Increased muscle calcium. A possible cause of mitochondrial dysfunction and cellular necrosis in denervated rat skeletal muscle

Bezafibrate attenuates immobilization-induced muscle atrophy in mice

Muscle atrophy due to fragility fractures or frailty worsens not only activity of daily living and healthy life expectancy, but decreases life expectancy. Although several therapeutic agents for muscle atrophy have been investigated, none is yet in clinical use. Here we report that bezafibrate, a drug used to treat hyperlipidemia, can reduce immobilization-induced muscle atrophy in mice. Specifically, we used a drug repositioning approach to screen 144 drugs already utilized clinically for their ability to inhibit serum starvation-induced elevation of Atrogin-1, a factor related to muscle atrophy, in myotubes in vitro. Two candidates were selected, and here we demonstrate that one of them, bezafibrate, significantly reduced muscle atrophy in an in vivo model of muscle atrophy induced by leg immobilization. In gastrocnemius muscle, immobilization reduced muscle weight by an average of ~ 17.2%, and bezafibrate treatment prevented ~ 40.5% of that atrophy. In vitro, bezafibrate significantly inhibited expression of the inflammatory cytokine Tnfa in lipopolysaccharide-stimulated RAW264.7 cells, a murine macrophage line. Finally, we show that expression of Tnfa and IL-1b is induced in gastrocnemius muscle in the leg immobilization model, an activity significantly antagonized by bezafibrate administration in vivo. We conclude that bezafibrate could serve as a therapeutic agent for immobilization-induced muscle atrophy.

Role of Ca2+ in proteolysis-inducing factor (PIF)-induced atrophy of skeletal muscle

Proteolysis-inducing factor (PIF) induces muscle loss in cancer cachexia through a high affinity membrane bound receptor. This study investigates the mechanism by which the PIF receptor communicates to intracellular signalling pathways. C(2)C(12) murine myoblasts were used as a model using PIF purified from MAC16 tumours. Calcium imaging was determined using fura-4-acetoxymethyl ester (Fura-4-AM). PIF induced a rapid rise in Ca(2+)(i), which was completely attenuated by a anti-receptor antibody, or peptides representing 20 mers of the N-terminus of the PIF receptor. Other agents catabolic for skeletal muscle including angiotensin II (AngII) tumour necrosis factor-α (TNF-α) and lipopolysaccharide (LPS) also induced a rise in Ca(2+)(i), but this was not attenuated by anti-PIF-receptor antibody. The rise in Ca(2+)(i) induced by PIF and AngII was completely attenuated by the Zn(2+) chelator D-myo-inositol-1,2,6-triphosphate, and this was reversed by administration of exogenous Zn(2+). The Ca(2+)(i) rise induced by PIF was independent of the presence of extracellular Ca(2+), and attenuated by the Ca(2+) pump inhibitor thapsigargin, suggesting that the Ca(2+)(i) rise was due to release from intracellular stores. This rise in Ca(2+)(i) induced by PIF was attenuated by both the phospholipase C inhibitor U73122 and 2-APB, an inhibitor of the inositol 1,4,5-triphosphate receptor, suggesting the involvement of a G-protein. Binding of the PIF to its receptor in skeletal muscle triggers a rise in Ca(2+)(i), which initiates a signalling cascade leading to a depression in protein synthesis, and an increase in protein degradation.

Cyclophilin-D is dispensable for atrophy and mitochondrial apoptotic signalling in denervated muscle

In the present study, we specifically determined whether the regulatory protein cyclophilin-D (CypD), and by extension opening of the permeability transition pore (PTP), is involved in the activation of mitochondria-derived apoptotic signalling previously described in skeletal muscle following loss of innervation. For this purpose, CypD-defficient (CypD-KO) mice and their littermate controls were submitted to unilateral sciatic nerve transection, and mitochondrial resistance to Ca2+-induced opening of the PTP, and muscle apoptotic signalling were investigated 14 days post-surgery. Denervation caused atrophy, facilitated Ca2+-induced opening of the PTP in vitro in permeabilized muscle fibres, and activation of the apoptotic proteolytic cascade in the whole muscle of both mouse strains. In CypD-KO mice, mitochondrial resistance to Ca2+-induced PTP opening was greater than in WT mice, in both the normal and the denervated state, indicating that lack of CypD desensitized to PTP opening. However, denervation in CypD-KO mice still resulted in a facilitation of PTP opening compared to normally innervated contralateral muscle, indicating that in vitro additional factors could poise mitochondria from denervated muscle toward PTP opening. At the whole muscle level, lack of CypD, despite conferring greater resistance to PTP opening, did not protect against atrophy, release of mitochondrial pro-apoptotic factors and activation of caspases following denervation. Altogether, these results provide direct evidence that CypD-dependent PTP opening is dispensable for atrophy and apoptotic signalling in skeletal muscle following denervation.

