Ca 2+ -specific removal of Z lines from rabbit skeletal muscle

Ultrastructural changes after concentric and eccentric contractions of human muscle

Four normal subjects performed a 20 min step test using a step of the same relative height. During the test the quadriceps muscle of one leg contracted concentrically throughout by stepping up, while the contralateral muscle contracted eccentrically by controlling the step down. Thus both muscles performed the same amount of work. Three subjects had bilateral needle biopsies just prior to exercise. All four had bilateral biopsies immediately after exercise, and 24-48 hours later when the muscles which had contracted eccentrically were painful. The samples were examined by light and electron microscopy. No abnormalities were seen in pre-exercise samples nor after exercise in muscles which had contracted concentrically. The muscles which had contracted eccentrically showed some damage immediately after exercise. In the samples taken 24-48 hours after exercise the damage was more marked and involved a greater percentage of fibres. In view of the known differences between these types of contractions it is suggested that the initial damage is mechanically induced. The exacerbation of damage with time could be due to mechanical or chemical factors.

Properties of soleus muscle Z-lines and induced Z-line analogs revealed by dissection with Ca2+-activated neutral protease

Rat soleus muscle Z-lines and Z-line anomalies induced by neostigmine methyl sulfate (NMS) and cat soleus muscle Z-lines and Z-line anomalies induced by tenotomy were examined by electron microscopy before and after dissection of muscle fibers with Ca2+-activated neutral protease (CAF) to elucidate structural properties of Z-lines and related Z-line-type structures. In both normal and treated muscles, interdigitation of thin (6-7 nm) filaments, which were continuous with I-filaments (actin) from adjacent sarcomeres, was observed at the Z-line in longitudinal section. Both neostigmine methyl sulfate and tenotomy treatments induced muscle atrophy associated with Z-line degradation, streaming, and irregular distribution and accumulation of Z-line material and Z-rod formation. Tenotomized muscle also was characterized by the presence of N-line-like bands and I-Z-I brushes. CAF digestion removed the electron-dense covering material from Z-rods and revealed a backbone of actin filaments. The origin of Z-rods, their structural similarity to Z-lines in longitudinal and cross section, and their susceptibility to CAF indicate that Z-rods are directly related to native Z-lines and are probably lateral polymers of a basic Z-line unit. The regular square net alignment (22 nm) of I-filaments (actin) in cross sections of I-Z-I brushes which contain no N-lines suggests that the I-square net arrangement near the Z-line is determined by Z-filament-actin filament interaction rather than by the N-line or other factors. The results suggest that I-filaments (actin) penetrate the mammalian Z-line and are Z-line constituents and that the width of Z-lines and the length of Z-rods are determined by the amount of overlap of actin filaments. The perpendicular periodicity of Z-rods and the zigzag-oblique arrowheadlike appearance seen in longitudinal sections of Z-lines are attributed to alpha-actinin.

Identification of filaggrin in cultured mouse keratinocytes and its regulation by calcium

Filaggrin is a histidine-rich cationic protein present in cells of the stratum corneum in vivo and derived from a precursor in keratohyalin granules. Biochemical and immunologic methods were used to determine the presence of filaggrin in keratinocytes cultured in vitro and induced to differentiate by increasing the extracellular calcium concentration from 0.07 to 1.2 mM. Indirect immunofluorescence using antibody to rat filaggrin was negative in cells cultured in low-calcium medium but positive in cells switched to high-calcium medium. Large immunofluorescent granules were identified in a perinuclear distribution starting at 6 hours after the shift in calcium concentration, coinciding with the time of appearance of phase-dense cytoplasmic granules. Radiolabeled histidine was preferentially incorporated into proteins of 95, 37, and 27 K. The 37 and 27 K bands were not adsorbed by DE52 cellulose and therefore are cationic. A 27 K cationic, histidine-labeled protein was readily extracted from frozen pellets of cells cultured in high-calcium medium. It comigrates with purified mouse filaggrin (27 K) and reacts with antibody to rat filaggrin on immunoautoradiography. Only trace amounts of this protein could be detected in cells cultured in low-calcium medium. Our observation of filaggrin-immunoreactive granules confirms the previous ultrastructural identification of keratohyalin granules after the shift to high-calcium medium. The results suggest that filaggrin synthesis is stimulated in keratinocytes induced to differentiate by the shift to high extracellular calcium concentration.

