Sarcoplasmic reticulum Ca2+ pump expression in denervated skeletal muscle

The Effect of SERCA Activation on Functional Characteristics and Signaling of Rat Soleus Muscle upon 7 Days of Unloading

Skeletal muscle abnormalities and atrophy during unloading are accompanied by the accumulation of excess calcium in the sarcoplasm. We hypothesized that calcium accumulation may occur, among other mechanisms, due to the inhibition of sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) activity. Consequently, the use of the SERCA activator will reduce the level of calcium in the sarcoplasm and prevent the negative consequences of muscle unloading. Wistar rats were randomly assigned into one of three groups (eight rats per group): control rats with placebo (C), 7 days of unloading/hindlimb suspension with placebo (7HS), and 7 days of unloading treated with SERCA activator CDN1163 (7HSC). After seven days of unloading the soleus muscle, the 7HS group displayed increased fatigue in the ex vivo test, a significant increase in the level of calcium-dependent CaMK II phosphorylation and the level of tropomyosin oxidation, as well as a decrease in the content of mitochondrial DNA and protein, slow-type myosin mRNA, and the percentage of slow-type muscle fibers. All of these changes were prevented in the 7HSC group. Moreover, treatment with CDN1163 blocked a decrease in the phosphorylation of p70S6k, an increase in eEF2 phosphorylation, and an increase in MuRF-1 mRNA expression. Nevertheless, there were no differences in the degree of fast and slow muscle fiber atrophy between the 7HS and 7HSC groups. Conclusion: SERCA activation during 7 days of unloading prevented an increase in soleus fatigue, the decrease of slow-type myosin, mitochondrial markers, and markers of calcium homeostasis but had no effect on muscle atrophy.

The expression of aquaporin-4 is regulated based on innervation in skeletal muscles

Aquaporin-4 (AQP4) is a selective water channel, which expresses on the plasma membrane of myofibers and regulates the osmotic pressure, energy metabolism and morphological changes in myofibers by modulating water transport across sarcolemma in skeletal muscles. Although the physiological roles of AQP4 have been gradually clarified in skeletal muscles, the regulatory mechanisms of AQP4 expression have been poorly understood in skeletal muscles. Recently, it was reported that the expression of AQP4 decreased in atrophied skeletal muscles following sciatic nerve transection, but not tail-suspension. Therefore, expecting that the nerve supply to myofibers would be one of the major regulatory factors regulating AQP4 expression in skeletal muscles, we investigated whether the expression patterns of AQP4 were changed in skeletal muscles by denervation and subsequent reinnervation. As a result, while the APQ4 expression levels were significantly decreased by sciatic nerve freezing-induced denervation, subsequently the expression levels of AQP4 were fully restored during reinnervation in skeletal muscles (p < 0.05, respectively). On the other hand, the expression levels of α1-syntrophin and AQP1, which are respectively structural and functional related AQP4 factors, were stably maintained during the denervation and subsequent reinnervation. Therefore, the present study demonstrated that the expression of AQP4 may be regulated depending on the innervation to skeletal muscles. Moreover, AQP4 regulatory mechanisms may be fundamentally different to those of AQP1 in skeletal muscles.

Properties of skeletal muscle in the teleost Sternopygus macrurus are unaffected by short-term electrical inactivity

Skeletal muscle is distinguished from other tissues on the basis of its shape, biochemistry, and physiological function. Based on mammalian studies, fiber size, fiber types, and gene expression profiles are regulated, in part, by the electrical activity exerted by the nervous system. To address whether similar adaptations to changes in electrical activity in skeletal muscle occur in teleosts, we studied these phenotypic properties of ventral muscle in the electric fish Sternopygus macrurus following 2 and 5 days of electrical inactivation by spinal transection. Our data show that morphological and biochemical properties of skeletal muscle remained largely unchanged after these treatments. Specifically, the distribution of type I and type II muscle fibers and the cross-sectional areas of these fiber types observed in control fish remained unaltered after each spinal transection survival period. This response to electrical inactivation was generally reflected at the transcript level in real-time PCR and RNA-seq data by showing little effect on the transcript levels of genes associated with muscle fiber type differentiation and plasticity, the sarcomere complex, and pathways implicated in the regulation of muscle fiber size. Data from this first study characterizing the acute influence of neural activity on muscle mass and sarcomere gene expression in a teleost are discussed in the context of comparative studies in mammalian model systems and vertebrate species from different lineages.

Remodelling of cardiac sympathetic re-innervation with thoracic spinal cord stimulation improves left ventricular function in a porcine model of heart failure

AIMS

Thoracic spinal cord stimulation (SCS) has been shown to improve left ventricular ejection fraction (LVEF) in heart failure (HF). Nevertheless, the optimal duration (intermittent vs. continuous) of stimulation and the mechanisms of action remain unclear.

