Motor and sensory reinnervation of fast and slow mammalian muscles

Summary 1. The formation of endplates outside the original endplate region of a muscle fibre was studied in slow and fast rat muscles. It was found that such new endplates are readily formed on the soleus muscle, whereas hardly at all in the fast extensor digitorum longus. Most new endplates appear to be morphologically normal within 2 months after nerve implantation.2. The time course of recovery of slow and fast cat muscles was followed after crushing the sciatic nerve. It was found that the slow soleus muscle recovers more rapidly than the fast flexor hallucis longus muscle.3. The endplates of reinnervated cat muscles are more complicated than those of the control muscles, but have nevertheless fewer nerve terminals per endplate. Reinnervated muscles are more sensitive to curare and it is suggested that this is due to a decrease in transmitter release, for it was found that less acetylcholine is released from reinnervated rat hemidiaphragms than from control ones.4. Motor and sensory reinnervation of spindles and tendon organs was studied. At the time when motor reinnervation is almost completed the sensory endings from spindles and tendon organs are highly abnormal. Thus sensory reinnervation proceeds much more slowly than motor.     

Differentiation of slow and fast muscles in chickens

1. The development of the characteristic histochemical appearance of the slow anterior latissimus dorsi (ALD) and fast posterior latissimus dorsi (PLD) was studied in chickens during embryonic development as well as during regeneration of minced muscle. 2. During embryonic development the activity of the oxidative enzyme succinic dehydrogenase (SDH) is higher in the slow ALD muscle already at 16 days of incubation. At this time the fast PLD has a higher activity of the glycolytic enzyme, phosphorylase. Although the histochemical appearance of the two types of muscle is already different at 16 days, their contractile speeds are still similar. No difference in myosin ATP-ase was found in the two muscles in young embryos but in 20-day old embryos the two muscles became distinctly different when stained for this enzyme. 3. When PLD muscles in hatched chickens redeveloped during regeneration in place of ALD the histochemical characteristics of the regenerated muscle resembled ALD, and when ALD regenerated in place of PLD it resembled PLD. 4. It is concluded that the histochemical characteristics of slow and fast muscles become determined during early development, even before any difference in contractile properties can be detected and that they are determined by the nerve.     

31P NMR study of phosphorus metabolites in fast and slow muscles

1. 31P NMR was used to characterize phosphate pools in perchloric acid extracts of muscles with various composition of muscle fibre types. 2. The white m. pectoralis major (MPM) of chickens 15 min post mortem is characterized by 1.6-times higher relative content of phosphocreatine (PCr) in comparison with mixed leg muscle (LM) of this species. The glycerophosphorylcholine (GPC) does not occur in MPM at NMR detectable level in contrast to the leg muscles. Relative amounts of other phosphates are similar in both muscles. 3. The intermediate MPM of pigeons as well as mixed LM of this species contain 15 min post mortem a very small amount of PCr and ATP but a large amount of inorganic phosphate. Relative content of GPC is higher in leg muscles than in intermediate MPM. 4. Muscles with higher occurrence of white fibres contain relatively more PCr than muscles with lower occurrence of white fibres. 5. The occurrence of GPC seems to be connected with metabolism of red muscle fibres.     

Effects of clofibrate on basal and insulin-activated glucose uptake in fast and slow skeletal muscles of rats.

1. The basal uptake of glucose was increased significantly in the extensor digitorum longus muscle (EDL) of rats by clofibrate administration. 2. The insulin-activated uptake of glucose was increased in the soleus muscle (Sol) by clofibrate. 3. The insulin-induced increment of glucose uptake was increased significantly in Sol and decreased significantly in EDL by clofibrate.     

A comparative study of muscle spindles in slow and fast neonatal muscles of normal and dystrophic mice

Muscle spindles from the slow-twitch soleus and the fast-twitch extensor digitorum longus (EDL) muscles of genetically dystrophic mice of the dy2J/dy2J strain were compared with age-matched normal animals at neonatal ages of 1-3 weeks according to histochemical, quantitative, and ultrastructural parameters. Intrafusal fibers in both the soleus and EDL exhibited similar regional differences in myosin ATPase activity, and conformed to those noted previously in various adult species. In distal polar regions, all nuclear bag fibers resembled extrafusal fibers of the type 1 variety, whereas in capsular zones they could be divided into two subtypes. Nuclear chain fibers possessed a staining pattern similar to type 2 extrafusal fibers, and in contrast to the bag fibers they exhibited no regional variations. These features were consistently observed in both the normal and dystrophic muscles at all ages. Spindles varied only slightly in their number and distribution in the two types of muscle, and their location followed the neurovascular branching pattern in each. Irrespective of age or genotype, spindles in the soleus were more homogeneously dispersed, but those in the EDL were concentrated along the dorsal aspect of the muscle. No significant differences were noted in the total number of spindles between normal and dystrophic muscles. In addition, no dramatic differences were observed in the muscle spindle index for soleus and EDL. The first obvious disease-related changes were noted in extrafusal fibers of the soleus of 3-week-old mice, and spindles were often located close to these areas of fiber degeneration. Despite alterations in the surrounding tissue, however, spindles appeared morphologically unaltered in dystrophy. These observations indicate that intrafusal fibers of spindles in neonatal mice appear enzymatically and histologically unaffected in incipient stages of progressive muscular dystrophy.     

