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PubMed1976

Connectin, an elastic protein from myofibrils.

The elastic protein isolated from myofibrils of chicken skeletal muscle was compared with extracellular non-collagenous reticulin prepared from chicken liver and skeletal muscle. The amino acid compositions of these proteins were similar except that their contents of Phe, Leu, Cys/2, and Hyp were different. The impregnations of the elastic protein and reticulin with silver were also different. The reticulin was not at all elastic. It also differed from reticulin in solubility and antigenicity. It is proposed to call the intracellular elastic protein connectin.

Amino Acids↗
PubMed1978

Fine structure of connectin nets in cardiac myofibrils.

A net-like structure extending through Z lines of myofibrils in frog cardiac muscle was clearly demonstrated under an electron microscope after the removal of myosin and actin. The diameter of the very thin filaments forming the nets was approximately 2 nm; this width was thinner than that of the actin filament. The net structure is ascribed to connectin, an elastic protein of muscle.

Actins↗
PubMed1993

An optical fiber transducer for single myofibril force measurement.

A force transducer has been developed for use in force measurement of skeletal muscle myofibrils. The transducer is suitable for measurement of passive and contractile forces in a range up to 200 micrograms, with 1 microgram resolution. It is based upon the operating principle of the deflection of an optical fiber of known compliance, sensed by the differential illumination of two phototransistors. Attractive features include ease of operation and specimen mounting, high bandwidth, adaptability for different force ranges, and simple and inexpensive construction.

Biomechanical Phenomena↗
PubMed1977

Effect of changing the composition of the bathing solutions upon the isometric tension-pCa relationship in bundles of crustacean myofibrils.

1. The relative isometric tension-pCa relationship has been determined for isolated bundles of barnacle myofibrils under a variety of ionic conditions using [Ca(2+)]-buffered solutions which also contained an ATP regenerating system (creatine phosphate and creatine kinase).2. The results are in better agreement with the ;consecutive' scheme of reaction rather than with the ;independent' alternative (Ashley & Moisescu, 1972) for the co-operative action of two Ca(2+) ions in the process of tension activation in crustacean skeletal muscle.3. Variations in the pH of the activating solutions did have a marked effect on the relative tension-Ca curve, although no effect was observed on the absolute maximum value for isometric tension. A shift in pH by 0.5 u. in the range 6.6-7.6 shifted the Ca(2+)-activation curve by 0.5 log u. towards lower free Ca(2+) concentrations.4. Changes in the free Mg(2+) concentration of the activating solutions in the millimolar range produced a pronounced shift of the relative tension-pCa curve along the pCa axis. Increasing [Mg(2+)] from 1 to 5 mM shifted the curve by about 0.7 log u. to higher free Ca(2+) concentrations, without significantly modifying its steepness.5. Changes in the MgATP concentration of the activating solutions in the range of 1-13 mM had no significant effect on the relative tension-pCa relationship.6. Varying the K(+) concentration in the activating solutions was also observed to have a marked effect upon the tension-pCa relationship in barnacle. An increase in the K(+) concentration from 90 to 170 mM shifted the curve by some 0.6 log u. towards higher free Ca(2+) concentrations.7. Cooling the standard activating solutions from room temperature to +4 degrees C made no apparent difference to the relative tension-pCa relationship, but decreased significantly the absolute tension responses.8. The results presented show that tonicity by itself has a marked effect upon the absolute steady-state tension levels in isolated bundles of myofibrils.9. Maximum isometric tension in this preparation was not simply related to ionic strength, or to the monovalent cation concentration, but it depended, as well, upon the anionic composition of the activating solution. In addition, a change in ionic strength of 25 mM over the range of 245-270 mM did not appear to modify the relative tension-pCa relationship.10. The effect of the physiologically occurring cations H(+), K(+), Mg(2+) upon the relative isometric tension-pCa relationship can be accounted for on the basis of a model of competitive inhibition between these cations and Ca(2+) for the functional unit for tension. This inhibitory effect appears to involve at least one H(+), one Mg(2+) and two K(+) per each Ca(2+) ion participating in the activation process of the functional unit for tension.

Adenosine Triphosphate↗
PubMed1964

CONTRACTION-BAND FORMATION IN BARNACLE MYOFIBRILS.

Contractions induced by adenosine triphosphate, in myofibrils isolated from the barnacle, Balanus aquilia, were observed with a phase microscope. The formation of contraction bands was well under way before the A-band came into contact with the Z-membrane. This is in disagreement with the hypothesis that contraction bands are formed when the A-band pushes against the Z-membrane.