Limited benefit to genetically dystrophic chickens from a synthetic proteinase inhibitor: Ep475

Chickens with inherited muscular dystrophy (Line 413) were treated in two separate trials with daily intraperitoneal injections of 10% DMSO-water solutions containing the proteinase inhibitors, Ep475 and E64. Drug therapy in each case significantly prolonged the functional ability of the treated chickens. Diluent control chickens around day 35 ex ovo characteristically reached a maximum ability to right from the supine position in a standardized functional test for muscle weakness. Subsequently, the control chickens were found to decline progressively in their ability to right. Treatment with the proteinase inhibitors had no effect on the typically elevated levels of plasma creatine kinase activity. In a histological analysis of the affected pectoralis major muscle, drug treatment had no effect on the relative distribution of degenerating, and vacuolated fibers, inflammatory cells, and abnormal fiber diameters. An exception was seen in decreased necrotic fibers of chickens treated with high doses of Ep475. Moreover, both inhibitors had positive effects on two biochemical abnormalities common to the dystrophic pectoralis muscle: increase in noncollagen protein, and reduction in total calcium.

Muscle fiber necrosis associated with human marathon runners

This study describes the events occurring in exercise-induced muscular necrosis. Biopsies of the gastrocnemius muscles of volunteer human marathon runners were extracted prior to and at intervals for 7 days following a marathon, and investigated ultrastructurally. Most of the preparations, including the pre-marathon samples, showed evidence of muscle fiber necrosis and inflammation. These preparations had many mitochondria, erythrocytes, leukocytes and other phagocytic cells within the extracellular and extravascular spaces. Less frequently observed were Z-line streaming and degeneration, contracture knots, disrupted sarcolemma, presence of erythrocytes within the muscle fibers, and empty basal lamina tubes in which the contents of the fibers and the sarcolemma had broken down to leave only the basal lamina outlining the former fiber. These abnormal conditions were most prevalent at 1 and 3 days after the marathon. These ultrastructural changes are compared and correlated with the reports of clinical manifestations of rhabdomyolysis and myoglobinuria. Because the abnormalities persist for the 7 day duration of these observations, and because many of these were observed in the pre-marathon biopsies, we conclude that both the intensive training for, and the marathon itself, induce inflammation and fiber necrosis which are manifested in the clinical symptoms for rhabdomyolysis and myoglobinuria. The inflammatory reaction that accompanies these activities may be a major factor in post-exercise soreness. The combined influences of training and necrosis are discussed in relation to muscle fiber type compositions of endurance athletes.

Mitochondrial calcium content and oxidative phosphorylation in heart and skeletal muscle of dystrophic mice

Mitochondrial calcium overloading was investigated in the genetically dystrophic mouse (strains 129/ReJ dy/dy) as a possible contributing factor to the development of muscle fiber necrosis. Mitochondrial calcium concentrations were significantly elevated in both skeletal muscle and heart organelles. Because mitochondria were isolated in the presence of ruthenium red this finding was not the result of an artefact of isolation. State 3 respiration rates and concomitantly the respiratory control ratios were slightly decreased in skeletal muscle, but not in heart mitochondria. This abnormality could result from calcium overloading in a small fraction of the mitochondria. Fractionation of skeletal muscle mitochondria on sucrose gradients gave two distinct populations of dystrophic organelles, one with high calcium, whereas normal skeletal muscle mitochondria and heart organelles showed only one broad band on the gradient. The results support the idea that both skeletal muscle and heart are affected in dystrophic mice, strain 129/ReJ dy/dy and also that in the dystrophic mouse the process of cell necrosis is associated with cellular calcium overloading.

Stringent requirement for Ca2+ in the removal of Z-lines and alpha-actinin from isolated myofibrils by Ca2+-activated neutral proteinase

Treatment of isolated myofibrils with Ca2+-activated neutral proteinase (CANP) results in specific removal of Z-line and of alpha-actinin. To investigate the ionic requirement for these processes, we measured Z-line removal by phase-contrast and interference microscopy and alpha-actinin removal by sodium dodecyl sulphate/polyacrylamide-gel electrophoretic analysis of myofibrillar proteins. The proteolytic digestion of native purified proteins was measured directly on polyacrylamide gels and by the fluorescamine technique. We found that the removal of Z-line and alpha-actinin as well as the release of proteolytic degradation products from isolated myofibrils by CANP occur only in the presence of Ca2+; Sr2+, Ba2+, Mn2+, Mg2+, Co2+ and Zn2+ are all ineffective. In contrast with this stringent requirement for Ca2+, the proteolytic activity of CANP measured with denatured casein, native and denatured haemoglobin, native actin and tropomyosin also occurs in the presence of other bivalent cations, in the following order: Ca2+ greater than Sr2+ greater than Ba2+. These data suggest that only Ca2+ can produce the conformational change in myofibrils that renders them susceptible to the action of CANP, whereas its proteolytic activity is stimulated by several bivalent ions.