A histochemical study of the muscles of Drosophila melanogaster.

The thoracic muscles of Drosophila melanogaster can be classified into two classes, the fibrillar and the tubular muscles, on morphological grounds. Histochemical techniques were used to characterize these two classes of muscle according to their content of various enzymes (alpha-glycerophosphate, NAD-dependent isocitrate, malate and succinate dehydrogenases, fumarase, acid phosphatase, adenosine triphosphatase and acetylcholinesterase) and of glycogen. These investigations showed that the two muslces types are histochemically very different and, further, that the morphologically similar tubular muscles are heterogeneous with respect to their enzyme content. In particular, the tergal depressor of the trochanter of the second leg, the largest of the tubular muslces, has considerably less of all the enzymes studied, with the exception of acetylcholinesterase, than all the other tubular muscles examined. The histochemical techniqes were also used to follow the changes in enzyme levels that occur during development of the indirect flight muscle fibres. All the enzymes that are present in adult flight muslces showed an increase in staining intensity throughout muscle development. Some minor differences were observed in the time of appearance and rate of increase of intensity of the different enzymes.     

The fine structure of fast and slow crustacean muscles

Known phasic and tonic muscle fibers of the crab Cancer magister were studied by electron microscopy. Phasic fibers have sarcomeres about 4.5 µ long, small polygonal myofibrils, and a well-developed sarcoplasmic reticulum. The thick myofilaments, disposed in hexagonal array, are each surrounded by six thin filaments. The tonic fibers have a sarcomere length of about 12 µ, larger myofibrils, a poorly developed sarcoplasmic reticulum, and a disorderly array of myofilaments. Each thick myofilament is surrounded by 10–12 thin filaments. The same morphological type of slow muscle has been found in the crustaceans, Macrocyclops albidus, Cypridopsis vidua, and Balanus cariosus, in each case in an anatomical location consistent with tonic action. A search of the literature indicates that this type of muscle is found in all classes of arthropods and is confined to visceral and postural muscles or specializations of these.     

Effect of thyroxine on basal and insulin-stimulated glucose uptake by fast and slow skeletal muscles of rats.

1. The sc injection of 1-thyroxine (2 mg/kg bw/day) for 8 days produced a significant decrease of body weight gain in young male Wistar rats. 2. In these hyperthyroid rats there was a significant decrease in the wet weight of the extensor digitorum longus (EDL) and soleus (Sol) muscles as compared with those of control rats. 3. The basal glucose uptake by the EDL and Sol muscles was unchanged in hyperthyroid rats using the wet weight of muscle as a reference. 4. In hyperthyroid rats, the insulin-stimulated uptake of glucose by both the EDL and Sol muscles was significantly decreased. This inhibition was stronger in Sol and there was no insulin stimulation of glucose uptake by Sol.     

Enzymatic activities in slow and fast denervated old rat muscles

The activities of five enzymes have been studied quantitatively in denervated extensor digitorum longus, gastrocnemius and soleus muscles of 24-month-old rats. The results have been compared with those obtained from normal muscles of a similar age group of rats. Three weeks after denervation, the activity of hexokinase was increased in gastrocnemius and extensor digitorum longus. Phosphofructokinase, lactate dehydrogenase, malate dehydrogenase and 3-hydroxyacyl-CoA-dehydrogenase showed decreased activities. These results suggest that enzyme which represents glucose uptake increased its activity in fast muscles and that enzymes for anaerobic glycolysis, lactate fermentation, citric acid cycle and beta-oxidation had a decreased activity in slow and fast muscles.     

Regulation of glucose uptake in rat slow and fast skeletal muscles

1. Regulation of glucose uptake was compared between extensor digitorum longus (EDL) and soleus (Sol) muscles in rats. 2. Insulin stimulated glucose uptake more in EDL than in Sol. 3. Under high concentrations of insulin, the glucose uptake was higher in EDL than Sol. 4. Inhibition of oxidative phosphorylation by anoxia or an uncoupler stimulated glucose uptake more in EDL than in Sol. 5. Anoxia abolished the effect of insulin on glucose uptake in both EDL and Sol. 6. The blocker to glucose transport system reduced glucose uptake more in Sol than in EDL.     