Adenosine Triphosphate↗
PubMed1985

Myofibrils bear most of the resting tension in frog skeletal muscle.

The tension that develops when relaxed muscles are stretched is the resting (or passive) tension. It has recently been shown that the resting tension of intact skeletal muscle fibers is equivalent to that of mechanically skinned skeletal muscle fibers. Laser diffraction measurements of sarcomere length have now been used to show that the exponential relation between resting tension and sarcomere length for whole frog semitendinosus muscle is similar to that of single fibers. Slack sarcomere lengths and the rates of stress relaxation in these muscles were similar to those in skinned fibers, and sarcomere length remained unchanged during stress relaxation, as in skinned fibers. Thus, in intact semitendinosus muscle of the frog up to a sarcomere length of about 3.8 micrometers, resting tension arises, not in the connective tissue as is commonly thought, but in the elastic resistance of the myofibrils.

Animals↗
PubMed1982

Congenital neuromuscular disease with type I fibre hypotrophy, ophthalmoplegia and myofibril degeneration.

We report a 7-year-old boy with progressive, early onset somatic and cranial muscle weakness associated with external ophthalmoplegia, facial weakness, type I fibre hypotrophy and myofibril degeneration. We separate this condition from congenital fibre type disproportion because of the facial weakness, ophthalmoplegia, central nucleation, and lysis in type I fibres. The case, which is similar to that described by Bender and Bender (1977), nosologically should be classified between the centronuclear myopathies and congenital fibre type disproportion, and most likely represents a congenital or neonatal disturbance of trophic interaction between nerve and muscle.

Blepharoptosis↗
PubMed1999

Cross bridge-dependent activation of contraction in cardiac myofibrils at low pH.

Striated muscle contracts in the absence of calcium at low concentrations of MgATP ([MgATP]), and this has been termed rigor activation because rigor cross bridges attach and activate adjacent actin sites. This process is well characterized in skeletal muscle but not in cardiac muscle. Rigor cross bridges are also thought to increase calcium binding to troponin C and play a synergistic role in activation. We tested the hypothesis that cross bridge-dependent activation results in an increase in contractile activity at normal and low pH values. Myofibrillar ATPase activity was measured as a function of pCa and [MgATP] at pH 7.0, and the data showed that, at pCa values of >/=5.5, there was a biphasic relationship between activity and [MgATP]. Peak activity occurred at 10-50 microM MgATP, and [MgATP] for peak activity was lower with increased pCa. The ATPase activity of rat cardiac myofibrils as a function of [MgATP] at a pCa of 9.0 was measured at several pH levels (pH 5.4-7.0). The ATPase activity as a function of [MgATP] was biphasic with a maximum at 8-10 microM MgATP. Lower pH did not result in a substantial decrease in myofibrillar ATPase activity even at pH 5.4. The extent of shortening, as measured by Z-line spacing, was greatest at 8 microM MgATP and less at both lower and higher [MgATP], and this response was observed at all pH levels. These studies suggest that the peak ATPase activity associated with low [MgATP] was coupled to sarcomere shortening. These results support the hypothesis that cross bridge-dependent activation of contraction may be responsible for contracture in the ischemic heart.

Acids↗
PubMed1992

Remodeling of myofibrils: subcellular distribution of myosin heavy chain mRNA and protein.

Myofibril remodeling occurs during the normal life, in growth and development, and in the special case when isoform switching occurs. In this review we concentrate on the ultrastructural aspects of how myosin is incorporated into the A-band. Anatomic methods of study are emphasized that include isotope and immunochemical labeling as well as in situ hybridization. We conclude that the mechanism of remodeling is one of continual orderly exchange between a monomeric myosin pool and the thick filament. Myosin mRNA distribution is intermyofibrillar and nonsarcomeric, which suggests that newly translated myosin is released before diffusion and insertion in the A-band of the myofibrillar lattice.

Animals↗
PubMed1976

Abnormalities in heart membranes and myofibrils during bacterial infective cardiomyopathy in the rabbit.

