Fine structural localization of adenosinetriphosphatase activity in heart muscle myofibrils

Activity of myofibrillar adenosinetriphosphatase was demonstrated histochemically at a fine structural level in isolated, unfixed or hydroxyadipaldehyde-fixed cardiac myofibrils in the rat, using a lead precipitation technique and either Ca⁺⁺ or Mg⁺⁺ as activating ion. Activity in relaxed myofibrils was found in the A band, but not the H, I, or Z bands. Deposits of final product frequently exhibited an axial periodicity of near 365 A, and bore a close relationship to filaments within the A band. Several patterns of distribution occurred in contracted myofibrils. In myofibrils which had shortened to the point of disappearance of the I band, final product was distributed throughout the sarcomere, except for the unreactive Z band. A second type of distribution occurred in strongly contracted fibers in which there was intensification of activity in the center of the sarcomere. These findings are discussed in the light of the recent morphological evidence and it is suggested that the distribution of final product is consistent with localization of enzyme activity to the cross-bridges between the thick and thin filaments.

The fine structure of some blood vessels of the earthworm, Eisenia foetida

The fine structure of the main dorsal and ventral circulatory trunks and of the subneural vessels and capillaries of the ventral nerve cord of the earthworm, Eisenia foetida, has been studied with the electron microscope. All of these vessels are lined internally by a continuous extracellular basement membrane varying in thickness (0.03 to 1 µ) with the vessel involved. The dorsal, ventral, and subneural vessels display inside this membrane scattered flattened macrophagic or leucocytic cells called amebocytes. These lie against the inner lining of the basement membrane, covering only a small fraction of its surface. They have long, attenuated branching cell processes. All of these vessels are lined with a continuous layer of unfenestrated endothelial cells displaying myofilaments and hence qualifying for the designation of "myoendothelial cells." The degree of muscular specialization varies over a spectrum, however, ranging from a delicate endowment of thin myofilaments in the capillary myoendothelial cells to highly specialized myoendothelial cells in the main pulsating dorsal blood trunk, which serves as the worm's "heart" or propulsive "aorta." The myoendothelial cells most specialized for contraction display well organized sarcoplasmic reticulum and myofibrils with thick and thin myofilaments resembling those of the earthworm body wall musculature. In the ventral circulatory trunk, circular and longitudinal myofilaments are found in each myoendothelial cell. In the dorsal trunk, the lining myoendothelial cells contain longitudinal myofilaments. Outside these cells are circular muscle cells. The lateral parts of the dorsal vessels have an additional outer longitudinal muscle layer. The blood plasma inside all of the vessels shows scattered particles representing the circulating earthworm blood pigment, erythrocruorin.

The nature of the muscle-relaxing factor. II. Some physicochemical properties

High speed centrifugal fractionation of homogenates of rabbit skeletal muscle has led to the discovery of a soluble muscle-relaxing factor in the homogenate. Assay of the relaxing activity with deoxycholate-treated myofibrils and reconstituted actomyosin systems has established that the activity is not produced by the presence of contaminants. Relaxing activity could be removed or destroyed by charcoal, dialysis, prolonged heating, and treatment with the chelating resin, chelex-100, making it improbable that the effect is due simply to calcium deficiency. Many of the characteristics of this muscle-relaxing factor suggest that it is very similar to or the same as the factor formed by the incubation of muscle granule fractions and ATP. Evidence is presented that some soluble protein component is involved in the stabilization of the factor. The relaxing activity could be separated from the high molecular weight material in the supernatant by the technique of gel filtration. On the basis of the gel used, the molecular weight of the active agent should be less than 4000.

Electron microscopic studies on the indirect flight muscles of Drosophila melanogaster. I. Structure of the myofibrils

The myofibrils in Drosophila have thick and thin types of myofilaments arranged in the hexagonal pattern described for Calliphora by Huxley and Hanson (15). The thick filaments, along most of their length in the A band, seem to be binary in structure, consisting of a dense cortex and a lighter medulla. In the H zone, however, they show more uniform density; lateral projections (bridges) also appear to be absent in this region. The M band has a varying number of granules (probably of glycogen) distributed between the myofilaments. The myofilaments on reaching the Z region appear to change their hexagonal arrangement and become connected to one another by Z filaments. The regular arrangement of the filaments found in most regions of the fibrils is not seen in the terminal sarcomeres of some flight muscles; the two types of filaments appear to be intermingled in an irregular pattern in these parts of the fibrils. The attachment of myofibrils to the cuticle through the epidermal cells is described.