External potassium and action potential propagation in rat fast and slow twitch muscles.

The role of extracellular K+ concentration in the propagation velocity of action potential was tested in isolated rat skeletal muscles. Different K+ concentrations were produced by KCl additions to extracellular solution. Action potentials were measured extracellularly by means of two annular platinum electrodes. Fibre bundles of m. soleus (SOL), m. extensor digitorum longus (EDL), red (SMR) and white (SMW) part of m. sternomastoideus were maximum stimulated. The conduction velocity (c.v.) was calculated from the distance between the electrodes and the time delay of the potentials measured at 22 degrees C. In Tyrode solution containing 5 mmol/l K+, the c.v. was close to 1 m.s-1. Bundles of the fast muscle type seemed to have a somewhat higher c.v. The differences observed in these studies were not significant. At higher temperatures, the c.v. increased (Q10 of approx. 2) and a dissociation between SMR and SMW muscles appeared. An elevation of K+ concentration to 10 mmol/l induced a drop of the c.v. by approx. 25% and 15% in EDL and SOL muscles, respectively. After return to normal solution, the recovery was not complete within 30 min. In K+ free solution the c.v. of EDL and SM muscles rose by a factor of 1.5, but less in SOL muscles. The weaker response of SOL to K+ modification was related to the higher resistance of this muscle to fatigue. This suggestion was supported by experiments on fatigued fibre bundles. Immediately after a tetanic stimulation producing fatigue, the c.v. of EDL and SOL muscles dropped similarly as in 10 mmol/l K+; again, the drop was less for SOL muscles. Adrenaline (0.5-10.0 mumol/l) enhanced both the c.v. and the twitch amplitude. The results support the suggestion that extracellular K+ accumulation during activity is an essential factor of muscle fatigue.     

Regulation of glucose uptake in fast and slow skeletal muscles with fasting and refeeding.

1. The decrease in wet weight and noncollagen protein (NCP) was faster and greater in extensor digitorum longus (EDL) during fasting than in soleus (Sol) muscles in rats. 2. During refeeding, recovery was completed faster in Sol than in EDL. 3. Glucose uptake in skeletal muscle increased significantly during fasting on both a per wet weight and NCP basis. 4. This increase was faster and greater in EDL than Sol. 5. The initial increase in glucose uptake was greater during refeeding than fasting only in EDL.     

Soluble phosphatidylinositol phosphodiesterase in normal and denervated fast and slow muscles of the rat.

Phosphatidylinositol phosphodiesterase activity was determined in cytosol prepared from rat slow (soleus) and fast (extensor digitorum longus) muscles. The substrate was prepared by incubation of sarcoplasmic reticulum with myo-[2-3H]inositol. The enzyme hydrolysed both membrane-bound and extracted phosphatidylinositol. The activity determined with the isolated phospholipid exhibited an optimum at pH 5.5. Ca2+ ions stimulated the activity. The enzyme specific activity was higher in cytosol prepared from soleus muscle than in that from extensor digitorum longus muscle. After section of the motor nerve, the activity of the enzyme increased in both muscles up to 36 h and then declined. A function for this enzyme in the control of acetylcholine sensitivity in muscle is discussed.     

Myosin transitions in developing fast and slow muscles of the rat hindlimb

Abstract. Myosin isozymes from the slow soleus and fast EDL muscles of the rat hindlimb were analyzed by pyrophosphate gel electrophoresis, by peptide mapping of heavy chains, and by antibody staining. At the earliest stage examined, 20 days gestation, distinctions between the developing fast and slow muscles were seen by all these criteria; all fibers in the distal hindlimb reacted strongly with antibody to adult fast myosin. Some fibers also reacted with antibody to adult slow myosin; these fibers had a precise, axial distribution in the hindlimb. This pattern of staining which includes the entire soleus, foreshadows the adult distribution of slow fibers and may indicate that the specific pattern of innervation of the limb is already determined. In the early developing soleus there are four fetal and neonatal isozymes plus two isozymes present in equal proportions in the 'slow' area of the pyrophosphate gel. The mobility of these two slow isozymes decreases with maturity and the slowest moving isozyme gradually becomes the dominant species. Thus early diversity between the soleus and EDL is expressed by myosins which are distinct from the mature isozymes. The relative proportion of slow isozymes significantly increases with development and as this occurs the fetal and neonatal isozymes are progressively eliminated. Transiently at least one mature fast isozyme appears in the soleus. This is present at 15 days postpartum and probably correlates with the population of fast, type II fibers, which comprise 50% of this muscle cell population at 15 days. The EDL contained three fetal and neonatal isozymes and only one slow isozyme which does not change in mobility with age. Slow isozymes in the soleus and EDL are thus not identical. Each muscle underwent a unique series of changes until the adult pattern of isozymes and heavy chains was reached about one month postpartum.