METHODS AND RESULTS

We performed chronic thoracic SCS at the level of T1-T3 (50 Hz, pulse width 0.2 ms) in 30 adult pigs with HF induced by myocardial infarction and rapid ventricular pacing for 4 weeks. All the animals were treated with daily oral metoprolol succinate (25 mg) plus ramipril (2.5 mg), and randomized to a control group (n = 10), intermittent SCS (4 h ×3, n = 10) or continuous SCS (24 h, n = 10) for 10 weeks. Serial measurements of LVEF and +dP/dt and serum levels of norepinephrine and B-type natriuretic peptide (BNP) were measured. After sacrifice, immunohistological studies of myocardial sympathetic and parasympathetic nerve sprouting and innervation were performed. Echocardiogram revealed a significant increase in LVEF and +dP/dt at 10 weeks in both the intermittent and continuous SCS group compared with controls (P < 0.05). In both SCS groups, there was diffuse sympathetic nerve sprouting over the infarct, peri-infarct, and normal regions compared with only the peri-infarct and infarct regions in the control group. In addition, sympathetic innervation at the peri-infarct and infarct regions was increased following SCS, but decreased in the control group. Myocardium norepinephrine spillover and serum BNP at 10 weeks was significantly decreased only in the continuous SCS group (P < 0.05).

CONCLUSIONS

In a porcine model of HF, SCS induces significant remodelling of cardiac sympathetic innervation over the peri-infarct and infarct regions and is associated with improved LV function and reduced myocardial norepinephrine spillover.

Modified cytoplasmic Ca2+ sequestration contributes to spinal cord injury-induced augmentation of nerve-evoked contractions in the rat tail artery

In rat tail artery (RTA), spinal cord injury (SCI) increases nerve-evoked contractions and the contribution of L-type Ca²⁺ channels to these responses. In RTAs from unoperated rats, these channels play a minor role in contractions and Bay K8644 (L-type channel agonist) mimics the effects of SCI. Here we investigated the mechanisms underlying the facilitatory actions of SCI and Bay K8644 on nerve-evoked contractions of RTAs and the hypothesis that Ca²⁺ entering via L-type Ca²⁺ channels is rapidly sequestered by the sarcoplasmic reticulum (SR) limiting its role in contraction. In situ electrochemical detection of noradrenaline was used to assess if Bay K8644 increased noradrenaline release. Perforated patch recordings were used to assess if SCI changed the Ca²⁺ current recorded in RTA myocytes. Wire myography was used to assess if SCI modified the effects of Bay K8644 and of interrupting SR Ca²⁺ uptake on nerve-evoked contractions. Bay K8644 did not change noradrenaline-induced oxidation currents. Neither the size nor gating of Ca²⁺ currents differed between myocytes from sham-operated (control) and SCI rats. Bay K8644 increased nerve-evoked contractions in RTAs from both control and SCI rats, but the magnitude of this effect was reduced by SCI. By contrast, depleting SR Ca²⁺ stores with ryanodine or cyclopiazonic acid selectively increased nerve-evoked contractions in control RTAs. Cyclopiazonic acid also selectively increased the blockade of these responses by nifedipine (L-type channel blocker) in control RTAs, whereas ryanodine increased the blockade produced by nifedipine in both groups of RTAs. These findings suggest that Ca²⁺ entering via L-type channels is normally rapidly sequestered limiting its access to the contractile mechanism. Furthermore, the findings suggest SCI reduces the role of this mechanism.

Laser capture microdissection of metachromatically stained skeletal muscle allows quantification of fiber type specific gene expression

Skeletal muscle contains various myofiber types closely associated with satellite stem cells, vasculature, and neurons, thus making it difficult to perform genetic or proteomic expression analysis with sufficient cellular specificity to resolve differences at the individual cell or myofiber type level. Here, we describe the combination of a simple histochemical method capable of simultaneously identifying Type I, IIA, IIB, and IIC myofibers followed by laser capture micro-dissection (LCM) to compare the expression profiles of individual fiber types, myonuclear domains, and satellite cells in frozen muscle sections of control and atrophied muscle. Quantitative RT-PCR (qPCR) was used to verify the integrity of the cell-specific RNAs harvested after histologic staining, while qPCR for specific genes of interest was used to quantify atrophy-associated changes in mRNA. Our data demonstrate that the differential myofiber atrophy previously described by histologic means is related to differential expression of atrophy-related genes, such as MuRF1 and MAFbx (a.k.a. Atrogin-1), within different myofiber type populations. This spatially resolved molecular pathology (SRMP) technique allowed quantitation of atrophy-related gene products within individual fiber types that could not be resolved by expression analysis of the whole muscle. The present study demonstrates the importance of fiber type specific expression profiling in understanding skeletal muscle biology especially during muscle atrophy and provides a practical method of performing such research.

