Fatigability of normal and dystrophic chicken muscle in vivo

Changes in the indirectly elicited isometric twitch tension, tetanic tension, twitch-to-tetanus ratio, and post-tetanic potentiation in response to fatigue were examined in the posterior latissimus dorsi muscles of normal and genetically dystrophic New Hampshire chickens. Contractile parameters were studied during 3-h fatigue stimulations and during continuous infusion of potassium chloride (0.24 m equiv/min) initiated at the conclusion of the fatigue period. Both twitch and tetanic tension of dystrophic muscles showed a relative resistance to fatigue; no significant changes in either twitch-to-retanus ratio or post-tetanic potentiation occurred during the fatigue period. In contrast, twitch and tetanic tension of normal muscles decreased more rapidly and to a greater extent in response to fatigue. The twitch-to-tetanus ratio decreased and post-tetanic potentiation increased such that after 30 min they were not significantly different from values seen in dystrophic muscles. Potassium chloride infusion produced a significant recovery (two- to ninefold improvement) of the fatigued twitch response to dystrophic muscle but did not have a significant effect on fatigued normal muscles. A comparison of directly and indirectly elicited twitch contractions indicated that part of the decrement of contractile response in dystrophic muscle was due to synaptic failure at the neuromuscular junction and that potassium chloride infusion resulted in restoration of neuromuscular transmission. It is suggested that the difference in fatigue pattern observed between normal and dystrophic muscle was a function of an altered distribution in the physiological types of motor units present in the diseased muscle.     

Effects of immobilization on the isometric contractile properties of embryonic avian skeletal muscle

Chicken embryos were chronically immobilized by applying a neuromuscular blocking agent, curare, to the chorioallantoic membrane from day 8 through day 16 of incubation to study the effects of a deficit in motor activity on the development of contractile properties of skeletal muscle. Compared with control embryos, spontaneous embryonic motor activity was depressed by 60 to 90% in the curare-treated animals during the treatment period. Growth of the posterior latissimus dorsi muscle, a fast-twitch muscle in the adult, was greatly affected by immobilization. The average blotted mass of the muscles from curare-treated 18- to 19-day embryos was approximately 20% of that from control embryos. The isometric contractile properties of posterior latissimus dorsi muscles isolated from control and curare-treated embryos were compared at 18 to 19 days of incubation. The times to peak tension and to one-half relaxation of the twitch and tetanic responses were significantly greater for the muscles from the immobilized embryos. The peak twitch and tetanic tensions, normalized for muscle cross-sectional area, were significantly less than control values for the muscles from curare-treated embryos. The maximal rate of tetanic tension production was, however, unaffected by immobilization. The results of this study demonstrate that the development of isometric contractile properties of embryonic skeletal muscle is significantly altered by an experimentally induced reduction of spontaneous motor activity. A disruption in the functional development of the sarcoplasmic reticulum following a similar decrease in motor activity, as reported by others, is discussed as a potential mechanism for the altered contractile properties of muscles from the curare-treated embryos.     

Potentiation of the isometric twitch and mechanism of tension recruitment in mammalian skeletal muscle.

Changes produced by bromide, nitrate, and iodide in the isometric myogram of the fast-twitch extensor digitorum longus and the slow-twitch soleus muscles of the rat were studied in vitro at 35°C. Replacement of chloride with bromide, nitrate, or iodide produced only small changes in the isometric tetanic tension, but it produced a reversible potentiation of the isometric twitch tension. The order of the potentiating influence was iodide > nitrate > bromide. The tension potentiation was associated with increases in rate of tension rise and the time to peak of the isometric twitch. The results from the two muscles were essentially similar and they were qualitatively similar to those reported from frog twitch muscle. The form of tension enhancement is qualitatively similar to that obtained during summation of mechanical responses, and the results predict with sufficient accuracy the characteristics of the fully summated two-stimuli mechanical response. We suggest that changes in excitation-contraction coupling, similar to those produced by these anions, occur in mammalian skeletal muscle fiber at 35 to 38°C during summation of isometric mechanical responses.     