Mechanism of activation of dsRNA-dependent protein kinase (PKR) in muscle atrophy

The role of Ca(2+) in the activation of PKR (double-stranded-RNA-dependent protein kinase), which leads to skeletal muscle atrophy, has been investigated in murine myotubes using the cell-permeable Ca(2+) chelator BAPTA/AM (1,2-bis (o-aminphenoxy) ethane-N,N,N',N'-tetraacetic acid tetra (acetoxymethyl) ester). BAPTA/AM effectively attenuated both the increase in total protein degradation, through the ubiquitin-proteasome pathway, and the depression of protein synthesis, induced by both proteolysis-inducing factor (PIF) and angiotensin II (Ang II). Since both protein synthesis and degradation were attenuated this suggests the involvement of PKR. Indeed BAPTA/AM attenuated both the activation (autophosphorylation) of PKR and the subsequent phosphorylation of eIF2alpha (eukaryotic initiation factor 2alpha) in the presence of PIF, suggesting the involvement of Ca(2+) in this process. PIF also induced an increase in the activity of both caspases-3 and -8, which was attenuated by BAPTA/AM. The increase in caspase-3 and -8 activity was shown to be responsible for the activation of PKR, since the latter was completely attenuated by the specific caspase-3 and -8 inhibitors. These results suggest that Ca(2+) is involved in the increase in protein degradation and decrease in protein synthesis by PIF and Ang II through activation of PKR by caspases-3 and -8.

Denervation-induced skeletal muscle atrophy is associated with increased mitochondrial ROS production

Reactive oxygen species (ROS), especially mitochondrial ROS, are postulated to play a significant role in muscle atrophy. We report a dramatic increase in mitochondrial ROS generation in three conditions associated with muscle atrophy: in aging, in mice lacking CuZn-SOD ( Sod1−/−), and in the neurodegenerative disease, amyotrophic lateral sclerosis (ALS). ROS generation in muscle mitochondria is nearly threefold higher in 28- to 32-mo-old than in 10-mo-old mice and is associated with a 30% loss in gastrocnemius mass. In Sod1−/− mice, muscle mitochondrial ROS production is increased >100% in 20-mo compared with 5-mo-old mice along with a >50% loss in muscle mass. ALS G93A mutant mice show a 75% loss of muscle mass during disease progression and up to 12-fold higher muscle mitochondrial ROS generation. In a second ALS mutant model, H46RH48Q mice, ROS production is approximately fourfold higher than in control mice and is associated with a less dramatic loss (30%) in muscle mass. Thus ROS production is strongly correlated with the extent of muscle atrophy in these models. Because each of the models of muscle atrophy studied are associated to some degree with a loss of innervation, we were interested in determining whether denervation plays a role in ROS generation in muscle mitochondria isolated from hindlimb muscle following surgical sciatic nerve transection. Seven days postdenervation, muscle mitochondrial ROS production increased nearly 30-fold. We conclude that enhanced generation of mitochondrial ROS may be a common factor in the mechanism underlying denervation-induced atrophy.

Muscle denervation promotes opening of the permeability transition pore and increases the expression of cyclophilin D

Loss of neural input to skeletal muscle fibres induces atrophy and degeneration with evidence of mitochondria-mediated cell death. However, the effect of denervation on the permeability transition pore (PTP), a mitochondrial protein complex implicated in cell death, is uncertain. In the present study, the impact of 21 days of denervation on the sensitivity of the PTP to Ca2+-induced opening was studied in isolated muscle mitochondria. Muscle denervation increased the sensitivity to Ca2+-induced opening of the PTP, as indicated by a significant decrease in calcium retention capacity (CRC: 111 +/- 12 versus 475 +/- 33 nmol (mg protein)(-1) for denervated and sham, respectively). This phenomenon was partly attributable to in vivo mitochondrial and whole muscle Ca2+ overload. Cyclosporin A, which inhibits PTP opening by binding to cyclophilin D (CypD), was significantly more potent in mitochondria from denervated muscle and restored CRC to the level observed in mitochondria from sham-operated muscles. In contrast, the CypD independent inhibitor trifluoperazine was equally effective at inhibiting PTP opening in sham and denervated animals and did not correct the difference in CRC between groups. This phenomenon was associated with a significant increase in the content of the PTP regulating protein CypD relative to several mitochondrial marker proteins. Together, these results indicate that Ca2+ overload in vivo and an altered expression of CypD could predispose mitochondria to permeability transition in denervated muscles.