On the structural continuities of the transverse tubular system of rabbit and human myocardial cells

An electron microscopic study of rabbit and human myocardium provides further evidence of the existence of two distinct components of the sarcoplasmic reticulum. A thin-walled tubular system (termed longitudinal system) is arranged in anastomosing channels sur-surrounding each sarcomere and has transverse and possibly also longitudinal connections with the tubules of adjacent sarcomeres. A thick-walled tubular system traverses the myofiber transversely at the level of the Z lines of the myofibrils. The structure of these tubules very closely resembles that of deep sarcolemmal invaginations. Indeed, the membranes of the tubules appear to be continuous with the sarcolemma in favorable sections so that there seems to be an extension of the cell membrane and extracellular fluid to all depths of the myocardial fiber. Certain physiologic data which support this concept are discussed. The calculations of A. V. Hill comparing the kinetics of diffusion and the time-distance relationships between excitation and activation in frog sartorius muscle are reconsidered for cardiac muscle.

Differential responses to endurance training in subsarcolemmal and intermyofibrillar mitochondria

To examine the effect of endurance training (6 wk of treadmill running) on regional mitochondrial adaptations within skeletal muscle, subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria were isolated from trained and control rat hindlimb muscles. Mitochondrial oxygen consumption (VO2) was measured polarographically by using the following substrates: 1 mM pyruvate + 1 mM malate (P+M), 10 mM 2-oxoglutarate, 45 microM palmitoyl-DL-carnitine + 1 mM malate, and 10 mM glutamate. Spectrophotometric assays of cytochrome-c reductase and NAD-specific isocitrate dehydrogenase (IDH) activity were also performed. Maximal (state III) and resting (state IV) VO2 were lower in SS than in IMF mitochondria in both trained and control groups. In SS mitochondria, training elicited significant 36 and 20% increases in state III VO2 with P+M and glutamate, respectively. In IMF mitochondria, training resulted in a smaller (20%), yet significant, increase in state III VO2 with P+M as a substrate, whereas state III VO2 increased 33 and 27% with 2-oxoglutarate and palmitoyl-DL-carnitine + malate, respectively. Within groups, cytochrome-c reductase and IDH activities were lower in SS when compared with IMF mitochondria. Training increased succinate-cytochrome-c reductase in both SS (30%) and IMF mitochondria (28%). IDH activity increased 32% in the trained IMF but remained unchanged in SS mitochondria. We conclude that endurance training promotes substantial changes in protein stoichiometry and composition of both SS and IMF mitochondria.

Beta-tropomyosin overexpression induces severe cardiac abnormalities

Tropomyosin, a coiled-coil dimer, stabilizes actin filaments and is central to the control of calcium-regulated striated muscle contraction. Striated muscle-specific alpha-tropomyosin is the predominant isoform in cardiac muscle, with low levels of beta-tropomyosin restricted to fetal development in the mouse. To understand the functional role of various tropomyosin isoforms during myofilament activation and regulation in the intact sarcomere, we generated transgenic mice that overexpress striated muscle-specific beta-tropomyosin in the adult heart. Our earlier results succinctly demonstrate that overexpression of beta-tropomyosin in the hearts of transgenic mice decreases endogeneous alpha-tropomyosin levels while altering diastolic function of the myocardium. To explore further the significance of altering the alpha- to beta-tropomyosin isoform ratio in developing murine myocardium, we generated transgenic mice which express beta-tropomyosin at high levels in the heart. The data show that higher levels of beta-tropomyosin expression are lethal with death ensuing between 10-14 days postnatally. A detailed histological analysis demonstrates that the hearts of these mice exhibit several pathological abnormalities, including thrombus formation in the lumen of both atria and in the subendocardium of the left ventricle. Other changes include atrial enlargement and fibrosis, and diffuse myocytolysis, Physiological analyses using ventricular muscle strip preparations from these mice reveal that both myocardial contraction and relaxation parameters are severely impaired. Thus, these results firmly demonstrate an essential difference in tropomyosin isoform function in physiologically regulating cardiac performance.

Microfabricated cantilevers for measurement of subcellular and molecular forces

We present two new microfabricated cantilever-beam force transducers. The transducers were fabricated from thin silicon-nitride films, and were used respectively to measure forces generated by two small-muscle preparations: the single myofibril, and the single actin filament in contact with a myosin-coated surface. A simple resonance method was developed to characterize the transducers. Because of the high reproducibility of lever dimensions and the consistency of the modulus of elasticity, few calibration measurements sufficed to characterize the stiffness of all the levers on a single wafer.

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.