Slowing of fast-twitch muscle in the dystrophic mouse

Isometric twitch tension was measured in fast-twitch and slow-twitch muscles of normal and dystrophic ( ) mice in vivo. In dystrophic mice more than 6 months old the fast-twitch extensor digitorum longus (EDL) showed a prolongation of the time to peak tension as well as the time to relax to one-half peak tension ( ) compared with age-matched controls. In younger dystrophic mice (4 to 6 weeks) the time to peak tension was prolonged but not significantly so. This apparent “slowing” of dystrophic fast-twitch muscle was accompanied by a reduction in both cooling potentiation and post-tetanic potentiation toward values typical of slow-twitch muscle. Slow-twitch soleus muscle (SOL) of old mice was almost unaffected by the dystrophic process with regared to its contractile characteristics. However, there appeared to be a slight, but significant “speeding” of young dystrophic SOL compared with age-matched control muscles. This was apparent in reduced times to peak tension and half-relaxation as well as an enhanced cooling potentiation. We suggest that the altered contractile characteristics result from a change in some intrinsic property of the muscle fibers rather than from extrinsic factors such as the additional perimysial connective tissue seen in these muscles.     

Functional properties of muscles transplanted between normal and dystrophic mice

Tibialis anterior muscles were transplanted between 12-week-old normal and dystrophic mice with intact or polydimethyl silicone-capped peroneal nerve. After 150 days the transplants were removed and their isometric twitch contraction properties were studied in vitro at 20 C. Intact normal and dystrophic muscles of equivalent age were used as controls. Dystrophic muscles developed lower twitch and tetanus tension than normal muscles and showed prolonged half relaxation time. The contraction time and twitch/tetanus ratio of both types of muscle were similar. Of all transplantations performed, only those in normal mice with intact nerve responded upon stimulation. Both normal and dystrophic transplants in normal hosts showed similar isometric properties. Although intact dystrophic muscles and viable dystrophic transplants in normal hosts were similar in weight, the transplants developed about three to four times more tension. In addition, dystrophic transplants showed relaxation times similar to normal muscles. It is suggested that the dystrophic lesion in mice may have a neural origin.     

Effect of low Ca2+ solution on muscle contraction of developing, preclinical dystrophic (dy2j) mice.

EDL muscles from normal and dystrophic (dy2j) mice aged 7 to 21 days of postnatal life were examined. Muscles were divided into 2 groups according to age, 7 to 14 days and 16 to 21 days postnatal, so as to assess age- and/or phenotype-related differences in the muscle response to low Ca2+ solution. Tension production was already much impaired in "predystrophic" muscles. At this stage, however, there was essentially no difference in twitch kinetics between normal and dystrophic muscles. Upon exposure to low Ca2+ solution, twitch responses of both normal and dystrophic muscles declined in a similar manner. In the youngest animals studied (7 to 14 days), the tetanic responses of both normal and dystrophic muscles to low Ca2+ solution were also similar. In animals 15 to 21 days old, however, the tetanic tension developed in low Ca2+ solution by dystrophic muscles, was significantly less than that of normal. Moreover, under these conditions (i.e., in low Ca2+ solution), and following tetanic stimulation, the membrane potential of dystrophic muscles in this age group was significantly more depolarized than that of normal muscles. Our results suggest that the ability of the cell to deal with extracellular Ca2+ is normal in predystrophic muscles up to 21 days of postnatal life. The results also clearly point to the fragility of the membrane in these muscles.     