Ca(2+)-dependent proteolysis in muscle wasting

Skeletal muscle wasting is a prominent feature of cachexia, a complex systemic syndrome that frequently complicates chronic diseases such as inflammatory and autoimmune disorders, cancer and AIDS. Muscle wasting may also develop as a manifestation of primary or neurogenic muscular disorders. It is now generally accepted that muscle depletion mainly arises from increased protein catabolism. The ubiquitin-proteasome system is believed to be the major proteolytic machinery in charge of such protein breakdown, yet there is evidence suggesting that Ca(2+)-dependent system, lysosomes and, in some conditions at least, even caspases are involved as well. The role of Ca(2+)-dependent proteolysis in skeletal muscle wasting is reviewed in the present paper. This system relies on the activity of calpains, a family of Ca(2+)-dependent cysteine proteases, whose regulation is complex and not completely elucidated. Modulations of Ca(2+)-dependent proteolysis have been associated with muscle protein depletion in various pathological contexts and particularly with muscle dystrophies. Calpains can only perform a limited proteolysis of their substrates, however they may play a critical role in initiating the breakdown of myofibrillar protein, by releasing molecules that become suitable for further degradation by proteasomes. Some evidence would also support a role for lysosomes and caspases in muscle wasting. Thus it cannot be excluded that different intracellular proteolytic systems may coordinately concur in shifting muscle protein turnover towards excess catabolism. Many different signals have been proposed as potentially involved in triggering the enhanced protein breakdown that underlies muscle wasting. How they are transduced to initiate the hypercatabolic response and to activate the proteolytic pathways remains largely unknown, however.

Novel aspects on the regulation of muscle wasting in sepsis

Muscle wasting in sepsis is associated with increased expression of messenger RNA for several genes in the ubiquitin-proteasome proteolytic pathway, indicating that increased gene transcription is involved in the development of muscle atrophy. Here we review the influence of sepsis on the expression and activity of the transcription factors activator protein-1, nuclear factor-kappaB (NF-kappaB), and CCAAT/enhancer binding protein, as well as the nuclear cofactor p300. These transcription factors may be important for sepsis-induced muscle wasting because several of the genes in the ubiquitin-proteasome proteolytic pathway have multiple binding sites for activating protein-1, nuclear factor-kappaB, and CCAAT/enhancer binding protein in their promoter regions. In addition, the potential role of increased muscle calcium levels for sepsis-induced muscle atrophy is reviewed. Calcium may regulate several mechanisms and factors involved in muscle wasting, including the expression and activity of the calpain-calpastatin system, proteasome activity, CCAAT/enhancer binding protein transcription factors, apoptosis and glucocorticoid-mediated muscle protein breakdown. Because muscle wasting is commonly seen in patients with sepsis and has severe clinical consequences, a better understanding of mechanisms regulating sepsis-induced muscle wasting may help improve the care of patients with sepsis and other muscle-wasting conditions as well.

Sepsis stimulates calpain activity in skeletal muscle by decreasing calpastatin activity but does not activate caspase-3

We examined the influence of sepsis on the expression and activity of the calpain and caspase systems in skeletal muscle. Sepsis was induced in rats by cecal ligation and puncture (CLP). Control rats were sham operated. Calpain activity was determined by measuring the calcium-dependent hydrolysis of casein and by casein zymography. The activity of the endogenous calpain inhibitor calpastatin was measured by determining the inhibitory effect on calpain activity in muscle extracts. Protein levels of μ- and m-calpain and calpastatin were determined by Western blotting, and calpastatin mRNA was measured by real-time PCR. Caspase-3 activity was determined by measuring the hydrolysis of the fluorogenic caspase-3 substrate Ac-DEVD-AMC and by determining protein and mRNA expression for caspase-3 by Western blotting and real-time PCR, respectively. In addition, the role of calpains and caspase-3 in sepsis-induced muscle protein breakdown was determined by measuring protein breakdown rates in the presence of specific inhibitors. Sepsis resulted in increased muscle calpain activity caused by reduced calpastatin activity. In contrast, caspase-3 activity, mRNA levels, and activated caspase-3 29-kDa fragment were not altered in muscle from septic rats. Sepsis-induced muscle proteolysis was blocked by the calpain inhibitor calpeptin but was not influenced by the caspase-3 inhibitor Ac-DEVD-CHO. The results suggest that sepsis-induced muscle wasting is associated with increased calpain activity, secondary to reduced calpastatin activity, and that caspase-3 activity is not involved in the catabolic response to sepsis.

Treatment of cultured myotubes with the calcium ionophore A23187 increases proteasome activity via a CaMK II-caspase-calpain–dependent mechanism

BACKGROUND

Previous studies suggest that increased ubiquitin-proteasome-dependent protein breakdown in various muscle-wasting conditions may at least in part be mediated by increased cellular calcium levels. The role of calcium in the regulation of proteasome activity, however, is not well understood.

METHODS

We treated cultured L6 myotubes with the calcium ionophore A23187 or thapsigargin, substances that increase intracellular calcium levels through different mechanisms, and measured proteasome activity by determining the degradation of the fluorogenic substrate LLVY-AMC.

RESULTS

Treatment of the myotubes with A23187 or thapsigargin resulted in a dose- and time-dependent increase in proteasome activity. When the myotubes were treated with metabolic inhibitors, results suggested that the A23187- and thapsigargin-induced activation of proteasome activity was at least in part regulated by calmodulin, calcium calmodulin-dependent kinase II, caspases, and calpains.