The effect of passive movement on denervated soleus highlights a differential nerve control on SERCA and MyHC isoforms

The sarco-endoplasmic reticulum Ca2+ ATP-ase (SERCA) and myosin heavy chain (MyHC) levels were measured in hindlimb-denervated and selectively denervated rat soleus muscles. Selective denervation allowed passive movement of the soleus, whereas hindlimb denervation rendered it to passivity. To minimize chronic effects, we followed the changes only for 2 weeks. Selective denervation resulted in less muscle atrophy, a faster slow-to-fast transition of MyHC isoforms, and less coordinated expressions of the slow vs fast isoforms of MyHC and SERCA. Generally, expression of the slow-twitch type SERCA2a was found to be less dependent, whereas the slow-twitch type MyHC1 was the most dependent on innervation. Our study shows that passive movement is able to ameliorate denervation-induced atrophy of the soleus and that it also accentuates the dyscoordination in the expression of the corresponding slow and fast isoforms of MyHC and SERCA.

Atrophy, but not necrosis, in rabbit skeletal muscle denervated for periods up to one year

Our understanding of the effects of long-term denervation on skeletal muscle is heavily influenced by an extensive literature based on the rat. We have studied physiological and morphological changes in an alternative model, the rabbit. In adult rabbits, tibialis anterior muscles were denervated unilaterally by selective section of motor branches of the common peroneal nerve and examined after 10, 36, or 51 wk. Denervation reduced muscle mass and cross-sectional area by 50-60% and tetanic force by 75%, with no apparent reduction in specific force (force per cross-sectional area of muscle fibers). The loss of mass was associated with atrophy of fast fibers and an increase in fibrous and adipose connective tissue; the diameter of slow fibers was preserved. Within fibers, electron microscopy revealed signs of ultrastructural disorganization of sarcomeres and tubular systems. This, rather than the observed transformation of fiber type from IIx to IIa, was probably responsible for the slow contractile speed of the muscles. The muscle groups denervated for 10, 36, or 51 wk showed no significant differences. At no stage was there any evidence of necrosis or regeneration, and the total number of fibers remained constant. These changes are in marked contrast to the necrotic degeneration and progressive decline in mass and force that have previously been found in long-term denervated rat muscles. The rabbit may be a better choice for a model of the effects of denervation in humans, at least up to 1 yr after lesion.

SERCA pump isoforms: Their role in calcium transport and disease

The sarcoendoplasmic reticulum (SR) calcium transport ATPase (SERCA) is a pump that transports calcium ions from the cytoplasm into the SR. It is present in both animal and plant cells, although knowledge of SERCA in the latter is scant. The pump shares the catalytic properties of ion-motive ATPases of the P-type family, but has distinctive regulation properties. The SERCA pump is encoded by a family of three genes, SERCA1, 2, and 3, that are highly conserved but localized on different chromosomes. The SERCA isoform diversity is dramatically enhanced by alternative splicing of the transcripts, occurring mainly at the COOH-terminal. At present, more than 10 different SERCA isoforms have been detected at the protein level. These isoforms exhibit both tissue and developmental specificity, suggesting that they contribute to unique physiological properties of the tissue in which they are expressed. The function of the SERCA pump is modulated by the endogenous molecules phospholamban (PLB) and sarcolipin (SLN), expressed in cardiac and skeletal muscles. The mechanism of action of PLB on SERCA is well characterized, whereas that of SLN is only beginning to be understood. Because the SERCA pump plays a major role in muscle contraction, a number of investigations have focused on understanding its role in cardiac and skeletal muscle disease. These studies document that SERCA pump expression and activity are decreased in aging and in a variety of pathophysiological conditions including heart failure. Recently, SERCA pump gene transfer was shown to be effective in restoring contractile function in failing heart muscle, thus emphasizing its importance in muscle physiology and its potential use as a therapeutic agent.

Reduced Expression of the Sarcoplasmic Calcium Pump SERCA2 in Skeletal Muscle From Patients With Chronic Obstructive Pulmonary Disease and Low Body Weight

OBJECTIVE

To compare the concentrations and extent of nitration of sarcoplasmic-endoplasmic reticulum Ca(2+) adenosine triphosphatase 2 (SERCA2) in biopsies of the quadriceps femoris from patients with chronic obstructive pulmonary disease (COPD) who have normal or low body mass index (BMI).

PATIENTS AND METHODS

The patients were divided into 2 groups (n=7, each group), one containing individuals with normal BMI (> 21 kg/m(2)) and the other with low BMI (< 21 kg/m(2)). Forced spirometry and blood gas analysis were performed in both groups and percutaneous needle biopsies of the lateral portion of the quadriceps femoris muscle were performed. Western blots were used to assess the concentration of SERCA2 in the biopsy material. To determine whether or not the protein was tyrosine-nitrated, immunoprecipitation of SERCA2 was performed with an antinitrotyrosine antibody followed by Western blotting to determine the concentration of the tyrosine-nitrated protein.

RESULTS

Expression of SERCA2 was significantly lower in patients with low BMI (4.2 [0.5] vs 8.1 [1.2] integrated optical density units, P < .05). SERCA2 was also tyrosine-nitrated in the patients with low BMI. Finally, a significant negative correlation was observed between the concentration of SERCA2 and that of inducible nitric oxide synthase (determined in a previous study using the same biopsy material) in patients with COPD and low BMI (r=-0.89, P=.007), while such a correlation was not observed in patients with COPD and normal BMI (r=0.35, P=.43).