Temperature effects on the kinetics of force generation in normal and dystrophic mouse muscles

The kinetics of isolated extensor digitorum longus and soleus muscles from normal and genetically dystrophic (129/ReJ dy/dy) mice were studied at temperatures from 8 to 38 degrees C. The rate constants for the exponential rise of tetanic force and for the exponential decay of force during an isometric twitch or short tetanus were similar in normal and dystrophic soleus muscles, but the decay rates were significantly reduced in dystrophic extensor digitorum longus muscles. The temperature dependence for several rate constants for isometric twitches and tetani was similar in all muscles studied, suggesting that the same rate limiting processes apply to fast and slow, normal and dystrophic muscles. Thus, the contractile proteins and those in the sarcoplasmic reticulum of dystrophic muscle are probably normal. The slower relaxation phase in dystrophic extensor digitorum longus muscles is compatible with a reduction in Ca2+-pumping sites in the sarcoplasmic reticulum, perhaps secondary to a change in motor unit composition. Some changes in the temperature dependence for measured times, toward those of soleus muscles, is consistent with the increased proportion of slow twitch motor units in dystrophic extensor digitorum longus muscles.     

Changes in isometric contractile properties of fast-twitch and slow-twitch skeletal muscle of C57BL/6J dy2J/dy2J dystrophic mice during postnatal development

Our primary aim was to determine if there exists a preferential involvement of the fast-twitch or slow-twitch skeletal muscle fibers in the dy2J/dy2J strain of murine dystrophy. The changes in the contractile properties of the slow-twitch soleus (SOL) and the fast-twitch extensor digitorum longus (EDL) muscles of normal and dystrophic mice were studied at 4, 8, 12, and 32 weeks of age. Isometric twitch and tetanus tension were decreased in the 4- and 8-week-old dystrophic EDL compared with controls, this situation being reversed in the older animals. At 12 weeks, the dystrophic EDL generated 15% more tetanic tension than normal EDL and by 32 weeks no significant difference was seen between normal and dystrophic EDL twitch or tetanus tension. By 8 weeks, dystrophic EDL exhibited a prolonged time-to-peak twitch tension (TTP) and half-relaxation time (1/2RT) of the isometric twitch which continued to 32 weeks. For the dystrophic SOL, decreased twitch and tetanus tension was observed from 4 to 32 weeks. At 8 and 12 weeks, TTP and 1/2RT of dystrophic SOL were prolonged. However, by 32 weeks there was no longer a significant difference seen in TTP or 1/2RT between normal and dystrophic SOL. Our results appear to indicate that a loss of the primary control which is determining the fiber composition of the individual muscles is occurring as the dystrophic process advances.     

Isometric contractile properties of the posterior latissimus dorsi muscle in normal and genetically dystrophic chickens

Various isometric contractile properties of posterior latissimus dorsi muscles from normal and genetically dystrophic New Hampshire chickens were examined. Compared to normal, the dystrophic muscles were lighter, the indirectly elicited twitch was weaker, and the twitch-to-tetanus ratio was reduced. The time course of the twitch contraction was not significantly different from normal. However, fusion frequency was higher in dystrophic muscle and the entire frequency-tension relationship was shifted to higher frequencies. The maximal rate of rise of tension was significantly reduced during both twitch and tetanus in dystrophic muscle. There was no difference in the decrement of the gross electromyogram during high-frequency stimulation. A significantly greater post-tetanic potentiation of the twitch contraction was observed in dystrophic muscle. We suggest that the modification of contractile properties observed in dystrophic chicken muscle represents a shift toward slow muscle characteristics. The paradoxical observation of an unchanged twitch time course in the presence of a reduced maximal rate of rise of tension is discussed in relation to an apparent reduction in duration of the plateau of the active state.     

Similar in vitro fatigue patterns of normal and BIO 40.54 dystrophic hamster extensor digitorum longus muscle.

Fatigue patterns of normal and BIO 40.54 dystrophic hamster extensor digitorum longus (EDL) muscles were studied in vitro (22°C) at 60, 120, 170, and 320 days of age. The diseased muscle showed similar rates of tension decline compared to their normal counterparts when stimulated intermittently (a twitch or tetanus every 90 s) for 3 h or in rapid succession (1-s tetanus every 5 s) until tetanic tension was decreased 50%. Electron microscopic observations revealed a subsarcolemmal accumulation of enlarged mitochondria in dystrophic muscle compared to normal EDL. These results suggest that the availability of adenosine triphosphate for cross-bridge formation may not be impaired in dystrophic hamster muscles.     