Evidence for actin involvement in cardiac Z-lines and Z-line analogues

Canine and feline cardiac Z-lines and Z-rods were examined by electron microscopy before and after digestion of muscle fibers with Ca2+-activated protease (CAF). Removal by CAF of electron-dense material which covers Z-lines and Z-rods exposed interdigitating longitudinal filaments (6-7 nm in diameter) apparently continuous with thin filaments of the respective I-bands. The newly exposed longitudinal filaments of CAF-treated Z-lines and of CAF-treated Z-rods bound heavy meromyosin and therefore are actin. The width of Z-lines and length of Z-rods are determined by the amount of overlap of actin filaments of opposite polarity. The oblique filaments in Z-lines and Z-rods are responsible for the perpendicular periodicity of Z-lines and Z-rods, and are attributed to alpha-actinin.

Calcium-dependent proteolysis in neural tissue is activated at physiologic intracellular Ca2+ levels and inhibited by some anticonvulsants

The level of calcium-activated neutral protease (CANP) activity in the brain and nerve cord of the crayfish Procambarus clarkii was assayed by measuring the degradation of casein yellow by tissue homogenates. When care was taken to maintain the ionic strength of all incubation media at 0.15M and to buffer the Ca2+ activity of the media with 5mM EGTA, CANP was found to be very sensitive to Ca2+; maximal activity was achieved at 1 X 10(-3)M Ca2+, with 50% of this maximum present at the physiologic intracellular Ca2+ activity of 1 X 10(-7)M. We found that the anticonvulsant agent phenytoin was without effect on CANP activity while pentobarbital and a relatively new anticonvulsant agent, valproic acid (an eight-carbon branched fatty acid), significantly inhibited CANP activity in a dose-dependent manner. The inhibitory effect of valproic acid was shared by a straight-chain eight-carbon fatty acid, caprylic acid. These findings demonstrate that inhibition of CANP activity is not limited to agents with a specific molecular structure. They also suggest that CANP plays a role in the normal turnover of proteins that control various cellular functions.

Calcium binding proteins and cellular regulation

An important feature of cellular regulation is the precise control of intracellular calcium levels. This is accomplished both by dynamic organelle release and sequestration of calcium and by specific calcium active transport mechanisms located in the plasma membrane. The actual calcium signal for mediation of a cellular response is carried out by specific intracellular proteins, the most widely studied examples are calmodulin and troponin C. The recent discovery of phospholipid protein kinase and calcimedins suggests receptor mediation via several independent proteins. The physiological importance of a particular protein as a calcium messenger rests on several features: 1) calcium binding is of the order of 1-10 microns, 2) the protein is known to be localized at the site of proposed action, 3) if translocation occurs upon activation, the time required is consistent with the time course of the physiologic response and 4) substrates or effectors at the next level of action when isolated can be demonstrated to have similar activation kinetics as in situ.

Comparison of low- and high-calcium-requiring forms of the calcium-activated protease with their autocatalytic breakdown products

The effect of controlled autocatalytic degradation on the subunit molecular weight, subunit composition and calcium sensitivity of the low-calcium-requiring and the high-calcium-requiring forms of the calcium-activated muscle protease was examined. Purified low- and high-calcium-requiring proteases coelectrophorese on SDS-polyacrylamide gels. Controlled autocatalysis of either form of the protease results in extensive degradation of their respective 30 kDa subunits and partial degradation of their 80 kDa subunits. In this electrophoresis system the electrophoretic banding pattern of the low-calcium-requiring protease is clearly different from that of the autocatalytically degraded high-calcium-requiring protease. Similar results were obtained using a nondenaturing polyacrylamide gel system. Both the high- and low-calcium-requiring proteases were made more sensitive to calcium by autocatalytic degradation. However, the results of this study strongly indicate that autocatalytic degradation does not result in conversion of high-calcium-requiring protease to the low-calcium-requiring protease as has been recently hypothesized (Suzuki et al. (1981) J. Biochem. 90, 275-278).