We studied hearts from sham-operated and uninfected catheterized rabbits as well as from rabbits at early and late stages of cardiomyopathy and failure after 3 and 6 days of infection with Streptococcus viridans. No ultrastructural abnormalities or biochemical changes in membrane and myofibrillar activities were seen in 3-day uninfected hearts. In 6-day uninfected hearts there were decreased sarcolemmal M2+ ATPase, Na+-K+ ATPase, adenylate cyclase and calcium binding, microsomal calcium binding and uptake, and myofibrillar Ca2+-stimulated ATPase as well as increased mitochondrial calcium uptake. Slight ultrastructural changes also were apparent in 6-day uninfected hearts. At both early and late stages of infective cardiomyopathy and failure there were varying degrees of depression in sarcolemmal Mg2+ ATPase, Na+-K+ ATPase, adenylate cyclase and calcium binding, microsomal calcium binding, calcium uptake and basal ATPase, and myofibrillar Ca2+-stimulated ATPase activities. However, sarcolemmal Ca2+ ATPase and myofibrillar Mg2+ ATPase activities were decreased only after 6 days of infection. Mitochondrial calcium binding and uptake were increased in early stages but decreased in late stages of disease. Furthermore in infected hearts there were defects in mitrochondrial respiration and phosphorylation. Generalized severe myocardial cell damage involving myofibrils, mitochondria, and the sarcotubular system was seen only in late stages of infection. The results demonstrate impairment of different membrane and contractile protein functions as well as ultrastructural abnormalities in bacterial cardiomyopathic hearts which were absent or of lesser magnitude in hearts with only hypertrophy. The findings reported here suggest to use that there is an association between heart failure and changes in function of cellular components during bacterial infective cardiomyopathy.

Adenosine Triphosphatases↗
PubMed1997

Constitutive and variable regions of Z-disk titin/connectin in myofibril formation: a dominant-negative screen.

The giant muscle protein titin (connectin) is assumed to play a crucial role in the control of Z-disk assembly. Analysis of the Z-disk region of titin/connectin revealed a novel 45 residue repeat that is spliced in variable copy numbers. The repeat region is coexpressed in normal human myocardium in size variants corresponding to between 5 and 7 repeats. Smaller isoforms can be detected in muscles with thinner Z-disks like M. psoas. Sequence analysis of chicken breast muscle titin/connectin reveals that in this tissue, where Z-disks are thin, only two Z-repeats are expressed. The greatly variable thickness of Z-disks is therefore correlated to a region of variable length in titin/connectin, constructed from a novel protein building block. Transfection experiments in myogenic cell lines demonstrate that overexpression of the entire integral Z-disk region of titin leads to a disruption of sarcomere assembly. The differentially expressed titin/connectin regions, however, show no dominant-negative effects on myofibril assembly and are not targeted to Z-disks. This supports the idea that the Z-repeat region of titin/connectin is responsible for the control Z-disk thickness as an element of highly variable length, due to extensive differential splicing.

Actins↗
PubMed2001

Ras/MAP kinase pathway is associated with the control of myotube formation but not myofibril assembly in quail myoblasts transformed with Rous sarcoma virus.

Tyrosine kinase activity of v-Src from Rous sarcoma virus (RSV) inhibits the differentiation of quail myoblasts. To clarify the inhibitory mechanism, we focused on the signaling pathways from v-Src. When the activation of the Ras/MAP (mitogen-activated protein) kinase pathway was inhibited by a dominant-negative mutant of Ras or PD98059, a specific inhibitor of p42 MAP kinase kinase, differentiation was restored; muscle specific proteins were expressed and myotubes formed even under active conditions of v-Src. Wortmannin, a specific inhibitor of phosphatidylinositol 3-kinase (P13-kinase), showed no effects on the inhibition by v-Src. These findings suggest that v-Src activates the Ras/MAP kinase signaling pathway, but not the P13-kinase pathway, and inhibits the differentiation. However, the myotubes derived from the dominant-negative Ras did not form actin fibers, suggesting that myofibril assembly is regulated by other pathway(s) from v-Src.

Animals↗
PubMed1977

Electron microscopic studies on the myofibrils in the epithelial cells of the Bowman's capsule and of proximal tubules in rat renal cortex (first report).

Two kinds of filaments in parallel arrangement have been observed in the basal part of the cytoplasm in the epithelial cells of a Bowman's capsule and in the epithelial cells of a proximal tubule of a glycerinated rat kidney. The first was thin and linear in shape, 55-70 A in diameter and exhibited arrow-head structures upon treatment with heavy meromyosin (HMM). The other was rod shaped, with thick filaments, 110-130 A in diameter, 0.14-0.16 micrometer in length, few in number scattered among the thin filaments. From the non-glycerinated kidney, bundles of fibers were attached to some electron-dense regions inside the cell membrane. In highly magnified pictures, two kinds of filaments could be observed. One kind was thin, attached to dense regions, and the other thick, and not attached to dense regions. It is conceivable that these filaments correspond to myofibrils from their characteristic electron microscopical features. An electron microscopical analogy detected in both smooth muscle cells and renal cells leads us to speculate that these thin and thick filaments consisted of actin and myosin respectively.

