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At least 1,207 records · Page 67Linked to original sources
PubMed1985

Structural and functional development of cricket ring muscles.

The sizes of the unifunctional dorsal longitudinal (DLM) and bifunctional subalar (SA) metathoracic flight muscles of the cricket Teleogryllus oceanicus increase by more than an order of magnitude between the second instar before the terminal molt and the tenth day of adult life. During the same developmental period isometric twitch duration (onset to 50% relaxation, 25 degrees C) varies little, while muscle mitochondrial content increased by a factor of ten as measured by stereological analysis of electron micrographs and citrate synthase activity (mumoles citrate . min-1 . gm protein-1, 25 degrees C). The wing muscles of adults have abundant sarcoplasmic reticulum (SR), narrow myofibrils, and a high volume density of mitochondria. At two molts from adulthood muscles that will later be used in flight behavior also have narrow myofibrils and abundant SR, but unlike muscles at later stages, nymphal muscles have a low volume density of mitochondria. At the terminal molt muscles have at least as much SR as is seen in muscles at the tenth day of adult life, and the myofibrils are also more narrow at the earlier stage. Since there is significant variation in muscle structure and little change in twitch duration during late development, the efficacy of the SR in releasing and resequestering CA2+ is seemingly lower in muscles at the terminal molt, a time of rapid muscle growth.

Animals↗
PubMed1977

Ca+2-accumulating components in developing skeletal muscle.

This ultrastructural study on the localization of Ca+2 in developing skeletal muscle indicates that the formation of calcium-accumulating components begins during embryonic development. Both oxalate and pyroantimonate techniques are used to localize Ca+2 in distinct cellular components of chick pectoral and sartorius muscles. Two major sites for Ca+2 accumulation are present in ultrathin sections of embryonic and post-embryonic muscles: the terminal cisternae of the sarcoplasmic reticulum and specific lines in the I-bands. Calcium oxalate-accumulating vesicles are present in the smallest recognizable myotubes at the twelfth day of incubation, but calcium-accumulating components are not seen at myofibrillar I-band sites until the fourteenth to seventeenth days of incubation. The fact that myofibrils first form and later in development accumulate a Ca+2-binding component suggests that this Ca+2-binding component is not necessary for the formation of myofibrils, but is added to myofibrils before hatching to serve a probable regulatory role in contraction.

Age Factors↗
PubMed1987

Calcium-activated protease activity in tenotomized muscle.

The purpose of this study was to investigate the possible role of calcium-activated neutral protease in the disorganization and dissolution of the myofibrils of the rat soleus that occurs following tenotomy. Rats were killed 3, 5, 7, 14, 21, and 42 days after tenotomy of the soleus, and the muscles were removed and assayed for calcium-activated protease activity. Maximal protease activity occurred 1 week after tenotomy, at the time when myofibril organization is completely disrupted. Activity was still high 2 and 3 weeks after the operation, but returned to normal levels by 6 weeks, when muscle histology had returned to normal. The time course of the calcium-activated protease activity corresponded closely to the time course of the morphological changes. Thus, calcium-activated neutral protease may play a major role in myofibrillar proteolysis following tenotomy and in making the myofibril susceptible to proteolytic attack by other, less specific proteases.

Animals↗
PubMed1993

Ultrastructure of developing flight muscle in Drosophila. II. Formation of the myotendon junction.