CONCLUSIONS

The present observations support a role of calcium in the regulation of proteasome-dependent protein breakdown in skeletal muscle and may explain why muscle wasting was reduced by calcium antagonists in previous studies.

Effect of 10-day cast immobilization on sarcoplasmic reticulum calcium regulation in humans

This study investigated the effects of 10-day lower limb cast immobilization on sarcoplasmic reticulum (SR) Ca2+ regulation. Muscle biopsies were analysed in eight healthy females for maximal rates of SR Ca2+ release, Ca2+ uptake and Ca2+ ATPase activity at control, during immobilization at day 3 (IM 3), day 6 (IM 6) and day 10 (IM 10). Quadriceps muscle cross-sectional area (CSA) and 1-repetition maximum (1RM) leg extension strength were measured to determine the extent of muscle size and strength adaptations. Muscle CSA and strength decreased following 10 days of immobilization (11.8 and 41.6%, respectively, P < 0.01). A decrease in SR Ca2+ uptake rate (analysed per g wet wt) was found at IM 3 (13.2%, P=0.05), with a further decrease at IM 10 (19.8% from control, P < 0.01). At IM 10, a decrease in SR Ca2+ uptake rate (per mg protein) also occurred (19.9%, P < 0.01). Sarcoplasmic reticulum Ca2+ ATPase activity and rate of Ca2+ release were not altered with 10 days of immobilization. This study observed a decrease in SR Ca2+ uptake rate, muscular atrophy and strength loss over 10 days of immobilization in humans.

Intracellular Ca2+ transients in mouse soleus muscle after hindlimb unloading and reloading

The objective of this study was to determine whether altered intracellular Ca(2+) handling contributes to the specific force loss in the soleus muscle after unloading and/or subsequent reloading of mouse hindlimbs. Three groups of female ICR mice were studied: 1) unloaded mice (n = 11) that were hindlimb suspended for 14 days, 2) reloaded mice (n = 10) that were returned to their cages for 1 day after 14 days of hindlimb suspension, and 3) control mice (n = 10) that had normal cage activity. Maximum isometric tetanic force (P(o)) was determined in the soleus muscle from the left hindlimb, and resting free cytosolic Ca(2+) concentration ([Ca(2+)](i)), tetanic [Ca(2+)](i), and 4-chloro-m-cresol-induced [Ca(2+)](i) were measured in the contralateral soleus muscle by confocal laser scanning microscopy. Unloading and reloading increased resting [Ca(2+)](i) above control by 36% and 24%, respectively. Although unloading reduced P(o) and specific force by 58% and 24%, respectively, compared with control mice, there was no difference in tetanic [Ca(2+)](i). P(o), specific force, and tetanic [Ca(2+)](i) were reduced by 58%, 23%, and 23%, respectively, in the reloaded animals compared with control mice; however, tetanic [Ca(2+)](i) was not different between unloaded and reloaded mice. These data indicate that although hindlimb suspension results in disturbed intracellular Ca(2+) homeostasis, changes in tetanic [Ca(2+)](i) do not contribute to force deficits. Compared with unloading, 24 h of physiological reloading in the mouse do not result in further changes in maximal strength or tetanic [Ca(2+)](i).

Alterations in hepatic gluconeogenesis, prostanoid, and intracellular calcium during sepsis

OBJECTIVE

The metabolic alterations observed during sepsis may be associated with changes in local concentrations of intracellular calcium (Ca2+) and prostanoid synthesis in the liver. The authors studied hepatocyte intracellular Ca2+ and the release of glucose and prostanoid in an in-vivo murine liver perfusion model.

METHODS

Sepsis was induced in anesthetized, fasted rats by cecal ligation and puncture (CLP, n = 42). Hepatic glucose release was studied in control (n = 10) and CLP (n = 10) groups using a non-recirculating liver perfusion model with and without lactate as gluconeogenic substrate. Hepatocyte intracellular Ca2+ (n = 11) was measured using the selective indicator Fura-2 under basal and epinephrine (10(-5) M) stimulated conditions. 6-Keto-prostaglandin F1alpha (6-Keto) and thromboxane B2 (TxB2) were determined from liver perfusate by radioimmunassay (n = 11). Data were analyzed using t-tests and repeated-measures ANOVA.

