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

Ca2+ homeostasis alterations induced by 2,4-dichlorophenoxyacetic butyl ester and 2,4-dichlorophenoxyacetic acid on avian skeletal muscle

Fertilized hen eggs were treated externally with 2,4-dichlorophenoxyacetic butyl ester (2,4-D b.e.) (3.1 mg/egg) before the start of the incubation. Actomyosin and sarcoplasmic reticulum adenosine triphosphatase (ATPase) activities from leg and complexus muscles of chicks hatched from treated eggs were measured. No significant variations were detected in the ATPase activities of actomyosin, but the sarcoplasmic reticulum Mg2(+)-activated ATPase and Ca2+, Mg2(+)-activated ATPase were inhibited 50 and 38% respectively. 45Ca2+ uptake into soleus muscle was increased by the 2,4-D b.e. treatment. The compartmental analysis of 45Ca2+ uptake kinetics showed increases in Ca2+ fluxes in sarcolemma and mitochondria and in the mitochondrial calcium pool. Isolated soleus muscles were treated with 2,4-dichlorophenoxyacetic acid (2,4-D) or 2,4-D b.e. [14C]2,4-D reached it highest level in these muscles after 1 hr of treatment. The in vitro treatment with 2,4-D or 2,4-D b.e. increased 45Ca2+ uptake into the muscles. 2,4-D b.e. produced greater alterations than 2,4-D. The compartmental analysis of the 45Ca2+ uptake kinetics also showed increases of the mitochondrial Ca2+ pool and Ca2+ fluxes through sarcolemma and mitochondria. These results led to a hypothesis based on Ca2+ permeability alterations for explaining the myopathic actions of these phenoxyherbicides.

Activation of the intramyofibral autophagic-lysosomal system in muscular dystrophy

Skeletal muscles obtained from myopathies with myofiber necrosis, including mdx dystrophic mice, plasmocid-induced myopathy in rats, and patients with Duchenne muscular dystrophy, were examined immunohistochemically with anticathepsin-peroxidase conjugates. Strong reactions for lysosomal cysteine proteinases, which can degrade myofibrillar proteins, were demonstrated in macrophages invading and surrounding the necrotic areas and some degenerative myofibers and also in intramyofibral portions of atrophic fibers of dystrophic mice and humans. Apparently normal and regenerating myofibers did not stain for lysosomal cathepsins. Abnormal increases of cathepsins L and B were seen even in the early stage of plasmocid myopathy and in a 20-day-old young mdx mouse before infiltration of macrophages, suggesting that autodigestion by intramyofibral lysosomal proteinases is an important event before digestion of the necrotic fibers by macrophage proteinases. Activation of the intramyofibral lysosomal system, as in muscular dystrophy, was also observed in distal myopathy with rimmed vacuoles without macrophageal infiltration (Am J Pathol 1986, 122:193-198). Thus, this activation seems to be an important, early response to myocellular damage.

Impaired muscle-nerve interaction (motility) characterizes the brachial region of dystrophic embryos

During development ex ovo, the avian mutant with an hereditary form of muscular dystrophy demonstrates biochemical, histochemical, and physiological (functional) abnormalities which may result from impaired muscle-nerve interaction. To investigate if impaired functional activity also characterizes the dystrophic process during development in ovo, limb motility, an index of embryonic functional muscle-nerve interaction, was compared between normal and dystrophic embryos from day 6E through day 16E. A highly significant reduction in this parameter was exhibited by dystrophic wings from day 11E to day 14E inclusive. In contrast, genotypically dystrophic hind limbs demonstrated values equivalent to normal legs. Thus, in the dystrophic embryo, impaired muscle-nerve interaction characterized the brachial region exclusively during a specific period of embryogenesis.

Abnormal expression of the calmodulin gene in muscle from the dystrophic chicken

Compared to that of genetically-related normal chickens, pectoralis muscle from the dystrophic chicken contained increased calmodulin measured by radioimmunoassay. Determined by the dot blot procedure, expression of the calmodulin gene was enhanced in muscle from affected animals. The bioactivity of the gene product was normal. Together with previous studies reporting increased cell Ca2+ content in dystrophic muscle, the current findings of increased sarcoplasmic calmodulin suggest the latter is a cellular response to defective Ca2+ transport at the level of cell efflux or intracellular organelle (sarcoplasmic reticulum) uptake.