Phagocytosis of the A band following Z line, and I band loss. Its significance in skeletal muscle breakdown

A band phagocytosis, following Z line and I band loss, is described in polymyositis, in epsilon amino caproic acid induced myopathy and in an experimental myositis of guinea pigs. It is far less prominent in Duchenne muscular dystrophy. This form of necrosis is always associated with lesions in the plasma membrane. It is argued that the ensuing ionic equilibration with the extracellular fluid allows activation of a calcium activated proteinase (CAP) which digests the Z line. Breakdown of the myofibrils into individual A bands promotes rapid phagocytosis of the myofibre contents and facilitates regeneration of the muscle.

Nemaline myopathy rod bodies. Structure and composition

Ca2+-activated protease (CAF) digestion of glycerinated nemaline myopathy muscle removed the electron-dense material covering rods and Z-lines and exposed longitudinal backbone filaments, 6-7 nm wide, which span the lengths of the original rods. Decoration of the exposed filaments (which are responsible for the periodicity parallel to the long axis of intact nemaline rods) with heavy meromyosin (HMM) proved they are actin filaments. After CAF treatment, cross-striated periodical patterns in longitudinal sections and Z-filament-like proteins connecting actin filaments seen in cross-section disappeared. This suggests that alpha-actinin may be involved in formation of this pattern because of the specificity of CAF toward alpha-actinin. Gel electrophoresis of CAF-treated nemaline muscle showed that most alpha-actinin is released into the supernatant, whereas the residue is mainly actin and myosin. Electron microscope examination of longitudinal sections of intact rods shows an oblique filament pattern, thin (7 nm) lines, thick (11 nm) lines, and an amorphous-appearance previously observed in normal Z-lines, patterns observed depend on sectioning angle and section thickness. In cross-section, rods show small square net (SS) and basket-weave (BW) forms. The SS form predominates and coexistence of the 2 forms, which also occur in normal Z-lines, is observed. Results support the idea that rods are lateral polymers of Z-line units. We think that the length of rods, as well as the width of Z-lines, is determined by the amount of overlap of actin filaments of opposite polarity. Initiation of rod formation may be due to deregulation of actin filament length.

Complement activation in muscle fiber necrosis: demonstration of the membrane attack complex of complement in necrotic fibers

The membranolytic C5b-9 complement membrane attach complex (MAC) is assembled after activation of either the classic or the alternative complement pathway. The quaternary configuration of the MAC macromolecule presents neoantigenic determinants not present on precursor molecules. Consequently, antibodies specific for these neoantigen(s) do not detect nonspecifically bound native complement precursors of MAC. By means of antibodies rendered specific for MAC neoantigen(s), MAC was localized by the immunoperoxidase reaction in cryostat sections of human muscle. In 66 biopsy specimens containing necrotic muscle fibers (Duchenne dystrophy, 13; other dystrophies, 15; inflammatory myopathies, 31; miscellaneous myopathies, 7) all of the necrotic fibers reacted for MAC neoantigen(s). C3 and C9 were also consistently localized in necrotic fibers, but localization of C1q, C4, and IgG was variable and often did not exceed background staining. None of the nonnecrotic fibers reacted for immunoglobulin or complement. Detection of MAC neoantigen(s) in necrotic fibers in a wide variety of muscle disease unambiguously shows that (1) the lytic complement pathway is consistently activated and participates in muscles fiber necrosis in vivo, and (2) complement reaction products are generated than can stimulate cellular infiltration and phagocytosis of the necrotic fiber. The findings also suggest that cell necrosis in general may involve participation of complement.

The Z-band: 85,000-dalton amorphin and alpha-actinin and their relation to structure

The conclusions arrived at as a result of this work can be summarized as follows: (a) We have found that there is an 85,000-dalton protein, which we have called 85K amorphin, associated with the Z-band of chicken pectoralis muscle myofibrils. We have isolated and purified this protein. It is not a structural component of the Z-filaments since it can be extracted completely without extraction of the Z-filaments. Extraction of 85K amorphin results in loss of specific staining of the Z-band with fluorescence specific anti-85K amorphin. (b) We have found that alpha-actinin is the structural component of the Z-filaments, since extraction of alpha-actinin is accompanied by loss of the Z-filament structure.