Myofibrillar myopathy. III. Abnormal expression of cyclin-dependent kinases and nuclear proteins

The pathological process in myofibrillar myopathy (MFM) (previously also referred to as "desmin storage" or "intermediate filament myopathy") results in dissolution of myofibrils, accumulation of products of the degradative process, and abnormal ectopic expression of desmin, dystrophin, gelsolin, NCAM, and N-terminal components of beta-amyloid precursor protein. We now demonstrate that the abnormal fiber regions in MFM immunoreact strongly for (a) CDC2 kinase, the mitotic kinase that phosphorylates and disassembles intermediate filaments; (b) cyclin-dependent kinases CDK2, CDK4, and CDK7, which are involved in regulation of the cell cycle; (c) lamin B, which normally supports the inner nuclear membrane; and (d) the nuclear matrix associated protein. The normal muscle fiber lies in a terminally differentiated state and is refractory to reentry into the cell cycle. The abnormal expression of multiple cyclin-dependent kinases in the terminally differentiated muscle fiber implies inappropriate activation of positive regulators of mitosis and may signal a mitotic catastrophe. The dissolution of myofibrils may be due to hyperphosphorylation occurring during this event.

Orientation of cross-bridges in skeletal muscle measured with a hydrophobic probe

Cis-parinaric acid (PA) binds to a hydrophobic pocket formed between the heavy chain of myosin subfragment-1 (S1) and the 41-residue N-terminal of essential light chain 1 (A1). The binding is strong (Ka = 5.6 x 10(7) M-1) and rigid (polarization = 0.334). PA does not bind to myofibrils in which A1 has been extracted or replaced with alkali light chain 2 (A2). As in the case of S1 labeled with other probes, polarization of fluorescence of S1-PA added to myofibrils depended on fractional saturation of actin filament with S1, i.e., on whether the filaments were fully or partially saturated with myosin heads. Because fluorescence quantum yield of PA is enhanced manyfold upon binding, and because PA binds weakly to myofibrillar structures other then A1, the dye is a convenient probe of cross-bridge orientation in native muscle fibers. The polarization of a fiber irrigated with PA was equal to the polarization of S1-PA added to fibers at nonsaturating concentration. Cross-linking of S1 added to fibers at nonsaturating concentration showed that each S1 bound to two actin monomers of a thin filament. These results suggest that in rigor rabbit psoas muscle fiber each myosin cross-bridge binds to two actins.

Modulation of cross-bridge affinity for MgGTP by Ca2+ in skinned fibers of rabbit psoas muscle

Previously we reported that saturation of cross-bridges with MgATP gamma S in skinned muscle fibers was calcium sensitive. In the present study we investigate whether this observation can be generalized to other nucleotides by studying saturation of cross-bridges with MgGTP. In solution, myosin-subfragment 1 (S1) in the presence of 10 mM MgGTP was found to bind to actin with low affinity, similar to that in the presence of MgATP and MgATP gamma S. In EGTA buffer, the equatorial x-ray diffraction intensity ratio I11/I10 recorded in single skinned fibers decreased upon increasing MgGTP concentration from 0 to 10 mM (1 degree C and 170 mM ionic strength). The I11/I10 ratio leveled off at 10 mM MgGTP, indicating full saturation of cross-bridges with the nucleotide. Under these conditions, the value of I11/I10 is indistinguishable from that obtained in the presence of saturating [MgATP]. In CaEGTA buffer, however, the decrease in I11/I10 occurs over a wider range of concentrations, and there is no indication of I11/I10 leveling off at 10 mM MgGTP, suggesting that full saturation is not reached. The Ca2+ dependence of GTP binding appears to be a direct consequence of the differences in the affinities of the strongly bound cross-bridges to actin versus weakly bound cross-bridges to actin. A biochemical scheme that could qualitatively explain the titration behavior of ATP gamma S and GTP is presented.

WATER TRANSFER AND CELL STRUCTURE IN ISOLATED CRAYFISH MUSCLE FIBERS

Changes in volume of crayfish single muscle fibers in response to changes in ionic or electrical conditions have been studied in conjunction with electrophysiological measurements and electron microscopic examinations. The occurrence of at least three mechanisms of water movements is revealed, two being processes which are superimposed on the normal osmotic water movement that results from a change in the concentration of solute in the medium. Differences between the time courses of the changes in volume and potential on changing K_i/K_o indicate that water may be distributed unequally for a time within compartments of the fiber. Electron micrographs reveal a selective accumulation of water at the periphery of the fiber under certain conditions. A correlation of H₂O transfer with a change in membrane potential is apparent in crayfish muscle fibers and is probably due to electroosmotic effects. Electrokinetic water movements are produced whenever the membrane potential is changed to a considerable degree by changing the level of K and/or Cl in the medium, or by applying currents with an intracellular microelectrode. Depolarizations cause shrinkage. Hyperpolarizations or repolarizations cause swelling. The volume changes are independent of the occurrence or absence of swelling in the anion-permselective transverse tubular system. They indicate that the fiber membrane along the surface is heterogeneous, not only with respect to the signs of its fixed charge sites, but also with respect to the sizes and relative permselectivities of these charged channels.