Animals↗
PubMed2005

Cardiac myofibril formation is not affected by modification of both N- and C-termini of sarcomeric tropomyosin.

Although the role of tropomyosin is well-defined in striated muscle, the precise mechanism of how tropomyosin functions is still unclear. It has been shown that extension of either N- or C-terminal ends of sarcomeric tropomyosin do not affect cardiac myofibrillogenesis, but it is not known whether simultaneous extension of both ends affects the process. For studying structural/functional relationships of sarcomeric tropomyosin, we have chosen the Ambystoma mexicanum because cardiac mutant hearts are deficient in sarcomeric tropomyosin. In this study, we have made an expression construct, pEGFP.TPM4alpha.E-L-FLAG, that, on transfection into normal and mutant axolotl hearts in organ culture, expresses GFP.TPM4alpha.E-L-FLAG fusion protein in which both the N- and C-termini of TPM4alpha are being extended. TPM4alpha is one of the three tropomyosins expressed in normal axolotl hearts. Both confocal and electron microscopic analyses show that this modified sarcomeric tropomyosin can form organized myofibrils in axolotl hearts.

Ambystoma mexicanum↗
PubMed1996

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and western blotting comparisons of purified myofibrils and whole muscle preparations for evaluating titin and nebulin in postmortem bovine muscle.

Purified myofibril (MF) and homogenized whole muscle (WM) samples were prepared from A maturity market steers. Samples were removed at 0, 1, 3, 7, 14, and 28 d postmortem. The MF and WM samples from all steers were analyzed by SDS-PAGE (5% gels) and by Western blot analysis using monoclonal antibodies to titin and nebulin. The rates of degradation of the intact forms of titin and nebulin, with regard to differences dependent on sample type (MF vs WM), were examined. The results showed that there was very little difference in the rate of postmortem degradation of the intact form of titin or of intact nebulin with respect to the two types of samples examined. Analysis of MF and WM preparations revealed that titin and nebulin were progressively degraded, each at its own rate, with nebulin degrading faster, as postmortem storage time increased. Examination of MF and WM samples showed that the intact form of titin (T1) was absent at the same time postmortem in both sample types. Intact nebulin was not detected in MF and WM preparations at the same time postmortem with respect to sample type examined. Our results indicate that either purified MF or WM samples can be used satisfactorily to analyze the rate of degradation of the intact forms of both titin and nebulin.

Animals↗
PubMed2001

Technical note: comparison of myofibril fragmentation index from fresh and frozen pork and lamb longissimus.

The myofibril fragmentation index (MFI) is strongly associated with indices of meat tenderness, such as Warner-Bratzler shear force and sensory tenderness. The MFI is normally determined on fresh muscle. It is not known whether this index can be determined on frozen muscle. The objective of this experiment was, therefore, to determine whether there is a difference between MFI values of fresh and frozen lamb and pork longissimus. To compare the effect of freezing on MFI, longissimus samples were obtained from eight lamb carcasses at 1, 3, and 15 d postmortem and longissimus samples were obtained from 12 pork carcasses at 3 d postmortem. For each sample, MFI was conducted on both fresh muscle and snap-frozen muscle (frozen in liquid nitrogen and stored 23 to 26 d at -70 degrees C). The R2 between MFI of fresh and frozen muscle was 0.94 and 0.92 for lamb and pork longissimus, respectively. The differences between fresh and frozen MFI were not significant for either species (P > 0.05). These results indicate that it is not necessary to determine MFI on fresh muscle.

Animals↗
PubMed2004

[Loss of the myofibrils as the manifestation of relative dedifferentiation of hypertrophied cardiomyocytes].

The material of intraoperational biopsies of right ventricle obtained from 54 patients aged 9-40 years with Fallot's tetrad was studied to analyze the changes of cardiomyocyte (CMC) ultrastructure and dimensions in the process of their hypertrophy. The results of the present investigation and the analysis of the literature indicate, that CMC ultrastructural remodeling in course of their hypertrophy follows the pattern of progressive dedefferentiation. Therefore, the loss of the myofibrils at the advanced stages of hypertrophy is not the manifestation of CMC degeneration, but is the result of their relative dedifferentiation. Relative dedifferentiation combines the processes of remodeling of hypertrophied CMC and CMC in a state of chronic hibernation. CMC dedifferentiation in the course of these processes may have a common nature and may reflect the variants of a single general biological phenomenon.

Adolescent↗