Using ultra-thin section electron microscopy, the development of the myotendon junction (MTJ) of Drosophila indirect flight muscle (IFM) is described for the first time. The MTJ is a cell-cell junction between the IFM and epithelial tendon cells. The terminal Z-band of each myofibril forms a uniform junction with a tendon cell; each junction shows a precise sequence of folding and elaboration in which microtubule arrays in both cell types play a prominent role. Upon IFM/tendon cell contact (by approximately 32 hr pupation), numerous flat, focal dense plaques form between muscle and tendon cell membranes. In the muscle, transient arrays of microtubules, which will form "sleeves" around the developing myofibrils, delineate the perimeter of these focal plaques. Each of the dense plaques enlarges and develops into the modified terminal Z-band (MT-Z) of a myofibril, linking the thick and thin filaments of the highly ordered terminal sarcomere to the membrane via a dense feltwork. As these plaques develop into the MT-Z, the perimeter of each plaque advances, leaving the central region deeply indented. Between 50-75 hr pupation, secondary folds appear in each MTJ, and tendon cell microtubules that will form the tendon elements attach to specific dense sites on the secondary folds of the junctional membrane opposite the MT-Z. By 100 hr pupation, each MTJ develops numerous sharp folds, thereby tightly interdigitating the muscle and tendon cell. Amorphous density associated with the junctional membranes assumes a crystalline array that includes the membrane cytoskeletons of both cells and the extracellular matrix. At the end of pupation (approximately 112 hr), the final link between tendon cell and cuticle is formed as the tiny, dense-tipped microvilli in contact with the cuticle are replaced by extracellular tonofibrils, dense shafts that fill deep pits in the tendon cell and extend deeply into the cuticle. The tendon cell microtubules become bundled and decorated by fine "feather" filaments and the free ends of these microtubule bundles become linked to the membrane surrounding the pits.

Animals↗
PubMed1995

Defects in the Drosophila myosin rod permit sarcomere assembly but cause flight muscle degeneration.

We have determined the molecular and ultrastructural defects associated with three homozygous-viable myosin heavy chain mutations of Drosophila melanogaster. These mutations cause a dominant flightless phenotype but allow relatively normal assembly of indirect flight muscle myofibrils. As adults age, the contents of the indirect flight muscle myofibers are pulled to one end of the thorax. This apparently results from myofibril "hyper-contraction", and leads to sarcomere rupture and random myofilament orientation. All three mutations cause single amino acid changes in the light meromyosin region of the myosin rod. Two change the same glutamic acid to a lysine residue and the third affects an amino acid five residues away, substituting histidine for arginine. Both affected residues are conserved in muscle myosins, cytoplasmic myosins and paramyosins. The mutations are associated with age-dependent, site-specific degradation of myosin heavy chain and failure to accumulate phosphorylated forms of flightin, an indirect flight muscle-specific protein previously localized to the thick filament. Given the repeating nature of the hydrophobic and charged amino acid residues of the myosin rod and the near-normal assembly of myofibrils in the indirect flight muscle of these mutants, it is remarkable that single amino acid changes in the rod cause such severe defects. It is also interesting that these severe defects are not apparent in other muscles. These phenomena likely arise from the highly organized nature and rigorous performance requirements of indirect flight muscle, and perhaps from the interaction of myosin with flightin, a protein specific to this muscle type.

Age Factors↗
PubMed1993

Opioid effects on contractility, Ca(2+)-transients and intracellular pH in cultured cardiac myocytes.

Morphine, the opioid-agonist, and the antagonists naloxone and levallorphan exerted direct effects on spontaneously-contracting cultures of cardiac myocytes from neonatal rats. Naloxone and levallorphan induced an increase in the amplitude of systolic cell motion (ASM) and in the size of [Ca2+]i-transients, measured as indo-1 fluorescence ratio (IFR), whereas morphine caused an increase in IFR with no change in ASM. Both morphine and naloxone caused a transient increase in 45Ca2+ influx into the cardiomyocytes. Analysis of the relationship between changes in ASM and IFR indicated dual action of the drugs: (a) An increase in [Ca2+]i-transients elicited by morphine and the antagonists, apparently resulting from a transient increase of Ca2+ influx. (b) Altered myofibril responsiveness to Ca2+; the agonists decreased it, and the antagonists increased it. Intracellular pHi measurements in cardiomyocytes loaded with the fluorescent indicator BCECF revealed that morphine caused acidosis and the antagonists caused alkalosis. These pH changes were inhibited by pertussis-toxin, protein kinase inhibitor K323a, phorbol-ester and ethylisopropyl-amiloride, indicating pathways mediated by GTP-binding proteins and altered activities of protein kinase C and Na+/H+ exchanger. Preincubation with pertussis toxin prior to the addition of morphine prevented the decrease in the myofibril responsiveness to Ca2+ as well as the decrease in pHi but did not affect the increase in [Ca2+]i-transients and the increase in the rate of Ca2+ influx. As a result, addition of morphine after preincubation with pertussis toxin caused a positive inotropic effect. Our results indicate that morphine acts by two different pathways distinguishable by their sensitivity to pertussis toxin (1), increased Ca2+ influx leading to increased Ca(2+)-transients and (2) decreased intracellular pH leading to reduced myofibril responsiveness to Ca2+.

