Early elevations of glycosidase, acid phosphatase, and acid proteolytic enzyme activity in denervated skeletal muscle

Effects of myotoxins on skeletal muscle fibers

This review highlights various aspects of a number of experimental myological alterations, induced by different chemical toxicants, including anticholinesterase, colchicine, vincristine, chloroquine, tetanus toxin, botulinum toxin, reserpine and emetine. Despite their chemical diversity and mechanism(s) of action, it is evident from the data discussed here that remarkably different toxic agents exert quite similar effects and induce toxic myopathies. The latter include preferential involvement of slow-twitch red muscle, mitochondrial derangement, denervation-like alterations, formation of membranous whorls, tubular aggregates, autophagic vacuoles and axonal sprouts. The non-invasive experimental models discussed here are valuable in studying various aspects of myopathology in the absence of any mechanical damage to the innervating elements from neurons to axonal terminals.

Effect of soluble factors from nerve and muscle on alpha-bungarotoxin binding sites in isolated sarcolemmal membranes of the rat

1. Sarcolemmal membranes were isolated from normal and from denervated muscles and the specific binding of [125I]-alpha-bungarotoxin to the membranes was determined. 2. Cytosol prepared from either slow (soleus) or fast (extensor digitorum longus) muscle increased the toxin binding. Similar effects were seen with cytosol from muscles which had been denervated 5 to 7 days previously and with cytosol from sciatic nerve. A purified preparation of a bacterial phospholipase C also produced this effect. 3. Bathing medium in which normal or denervated muscles had been incubated, increased the toxin binding to normal membranes, but the normal bathing medium decreased the binding to membranes prepared from denervated muscles. 4. The possibility that pre-existing latent acetylcholine receptors can be activated by intracellular factors released from muscles is discussed.

Release of hydrolases and acetylcholine sensitivity in rat skeletal muscle

Cytosol prepared from the hind-limb muscles of the rat contained factors which increased the acetylcholine (ACh) contractures in intact normally innervated muscles in vitro. Muscles which had been denervated 4-6 days previously and normally innervated muscles released a factor upon incubation which increased the ACh responses of normal muscles. The normal muscles, however, also released an inhibitor of this effect. The medium obtained after incubation of the muscles contained a variety of hydrolytic enzymes including phospholipase C, acid protease and calcium-activated neutral protease. The amounts of phospholipase C released by normal and denervated muscles were similar, but approximately four times more of the two proteases was released from the denervated muscles than from normal muscles. The possibility that these hydrolases could be the factors which increased the ACh sensitivity in normal muscles is discussed.

The effect of denervation on mammalian sarcolemmal proteins and glycoproteins

The effects of surgical denervation on proteins and glycoproteins of sarcolemmal membranes have been investigated using sarcolemmal fractions prepared from mixed rat muscles. Denervation did not cause any gross change in the protein composition but some consistent quantitative changes were detected. Denervation caused a significant increase in the binding of 125I-labeled concanavalin A (ConA), Ricinus communis agglutinin (RCA120), and wheat germ agglutinin (WGA) to intact membranes. This increased binding appears to be brought about by two mechanisms: the synthesis of more binding-sites with the same apparent KD and the unmasking of previously cryptic binding sites. The activity of sialyl-, galactosyl-, and N-acetylglucosaminyltransferases in the sarcolemmal fractions increased, whereas fucosyl-glycoprotein-transferase activity decreased following denervation. Kinetic analysis of the sialyl- and galactosyltransferase activities showed that the change was due to an increase in Vmax with no change in Km. These results are consistent with an increase in the turnover of sarcolemmal glycoproteins following denervation.

Nerve crush induced changes in molecular forms of acetylcholinesterase in soleus and extensor digitorum muscles

The molecular forms of acetylcholinesterase (AChE, EC 3.1.1.7) extracted from the fast twitch extensor digitorum longus (EDL) and slow twitch soleus (SOL) muscles, were separated by velocity sedimentation after sciatic nerve crush. Three molecular forms were routinely separated from EDL and four from SOL muscle. In the EDL, the 4S and 10S represented the greatest amounts of AChE, and in the SOL the 12S and 16S were the major constituent forms of the enzyme. Total AChE activity in EDL and SOL muscles rapidly decreased after nerve crush. During reinnervation (2 weeks postcrush), total AChE activity in the EDL gradually recovered, whereas the SOL exhibited a 2.5-fold transient increase above control. Immediately after denervation, decreases in the three AChE forms from the EDL (4, 10, and 16S) were evident, whereas the SOL exhibited both rapid increases (4 and 10S) and decreases (12S and 16S). In both muscles the 4S form reappeared before the 16S and 10S molecular forms, suggesting that the light form, 4S, may be a precursor of the heavier molecules. Transient increases during reinnervation occurred in the 16S AChE form in both muscles; however, they were approximately five times greater in the SOL than in the EDL. In the SOL all other molecular forms showed similar increases, whereas none was seen in the EDL. Possible mechanisms are differences in synthesis and catabolism, and in release of molecular forms from these muscles.