Post-tetanic depression of twitch tension in the cat soleus muscle

Changes produced by a preceding tetanus in the isometric twitch myogram were examined in cat soleus (slow twitch) muscle in situ, at 37°C, and with supramaximal nerve stimulation. A tetanus produced a depression in the twitch tension (P) accompanied by a shortening of time to peak (TP) and time to half-relaxation (HR), and a slight depression in the maximum rate of tension rise (MR). Recovery of P, TP, and HR, after a standard tetanus of 100 Hz for 300 ms, showed an early-fast-phase (time constant of 0.8 to 0.9 s) and a late-slow-phase (time constant of 70 to 80 s), whereas that of MR showed only the late phase of recovery. Analysis of the data indicated the occurrence of two post-tetanic influences which produce different maxima of tension depression at zero test interval and have different rates of recovery. The twitch tension depression, when examined at the same test interval (0.7 s), increased with increase in the number of stimuli in the preceding tetanus. The relationship between the post-tetanic twitch tension and the number of stimuli in the tetanus was approximately logarithmic and was similar for tetanic frequencies of 50, 100, and 200 Hz. Unlike in the fast-twitch muscles, there was no clear evidence of a post-tetanic influence which may potentiate the twitch tension: the most prominent post-tetanic effect, in this muscle, was an acceleration of the tension relaxation in the twitch.     

The isometric responses of fast and slow twitch muscles from normal and genetically dystrophic hamsters

There has been a need for some time to examine the effects of the dystrophic process upon the mechanical properties of the limb muscles of the dystrophic hamster. This paper reports the findings of such a study by using an in vivo technique to record the isometric twitch characteristics of the extensor digitorum longus and soleus muscles. Normal and dystrophic animals have been examined at various ages as it has been reported that the disease is progressive. Normal muscle characteristics fall in line with those already published for small mammals. Dystrophic muscles show little difference from normal muscles with respect to their twitch characteristics. Only at the 60 day age point was the time to half relaxation of dystrophic muscles longer than its normal counterpart. Differences were found between normal and dystrophic muscle weights and between the ratio of muscle weight to body weight. It is proposed that these result probably from increased hydration of the muscles caused secondarily to the cardiomyopathy.     

Influence of temperature on the characteristics of summation of isometric mechanical responses of mammalian skeletal muscle

Characteristics of summation of isometric mechanical responses elicited by two supramaximal nerve stimuli were examined at different temperatures (18 to 38°C) in a fast twitch (flexor digitorum longus) and a slow twitch (soleus) muscle, in situ, in the cat hind leg. At 37 to 38°C, both muscles developed a peak tension that was more than twice the twitch tension when a second stimulus was applied shortly after the first. The rate of rise of tension in such a summated response was also about, or more than, twice that of the twitch tension, and the time to peak tension was about 1.5 times the time to peak of the twitch. The peak tension in summated responses decreased with increasing the stimulus interval so that, when the interval was equal to the time to peak twitch, the tension developed was significantly less. Cooling significantly alters these characteristics in the fast twitch muscle, so that, at 20 to 22°C, the tension, the rate of rise of tension, and the time to peak tension enhancement produced by a second stimulus are markedly reduced; little or no change in the characteristics is seen with cooling of the slow twitch muscle. These results are examined in the light of some observations made on the pattern of summation in frog sartorius muscle at different temperatures (2 to 28°C). Cooling significantly increases the degree of activation occurring during the twitch in the fast twitch muscle fibers, whereas it has little or no effect on the degree of mechanical activation in the slow twitch muscle fibers. Furthermore, the results suggest that the tension, the rate of rise of tension, and, perhaps the time to peak tension of the isometric twitch in both the fast twitch and the slow twitch muscle fibers at 37 to 38°C are limited by the degree of mechanical activation and that a second stimulus applied shortly after the first can greatly overcome the effect of that limitation.     

New muscle transplant method produces normal twitch tension in dystrophic muscle.