Acidosis↗
PubMed1994

Spectrin in developing normal and cardiomyopathic hamster heart.

In the present study, we compare fetal, newborn and adult normal and cardiomyopathic hearts for the presence of spectrin. Spectrin is a cytoskeletal-membrane protein found associated with the membranes and cortical regions of a variety of cell types. Its function in non-erythroid tissue is not fully understood. We determined previously that spectrin is associated with the membranes, myofibrils, intercalated discs and possibly the T-tubules of developing normal Syrian hamsters. Thus, it is a good candidate for comparison with cardiomyopathic (CM) Syrian hamster hearts since this model system exhibits myofibril and membrane abnormalities. Normal and CM hamsters were analysed at three stages of fetal development, as well as newborn, and adult stages using immunohistochemical, electron microscopic, and electrophoretic techniques. The data presented here indicate that no obvious difference exists in the distribution of spectrin between normal and CM hearts. Thus, the isoform of spectrin recognized by our antibody with the present protocols is not affected by the cardiomyopathic condition and can be ruled out as a contributor to the myofibril disarray and membrane-associated defects seen in these animals.

Animals↗
PubMed1999

The N-terminal region of troponin T is essential for the maximal activation of rat cardiac myofilaments.

Troponin T (TnT) is an essential protein in the transduction of the Ca2+-binding signal that triggers striated muscle contraction. Functional diversity among various TnT isoforms found in cardiac and skeletal muscles has been correlated with the sequence heterogeneity at the amino (N-) and the carboxyl (C-) terminal regions. The most striking difference between cardiac TnT (cTnT) and skeletal TnT (sTnT) is that cTnT has an extended N-terminus, which is rich in negatively charged amino acids. To investigate the role of this region in cTnT, we deleted the first 76 amino acids in rat cTnT (cTnT77-289) by site-directed mutagenesis. We exchanged the native troponin complex in rat cardiac myofibrillar preparations and detergent skinned cardiac fiber bundles by treatment with excess cTnT or cTnT77-289. After reconstituting the cTnT77-289 containing myofibrils with cardiac troponin I-cardiac troponin C (cTnI-cTnC), the MgATPase activity was 70% of the cTnT treated myofibrils in the relaxed state and 83% of the cTnT treated myofibrils in the maximal Ca2+-activated state. These observations were supported by force measurements in which cTnT and cTnT77-289 were exchanged into skinned fiber bundles. Prior to reconstitution with cTnI-cTnC, the Ca2+-independent maximal force developed by the cTnT77-289 containing fiber was 45% of the force developed by the cTnT containing fiber. After reconstituting with cTnI-cTnC, the Ca2+-activated maximal force of the cTnT77-289 containing fiber was 62% of the force developed by the cTnT containing +cTnI-cTnC reconstituted fiber. In both assays, no significant changes in the normalized Ca2+-activity relation or in co-operativity were observed. Fluorescence experiments using pyrene-labeled Tm demonstrated that the binding of cTnT77-289 to Tm was 3-4 fold stronger than that of cTnT. Our results suggest that strong interactions between cTnT77-289 and Tm stabilize cardiac myofilaments in a sub-maximally activated state. Our findings also indicate that the N-terminus of cTnT is essential for maximal activation of cardiac myofilaments.

Actin Cytoskeleton↗
PubMed1982

Mitochondrial creatine kinase in mammalian myocardial cells in culture.