CONCLUSIONS

In patients with COPD, SERCA2 concentration is reduced and the protein is tyrosine-nitrated in skeletal muscle from patients with low BMI compared to those with normal BMI. These results indicate the presence of a previously unrecognized cellular alteration in skeletal muscle from patients with COPD and low muscle weight.

The denervated muscle: facts and hypotheses. A historical review

Denervation changes in skeletal muscle (atrophy; alterations of myofibrillar expression, muscle membrane electrical properties, ACh sensitivity and excitation-contraction coupling process; fibrillation), and their possible causes are reviewed. All changes can be counteracted by muscle electrostimulation, while denervation-like effects can be caused by the complete conduction block in muscle nerve. These results do not support the hypothesis that the lack of neurotrophic, non-motor factors plays a role in denervation phenomena. Instead they support the view that the lack of neuromotor discharge is the only cause of the phenomena and that neuromotor activity is an essential factor in regulating muscle properties. However, some experimental results cannot apparently be explained by the lack of neuromotor impulses, and may still suggest that neurotrophic influences exist. A hypothesis is that neurotrophic factors, too feeble to maintain a role in completely differentiated, adult muscles, can concur with neuromotor activity in the differentiation of immature, developing muscles.

Denervation produces different single fiber phenotypes in fast- and slow-twitch hindlimb muscles of the rat

Using a single, mechanically skinned fiber approach, we tested the hypothesis that denervation (0 to 50 days) of skeletal muscles that do not overlap in fiber type composition [extensor digitorum longus (EDL) and soleus (SOL) muscles of Long-Evans hooded rats] leads to development of different fiber phenotypes. Denervation (50 day) was accompanied by 1) a marked increase in the proportion of hybrid IIB/D fibers (EDL) and I/IIA fibers (SOL) from 30% to >75% in both muscles, and a corresponding decrease in the proportion of pure fibers expressing only one myosin heavy chain (MHC) isoform; 2) complex muscle- and fiber-type specific changes in sarcoplasmic reticulum Ca(2+)-loading level at physiological pCa approximately 7.1, with EDL fibers displaying more consistent changes than SOL fibers; 3) decrease by approximately 50% in specific force of all fiber types; 4) decrease in sensitivity to Ca(2+), particularly for SOL fibers (by approximately 40%); 5) decrease in the maximum steepness of the force-pCa curves, particularly for the hybrid I/IIA SOL fibers (by approximately 35%); and 6) increased occurrence of biphasic behavior with respect to Sr(2+) activation in SOL fibers, indicating the presence of both slow and fast troponin C isoforms. No fiber types common to the two muscles were detected at any time points (day 7, 21, and 50) after denervation. The results provide strong evidence that not only neural factors, but also the intrinsic properties of a muscle fiber, influence the structural and functional properties of a particular muscle cell and explain important functional changes induced by denervation at both whole muscle and single cell levels.

Adaptive expression pattern of different proteins involved in cellular calcium homeostasis in denervated rat vas deferens

The activity and protein expression of plasma membrane and sarco(endo)plasmic reticulum (Ca2+-Mg2+)ATPases and ryanodine receptors were investigated in surgically denervated rat vas deferens. The function of thapsigargin-sensitive but not thapsigargin-resistant (Ca2+-Mg2+)ATPase (from sarco(endo)plasmic reticulum and plasma membrane, respectively), evidenced by enzyme activity and Ca2+ uptake experiments, was significantly depressed by 30-50% when compared to innervated vas. Western blots showed that such reduction in sarco(endo)plasmic reticulum (Ca2+-Mg2+)ATPase performance was accompanied by a decrement of similar magnitude in sarco(endo)plasmic reticulum (Ca2+-Mg2+)ATPase type 2 protein expression, without any significant change in plasma membrane (Ca2+-Mg2+)ATPase expression. Finally, [3H]ryanodine binding revealed that the density of ryanodine binding sites was reduced by 45% after denervation without modification in affinity. The present findings demonstrate that sarco(endo)plasmic reticulum proteins involved in intracellular calcium homeostasis are clearly down-regulated and brings further evidence of a modified calcium translocation in denervated rat vas deferens.

Alteration of cyclopiazonic acid-mediated contracture of mouse diaphragm after denervation

As a major Ca(2+) source for muscle contraction, the sarcoplasmic reticulum (SR) of skeletal muscle maintains its Ca(2+) content by uptake of myoplasmic Ca(2+) and by replenishment with extracellular Ca(2+). Since transection of motor nerve alters the functions of SR Ca(2+) pump and sarcolemma ion channels, this study explored the effect of denervation on the contracture evoked by cyclopiazonic acid (CPA), an inhibitor of SR Ca(2+) pump. In innervated hemidiaphragm, CPA elicited a bimodal elevation of muscle tone, which was dependent on extracellular Ca(2+) and differentially inhibited by pretreatment with 2-aminoethoxydiphenylborane (APB) and U73122. Activation of muscle Na(+) channels to simulate denervation-induced membrane depolarization did not change the contracture profile. After denervation for 5-14 days when the contracture induced by caffeine was not yet depressed, CPA elicited only APB-sensitive monophasic contracture. Stimulation of ATP-regulated K(+) channels with lemakalim hyperpolarized muscle membrane and attenuated CPA contracture in denervated, but not innervated, hemidiaphragm. The effects of lemakalim were antagonized by glybenclamide. It is inferred that the bimodal CPA contracture is resulted from distinct recruitments of Ca(2+) entry and that denervation alters the voltage dependence and down-regulates CPA-mediated Ca(2+) influx.