ISOLATION OF RELAXING PARTICLES FROM RAT SKELETAL MUSCLES IN ZONAL CENTRIFUGES

Supernatants of rat skeletal muscle homogenates were fractionated by differential centrifugation and by zonal centrifugation in sucrose density gradients. Cytochrome oxidase was employed as an enzymatic marker for locating mitochondria. The subcellular fractions were also assayed for their ability to prevent the ATP-induced contraction of myofibrils. Both the mitochondrial and microsomal fractions obtained by differential fractionation were found to be rich in such relaxing activity, and the microsomal fraction was appreciably contaminated by mitochondria. In contrast to this, when fractionation was carried out by means of zonal centrifugation (4200 RPM x 205 min. to 40,000 RPM x 60 min.), relaxing activity was found to be associated only with particles having the sedimentation characteristics of microsomes (s_20,w estimated to be between 370 and 1880S). Relaxing activity was not detected in the regions of the gradient containing either the starting sample zone (soluble phase) or the mitochondrial peak. The microsomal relaxing particles showed negligible cytochrome oxidase activity.

OBSERVATIONS ON THE FINE STRUCTURE OF PHARYNGEAL MUSCLE IN THE PLANARIAN DUGESIA TIGRINA

Pharyngeal muscle of the planarian Dugesia tigrina was studied by electron microscopy after osmium tetroxide fixation. The muscle cell was observed to contain one myofibril or bundle of myofilaments parallel to its longitudinal axis. The myofilaments were of two types, different in size and distribution. No Z lines or myofilament organization into cross or helical striations were seen. Dense bodies were seen as projections from an invagination of the plasma membrane and as dense lines parallel to the myofilaments. The muscle cells are surrounded by a plasma membrane which is structurally associated with dense body projections, with vesicles and cisternae of sarcoplasmic reticulum, and with synaptic nerve endings. The cell has sarcoplasmic projections perpendicular to its long axis; these projections are seen to contain the nucleus or mitochondria and granules. Mitochondria and granules are also seen in a sarcoplasm rim around the fibril. The dense bodies may serve as attachment for thin myofilaments and function in transmission of stimuli from plasma membrane to the interior of the fibril.

AN ANALYSIS OF THE STRIATED MUSCLE FIBER ACTION CURRENT

A method is presented for determining the magnitude of the ionic current associated with the propagated spike potential. Parameters of the action current are then compared to various aspects of the response as recorded in a phase plane. The findings also include evidence for (a) Na(+) ion as the major inward current carrier, (b) a fairly constant membrane conductance during the terminal phase ( approximately 1 msec.) of the spike potential, and (c) the influence of Ca(++) ion concentration on the action current.

SARCOLEMMAL INVAGINATIONS CONSTITUTING THE T SYSTEM IN FISH MUSCLE FIBERS

Striated muscle fibers from the body and tail myotomes of a fish, the black Mollie, have been examined with particular attention to the sarcoplasmic reticulum (SR) and transverse tubular (or T) system. The material was fixed in osmium tetroxide and in glutaraldehyde, and the images provided by the two kinds of fixatives were compared. Glutaraldehyde fixes a fine structure that is broadly comparable with that preserved by osmium tetroxide alone but differs in some significant details. Especially significant improvements were obtained in the preservation of the T system, that is, the system of small tubules that pervades the fiber at every Z line or A-I junction level. As a result of this improved glutaraldehyde fixation, the T system is now clearly defined as an entity of fine structure distinct from the SR but uniquely associated with the SR and myofibrils. Glutaraldehyde fixation also reveals that the T system is a sarcolemmal derivative that retains its continuity with the sarcolemma and limits a space that is in direct communication with the extracellular environment. These structural features favor the conclusion that the T system plays a prominent role in the fast intracellular conduction of the excitatory impulse. The preservation of other elements of muscle fine structure, including the myofibrils, seems for reasons discussed, to be substantially improved by glutaraldehyde.