Previous studies on the energy metabolism of rat myocardial cells in culture supported the hypothesis that the creatine-phosphorylcreatine-creatine kinase system is essential for intracellular transport of energy from the mitochondria to the myofibrils and in the regulation of energy production to meet energy utilization. Effective functional compartmentation of ATP could result from the binding of creatine kinase to cellular organelles (e.g., myofibrils and mitochondria) so that the high-energy charge at the myofibrils is maintained by the reverse creatine kinase reaction, whereas phosphorylcreatine is synthesized mainly at the mitochondria in the forward creatine kinase reaction. It was essential to demonstrate the presence of mitochondrial creatine kinase to support the hypothesis. Using polyacrylamide gel electrophoresis and electrophoresis on cellulose acetate strips, the mitochondrial creatine kinase isozyme, as well as MM, MB, and BB isozymes, has now been demonstrated in myocardial cells in culture. Nonmuscle cells in culture also derived from neonatal rat hearts lack the mitochondrial creatine kinase isozyme. Total creatine kinase in myocardial cells is greatly decreased by treatment of the cells with adriamycin, a cardiotoxic chemotherapeutic agent, and the relative amounts of the isozymes are altered. The mitochondrial creatine kinase seems to be reduced less than either the BB or MM isozymes.

Adenosine Triphosphate↗
PubMed1993

Contraction-induced movements of water in single fibres of frog skeletal muscle.

Although X-ray diffraction measurements imply almost constant filament separation during isometric contraction, such constancy does not hold at the level of the isolated cell; cell cross-section increases substantially during isometric contraction. This expansion could arise from accumulation of water drawn from other fibre regions, or from water drawn into the cell from outside. To distinguish between these hypotheses, we froze single fibres of frog skeletal muscle that were jacketed by a thin layer of water. Frozen fibres were freeze-substituted, sectioned transversely, and examined in the electron microscope. In fibres frozen during contraction, we found large amounts of water just beneath the sarcolemma, less in deeper regions, and almost none in the fibre core. Such gradients were absent or diminished in fibres frozen in the relaxed state. The water was not confined to the myofibril space alone; we found large water spaces between myofibrils, particularly near mitochondria. Accumulation of water between myofibrils and around mitochondria implies that the driving force for water movement probably lies outside the filament lattice, and may therefore be osmotic. The fact that the distribution was nonuniform-highest near the sarcolemma and lowest in the core--implies that the water was likely drawn from the thin jacket surrounding the cell. Thus, the contractile cycle appears to be associated with water entry into and exit from the cell.

Animals↗
PubMed1996

Development of cardiac musculature in the cranial vena cava of rat embryos.

Development of cardiac musculature in the rat cranial vena cava (common cardinal vein or duct of Cuvier) was examined by immunohistochemistry and transmission electron microscopy. Undifferentiated cardiac myocytes were detected in the cranial vena cava wall of rat embryos after 12.5 days post-coitum (dpc). The tunica media of the cranial vena cava was composed of cardiac myocytes after formation of the endothelium. Therefore, the cranial vena cava may be not only a part of the venous system but also of the heart. Myocytes in the cranial vena cava contained developing myofibrils, mitochondria and intercalated discs similar to those found in the myocytes in heart. Striated myofibrils began to differentiate as soon as myocytes appeared in the vena cava wall, and myocytes with differentiating myofibrils occur in the wall as the first component of the tunica media at 12.5 dpc. We concluded that the cardiac musculature in the vena cava is not a secondary extension into the tunica media after birth only in the rat, but a basic structure formed in all mammals during early embryonic development.

Animals↗
PubMed1983

An ultrastructural morphometric study of the papillary muscle of the right ventricle of the cat.

The papillary muscle of the cat heart's right ventricle has not been studied previously with quantitative ultrastructural techniques despite its wide use for functional studies. This tissue was perfusion-fixed, processed for electron microscopy, and morphometric techniques were used to assess the ultrastructural characteristics of the papillary muscle as well as the working myocardial cells. The results of this study were that 73.5% of the papillary muscle was composed of muscle cells, 9.7% of blood vessels, and the remainder of interstitial connective tissue. In the muscle cell the volume fraction of mitochondria was 17.3%, that of myofibrils was 49.8%, and that of the nucleus was 2.0%. The mitochondria to myofibrils ratio was 0.36 and the surface to volume ratio was 0.309. In a quantitative ultrastructural comparison of perfusion and immersion fixed tissue it was found that significant differences in the volume density of the blood vessel lumen existed between the two groups. In addition, there were significant differences in the volume fraction of mitochondria and nucleus between perfusion-fixed and immersion-fixed muscle cells. A concurrent significant decrease between the two groups was also found for the ratio of mitochondria to myofibrils. The perfusion-fixed tissue can be considered to provide only normal baseline data for the papillary muscle of the right ventricle. These data are important as they can be used in future structure-function studies on normal and pathological heart tissue.