Increased concentration of spectrin is observed in avian dystrophic muscle

A significant increase in the concentration of spectrin has been observed in dystrophic chicken pectoralis major muscle when compared to normal fast-twitch muscle. In normal muscle, alpha-spectrin-specific immunofluorescence delineates each myofiber with a network pattern of staining at the sarcolemma with little staining within the cytoplasm. In dystrophic fibers, numerous intensely stained areas occur within the cytoplasm and staining at the sarcolemma is increased, thereby obscuring or eliminating the highly regular network arrangement of spectrin usually seen in this region. When immunofluorescence experiments are performed on microsomal vesicles isolated from normal and dystrophic tissues, only a small fraction of normal vesicles are stained, whereas most of the dystrophic vesicles are associated with spectrin. An increase in spectrin concentration is observed using immunoautoradiography of whole muscle and isolated microsomes, thus supporting the immunofluorescent observations described above. The early-age post-hatching when increases in spectrin concentration can be detected and the simplicity of the immunofluorescent technique make this observation useful as a new diagnostic parameter. This observation also shows that the distribution of spectrin and its concentration within nonerythroid cells can be modified by abnormal physiological states; this modification may contribute to subsequent symptoms, such as increased rigidity and abnormal calcium metabolism, that are observed in dystrophy.

Elevation of calmodulin in avian muscular dystrophy

In an attempt to understand the mechanism of calcium accumulation in myopathies, changes in the major calcium-binding protein, calmodulin, was studied in genetically dystrophic chickens. Measurements by radioimmunoassay revealed an increase in the calmodulin concentration of dystrophic chicken muscles. Poly A-containing RNA(s) of fast and slow muscles from the normal and dystrophic chicks were hybridized with [32P]-labeled calmodulin cDNA probe by the dot-hybridization technique. Densitometric scan of the autoradiogram showed that the calmodulin mRNA levels of dystrophic fast muscles (pectoralis and posterior latissimus dorsi) were approximately two-fold higher than those of the corresponding normal muscles. No significant change in calmodulin and calmodulin messenger RNA of slow muscle (ALD) was found in dystrophic chickens. Our results suggest that increased calcium flux within the dystrophic muscle may be modulated by calmodulin.

Effects of percutaneous electrical stimulation on functional ability, plasma creatine kinase, and pectoralis musculature of normal and genetically dystrophic chickens

The breast musculature of genetically dystrophic Line 413 and genetically related normal Line 412 chickens were treated in three separate trials with high-frequency electrical stimulation (ES). Beginning on days 7 or 14 ex ovo, each bird received three ES treatments per week. Each stimulation cycle repeated five times per day consisted of 15 s "on" followed by 50 s "off". In the third trial only, the birds were additionally treated beginning day 3 ex ovo with either leucine (100 mg/kg) or the proteinase inhibitor Ep475 (10 mg/kg). ES significantly delayed the onset of righting disability in the dystrophic chickens. However, this improvement was temporary and could be masked by single treatments of either leucine or Ep475. Plasma creatine kinase activities were increased generally in both the stimulated normal and dystrophic birds. In two trials ES increased the relative muscle mass, and in one trial increased protein. ES had little effect on normal muscle mass or protein. However, ES treatment together with either leucine or Ep475 appeared to improve both normal and dystrophic muscle mass and protein. Furthermore ES decreased dystrophic muscle calcium but not acetylcholinesterase activity. On the other hand, ES had no effect on the total normal muscle calcium but increased normal acetylcholinesterase values. In both normal and dystrophic muscle samples, ES treatment in combination with leucine appeared to increase the mean muscle fiber diameters and number of myonuclei, and in the case of the dystrophic muscle, appeared to decrease the relative proportion of vacuolated, degenerating, and intensely oxidative histochemical fibers. In general, stimulation (especially in combination with leucine) appears to alter in varying degrees the phenotypic expression of the muscle disease exhibited in the dystrophic chicken.

In vivo effects of three calcium blockers on chickens with inherited muscular dystrophy

Genetically homozygous Line 413 dystrophic chickens were given in separate trials daily i.p. injections of aqueous solutions of the calcium blocker drugs, diltiazem, verapamil, or nifedipine. At a dosage of 20 mg/kg/day, drug therapy in each case significantly prolonged the functional ability of the dystrophic chickens as quantitated regularly by a standardized test for righting ability. Enhanced functional ability, however, was not generally accompanied by a decrease in the usually high plasma creatine kinase activity. In addition, there was no change in the pectoralis muscle mass or protein with any of the drug treatments. Moreover, no significant reduction in the abnormally high total muscle calcium was found with calcium blocker treatment. Also, there was no marked change in the histopathology of muscle from the drug-treated dystrophic chickens. We concluded that drugs with calcium entry blocker activity offer only limited benefit in retarding dystrophic symptoms expressed in the chicken (viz., short-term enhancement in righting ability).