RESULTS

Plasma glucose was significantly lower in CLP groups compared with controls (74.9+/-6.6 vs 115.7+/-4.6 mg/dL, p < 0.05). Plasma lactate was significantly higher in CLP vs controls (3.7+/-0.4 vs 1.4+/-0.1 mM, p < 0.05). Glucose release in isolated perfused livers was significantly lower in CLP vs controls (8.5 vs 16+/-1.2 microM/g/hr, p < 0.001). With the addition of lactate + pyruvate to the perfusate, glucose output in CLP livers was significantly lower following 5 (9.9+/-0.7 vs 17.7+/-1.1 microM/g/hr, p < 0.05) and 10 (11.9+/-1.2 vs 20.6+/-1.3 microM/g/hr, p < 0.001) minutes of perfusion. The basal level of intracellular calcium ([Ca2+]i) in CLP rats (460.1+/-91.6 nM) was significantly higher than in control rats (196.3+/-35.5 nM) (p < 0.05). A significant increase (p < 0.05) in [Ca2+]i occurred after the addition of epinephrine in hepatocytes in control (196.3+/-35.5 vs 331.8+/-41.4 nM) but not CLP (460.1+/-91.6 vs 489.4+/-105 nM) rats. 6-Keto was significantly lower in CLP compared with controls at 30 minutes (25.7+/-3.9 vs 33.4+/-5.5 pg/mL, p < 0.05), whereas TxB2 was not significantly altered (52.1+/-34.7 vs 87.5+/-43.2 pg/mL).

CONCLUSION

These results demonstrate that CLP sepsis is associated with an increase in hepatocyte intracellular free Ca2+ concentration along with attenuation of hormone-mediated Ca2+ mobilization and hepatic gluconeogenesis.

Succinylcholine-induced fasciculations in denervated rat muscles as measured using 31P-NMR spectroscopy: the effect of pretreatment with dantrolene or vecuronium

BACKGROUND

We have previously demonstrated by 31P nuclear magnetic resonance (NMR) that succinylcholine (SCh) induces metabolic changes in denervated muscle. To specify those changes, we attempted to inhibit them using two different kinds of drugs, dantrolene and vecuronium.

METHODS

Three weeks after unilateral sciatic nerve section, 75 male Wistar rats were randomly assigned to one of the following 5 groups: (1) non-pretreated normal muscle group; (2) non-pretreated denervated muscle group; (3) denervated muscle group pretreated with a low dose of vecuronium (0.02 mg.kg(-1)); (4) denervated muscle group pretreated with a high dose of vecuronium (0.2 mg.kg(-1)); (5) denervated muscle group pretreated with dantrolene (2 mg.kg(-1)). The change of the inorganic phosphate/phosphocreatine (Pi/PCr) ratio of each muscle was measured by 31P-NMR before and after SCh (1 mg.kg(-1)) administration and the corresponding peak amplitude of the electromyograms (EMG) was determined.

RESULTS

The high dose of vecuronium totally inhibited SCh-induced fasciculation on EMG (100%-->2%). In this group, though the Pi/PCr ratio significantly increased 10 min after SCh, the peak after 5 min disappeared. The inhibition with dantrolene was about the same order of magnitude as with the low dose of vecuronium (35%:21%). However, the increase in the Pi/PCr only lasted about 10 min, in contrast to the other drugs.

CONCLUSION

Our findings indicate that the Pi/PCr increases 5 and 10 min after SCh, respectively, as a result of two different processes. The first peak is caused by an excessive energy consumption in response to excessive muscle contraction. This in turn triggers the second peak, caused by breakdown of glycogen, initiated by an increased Ca2+ concentration.

Effects of tetrodotoxin-induced neural inactivation on single muscle fiber metabolic enzymes

Selected enzymes were measured in mixed-fiber bundles and individual fibers from rat plantaris (PL) and soleus (Sol) muscles that had undergone either 2 wk of tetrodotoxin (TTX) inactivation of the sciatic nerve, a sham operation, or were contralateral to the TTX limb. TTX disuse caused severe wasting of PL (46%) and Sol (26%) muscles and of single fibers (50% and 40%, respectively). TTX PL and Sol also had reduced (50%) glycogen content. In TTX, PL, and Sol macro samples and single fibers, the activities (mol.h-1.kg dry wt-1) of hexokinase, glycogen phosphorylase, and lactate dehydrogenase were higher, lower, and unchanged, respectively, compared with controls. Single-fiber data showed that these changes occurred in all fibers. In TTX PL macro samples, activities of glycerol-3-phosphate dehydrogenase (GPDH), pyruvate kinase (PK), malate dehydrogenase (MDH), citrate synthase (CS), beta-hydroxyacyl-CoA dehydrogenase (BOAC), and thiolase were, or tended to be, lower. Single-fiber data showed a disappearance of high-oxidative moderate glycolytic fibers (i.e., usually fast-twitch oxidative in control) and the appearance of more fibers with a metabolic enzyme profile approaching that of control slow-oxidative fibers. In TTX Sol macro samples, GPDH and PK tended to be higher, and thiolase, BOAC, CS, and MDH lower. Single-fiber data corroborated these findings and suggested the appearance of fast fibers with downregulated oxidative enzyme profiles. Our results suggest that neuromuscular activity is a major, but not the sole, determinant of the size and metabolic heterogeneity that exists in muscle cells.

