Bernard H. Bressler

Changes in isometric contractile properties of fast-twitch and slow-twitch skeletal muscle of dystrophic mice during postnatal development

Abstract Our primary aim was to determine if there exists a preferential involvement of the fast-twitch or slow-twitch skeletal muscle fibers in the dy 2J dy 2J 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 2 RT ) 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 2 RT of dystrophic SOL were prolonged. However, by 32 weeks there was no longer a significant difference seen in TTP or 1 2 RT 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.

Effect of neonatal denervation on the distribution of fiber types in a mouse fast-twitch skeletal muscle

SummaryThis study was designed to assess the changes in fiber-type distribution of the extensor digitorum longus (EDL) muscle of the mouse during the first 21 days of age following neonatal sciatic neurectomy. Denervated and normal muscles were compared at 7, 14, and 21 days of age and the normal EDL was also studied at 1 day of age. Frozen sections of the EDL were treated histochemically to detect NADH-tetrazolium reductase and myosin ATPase reactions. Quantitative assessment included measurements of cross-sectional areas and fiber counting. Denervation resulted in muscle atrophy which was due primarily to a decrease in individual fiber area as opposed to fiber loss. Histochemical maturation of the EDL was severely affected by neonatal denervation during the first three postnatal weeks. By 21 days, two extrafusal fiber types which were both oxidative could be distinguished. One type was highly atrophied and resembled an immature fiber exhibiting myosin ATPase staining at both acid and alkaline preincubation conditions, whereas another type was less atrophied and showed myosin ATPase staining resembling fast-twitch (type HA) fibers. These findings emphasize the importance of an intact nerve supply in determining the phenotypic expression of skeletal muscle, and point to the early postnatal period as a critical stage in fiber type differentiation.

Myotrophic effects on denervated fast-twitch muscles of mice: Correlation of physiologic, biochemical, and morphologic findings

Certain morphological, biochemical, and physiological parameters were assessed in fast-twitch muscles of 6-week-old mice with unilateral hindlimb denervation for 4 weeks. Some of the mice received daily injections (i.p.) of nerve extract throughout the period of denervation. Values from treated and untreated denervated muscles were compared with each other and with those from contralateral, innervated controls. The cross-sectional areas of denervated types IIA and IIATy muscle fibers were 45% and 28% greater, respectively, in muscles of treated than of untreated mice, which resulted in greater maximal tetanic tension. Injection with nerve extract did not influence the postdenervation reduction of phosphorylation of myosin light chain 2-fast nor the loss of posttetanic twitch potentiation, two parameters thought to be related. Denervation produced a significant decrease in relative content of cytosolic parvalbumin; however, this change was completely prevented by administration of nerve extract. This latter finding correlated with the amelioration of greater than 50% of the postdenervation prolongation of half-relaxation time of the twitch in treated than in untreated muscles. More than half of the prolongation of time-to-peak of the twitch was also prevented in denervated muscles of treated than of untreated mice.

Isometric contractile properties of extensor digitorum longus muscle of the dystrophic hamster BIO 40.54.

Abstract The contractile properties, including instantaneous stiffness, of normal and the genetically dystrophic BIO 40.54 hamster extensor digitorum longus muscle were compared at 60, 120, 170, 220, and 320 days of age. A general impairment in tension-generating ability of the dystrophic muscles was observed which correlated well with a corresponding change in the measured stiffness. A decrease in the mechanical half-relaxation time was observed at all age groups except 120 and 170 days of age. The data presented are discussed in terms of a defect in the Ca 2+ regulatory functions and/or energy production and utilization in the dystrophic skeletal muscle cell.

Extracts from MDX and normal mouse skeletal muscle contain similar levels of mitogenic activity for myoblasts

The mdx mouse has been used as an animal model for human Duchenne muscular dystrophy (DMD). Unlike DMD, skeletal muscles of mdx mice undergo successful regeneration and do not show extensive fibrosis and functional impairment. Growth factors have been proposed to be involved in muscle growth and regeneration. We compared mitogenic activity for skeletal myoblasts released after injury in mdx and control mice, using crushed muscle extract (CME) as a model system. We found that CMEs from normal and mdx mice contained similar mitogenic activities per microgram protein, and produced similar maximal levels of mitogenic stimulation. Skeletal muscles from mdx mice, however, released higher amounts of CME protein per gram of muscle weight compared to controls, possibly as a result of histological or physiological alterations in mdx muscle tissue. Adequate mitogenic activity in CME from mdx muscles may be related to successful muscle regeneration in mdx mice.

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

Abstract 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.

Alterations in contractile properties of extensor digitorum longus muscle from C57BL6J mice following denervation at 1 day of age

Changes in contractile properties of developing fast-twitch skeletal muscle of the C57/BL6J mouse were studied following neonatal denervation. A sciatic neurectomy was performed at 1 day of age and then denervated and control muscles were examined at 7, 14, and 21 days postdenervation. In addition, normal muscles were studied at 1 day of age. The denervated muscles exhibited prolongation of time-to-peak twitch tension and half-relaxation time, a slowing of the maximum velocity of shortening, and a marked increase in resistance to fatigue compared with controls. Isometric tetanus tension was reduced compared to the control muscle both in absolute terms and when expressed relative to body weight at all ages studied. The absolute isometric twitch tension was reduced at 7 and 14 days, but was reduced only at 21 days when expressed as a fraction of the muscle weight. Post-tetanic twitch potentiation failed to appear in the denervated muscle. It would appear that neonatal denervation results in an uncoupling of the developmental pattern of skeletal muscle.

A Comparison of Muscle Stiffness Measurements Obtained with Rapid Releases or Stretches of Frog Semitendinosus Fibers

The stiffness-tension relationship has been studied during the isometric twitch. All experiments were carried out at 0 degrees C using small fiber bundles from frog semitendinosus. Rapid shortening and lengthening steps of 3-6 nm/hs, complete in 500 mu sec were given at various times during the twitch. Instantaneous stiffness was measured as the ratio of delta P to delta L and expressed as a relative change to the maximum values recorded at the plateau of an isometric tetanus. The time course of the stiffness changes followed the tension for both releases and stretches. However, the stiffness measured with a rapid stretch was higher than when measured with a rapid release. This raises the possibility that the instantaneous elasticity is non-linear in stretches.

Measurement of the position and displacement of a light beam with a spiral scanner

A rotating spiral scanner interrupts an incident light beam for time intervals which are proportional to the position of the beam. This produces an electrical signal via a photodetector system which is independent of beam intensity. The device was used to measure the relative change in sarcomere spacing during the initial development of tension in a contracting frog sartorius muscle.