Myosin components of the latissimus dorsi and the pectoralis major muscles in the dystrophic chicken

The myosin composition of the anterior latissimus dorsi, the posterior latissimus dorsi, and the pectoralis major muscles was examined in the inbred White Leghorn dystrophic chicken and its isogenic normal line at different ages during development and maturation. Using the biochemical methods of native gel electrophoresis, one- and two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE), and peptide mapping, it was found that myosin isozyme changes occurred normally in the anterior latissimus dorsi muscle. However, in the posterior latissimus dorsi muscle, slow myosin components which were not present in the adult normal muscle were present in the adult dystrophic muscle. In addition, the pectoralis major muscle of the dystrophic chicken failed to undergo the neonatal to adult fast myosin isozyme transition. Our data also showed that muscle cell cultures derived from the pectoralis major muscle of dystrophic chickens expressed identical myosin components to cultures derived from normal embryos. However, since these cultures only produced embryonic myosins even after 1 month in culture, it implied that cells in tissue culture were phenotypically normal because present cell culture conditions were insufficient to induce the fetal to adult isozyme changes.

Sex differences in the phenotypic expression of avian dystrophy

Although the gene for muscular dystrophy in chickens is not sex-linked, results from clinical tests suggest that it is expressed differently in males and females. As measurement of muscle contractile responses provides a quantitative index for the severity of the disease, the contractile properties of the extensor digitorum communis muscle were examined in normal and dystrophic chickens with respect to sex. Furthermore, these differences were examined in young (6 to 9 weeks) and old (greater than 6 months) chickens. Results showed that age-related sex differences were apparent for those mechanical parameters of the muscle (in particular the posttetanic potentiation and posttetanic contracture) known to distinguish normal and dystrophic birds. The sex differences observed in the younger group indicate that the female birds were more severely affected by the disease than were the male. In the older group, the male were affected by the disease more severely than age-matched female birds. If the inheritance pattern is truly autosomal then it is likely that one or more developmental factors interact with the dystrophic genotype and alter the dystrophic phenotype.

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.

Electrophysiological observations in normal and dystrophic chicken muscles

Intracellular recordings were made on the fast posterior latissimus dorsi muscles of normal and dystrophic chickens. In the dystrophic chickens, the rate of rise of the action potential was decreased. With repetitive indirect stimulation, the action potentials decreased in size and disappeared; only an end-plate potential remained. Membrane resistance, membrane capacitance, and duration of miniature end-plate potentials were increased. A decrease in sodium permeability may be in part responsible for the observed alterations in the electrical properties of the nerve terminal and postsynaptic muscle membrane.

In vivo effect of uncoupling agents on the incorporation of calcium and strontium into mitochondria and other subcellular fractions of rat liver

After injection of (45)Ca(++) or (89)Sr(++) into rats, the largest part of the radioactivity in the liver cell is associated with the subcellular structures, only negligible amounts of it being found in the soluble hyaloplasm. 50 % or more of the (45)Ca(++) and (89)Sr(++) in the liver cell is recovered in the mitochondrial fraction. The specific activity of Ca(++) after injection of (45)Ca(++) is far greater in mitochondria than in microsomes. Pretreatment of the rats with uncouplers of oxidative phosphorylation markedly decreases the amount of radioactivity associated with the mitochondrial fraction. The amount of radioactivity recovered in the microsomes and in the final supernatant on the contrary increases. These effects are present only when mitochondrial oxidative phosphorylation is completely uncoupled. The Ca(++) content of mitochondria from the livers of rats pretreated with uncouplers is sharply decreased with respect to the controls. It is concluded that in the liver cells of the intact animal energy-linked movements of Ca(++) and Sr(++) take place in mitochondria.

The intracellular distribution of calcium in the mucosa of the avian shell gland

The intracellular distribution of calcium has been studied in the mucosa of the avian shell gland, a tissue which transports large quantities of calcium during discrete time intervals. Ca(45) was administered to hens either in a single dose followed by sacrifice 5 min later or in repeated doses over an extended period followed by sacrifice 2 hr or 24 hr after the last injection. Subcellular fractions were isolated by differential centrifugation and analyzed for Ca(45). The Ca(45) was located principally in the particulate fractions; the concentration (CPM Ca(45)/mg N) was highest in the mitochondrial fraction. Comparisons of (1) the Ca(45) distribution in shell gland cells with that of liver cells, (2) the alterations which occur due to the phase of the egg laying cycle, (3) the effects due to the time elapsed since the last injection of Ca(45), and (4) the Ca(45) distribution of the short term experiments with that of the long term experiments revealed that the mitochondrial fraction of the shell gland appeared to be active in the movement of calcium. The microsomal fraction showed increased values in CPM Ca(45)/mg N when calcification was occurring, which may indicate that the subcellular components of this fraction have a role in calcium transport. The nuclear and supernatant fractions did not seem to be involved in the transport process. The implications of these results concerning the manner by which calcium may be controlled on a cellular level in this system are discussed.