Cell proliferation in denervated muscle: time course, distribution and relation to disuse

The effects of denervation on skeletal muscle fibers have been intensively investigated, but the effects on other cell types within muscle tissue are not well understood. In the present experiments, cell proliferation was analyzed in mouse extensor digitorum longus muscles denervated for periods of one day to six weeks. Incorporation of tritiated thymidine into DNA increased 36 h after denervation, reached a maximum at a level twenty times control at 4 days, and returned towards control values by 7 days. Incorporation first increased in the endplate area, but 12 h later involved the entire muscle. Six weeks after denervation, muscles labeled at 4 days had lost 90% of the total label. Muscle disuse, produced by tetrodotoxin block of the nerve for up to 4 days, did not result in a proliferative response. Thus, cell proliferation after denervation is not a response to simple disuse, but rather to a nerve- or muscle-related mitogen. Since the response is mostly distributed throughout the entire muscle, the mitogen probably emanates from muscle fibers.

Neurotrophic regulation of rat muscle glucose-6-phosphate dehydrogenase in vitro

In organ culture of rat diaphragm, the presence of a 2-2.5 cm phrenic nerve stump delays the time of failure of miniature endplate potentials and eliminates the increase in glucose-6-phosphate dehydrogenase (G6PD) activity which otherwise occurs at 16.5 h in muscles cultured without nerve stumps. The nerve stump effect persists in the presence of blocking doses of D-tubocurarine but is eliminated by nerve crush. As shown by studies of amino acid incorporation into protein, the effect does not involved an overall change in protein synthesis. Effluents collected over 1-2 h from unstimulated or stimulated phrenic nerve-muscle preparations had no effect on G6PD activity when applied to muscles cultured without nerve stumps. However, medium conditioned by use in organ cultures with long nerve stumps partially countered the denervation-like effect in host cultures. Thus, the nerve maintains muscle G6PD by a humoral mechanism probably unrelated to impulse activity or nicotinic receptor activation.

Effects of acute and chronic denervation on release of acetylcholinesterase and its molecular forms in rat diaphragms

Hemidiaphragms were removed from rats at various times after intrathoracic transection of the left phrenic nerve and were incubated in organ baths containing 1.5 ml of oxygenated, buffered physiologic saline solution, with added glucose and bovine serum albumin. After incubation, the acetylcholinesterase (AChE: EC 3.1.1.7) activities of the bath fluid and of the muscle were determined. Innervated left hemidiaphragms were found to release 107 units of AChE over a 3-h period, corresponding to 1.9% of their total AChE activity. Denervation led to a rapid loss of AChE from the muscle coincident with a transient increase in the outpouring of enzyme activity into the bath fluid. Thus, 1 day after nerve transection the left hemidiaphragm contained only 68% of the control amounts of AChE activity, but released 140% as much as control. After 3 or 4 days of denervation, the AChE activity of the diaphragm stabilized at 35% of the control value. Release also fell below control by this time, but not as far. One week after denervation the release, 69 units per 3 hr, correspond to 3.3% of the reduced content of AChE activity in the muscle, indicating that denervation caused an increase in the proportion of AChE released. Sucrose density gradient ultracentrifugation showed that 10S AChE accounted for more than 80% of the released enzyme activity at all times. The results did not rule out the possibility, however, that the released enzyme originally stemmed from 4S or 16S AChE in the diaphragms.

