adherens being the most frequent junctional type observed; occasionally a desmosome was seen. Nexus type junctions are present but are unassociated with the intercalated discs. There are no t-tubules evident but the plasmalemma exhibits numerous caveolae which rarely form couplings with the sarcoplasmic reticulum. A morphometric analysis of the volume percent of mitochondria and myofibrils showed that the myocardial cells in the spongy layer of the heart have a significantly greater volume percentage of mitochondria than the compact layer. No significant differences were found between myocardial regions when the volume percentages of myofibrils were compared. The physiological studies revealed that the albacore tuna has heart rates (120 bpm) and ventricular blood pressures (100 mmHg) that are among the highest reported for fish.

Animals↗
PubMed1980

A correlative transmission and scanning electron microscopic study of the pigeon myocardial cell.

A comparative study of the pigeon ventricular myocardial cell has been performed by transmission electron microscopy (TEM) and by scanning electron microscopy (SEM). Three-dimensional access to the cell interior was obtained by cryo-fracturing paraffin-embedded tissue immersed in liquid nitrogen. The TEM studies revealed parallelly arranged myofibils separated by rows of mitochondria. The sarcoplasmic reticulum is represented by a well-developed network of tubules which, at the Z- and H-band level of the sarcomere, expands to form belt-like cisternae. The cisternae at the Z-band level lie in close proximity to both myofilaments and mitochondria. Transverse tubules are absent and thus only peripheral couplings are present. SEM observations of the fractured tissue revealed the spatial relationship between the different cell organelles, the most important of these being the parallel myofibrils and the mitochondria. The conspicuous ridges transversing the myofibril at the Z-band level consist mainly of expanded Z-bands, but overlying SR-tubules also contribute to these ridges. Traces of the SR can sometimes be seen covering the myofibrils. The close proximity between the SR and the mitochondria was also confirmed in the SEM. Preparation and examination of SEM prepared tissue in the TEM confirmed that no essential damage or reorganization of cell organelles had taken place during tHE SEM procedure. On the other hand some shrinkage of the tissue, which was probably caused by critical point drying, was noticed.

Animals↗
PubMed1981

Quantitative analysis of muscle breakdown during starvation in the marine flatfish Pleuronectes platessa.

The present study describes the effects of starvation for a duration of four months on the ultrastructure of skeletal muscles from the marine flatfish (Pleuronectes platessa L.). Starvation is associated with a decrease in resting metabolic rate from 20.1 +/- 2.2 to 11.6 +/- 1.5 mg . O2/kg/h (P less than 0.05) and muscle wasting. Median fibre size fell from 700 micrometer 2 to 500 micrometer 2 in intermediate (fast oxidative) and from 1,800 micrometer 2 to 600 micrometer 2 in starved, white (fast-glycolytic) muscle fibres. In contrast, median fibre size in red (slow oxidative) muscle remained within the range 300-400 micrometer 2. The fraction of red fibre volume occupied by myofibrils (58.6%) and mitochondria (24.5%) did not change significantly with starvation. There was, however, a decrease in stored lipid 110.7% to 3.2%) and an alteration in the structure of the cristae in mitochondria from red muscle. Atrophy of white muscle fibres is associated with a decrease in both the diameter and fractional volume occupied by myofibrils (85.7% to 61.9% P less than 0.01). In a high proportion of white fibres peripheral degeneration of Z-discs is evident causing an unravelling of the thin filament lattice. It is suggested that this allows a partial decrease in myofibril diameter and hence the maintenance of contractile function in muscle from starved fish. In severely degenerating white fibres, disorganised thick and thin filaments and numerous multi-membrane lysosome-like vesicles are observed. Starvation results in an increase in the average content of mitochondria in white fibres from 2.2 to 6.7% (P less than 0.01). In fed plaice mitochondria constitute less than 1% of the volume of the white fibre in 43.5% of the fibres. The proportion of white fibres containing more than 6% mitochondria increases from 6.5% to 58% with starvation.

Animals↗
PubMed1994

Muscle fiber degradation in distal myopathy with rimmed vacuoles.

Late-onset distal myopathy showed numerous rimmed vacuoles with the same properties as autophagic vacuoles. Electron microscopy showed numerous degenerated mitochondria, glycogen, or cell membranes in rimmed vacuoles, but no evidence that these vacuoles engulfed and contained intact or partially disrupted myofibrils. Immunostaining for myosin, alpha-actinin, and actin, however, was sometimes positive within the vacuoles. Compared to the control muscle, there was increased staining activity by calpain around the rimmed vacuoles or in the cytoplasm of mainly atrophic fibers. The result seems to indicate an increase of calpain activity in these muscle fibers. We hypothesize that the myofibrils as well as mitochondria, glycogen, or cell membranes in this myopathy are degraded finally through a lysosomal autophagic process. However, the breakdown of the myofibrils may be not initiated by lysosomal activation; rather it may be the result of extralysosomal processes such as the calpain system.

Actinin↗
PubMed1982

Oculopharyngeal dystrophy: ultrastructure of muscles distinct from the primary myopathy.