Animals↗
PubMed1985

Immunofluorescence staining of thin-filament sections not participating in actomyosin crossbridges: studies by use of a monoclonal antibody specific to actin.

Monoclonal antibodies (mcab) were produced in vitro by fusing mouse X63-Ag8.653 plasmacytoma cells with spleen cells from a Balb/c mouse immunized with primary cultures of chick skeletal muscle (pmcc). After cloning on agar, stable clones were obtained, the antibodies of which stain specifically the I-band of myofibrils in the immunofluorescence (IF) procedure. For further characterization of these mcab their affinities to muscle proteins were tested by immunoblotting and by enzyme-linked immunosorbent assay (ELISA). Mcab specific for actin were revealed by these criteria. One of the anti-actin antibodies, mcab 647, reveals a variety of IF-staining patterns on myofibrils. On rest-length myofibrils the I-band is labeled only. However, at sarcomere lengths below 2 micrometers, where the thin filaments meet in the middle of the A-band and form a region of double overlap, an additional fluorescent band appears in this position. The fluorescence intensity of this band is increased significantly in shorter sarcomeres. Finally, when the I-band has disappeared at a sarcomere length of 1.5 micrometer, fluorescence is located exclusively in the middle of the A-band. These IF-staining patterns suggest that only those sections of the thin filament are stained that do not participate in actomyosin crossbridges.

Actins↗
PubMed1984

Morphological studies of different mitochondrial populations in monkey myocardial cells.

The ultrastructure of mitochondria in monkey myocardial cells was investigated by scanning electron microscopy, thin sections and freeze-fracturing. Mitochondria with well-developed cristae were distributed around the nucleus, between the myofibrils and beneath the sarcolemma. Those clustered near the poles of the nucleus were generally spherical in shape. Interfibrillar mitochondria were arranged in longitudinal rows between the myofibrils, were elongated and usually about the same length as a sarcomere. Subsarcolemmal mitochondria varied in size and shape, being rod-like, spherical, polygonal or horseshoelike. There were usually two profiles of subsarcolemmal mitochondria in each section of sarcomere, although sometimes one or three occurred, and they were typically oriented perpendicularly to the myofibrils. These morphological differences among mitochondria could reflect functional and/or mechanical properties in the various cellular locations.

Animals↗
PubMed1987

Differential effects of carbohydrate intake on cardiac myosin isoform expression in normal weanling and adult rats.

Dietary manipulations involving high carbohydrate feeding increase V1 cardiac myosin isoform expression in hormonally deficient rats. The purpose of this study was to determine if extremes in dietary carbohydrate availability could alter cardiac myosin isoform patterns in normal weanling and adult rats. Three and six weeks of dietary manipulations (either high or low carbohydrate diets) failed to change calcium-activated myofibril ATPase activity, calcium regulated myofibril ATPase activity, or the myosin isoform distribution in the adult. In contrast, a four week, high carbohydrate diet reduced calcium activated myosin ATPase activity by 33%, calcium regulated myofibril ATPase activity by 10%, and V1 isoform expression by 66% in weanling rats. Although the low carbohydrate diet caused no change in the myosin ATPase properties, it decreased V1 isoform expression by 17%. These results show that carbohydrate availability can alter cardiac myosin isoform expression in normal rats, but only at weanling age. The reason for this age-related contrast in response to dietary manipulations is unknown at this stage. The dietary manipulations may have acted directly on the heart by creating a state of malnutrition, or indirectly, by altering some developmental process which links maturation of the sympathetic nervous system with myosin isoform expression.

Adenosine Triphosphatases↗
PubMedDate not supplied

E-1020, a water soluble imidazopyridine, has direct effects on Ca(2+)-dependent force and ATP hydrolysis of canine and bovine cardiac myofilaments.