Successful treatment of murine muscular dystrophy with the proteinase inhibitor leupeptin

Mice with genetic muscular dystrophy were treated with intraperitoneal injections of the proteinase inhibitor leupeptin, beginning before the onset of weakness. A significant number of the treated animals failed to develop histological evidence of dystrophy, compared with controls. Leupeptin treatment prevented (or delayed) the onset of muscular dystrophy in this experiment.

Dynamic aspects of structural proteins in vertebrate skeletal muscle

In this review, our current knowledge on the structural proteins of vertebrate skeletal muscle is briefly outlined. Structural proteins include the contractile proteins (actin and myosin), the major regulatory proteins (troponin and tropomyosin), the minor regulatory proteins (M-protein, C-protein, F-protein, I-protein, and actinins), and the scaffold proteins (connectin, desmin, and Z-protein). In addition, the relative turnover rates of the muscle proteins (M-protein greater than or equal to troponin greater than soluble protein as a whole greater than tropomyosin not equal to alpha-actinin greater than myosin greater than 10S-actinin greater than actin) are discussed. The changes in the turnover of muscle proteins are compared in denervated and dystrophic muscles. The properties of the various proteases in muscle, including alkaline protease, calcium-activated neutral protease (CANP), and acidic protease (cathepsins), and the structural alterations of myofibrils by these proteases are also described. Finally, the role of proteases and their inhibitors in diseased muscle is summarized, with focus on CANP and its inhibitors, leupeptin and E-64.

The contractile apparatus of striated muscle in the course of atrophy and regeneration. I. Myosin and actin filaments in the denervated rat soleus

The ultrastructure of the contractile apparatus of the rat soleus muscle during the course of denervation atrophy was investigated. It was found that the ratio of thin the thick filaments increased in myofibrils of atrophying muscle fibers. Elevation of the ratio was observed as early as the second day after denervation, and became more pronounced with the progress of atrophy. Parallel measurements of the amounts of actin and myosin in the myofibrils and in the muscle protein extracts revealed a lower proportion of myosin heavy chains to actin in the fractions from denervated muscles, compared with the control values. Both the electron-microscopic observations and the biochemical evaluation of the actin content of the muscle, suggests that the elevated ratio of thin of thick filaments seen in the course of the muscle atrophy appears as the results of an earlier and more intensive disappearance of thick filaments. Thin filaments disappeared more slowly, in parallel to the decrease in muscle weight. On the basis of the results presented a mechanism of progress of "simple atrophy" of muscle in suggested.

A light and electron microscopic histochemical study on the mechanism of DFP-induced acute and subacute myopathy

The histochemical changes occurring in association with the development of acute and subacute myopathy have been studied in the rat diaphragm 30 min-48 h after a single i.p. injection of 1.82 mg/kg of the irreversible cholinesterase inhibitor organophosphate, diisopropylfluorophosphate (DFP). In addition to a considerable inhibition of the AChE activity of the motor end-plates, accumulation of ionic Ca2+ and an increase in neutral protease activity in the subjunctional sarcoplasm have been demonstrated. A temporal and causal relationship has been established between the histochemical changes and the development of the ultrastructural signs of myopathy.

A calcium-activated protease possibly involved in myofibrillar protein turnover. Isolation of a low-calcium-requiring form of the protease

Two forms of calcium-activated neutral protease were isolated and purified from porcine skeletal muscle. The two forms of the protease differ markedly in their requirement for calcium with the low-calcium-requiring form showing one-half maximal activation at 45 micro M calcium while the high-calcium-requiring form shows one-half maximal activation at 0.74 micro M calcium. Additionally, they chromatograph differently on DEAE-cellulose, exhibit different mobilities in electrophoresis in a nondenaturing buffer, are affected differently by certain divalent cations, and have slightly different pH dependencies. Despite these differences, the purified forms of the calcium-activated protease co-chromatograph in gel permeation chromatography, have identical banding patterns on sodium dodecyl sulfate (SDS)-polyacrylamide gels, cross-react with an antibody directed against the 80 000-dalton subunit of the calcium-activated protease we originally purified from skeletal muscle (Dayton, W.R., Goll, D.E., Zeece, M.G., Robson, R.M. and Reville, W.J. (1976) Biochemistry 15, 2150-2158), and have identical effects on the ultrastructure of myofibrils. THe high-calcium-requiring protease purified in this study is very likely identical to the calcium-activated protease we originally purified from skeletal muscle. The properties of the low-calcium-requiring form of the protease suggest that it is the form of the enzyme that is active in vivo.