Grafting newborn muscle is an innovative method of muscle transplant. This method overcomes hypoxia in the deeper fibers and facilitates reinnervation and revascularization of the grafted muscle fibers, thus promoting the survival and development of the characteristics of the donor muscle. The result achieved is superior to that obtained from mature muscle grafts or from minced muscle transplants. When an intact soleus from a 1-day-old normal mouse was grafted into a recipient soleus of a 20-day-old dystrophic C57BL/6J-dy2J mouse, the actively developing normal graft helped to improve the structure and function of the dystrophic muscle. When compared to the intact dystrophic solei, the test dystrophic muscles five to six months after operation showed increases in cross-sectional area, in wet weight, in twitch and tetanic tension, and in the number of muscle fibers with high resting membrane potentials. This is the first procedure to have raised the muscle twitch tension in an adult dystrophic mouse to the normal level.     

Isometric contractile properties of fast- and slow-twitch muscles in normal and spastic mice

Isometric contractile properties of the fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles of 24 male C57 mice were studied in vitro at 35°C. Ten animals exhibited the hereditary movement disorder known as spasticity. EDL muscles of spastic and normal mice developed similar specific tensions and they had similar twitch tension:tetanic tension ratios. The time-to-peak tension and the time to half relaxation in the twitch of spastic EDL muscles were, on average, slightly longer. SOL muscles of spastic and normal mice also developed similar specific tensions and their twitch contractions had similar times to half-relaxation. The time-to-peak tension in the twitch and the twitch tension: tetanic tension ratio of spastic SOL muscles were, on average, higher than in normal SOL. The significance of our findings remains somewhat uncertain.     

Changes in genotypic expression, development, and the effects of chronic penicillamine treatment on the electrical properties of the posterior latissimus dorsi muscle in two lines of normal and dystrophic chickens.

When line 304 dystrophic chickens were outcrossed, a new line of genetically dystrophic chickens designated 413 was produced together with a genetically related normal animal (line 412). The expected alteration in membrane resistance in the surface fibers of the posterior latissimus dorsi (PLD) muscle was eliminated in this new line of dystrophic chickens. Although the input resistance of PLD fibers from both lines 200 and 304 chickens declined simultaneously from 1 to 2 Mω at 5 weeks ex ovo to 0.3 Mω at 17 to 33 weeks ex ovo, specific membrane resistance, membrane space and time constants, and fiber radius of line 304-dystrophic fibers were significantly greater than in line 200-normal fibers at various times ex ovo. Only the membrane time constant and capacitance of PLD fibers of line 413-dystrophic chickens were greater than in line 412-normal at 8 weeks ex ovo. Other differences in electrical properties were also seen. Only in line 413-dystrophic chickens were the resting membrane potentials and action potential properties significantly different from line 412. Outcrossing of line 304 chickens [has thus] revealed that various electrical properties of muscle such as membrane resistance are under genetic control but did not influence the development of dystrophy. On the other hand, alterations in action potential activity persists after outcrossing and appear to have a greater role in the symptomatology of muscular dystrophy. A therapeutic approach to the disease using oral d-penicillamine treatment was initiated 9 days ex ovo in normal (line 412) and dystrophic (line 413) chickens of either sex and continued 7 to 10 weeks. An improvement in the righting ability of treated vs. untreated dystrophic birds was first noted at 6 weeks ex ovo, and by 8 weeks, treated dystrophic chickens were always able to right themselves whereas untreated dystrophic chickens failed 75% of the time. The body weights and rates of growth in untreated normal and dystrophic chickens were similar but that of treated birds were 35 to 40% lower. The drug had no effect on the resting membrane potential of PLD surface fibers in treated chickens. The beneficial effect of the drug was initially correlated with the ability of the PLD muscle to respond with a twitch to single indirect stimulation and to the absence of a decrementing response in muscle to repetitive nerve stimulation. Penicillamine only decreased the action potential threshold in dystrophic chickens. Penicillamine treatment reduced the depression of the probability of release of transmitter in dystrophic muscle. The higher threshold for excitation and greater resting membrane potential in the newer line of dystrophic chickens may account in part for the inability of these chickens to right themselves when placed on their backs. It is suggested that the beneficial effect of penicillamine is related to an alteration of synaptic function and lowering of the threshold for excitation in muscle.     