Modulation of prostaglandin E2 synthesis in rat skeletal muscle

The effect of muscle denervation, inhibitors of protein synthesis, G proteins, and sphingolipids on prostaglandin E2 (PGE2) release by rat soleus muscle in vitro was investigated. To assess the effect of muscle denervation, the sciatic nerve in one hindlimb of rats was interrupted, and soleus muscles from the denervated hindlimb and the contralateral sham (control) hindlimb were excised 1-5 days after surgery. Compared with corresponding sham muscles, PGE2 release by denervated muscles was increased 56, 230, and 435% at 1, 3, and 5 days after denervation, respectively. Protein synthesis inhibitors cycloheximide (10 microM) and puromycin (10 microM) lowered PGE2 release by sham and denervated muscles 62-80%. The release of PGE2 by sham and denervated muscles was not altered by pertussis toxin (1 microgram/ml) but was inhibited 30-51% by AlF4-. Addition of 100 microM guanosine 5'-O-(3-thiotriphosphate) to saponin-permeabilized sham and denervated muscles had only a moderate, if any, stimulatory effect on PGE2 release. This effect was not counteracted by 1 mM guanosine 5'-O-(2-thiodiphosphate). Increasing muscle ceramide concentration by incubation with sphingomyelinase (100 mU/ml) increased PGE2 release by sham and denervated muscles 43 and 157%, respectively. Because degradation of ceramides yields sphingosine, the effect of sphingosine was also tested. Sphingosine (25 microM) increased PGE2 release by sham and denervated muscles 139 and 187%, respectively, without affecting muscle viability, as assessed by the release of lactate dehydrogenase. The data indicate that muscle denervation, treatment with sphingomyelinase, and sphingosine stimulate, whereas inhibitors of protein synthesis inhibit PGE2 synthesis by muscle.

Human muscle cell denervation: the results of a 31-phosphorus magnetic resonance spectroscopy study

The results presented here demonstrate that there is a major abnormality of high and low energy phosphate metabolism in muscle following peripheral nerve damage. Using 31-phosphorus magnetic resonance spectroscopy the changes in phosphocreatine, adenosine triphosphate, inorganic phosphate and metabolites of membrane metabolism could be observed in vivo in human subjects. The data indicate that there may be a metabolic myopathy in the muscle cells after nerve injury. Further, the metabolic changes did not always return to the control level, indicating a persistence of the abnormality. This failure of the metabolic function of the cells may be important in determining the ultimate outcome of peripheral nerve surgery.

Denervation of the rabbit hind limb studied by 31-phosphorus magnetic resonance spectroscopy

An animal model of muscle denervation was examined by 31P. magnetic resonance spectroscopy. The experiments demonstrated that there is a significant alteration in high and low energy phosphate metabolites in rabbit muscle after nerve section. The data show that there is an early change in the metabolites which appears to plateau at about six weeks. High resolution spectra of muscle cell extracts demonstrate qualitative alterations in the phosphate resonances found in the phosphodiester and phosphomonoester regions of the spectra. There would seem to be a time-related alteration in these components.

Intracellular calcium and pH alterations induced by Escherichia coli endotoxin in rat hepatocytes

In this study, the fluorescent Ca2+ probe fura-2 and the fluorescent pH indicator BCECF have been used to monitor cytosolic free Ca2+ and intracellular pH (pHi), respectively, in isolated and cultured hepatocytes treated with Escherichia coli O111:B4 endotoxin. Uptake of 45Ca2+ was also measured to study the effect of endotoxin on the extracellular calcium influx. Endotoxin treatment produced a progressive increase of cytosolic Ca2+ in a dose-dependent manner caused by both induction of a significant release of Ca2+ from intracellular stores and stimulation of the extracellular calcium influx. The perturbation of Ca2+ homeostasis by endotoxin may cause an abnormal stimulation of physiological processes, developing lethal cell injury. Endotoxin also produced a significant decrease in the pHi of hepatocytes which can justify important metabolic alterations during endotoxicosis.

Effect of denervation on the expression of glycogen phosphorylase in mouse skeletal muscle

After sciatectomy of the left hind-limb of C57BL/J mice, a denervation-induced muscular atrophy ensued and was accompanied by a decrease in the specific activity of glycogen phosphorylase to approx. 25% of control values. The cofactor of phosphorylase, pyridoxal 5'-phosphate, was used as a specific label in the determination of the degradation rate of the enzyme following nerve section. After a delay of 3-4 days, phosphorylase was degraded approx, twice as rapidly in the denervated gastrocnemius (0.20 day-1) as in the control muscle (0.12 day-1). The effect of denervation on phosphorylase mRNA was measured by quantitative Northern-blot analysis using a rat skeletal-muscle phosphorylase cDNA probe. After an initial rapid decline, phosphorylase mRNA levels stabilized in denervated muscle at 50% of the value measured in the contralateral control muscle.