Expression of SERCA2a is independent of innervation in regenerating soleus muscle

The speed of contraction of a skeletal muscle largely depends on the myosin heavy chain isoforms (MyHC), whereas the relaxation is initiated and maintained by the sarcoplasmic reticulum Ca2+-ATPases (SERCA). The expression of the slow muscle-type myosin heavy chain I (MyHCI) is entirely dependent on innervation, but, as we show here, innervation is not required for the expression of the slow-type sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) in regenerating soleus muscles of the rat, although it can play a modulator role. Remarkably, the SERCA2a level is even higher in denervated than in innervated regenerating soleus muscles on day 7 when innervation is expected to resume. Later, the level of SERCA2a protein declines in denervated regenerated muscles but it remains expressed, whereas the corresponding mRNA level is still increasing. SERCA1 (i.e., the fast muscle-type isoform) expression shows only minor changes in denervated regenerating soleus muscles compared with innervated regenerating controls. When the soleus nerve was transected instead of the sciatic nerve, SERCA2a and MyHCI expressions were found to be even more uncoupled because the MyHCI nearly completely disappeared, whereas the SERCA2a mRNA and protein levels decreased much less. The transfection of regenerating muscles with constitutively active mutants of the Ras oncogene, known to mimic the effect of innervation on the expression of MyHCI, did not affect SERCA2a expression. These results demonstrate that the regulation of SERCA2a expression is clearly distinct from that of the slow myosin in the regenerating soleus muscle and that SERCA2a expression is modulated by neuronal activity but is not entirely dependent on it.

Ca2+-regulatory muscle proteins in the alcohol-fed rat

Alcoholic myopathy is characterized by muscle weakness and difficulties in gait and locomotion. It is one of the most prevalent skeletal muscle disorders in the Western hemisphere, affecting between 40% and 60% of all chronic alcohol misusers. However, the pathogenic mechanisms are unknown, although recent studies have suggested that membrane defects occur as a consequence of chronic alcohol exposure. It was our hypothesis that alcohol ingestion perturbs membrane-located proteins associated with intracellular signalling and contractility, in particular those relating to calcium homeostasis. To test this, we fed male Wistar rats nutritionally complete liquid diets containing ethanol as 35% of total dietary energy. Controls were pair-fed identical amounts of the same diet in which ethanol was replaced by isocaloric glucose. At the end of 6 weeks, rats were killed and skeletal muscles dissected. These were used to determine important ion-regulatory skeletal muscle proteins including sarcalumenin (SAR), sarcoplasmic-endoplasmic reticulum Ca(2+)-adenosine triphosphatase (ATPase) (SERCA1), the junctional face protein of 90 kd (90-JFP), alpha(1)- and alpha(2)-dihydropyridine receptor (alpha(1)-DHPR and alpha(2)-DHPR), and calsequestrin (CSQ) by immunoblotting. The relative abundance of microsomal proteins was determined by immunoblotting using the enhanced chemiluminescence (ECL) technique. The data showed that alcohol-feeding significantly reduced gastrocnemius and hind limb muscle weights (P <.05 in both instances). Concomitant changes included increases in the relative amounts of SERCA1 (P <.05) and Ca(2+)-ATPase activity (P <.025). However, there were no statistically significant changes in either SAR, 90-JFP, alpha(1)-DHPR or alpha(2)-DHPR (P >.2 in all instances). Reductions in CSQ were of marginal significance (P =.0950). We conclude that upregulation of SERCA1 protein and Ca(2+)-ATPase activity may be an adaptive mechanism and/or a contributory process in the pathology of alcohol-induced muscle disease.

Myosin and SERCA isoform expression in denervated slow-twitch muscle of euthyroid and hyperthyroid rabbits

Changes in the isoform patterns of myosin heavy chains (MHC) and Ca2+-ATPase (SERCA) were studied in long-term (72 days) denervated slow-twitch soleus muscles of euthyroid and hyperthyroid rabbits. MHC isoforms were separated electrophoretically. SERCA isoforms were assessed by electrophoretic separation of the trypsin-digested and phosphorylated fragments. Denervation led to pronounced slow-to-fast transitions with increases in the fast MHC and fast SERCA1a isoforms. Hyperthyroidism had no significant effect on the denervation-induced changes in SERCA isoforms, but led to slight increases in the relative concentrations of the fast MHC isoforms. A high correlation existed between the relative concentrations of slow myosin and SERCA2a in both innervated euthyroid and hyperthyroid soleus muscles. This correlation, however, seems to be less tight in denervated soleus muscles.