Characteristics of tension recruitment and mechanical activation in mammalian skeletal muscle

Characteristics of summation of isometric mechanical responses were studied in in situ flexor digitorum longus (FDL) (fast-twitch) and soleus (slow-twitch) muscles of the cat at 37°C. The pattern of tension recruitment in short tetanic contractions was examined at different stimulation frequencies. At frequencies leading to the development of maximum tetanic tension, the second stimulus activation made the largest individual contribution to tension development. The tension developed with two stimuli was dependent on the interval separating them. When the first two stimuli were fixed at a short interval such that the muscle developed the highest two-response tension, the tension developed following application of the third stimulus was independent over a wider range of two-to-three stimulus intervals. Application of a stimulus shortly after another, in general, lengthened the contraction. The results from the two muscles were basically similar when allowance was made for the difference in their speed of contraction. The marked tension potentiation seen in the twitch of FDL, with staircase effect and with cooling, was largely absent in the well-fused double response. We suggest that the mechanical activation occurring during the isometric twitch is incomplete in both muscle fiber types and that a second stimulus applied shortly after the first brings about nearly complete mechanical activation.     

Electrical and mechanical responses in the platysma and in the adductor pollicis muscle: in normal subjects.

In the platysma of 34 normal subjects the amplitude of the action potential and twitch tension and tetanic tension were lower, the contraction time of the isometric twitch was 1.4 times shorter, and the potentiation of twitch tension in a staircase and after tetanus was two to four times greater than in m. adductor pollicis. Differences in twitch kinetics and potentiation were related to the four times higher incidence of fast fibres in the platysma than in the adductor pollicis muscle, as determined by histochemistry. Ninety-five per cent confidence limits were established for comparison with patients with myasthenia gravis.     

The effect of electrical stimulation on reinnervation of rat muscle: contractile properties and endplate morphometry

Denervated extensor digitorum longus muscles of Wistar rats were electrically stimulated in vivo for 4 days (2h per day) after peroneal nerve crush 1 cm from the muscle. Isometric contractile properties and endplate ultrastructure were measured on days 11 and 18. On day 11, the time to peak (116% of control) and 1/2-relaxation time (136% of control) for the twitch tensions of stimulated muscles measured in vivo were significantly less than those (127% and 157% of controls, respectively) of non-stimulated muscles. Peak twitch and tetanic tensions were not significantly different. The postsynaptic area of endplates for stimulated muscles were closer in size to controls than those for the non-stimulated ones. On day 18, no difference was found in the contractile responses between stimulated and non-stimulated groups. Similarly, the postsynaptic areas were the same for both groups. These results demonstrate that denervated muscle stimulated electrically for 4 days prior to reinnervation can preserve the structure of the endplate as well as accelerate recovery of normal function in reinnervated muscle fibers after 11 days of denervation.     

Porcine malignant hyperthermia: cell injury enhances halothane sensitivity of biopsies

A possible relationship between muscle cell injury or deterioration and enhanced halothane sensitivity was studied by monitoring mechanical responses of skeletal muscles from normal pigs and pigs susceptible to malignant hyperthermia (MH). Increased time postbiopsy and decreased maximum control tetanic tension both correlated significantly with enhanced sensitivity to halothane. In both normal and MH-susceptible (MHS) muscles, greater halothane sensitivity was observed in cut cell than in intact cell bundles and in low tetanic tension as compared to high tension preparations. Subsequent to halothane exposure, twitches of high tension (greater than or equal 1.75 kg . cm-2) intact bundles of both normal and MHS muscles were potentiated. Tetani of normal intact bundles were not altered, whereas those of MHS bundles were depressed after halothane exposure. Control twitch-to-tetanus ratios (twitch ratios) were higher in MHS (0.23) than in normal (0.12) intact bundles. According to discriminant analysis, the best distinction between normal and MHS muscles, either cut or intact, was obtained by comparing halothane-induced changes in tetanic tension and control twitch ratios.