The pectoral and psoas muscles from a 72-year-old man afflicted with oculopharyngeal muscular dystrophy were processed at autopsy for electron microscopy. The ultrastructural analyses of the pectoral muscle showed myofibrils which exhibited Z line streaming and a general breakdown in the organization of the sarcomere. In addition, some of the myofibrils displayed sites of degeneration at the center of the A band. The changes in the psoas muscles which are distant from the primary myopathic loci of oculopharyngeal muscular dystrophy were more extensive than that seen in the pectoral muscle. Control tissues showed intact myofibrils ad little postmortem alteration.

Aged↗
PubMed1984

Structural studies of the waves in striated muscle fibres shortened passively below their slack length.

Isolated skeletal muscle fibres of Rana pipiens were shortened below their slack length by longitudinal compression in a gelatine block, and examined by light and electron microscopy. Waves appeared sharply when the striation spacing (S) reached a critical value (about 2 microns) and increased in height with further compression down to S = 1.6 microns while the resting band pattern was maintained. The waves were plane, helical or irregular, with wave lengths of 5-15 striations. The Z lines usually ran perpendicular to the direction of the myofibrils to form wedge-shaped sarcomeres. The bending occurred mainly in the I band. The thin filaments ran stiffly for about 30 nm from the Z line and then bent toward the A band. The thick filaments bent very slightly, particularly at their tips. The edges of the A band were indistinct, and there were no dense lines at the A-I junction. The appearance of the individual sarcomeres resembled those in relaxed myofibrils at slack length, with no Cm bands. The H zone was only seen occasionally in the slack and wavy fibres examined. In very thin sections the individual thin filaments were seen to end in the pseudo-H zone, and not to cross the M line. There was a single array of not more than six thin filaments round each thick one in transverse sections of the M-line region. These observations suggest that the narrowing of the bands observed in fresh wavy fibres is due mainly to the obliquity of the myofibrils, and that the sarcomere length measured parallel to their axis is longer than the striation spacing. The relationship between sarcomere length and the length of the thin-filament complex is discussed.

Animals↗
PubMedDate not supplied

Myofibrillar creatine kinase and cardiac contraction.

This article is a review on the organization and function of myofibrillar creatine kinase in striated muscle. The first part describes myofibrillar creatine kinase as an integral structural part of the complex organization of myofibrils in striated muscle. The second part considers the intrinsic biochemical and mechanical properties of myofibrils and the functional coupling between myofibrillar CK and myosin ATPase. Skinned fiber studies have been developed to evidence this functional coupling and the consequences for cardiac contraction. The data show that creatine kinase in myofibrils is effective enough to sustain normal tension and relaxation, normal Ca sensitivity and kinetic characteristics. Moreover, the results suggest that myofibrillar creatine kinase is essential in maintaining adequate ATP/ADP ratio in the vicinity of myosin ATPase active site to prevent dysfunctioning of this enzyme. Implications for the physiology and physiopathology of cardiac muscle are discussed.

Actin Cytoskeleton↗
PubMed2002

Isometric force kinetics upon rapid activation and relaxation of mouse, guinea pig and human heart muscle studied on the subcellular myofibrillar level.

The kinetics of force development and relaxation upon rapid application and removal of Ca2+ was measured in bundles of few myofibrils isolated from triton X-100 skinned left ventricular trabeculae of mice (M), guinea pigs (G) and humans (H). Upon rapidly switching from relaxing solution (pCa 7.5) to activating solution (pCa 4.5) at 10 degrees C, force rose by a single exponential with a rate constant k(act) of 5.2 s(-1) (M), 1.7 s(-1) (G) and 0.3 s(-1) (H) to a plateau of 0.14 microN/microm2 (M), 0.16 microN/microm2 (G) and 0.15 microN/microm2 (H). A rapid release followed by a rapid restretch to the original length applied during steady-state Ca2+ activation at pCa 4.5 induced an exponential force redevelopment with a rate constant k(redev) that was indistinguishable from k(act), indicating that k(act) is limited by cross-bridge turnover kinetics rather than by the Ca2+-induced activation of the regulatory system. Upon rapidly switching from pCa 4.5 to pCa 7.5, force decayed in a pronounced biphasic manner. Thus a slow initial, almost linear decay with a rate constant k(lin) of 1.8 s(-1) (M), 0.6 s(-1) (G) and 0.15 s(-1) (H) and a duration t(lin) of 0.06 s (M), 0.11 s (G) and 0.3 s (H) was followed by a rapid exponential decay with a rate constant k(rel) of 18 s(-1) (M), 11 s(-1) (G) and 4.6 s(-1) (H). The pronounced biphasic shapes of the force decays determined here for the first time in cardiac myofibrils differs from the force decays that had been reported for multicellular skinned trabeculae in which relaxation was induced by rapid removal of Ca2+ by flash photolysis of caged Ca2+ chelators. In the skinned trabeculae, no pronounced initial slow phase was observed. The force decays shown here are much more similar to those reported for single skeletal myofibrils. The kinetics of isometric relaxation of skinned trabeculae (i.e., multicellular preparations), therefore, do not reflect the kinetics of force relaxation at the cardiac myofibrillar level.

Animals↗