Protease inhibitors reduce effects of denervation on muscle end-plate acetylcholinesterase

The effects of certain protease inhibitors on end-plate acetylcholinesterase (AChE) activity, as well as on wet weight and total protein, were studied in vivo in intact and denervated anterior gracilis muscles from the rat. A combination of leupeptin, pepstatin, and aprotinin, administered intraarterially, partly prevented the early (24 h) denervation-induced decrease in muscle weight and protein content. In turn, leupeptin and aprotinin, either alone or in combination, markedly reduced the decay of AChE activity in the denervated muscles, whereas pepstatin alone was ineffective. Such effects were additive in that the inhibitors in combination were more effective than when they were used separately. Additional experiments indicated that none of the inhibitors, at the concentrations used, affected AChE activity directly, nor did they have a significant effect during processing of the muscle samples. These findings indicate that the initial decay of AChE activity with denervation was effectively reduced by the inhibitors, probably through inactivation of proteolytic enzymes which, otherwise, would be increase in denervated muscle.

Connective tissue metabolism in normal and atrophic skeletal muscle

The first two enzymes of the pentose-phosphate pathway (G6PD and 6PGD) were studied in rabbit skeletal muscle. Their activities closely paralleled the connective tissue levels in each individual muscle, and they increased in atrophying muscles at the same rate as connective tissue. Skeletal muscle tendons and subcutaneously implanted polyvinyl sponges, both of which consist of relatively pure connective tissue, displayed much higher activity of these enzymes than the muscle homogenates. These results suggest that pentose synthesis in muscle is mostly confined to its connective tissue elements, raising the possibility of a role for connective tissue in the biosynthetic processes of skeletal muscle. The contribution of connective tissue to the results of muscle quantitative studies may be significant, and even overshadow other values in atrophic muscles, where the muscle cell population may be greatly reduced. The interpretation of quantitative biochemical studies on pathological muscle samples should take this factor into consideration.

Effects of denervation on the composition and synthesis of sarcolemmal proteins

In vitro synthesis of proteins and changes in polypeptide composition of sarcolemma were studied in innervated and denervated extensor digitorum longus muscle of the rat. A technique of evacuating myoplasm from muscle slices was used as a preliminary step in the preparation of three membrane fractions, M, H and S, containing sarcolemma. On the basis of findings from the previous study and the present investigation, it was concluded that the M fraction was most enriched with extrajunctional sarcolemma. In vitro incorporation of [3H]leucine into membrane proteins of the M fraction showed an apparent linear increase in the rate of protein synthesis from 1-10 days after denervation. The relative increase at 10 days was 137% greater than that of innervated controls. Fractions H and S showed a smaller relative increase. Polypeptide composition of M, H and S fractions based on SDS gel electrophoresis of innervated and denervated muscle, showed qualitative and quantitative changes. The most striking difference was a nominal 29 000 component in M that constituted a disproportionately large peak. Following 10 days of denervation the M fraction underwent significant compositional changes in its electrophoretic profile, the most dramatic of which was a large reduction in the proportion of the 19 000 component. The denervation-induced compositional change is discussed in light of known alterations in the chloride conductance of the muscle plasmalemma.

Neuromuscular junction macromolecules in the pathogenesis of amyotrophic leteral sclerosis

An hypothesis regarding the pathogenesis of amyotrophic lateral sclerosis is presented, which places emphasis on extraneural cells. Classical experimental denervation is compared and contrasted with motor neuron disease, both from information in the literature as well as concepts deriving from the hypothesis. Background information regarding neuromuscular junction-specific (16S) acetylcholinesterase and a basal lamina-enriched surface glycoprotein (fibronectin) are presented, which suggest not only their mutual interaction, but likely parallel regulation on muscle cell surfaces by the motor nerve. Since 16S acetylcholinesterase likely contains basal lamina-type collagen and fibronectin specifically associates with collagen, a model relating activation of latent collagenase enzyme in amyotrophic lateral sclerosis is described. It is suggested that continued degeneration, including transneuronal effects, of the motor system ensues from random, continuous loss of nerve-muscle adherence resulting from collagen resorption at the neuromuscular junction.

Early changes in muscle glucose-6-phosphate dehydrogenase activity after denervation: locus and dependence on nerve stump length

Glucose-6-phosphate dehydrogenase (G6PD) activity of rat diaphragm muscle increased significantly 12 h after intrathoracic ('distal') denervation. Since this change represented one of the earliest known events after loss of innervation, the mechanism of neuronal regulation and the nature of the muscle response were further investigated. When, instead of distal denervation, the motor nerve was cut about 3.5 cm from the diaphragm, G6PD activity slightly decreased 12 h later. Denervations in which part of the same hemidiaphragm had a short nerve stump and another part, a long nerve stump, also showed a differential increase of G6PD activity dependent on length of nerve stump. In biochemical studies of the locus of the early post-denervation elevation of G6PD, enzyme activity was increased both in endplate and non-endplate regions of the muscle. Finally, the histochemical reaction for G6PD was more intense in both muscle fibers and interstitial elements, although the former made the major contribution to the biochemical results. In sum, muscle G6PD enzyme activity is closely regulated by a neurotrophic mechanism partly independent of nerve or muscle electrical activity.