The role of proteases in experimental glucocorticoid myopathy

Mature, male, New Zealand white rabbits were treated with the synthetic glucocorticoid betamethasone (0.3 mg/kg body weight/day) for 2 weeks. The glucocorticoid treatment caused a 30% decrease in muscle weight of the type 2 psoas muscle, but had no apparent effect on the type 1 soleus muscle. Cathepsin D activity was elevated twofold in the psoas muscle of treated rabbits, a finding suggesting an active role for lysosomes in mediating muscle breakdown in glucocorticoid-induced myopathy of the rabbit. There was no detectable alkaline serine protease activity in the muscles from either treated or control rabbits. Alkaline protease is localized in mast cells in some species, particularly the rat. Toluidine blue staining for mast cells was absent in rabbit muscles, a finding indicating that this species does not contain these cells. This protease, previously implicated in glucocorticoid myopathy, apparently plays no role in rabbit myopathy. There was no detectable elevation of the Ca-activated protease in muscles from glucocorticoid-treated animals. This finding suggests that if this protease plays a role in muscle degradation, its activity is controlled in vivo by special conditions (such as elevated CA levels, inhibitors, and compartmentalizations).

Monoclonal antibodies against myofibrillar components of rat skeletal muscle decorate the intermediate filaments of cultured cells

Monospecific antibodies were produced in vitro by fusing mouse myeloma cells with spleen cells from a BALB/c mouse immunized with rat skeletal myofibrils. After cloning 3 times on agarose, two stable clones were obtained and chosen for further characterization. The first clone, JLB1, produced an antibody that recognizing an antigen distributed in the M-line region and on either site of the Z line of myofibrils. The second clone, JLB7, produced an antibody reacting only with an antigen located at the M-line region of myofibrils. Both JLB1 and JLB7 antibodies decorate the typical intermediate filaments of a variety of cultured cells. Colcemid treatment of cells before reaction with both antibodies resulted in the coiling or capping (or both) of the fibers around the nucleus. Brief treatment of cells with cytochalasin B did not affect the integrity of the fibers stained by both antibodies whereas, under the same conditions, microfilament bundles visualized by another monoclonal antibody (JLA20) against actin were disassembled into many aggregates in the cytoplasm. Identical staining patterns of the intermediate filaments are obtained by double-label immunofluorescence microscopy of the same cell stained with these monoclonal antibodies and rabbit autoimmune serum (which has been shown to react with the components of the intermediate filaments). By using immunoprecipitation, protein bands at 210,000 and 95,000 daltons from chicken embryo fibroblasts were identified as the potential antigens recognized by JLB1 and JLB7 monoclonal antibodies, respectively. The widespread occurrence of these antigenic determinants in different cultured cells suggests the highly conservative property of these intermediate-filament components.

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.

Adult mammalian cardiac muscle cells in culture

Adult rat cardiac muscle cells were isolated from the ventricle by a retrograde perfusion technique through the aorta (Nag and Zak, 1979). These single, isolated cardiac muscle cells were cultured for 4 weeks. Throughout the culture period, a small number of muscle cells retained their cylindrical shape, while the rest exhibited alterations in shape and size assuming a flattened body of irregular shape with pseudopodia-like processes and thereby resembling embryonic/neonatal cardiac muscle cells in culture. Transmission electron microscopy revealed that the cylindrical muscle cells contained compactly arranged myofibrils and cellular organelles, similar to those of freshly isolated and in vivo cells. A few irregularly shaped cardiac muscle cells were similar to the cylindrical cells in their internal structural organization. Most of the irregular cells exhibited less myofibrillar content than that of the freshly dissociated and in vivo cells. Myofibrils in the irregular cells were widely spaced and myofilament of some of the myofibrils were loosely bunched. In addition, scattered patches of myofibrils and free myofilaments were observed in many of these cells. The internal structural organization of these irregularly shaped cardiac muscle cells closely resembled the embryonic and neonatal cardiac muscle cells in vitro and in vivo. Most of the muscle cells in culture continued to contract spontaneously, and electron microscope studies clearly indicated that they underwent dedifferentiation. Autoradiography studies demonstrated that the cylindrical and irregularly shaped cardiac muscle cells underwent DNA synthesis and cell division in culture.