Early changes of type 2B fibers after denervation of rat EDL skeletal muscle

Skeletal muscle type 2B fibers normally receive a moderate level of motoneuron discharge. As a consequence, we hypothesize that type 2B fiber properties should be less sensitive to the absence of the nerve. Therefore, we have investigated the response of sarcoplasmic reticulum and myofibrillar proteins of type 2B fibers isolated from rat extensor digitorum longus muscle after denervation (2 and 7 days). Single fibers were identified by SDS-PAGE of myosin heavy chain isoforms. Electrophysiological and isometric contractile properties of the whole muscle were also analyzed. The pCa-tension relationship of type 2B single fibers was shifted to the left at 2 days and to right at 7 days after denervation, with significant differences in the Hill coefficients and pCa threshold values in 2- vs. 7-day-denervated fibers. The sarcoplasmic reticulum Ca2+ uptake capacity and rate significantly decreased after 2 days of denervation, whereas both increased at 7 days. Caffeine sensitivity of sarcoplasmic reticulum Ca2+ release was transitory and markedly increased in 2-day-denervated fibers. Our results indicate that type 2B fiber functional properties are highly sensitive to the interruption of nerve supply. Moreover, most of 2-day-denervated changes were reverted at 7 days.

Denervation alters myosin heavy chain expression and contractility of developing rat diaphragm muscle

We hypothesized that unilateral denervation (DNV) of the rat diaphragm muscle (Dia(m)) in neonates at postnatal day 7 (D-7) alters normal transitions of myosin heavy chain (MHC) isoform expression and thereby affects postnatal changes in maximum specific force (P(o)) and maximum unloaded shortening velocity (V(o)). The relative expression of different MHC isoforms was analyzed electrophoretically. With DNV at D-7, expression of MHC(neo) in the Dia(m) persisted, and emergence of MHC(2X) and MHC(2B) was delayed. By D-21 and D-28, relative expression of MHC(2A) and MHC(2B) was reduced in DNV compared with control (CTL) animals. Expression of MHC(neo) also reappeared in adult Dia(m) by 2-3 wk after DNV, and relative expression of MHC(2B) was reduced. At each age, P(o) was reduced and V(o) was slowed by DNV, compared with CTL. In CTL Dia(m), postnatal changes in P(o) and V(o) were associated with an increase in fast MHC isoform expression. In DNV Dia(m), no such association existed. We conclude that, in the Dia(m), DNV induces alterations in both MHC isoform expression and contractile properties, which are not necessarily causally linked.

Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease

Mammalian skeletal muscle shows an enormous variability in its functional features such as rate of force production, resistance to fatigue, and energy metabolism, with a wide spectrum from slow aerobic to fast anaerobic physiology. In addition, skeletal muscle exhibits high plasticity that is based on the potential of the muscle fibers to undergo changes of their cytoarchitecture and composition of specific muscle protein isoforms. Adaptive changes of the muscle fibers occur in response to a variety of stimuli such as, e.g., growth and differentition factors, hormones, nerve signals, or exercise. Additionally, the muscle fibers are arranged in compartments that often function as largely independent muscular subunits. All muscle fibers use Ca(2+) as their main regulatory and signaling molecule. Therefore, contractile properties of muscle fibers are dependent on the variable expression of proteins involved in Ca(2+) signaling and handling. Molecular diversity of the main proteins in the Ca(2+) signaling apparatus (the calcium cycle) largely determines the contraction and relaxation properties of a muscle fiber. The Ca(2+) signaling apparatus includes 1) the ryanodine receptor that is the sarcoplasmic reticulum Ca(2+) release channel, 2) the troponin protein complex that mediates the Ca(2+) effect to the myofibrillar structures leading to contraction, 3) the Ca(2+) pump responsible for Ca(2+) reuptake into the sarcoplasmic reticulum, and 4) calsequestrin, the Ca(2+) storage protein in the sarcoplasmic reticulum. In addition, a multitude of Ca(2+)-binding proteins is present in muscle tissue including parvalbumin, calmodulin, S100 proteins, annexins, sorcin, myosin light chains, beta-actinin, calcineurin, and calpain. These Ca(2+)-binding proteins may either exert an important role in Ca(2+)-triggered muscle contraction under certain conditions or modulate other muscle activities such as protein metabolism, differentiation, and growth. Recently, several Ca(2+) signaling and handling molecules have been shown to be altered in muscle diseases. Functional alterations of Ca(2+) handling seem to be responsible for the pathophysiological conditions seen in dystrophinopathies, Brody's disease, and malignant hyperthermia. These also underline the importance of the affected molecules for correct muscle performance.