Histochemistry of some acid hydrolases in striated muscles of the rat

The distribution of acid phosphatase, beta-N-acetylglucosaminidase, beta-glucuronidase, and acid beta-galactosidase was studied in mm. extensor digitorum longus, soleus, and diaphragm of rats. Using the technic of semipermeable membranes activities of these enzymes were demonstrated beside cells of the interstitial tissue in muscle fibers themselves as well. Acid phosphatase displayed the highest activity which appeared in many small dots dispersed in the fiber. The activity of acid phosphatase was about 1.2 X higher in the m. soleus than in the m. extensor digitorum longus. In the latter muscle a somewhat higher activity was often found in muscle fibers displaying a higher staining for NADH tetrazolium reductase. The activity of beta-N-acetylglucosaminidase was slightly lower, that of beta-glucuronidase very weak but still discernible. The activity of acid beta-galactosidase was not ascertained in the majority of fibers. The ratio of activities measured in an area of the same size in cells of the interstitial tissue and in muscle fibers amounted in average to 2.6:1 in the case of acid phosphatase, 2.5:1 in the case of beta-N-acetylglucosaminidase, 5.7:1 in the case of beta-glucuronidase, and 44.3:1 in the case of acid beta-galactosidase. The importance of the histochemical technic in studies concerned with acid hydrolases in striated muscle fibers in normal and pathological conditions is pointed out.

Changes of acid phosphatase activity of fast and slow rat muscles during ontogenetic development

1. The activity of acid phosphatase (AP) were studied both bio- and histochemically in fast and slow muscles of the rat during postnatal development. 2. Both biochemical and histochemical methods show at birth a similar, high AP activity in both, fast and slow muscles. From 20 days onward AP activity is higher in the slow soleus than in the fast extensor digitorum longus (EDL) muscle. 3. Fibres with high AP activity have also higher activities of oxidative enzymes and of non-specific esterase (NSE). This property is not coupled to degree of ATP-ase activity; fibres with high or low ATP-ase activity reveal similar activities with respect to AP. 4. There is a "two-phase" developmental change in the degree of enzyme activity in both the fast EDL and slow soleus muscles. AP activity is high at birth, decreases during later postnatal development and increases again in senescent muscles.

The effect of muscle extracts on the contracture response of skeletal muscle to acetylcholine

Preincubation of normal rat soleus muscles in vitro with homogenates prepared from mixed leg muscles which had been denervated 4 days previously resulted in an increase in the contracture response to acetylcholine. After 30 min incubation a 1.5-fold increase was observed. Homogenates of normally innervated muscles did not increase the response. The active principles of the denervated muscles were found to reside in the "cytosol" fraction. An approximately 2-fold increase was observed upon incubation with the cytosol for 30 min; incubation for longer periods resulted in a subsequent decrease in the response. The effect of the denervated muscle cytosol was concentration-dependent and heat-labile. Normal muscle cytosol also increased the soleus muscle response to acetylcholine but this fraction was less effective than denervated muscle cytosol. The response of control muscles incubated in Krebs-Henseleit solution was found to decrease with time. Commercially obtained phospholipases C and D increased the response of normal soleus muscles approximatley 2-fold. Phospholipase A, lipase, trypsin, collagenase and a bacterial protease had no effect, lysozyme produced a small but consistent increase in the response to acetylcholine.

Appearance of acetylcholinesterase and creatine kinase in plasma of normal chickens after denervation

Evidence that acetylcholinesterase (AChE) activity is released from normal chick embryonic muscle fibers and from muscles of chickens with inherited muscular dystrophy suggested that denervated chick muscles, which have AChE properties similar to dystrophic muscles, would also release AChE. Bilateral denervation of the breast and wing muscles of normal chickens was followed by the appearance of AChE activity, distinguished from plasma cholinesterase by differential substrate hydrolysis, inhibitor sensitivity, and electrophoretic migration. Plasma creatine kinase (CK) activity was also elevated after denervation.