Different mechanisms of increased proteolysis in atrophy induced by denervation or unweighting of rat soleus muscle

Mechanisms of accelerated proteolysis were compared in denervated and unweighted (by tail-cast suspension) soleus muscles. In vitro and in vivo proteolysis were more rapid and lysosomal latency was lower in denervated than in unweighted muscle. In vitro, lysosomotropic agents (eg, chloroquine, methylamine) did not lessen the increase in proteolysis caused by unweighting, but abolished the difference in proteolysis between denervated and unweighted muscle. Leucine methylester, an indicator of lysosome fragility, lowered latency more in denervated than in unweighted muscle. 3-Methyladenine, which inhibits phagosome formation, increased latency similarly in all muscles tested. Mersalyl, a thiol protease inhibitor, and 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride (TMB-8), which antagonizes sarcoplasmic reticulum release of Ca2+, reduced accelerated proteolysis caused by unweighting without diminishing the faster proteolysis due to denervation. Calcium ionophore (A23187) increased proteolysis more so in unweighted than control muscles whether or not Ca2+ was present. Different mechanisms of accelerated proteolysis were studied further by treating muscles in vivo for 24 hours with chloroquine or mersalyl. Chloroquine diminished atrophy of the denervated but not the unweighted muscle, whereas mersalyl prevented atrophy of the unweighted but not of the denervated muscle, both by inhibiting in vivo proteolysis. These results suggest that (1) atrophy of denervated, but not of unweighted, soleus muscle involves increased lysosomal proteolysis, possibly caused by greater permeability of the lysosome, and (2) cytosolic proteolysis is important in unweighting atrophy, involving some role of Ca2(+)-dependent proteolysis and/or thiol proteases.

Resistance of protein and glucose metabolism to insulin in denervated rat muscle

Denervated (1-10 days) rat epitrochlearis muscles were isolated, and basal and insulin-stimulated protein and glucose metabolism were studied. Although basal rates of glycolysis and glucose transport were increased in 1-10-day-denervated muscles, basal glycogen-synthesis rates were unaltered and glycogen concentrations were decreased. Basal rates of protein degradation and synthesis were increased in 1-10-day-denervated muscles. The increase in degradation was greater than that in synthesis, resulting in muscle atrophy. Increased rates of proteolysis and glycolysis were accompanied by elevated release rates of leucine, alanine, glutamate, pyruvate and lactate from 3-10-day-denervated muscles. ATP and phosphocreatine were decreased in 3-10-day-denervated muscles. Insulin resistance of glycogen synthesis occurred in 1-10-day denervated muscles. Insulin-stimulated glycolysis and glucose transport were inhibited by day 3 of denervation, and recovered by day 10. Inhibition of insulin-stimulated protein synthesis was observed only in 3-day-denervated muscles, whereas regulation by insulin of net proteolysis was unaffected in 1-10-day-denervated muscles. Thus the results demonstrate enhanced glycolysis, proteolysis and protein synthesis, and decreased energy stores, in denervated muscle. They further suggest a defect in insulin's action on protein synthesis in denervated muscles as well as on glucose metabolism. However, the lack of concurrent changes in all insulin-sensitive pathways and the absence of insulin-resistance for proteolysis suggest multiple and specific cellular defects in insulin's action in denervated muscle.

Decrease in force potentiation and appearance of alpha-adrenergic mediated contracture in aging rat skeletal muscle

The effect of increasing age on contractile performance and catecholamine receptor activity was investigated in a distal, predominantly fast twitch oxidative glycolytic (FOG) muscle from the plantar surface of the rat hindfoot. The ability of the flexor digitorum brevis (FDB), isolated from anesthetized rats and maintained in vitro, to undergo post-tetanic potentiation and a staircase response declined with age. Potentiation following repetitive stimulation was reduced by 50% in 2 year old rats and eliminated in 3 year old animals. The rate of muscle fatigue during intermittent tetanic stimulation also increased in aging muscles. FDB, regardless of age, did not develop a positive inotropic response to 10(-6) M epinephrine applied in vitro, but 3 year old FDB generated a prolonged contracture. Contracture tension was approximately 25% of twitch tension and was maintained for 2-10 min in the continued presence of catecholamine. Contractures were eliminated by pretreatment with alpha-adrenergic antagonists or by removing Ca2+ from the bathing medium. In addition to decreased contractile capacity, aging muscles acquire a population of alpha-adrenergic receptors which may underlie some of the metabolic and structural changes associated with increasing age.