Corticosteroids decrease mRNA levels of SERCA pumps, whereas they increase sarcolipin mRNA in the rat diaphragm

1. In order to explore the potential role of the sarcoplasmic-endoplasmic reticulum Ca2+-ATPase (SERCA)-type pumps and of their modulators phospholamban (PLB) and sarcolipin (SLN) in the functional alterations of the diaphragm induced by corticosteroid treatment, expression of SERCA, PLB and SLN was assessed by RT-PCR in the diaphragm of rats treated daily for 5 days either with triamcinolone (80 mg kg-1, n = 8) or with saline (control; 0.6 ml, n = 8). 2. Triamcinolone treatment reduced the normalised overall amount of all SERCA mRNA in diaphragm by 70 % compared to controls (P < 0.05). This reduction was accounted for by a relatively larger decrease in the SERCA1 mRNA (-69 %, P < 0.05) whilst the decrease in SERCA2 mRNA (-49 %, P = 0.09) did not reach statistical significance. As a result the relative proportion of SERCA2 mRNA was increased from 43 +/- 7 % in control diaphragm to 52 +/- 4 % after triamcinolone treatment (P < 0.05). 3. Only the adult isoform of SERCA1 (i.e. SERCA1a) mRNA was found in the diaphragm of the 15-week-old control rats. Furthermore, triamcinolone treatment resulted in reduced levels of SERCA2a (-40 %, P < 0.05) and increased levels of SLN mRNA (+100 %, P < 0.05), while the decrease in PLB mRNA (-31 %, P = 0.277) did not reach statistical significance. SERCA1b, SERCA2b and SERCA3 mRNA levels fell below the detection limit in the diaphragm of both control and triamcinolone-treated rats. 4. Compared to control diaphragm, control rat heart showed a relatively high PLB/(SERCA1 + SERCA2) mRNA ratio of 7.88 while this ratio amounted only to 0.16 in control extensor digitorum longus (EDL) muscle. Remarkably, the SLN/(SERCA1 + SERCA2) mRNA ratio in normal cardiac muscle (0.96) was nearly the same as in diaphragm, but in EDL it amounted to only 0.05 that in diaphragm. This indicates the very low expression of SLN in rat EDL. 5. These data reveal that considerable alterations in SERCA mRNA levels accompany the functional changes seen in diaphragm after corticosteroid treatment. The relatively larger decrease in SERCA1 mRNA is in agreement with the selective type II fibre atrophy previously observed in the diaphragm of triamcinolone-treated rats, but the magnitude of SERCA alterations is more pronounced than expected on the basis of the structural changes in the diaphragm. The increase in SLN mRNA levels may represent a compensatory mechanism.

Myosin heavy chains in fibers of TTX-paralyzed rat soleus and medial gastrocnemius muscles

The expression of five myosin heavy chain (MHC) isoforms was analyzed in the rat soleus (Sol) and the deep and superficial medial gastrocnemius (dGM, sGM) muscle after 2 and 4 wk of TTX paralysis by using immunohistochemical techniques. In Sol, after 4 wk of paralysis, fibers containing type I MHC were either pure type I (14%) or also contained developmental (D; 76%), IIa (26%), or IIx (18%) MHC. Values for corresponding fibers in dGM were 8.5, 65, 38, and 22%. Also, by 4 wk an increase was seen in the proportions of fibers expressing IIa MHC in Sol (from 16 to 38%) and dGM (from 24 to 74%). In a region of sGM in control muscles containing pure IIb fibers, a major proportion (86%) remained pure after 4 wk of paralysis, with the remainder coexpressing IIb and IIx. The results indicate that TTX-induced muscle paralysis results in an increase in fibers containing multiple MHC isoforms and that the D isoform appears in a major proportion of these hybrid fibers.

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.

Dihydropyridine receptor and ryanodine receptor gene expression in long-term denervated rat muscles

Following disruption of the nerve supply, extensor digitorum longus (EDL) and soleus (SOL) muscles in rats are known to exhibit alterations in excitation-contraction coupling. After total RNA isolation from the denervated and the contralateral control muscles performed at 25 and 50 days following denervation, RNase protection assays were carried out with four cDNA probes specific for the skeletal and cardiac isoforms of both the DHPR alpha 1-subunit and the RyR. Longterm denervation increased the expression of the mRNA for skeletal DHPR and skeletal RyR in SOL muscle, but it also significantly increased the expression of the mRNA for the cardiac isoform of the DHPR alpha 1 subunit in EDL muscle.