SMOOTH MUSCLE: AN ULTRASTRUCTURAL BASIS FOR THE DYNAMIC OF ITS CONTRACTION

Electron micrographs of vertebrate and invertebrate smooth muscle indicate that the myofilaments are oriented obliquely to the long axis of the muscle fibers containing them and insert along the sides of the fibers. As a result, a greater proportion of the contractile elements are in parallel with one another and a smaller proportion are in series than would be possible if the myofilaments were strictly parallel to the fiber axis. From this ultrastructural organization it is possible to predict several well-known, but previously unexplained, physiological properties of smooth muscle.

THE RELATION BETWEEN THE LATE AFTER-POTENTIAL AND THE SIZE OF THE TRANSVERSE TUBULAR SYSTEM OF FROG MUSCLE

This is an investigation of the effects on the late after-potential of immersing frog sartorius muscles in three kinds of modified Ringer's fluid; hypertonic, low chloride, and potassium-free. The late after-potential has been attributed to the depolarizing effect of an accumulation of potassium, during a preceding train of impulses, in the intermediary space of the model of a muscle fiber proposed by Adrian and Freygang. Both the hypertonic and low chloride solutions prolonged the late after-potential reversibly and the potassium-free solution shortened it. The effect of the low potassium solution fitted those data calculated from the model, but the effect of the hypertonic and low chloride solutions required an increase in size of the intermediary space of the model in order to fit the calculated data. An electron microscopic study of the muscles showed that the size of the transverse tubular system changed reversibly in the hypertonic and low chloride solutions in almost the amount necessary to fit the experimental data to the calculated data. This agreement between the change in size of the transverse tubular system and that of the intermediary space indicates that the intermediary space may be the transverse tubular system.

SPECIFIC GRANULES IN ATRIAL MUSCLE CELLS

Large populations (up to 600/cell) of spherical, electron-opaque granules ∼0.3 to 0.4 µ in diameter are characteristically found in muscle fibers of mammalian atria. They are absent in muscle fibers of the ventricles. The granules are concentrated in the sarcoplasmic core and occur in lesser numbers in the sarcoplasmic layers between myofibrils and under the plasma membrane. Their intimate association with a central voluminous Golgi complex and the frequent occurrence of material reminiscent of the granular content within the cisternae of the Golgi complex suggest that the latter is involved in the formation of the atrial granules. Atrial granules are larger and more numerous in smaller species (rat, mouse), and generally smaller and less numerous in larger mammals (dog, cat, human); they are absent from the atrial fibers of very young fetuses (rat) but are present in those of newborn animals. A small population of bodies containing glycogen particles and remnants of the endoplasmic reticulum and mitochondria occurs in the sarcoplasmic cores of atrial as well as ventricular muscle fibers in the rat; they contain acid phosphatase and thus appear to be residual bodies of autolytic foci. Their frequency increases with the age of the animal. Typical lipofuscin pigment granules, which are known to contain acid phosphatase and are found in the sarcoplasmic cores in old animals (cat, dog and human), are presumed to arise by progressive aggregation and fusion of small residual bodies.

INTRACELLULAR DISTRIBUTION OF CALCIUM IN DEVELOPING BREAST MUSCLE OF NORMAL AND DYSTROPHIC CHICKENS

To follow the intracellular distribution of calcium in the breast muscles of developing chickens, Ca(45) was injected into the albumen of predeveloped eggs. Since the embryos were grown in a radioactive medium, a complete exchange of the isotope for its non-radioactive counterpart in muscles was accomplished. Subcellular particulates of the muscle cells were separated by the method of differential centrifugation. Analysis of the separated fractions showed that in the muscles of the 13-day embryo, when the nuclear-myofibrillar ratio is high, 65 per cent of the muscle calcium is in the nuclei. With the increased synthesis of myofibrils, the nuclear-myofibrillar ratio decreases with a concomitant fall in radioactivity. Thus, calcium was not associated with the developing myofibrils. At the time of hatching, when myofibrils perform physiological work, the highest level of calcium is in the mitochondria. This suggests that the mitochondria play a key role in the physiological activities of calcium in the cell. The microsomal fraction reaches a maximal level of calcium when the adult composition of muscle is attained. Results of investigations on dystrophic muscles show changes in the calcium distribution of the fractions as early as the 3rd week of embryonic development, which are interpreted to indicate an alteration in the protein metabolism of the cell, or an early destruction of muscle tissue. Further, alterations in the calcium content of fractions which seem to regulate the movements of this ion in the cell are discussed. A new technique for homogenizing tissues from embryos of different ages is presented.