The activation of protein degradation in muscle by Ca2+ or muscle injury does not involve a lysosomal mechanism

By use of different inhibitors, we distinguished three proteolytic processes in rat skeletal muscle. When soleus muscles maintained under tension were exposed to the calcium ionophore A23187 or were incubated under no tension in the presence of Ca2+, net protein breakdown increased by 50-80%. Although leupeptin and E-64 inhibit this acceleration of protein breakdown almost completely, other agents that prevent lysosomal function, such as methylamine or leucine methyl ester, did not inhibit this effect. A similar increase in net proteolysis occurred in muscle fibres injured by cutting, and this response was also inhibited by leupeptin, but not by methylamine. In contrast, all these inhibitors markedly decreased the 2-fold increase in protein breakdown induced by incubating muscles without insulin and leucine, isoleucine and valine. In addition, the low rate of proteolysis seen in muscles under passive tension in complete medium was not affected by any of these inhibitors. Thus the basal degradative process in muscle does not involve lysosomes or thiol proteinases, and muscle can enhance protein breakdown by two mechanisms: lack of insulin and nutrients enhances a lysosomal process in muscle, as in other cells, whereas Ca2+ and muscle injury activate a distinct pathway involving cytosolic thiol proteinase(s).

Differential effect of denervation on free-radical scavenging enzymes in slow and fast muscle of rat

To determine the effect of denervation on the free-radical scavenging systems in relation to the mitochondrial oxidative metabolism in the slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles, the sciatic nerve of the rat was crushed in the midthigh region and the muscle tissue levels of five enzymes were studied 2 and 5 weeks following crush. Recently developed radioimmunoassays were utilized for the selective measurement of cuprozinc (cytosolic) and mangano (mitochondrial) superoxide dismutases. Total tissue content of cuprozinc superoxide dismutase showed a mild decrease after denervation in slow but not in fast muscle. Manganosuperoxide dismutase and fumarase decreased markedly at 2 weeks and returned toward control levels by 5 weeks, the changes appearing to be greater in slow than in fast muscle. At 2 weeks, cytochrome c oxidase decreased significantly in slow, but not in fast muscle. GSH-peroxidase at baseline was 10-fold higher in slow than in fast muscle, markedly decreased at 2 weeks in slow muscle, and returned toward control levels at 5 weeks, whereas the total enzyme activity in fast muscle did not change through 5 weeks. These data represent the first systematic report of free radical scavenging systems in slow and fast muscles in response to denervation. Selective modification of cuprozinc and manganosuperoxide dismutases and differential regulation of GSH-peroxidase was demonstrated in slow and fast muscle.

[Detection of peranesthetic malignant hyperthermia by muscle contracture tests and NMR spectroscopy]

To diagnose malignant hyperthermia susceptibility (MHS), caffeine and halothane contracture tests were performed on six patients. One of them, who presented a peroperative crisis, was recognized as MHS; the five others were negative (MHN). By means of 31P-NMR spectroscopy, the muscular energetic metabolism of these patients was studied during and after moderate exercise in normal and moderate ischaemic conditions. Metabolic abnormalities appeared in the MHS patient. It must be concluded therefore that malignant hyperthermia is a latent myopathy. 31P-NMR spectroscopy appeared to be a useful non-invasive tool for screening for this affliction.

The free cytoplasmic Ca2+ levels in duchenne muscular dystrophy lymphocytes

An increased cellular Ca2+ content has been associated with Duchenne muscular dystrophy (DMD). However, estimates of the free cytoplasmic Ca2+ concentration ([Ca2+]i) in cells of DMD patients were not available. We compared the [Ca2+]i levels of normal and DMD peripheral blood lymphocytes and Epstein-Barr virus-transformed lymphoblasts using the novel probe, quin 2, an internally trapped fluorescent indicator. The [Ca2+]i levels of normal and DMD cells were not significantly different.

Respiratory activities of subsarcolemmal and intermyofibrillar mitochondrial populations isolated from denervated and control rat soleus muscles

Ultraturrax and Nagarse released populations of mitochondria isolated from control and day 21 denervated rat soleus muscle were characterized with respect to their oxidative phosphorylation, ADP translocase and ATPase activities. Both Ultraturrax and Nagarse released mitochondrial populations displayed lower capacities for oxidative phosphorylation; lower ADP translocase activities and higher Mg2+ stimulated ATPase activities than their corresponding controls. For both the denervated and control states, the Nagarse-released mitochondrial populations displayed significantly higher respiratory activities than the Ultraturrax released fractions. The significance of these findings is discussed with regard to the process of mitochondrial respiratory control. In addition the role of mitochondrial dysfunction in denervation muscular atrophy is assessed.

Ca2+-uptake properties of two populations of mitochondria from normal and denervated rat soleus muscle

Ultraturrax- and Nagarse-released populations of mitochondria were characterized with respect to their Ca2+-uptake activities (i) by means of the indirect polarographic technique and (ii) directly by the 45Ca Ruthenium Red-quench method of Reed & Bygrave [(1974) Biochem. J. 140, 143-155]. The denervated-muscle subsarcolemmal and intermyofibrillar mitochondrial fractions displayed markedly decreased rates and capacities for Ca2+ uptake compared with their respective controls. The implications of these findings with respect to the process of cell necrosis are discussed.