E-1020 is a cardiotonic agent that acts as a cyclic-AMP phosphodiesterase inhibitor but also may have actions which alter myofilament response to Ca2+. To identify direct actions of E-1020 on cardiac contractile proteins, effects of E-1020 on myofibrillar Ca2+ dependent MgATPase and force generation in chemically skinned fiber bundles were measured. In bovine cardiac myofibrils, E-1020 (100 microM) significantly increased myofilament Ca2+ sensitivity and Ca(2+)-dependent ATPase activity at submaximal pCa values. At pCa 6.75, E-1020 significantly increased ATPase activity in bovine (10-100 microM) and canine (1-100 microM) cardiac myofibrils but had no effect on rat cardiac myofibrils. Moreover, in one population of canine ventricular fiber bundles, E-1020 (0.01-10 microM) significantly increased isometric tension at pCa 6.5 and 6.0, whereas in another population of bundles E-1020 had no effect on tension. In no case was resting (pCa 8.0) or maximal tension (pCa 4.5) increased by E-1020. Measurements of Ca2+ binding to canine ventricular skinned fiber preparations demonstrated that E-1020 does not alter the affinity of myofilament troponin C for Ca2+. We conclude that part of the mechanism by which E-1020 acts as an inotropic agent may involve alterations in the responsiveness of contractile proteins to Ca2+. The lack of effect of E-1020 on some preparations may be dependent on isoform populations of myofilament proteins.

Actin Cytoskeleton↗
PubMed1992

The distribution of desmin and titin in normal and dystrophic human muscle.

We have used monoclonal antibodies to desmin and titin, and a combination of immunofluorescence and immunogold labelling to study the disposition of these two proteins in normal human muscle fibres and in fibres at various stages of degeneration in dystrophic muscle. The normal pattern of desmin labelling, in particular the subsarcolemmal labelling, became disrupted at an early stage of fibre breakdown. There was a change from a transverse to a longitudinal orientation of the labelled intermediate filaments as the myofibrils sheared relative to one another. Thus, while it is probable that the desmin filaments are able to play a role in the mechanical integration of the myofibrils in healthy muscle, our results suggest that they cannot withstand the excessive forces generated by the hypercontraction and stretching of dystrophic muscle. However, small accumulations of desmin persisted between the damaged myofibrils until necrosis reached an advanced stage. In general, the degradation of titin appeared to occur before the degradation of desmin, and at the ultrastructural level, labelling with antibodies to epitopes from parts of the titin molecule close to the A-I-band junction was lost before labelling with an antibody to an epitope in the A-band. This suggests that different regions of the titin molecule break down at different stages in the breakdown of the fibre. We propose that lysis of titin in the I-band may underlie 'slippage', an abnormality often seen in dystrophic muscle, in which the A-band slips to one pole of the sarcomere such that it abuts onto the Z-line. Breakdown of the A-band section of titin may facilitate the disassembly of the A-filaments.

Connectin↗
PubMed1991

Combined inhibitory actions of acidosis and phosphate on maximum force production in rat skinned cardiac muscle.

Possible interactions between the effects of pH and phosphate (Pi) on the maximum force development of cardiac myofibrils were investigated in rat skinned trabeculae in solutions of different pH (7.4-6.2) and [Pi] (where [] denote concentration). At pH 7.0 there was an inverse linear relationship between force and log [Pi] over the [Pi] range 0.2-20 mM; its slope (-0.46/decade) was twice that found previously for skeletal muscle [21]. Acidosis depressed force substantially, but the relative change of force was unaffected by Pi addition (0, 5, 20 mM); there was no evidence for the synergism between acidosis and Pi that would be expected if some of the inhibition by acidosis was due to protonation of Pi to the putative inhibitory form, H2PO4-. It was taken into account that even without Pi addition, there was enough Pi inside the muscle from various sources to produce significant changes in [H2PO4-] as the pH was varied. The results suggest that H+ and Pi inhibit maximum force development of cardiac myofibrils independently, by different mechanisms. From this it is argued that H+ and Pi may be released at different steps in the crossbridges cycle. In the myocardium Pi and H+ probably exert tonic inhibitory influences on cardiac myofibrils under all conditions.

Acidosis↗