Diaphragm disuse reduces Ca2+ uptake capacity of sarcoplasmic reticulum

Chronic phrenic tetrodotoxin (TTX) blockade and phrenic denervation (Dnv) of hamster diaphragm result in decreased maximum specific tension, prolonged contraction time, and improved fatigue resistance (W. Z. Zhan and G. C. Sieck, J. Appl. Physiol. 72: 1445-1453, 1992). An underlying increased relative contribution of type I fibers to total muscle mass appears to be consistent with, but does not completely account for, changes in contractile and fatigue properties. The present study was designed to evaluate a potential role for altered cellular Ca2+ metabolism in the adaptive response of the diaphragm to chronic disuse. An analytic method based on simulation and modeling of long-term 45Ca2+ efflux data was used to estimate Ca2+ contents (nmol Ca2+/g wet wt tissue) and exchange fluxes (nmol Ca2+.min-1.g-1) for extracellular and intracellular compartments in the in vitro hamster hemidiaphragm after prolonged disuse. Three groups were compared: control (Con, n = 5), phrenic TTX blockade (TTX, n = 5), and phrenic denervation (Dnv, n = 5). Experimental muscles were loaded with 45Ca2+ for 1 h, and efflux data were collected for 8 h by using a flow-through tissue chamber. Compartmental analysis of efflux data estimated that the Ca2+ contents and Ca2+ exchange fluxes of the largest and slowest intracellular compartment (putative longitudinal reticulum) were reduced by approximately 50% in TTX and Dnv muscle groups compared with Con. In addition, the kinetic model predicted significant decreases in total intracellular Ca2+ and total diaphragm Ca2+ in TTX and Dnv muscles. We conclude that the data support the hypothesis that the capacity of the sarcoplasmic reticulum for Ca2+ sequestration is reduced in chronic diaphragm disuse. The impact of this effect on diaphragm contractile and fatigue properties is discussed.

The recovery of long-term denervated rat muscles after Marcaine treatment and grafting

Disruption of the nerve supply results in the rapid loss of mass and contractile force in skeletal muscles. These losses are reversible to a high degree in short-term denervated muscles with grafting and nerve implantation. However, return is much poorer in long-term denervated muscles. This study examined the basis for the differences in the recovery of non-denervated and 7-month denervated rat extensor digitorum longus (EDL) muscles after grafting and nerve implantation. We found that the level of recovery is related to the ability of muscle fibers to degenerate and regenerate after grafting. Fibers within long-term denervated muscles do not degenerate and regenerate as well as those within muscles which are not denervated prior to grafting. The functional recovery of the denervated muscles is significantly improved when their fibers are induced to degenerate with the myotoxic anesthetic, Marcaine, Degeneration of these fibers is followed by massive regeneration. The finding that denervated muscles are capable of being restored to a significant level by inducing regeneration may be useful in the clinical treatment of denervated muscles.

Changes in mRNA levels of the sarcoplasmic/endoplasmic-reticulum Ca(2+)-ATPase isoforms in the rat soleus muscle regenerating from notexin-induced necrosis

The relative mRNA levels corresponding to the different sarcoplasmic/endoplasmic-reticulum Ca(2+)-ATPase isoforms (SERCA1a, SERCA1b, SERCA2a, SERCA2b and SERCA3) were measured by reverse transcriptase-PCR in rat soleus muscles regenerating after notexin-induced necrosis. The succession of appearance of the different types of SERCA mRNA species in regenerating muscle largely recapitulates those observed during normal ontogenesis. The mRNA levels of the muscle-specific isoforms SERCA1a and SERCA2a became very low on the first and third days after injection of the snake venom. It was only on the fifth day of regeneration that the mRNA of the neonatal variant of the fast-twitch skeletal SERCA1b isoform began to rise, well before the other SERCA transcripts. At 7 and 10 days, i.e. at a time when the new myofibres normally become reinnervated, the mRNA level of SERCA1a and SERCA2a increased markedly, but the fast-twitch skeletal SERCA1a isoform was still the most prominent. On day 21, in the advanced stage of regeneration, a switch in the relative expression levels of SERCA1a and SERCA2a mRNA was observed and the ratio of both isoforms became similar to that found in the normal soleus muscles. This was followed by a decline in the level of all SERCA mRNA species, so that on day 28 the levels of the sarcoplasmic/endoplasmatic-reticulum Ca(2+)-pump RNAs was again lower but their ratio remained similar to that of the untreated control soleus.

Interactive effects of denervation and malnutrition on diaphragm structure and function

The purpose of this study was to examine the interactive effects of unilateral denervation (DN) and prolonged malnutrition (MN) on the structure and function of the diaphragm muscle (Dia). Four groups of rats were studied: control (Con), MN, DN, and DN-MN. MN began 2 wk after DN and lasted 4 wk. In both the DN and DN-MN groups, the relative loss in Dia weight exceeded the relative change in body weight. Compared with the Con group, Dia specific force was reduced by approximately 40% in both the DN and DN-MN groups but was unaffected in the MN group. Dia fatigue resistance improved in all experimental groups but to a greater extent in the DN and DN-MN groups. In both the DN and DN-MN groups, approximately 50% of Dia fibers were classified as type IIc, whereas fiber type proportions did not change in the MN group. In the DN group, only type IIb/x fibers atrophied, whereas all fiber types atrophied in the MN and DN-MN groups. We conclude that in the DN-MN group the reduction in specific force combined with the reduction in total cross-sectional area of the muscle significantly curtails Dia force-generating capacity.

Morphological changes in the triads and sarcoplasmic reticulum of rat slow and fast muscle fibres following denervation and immobilization

We observed the morphological features of the membrane systems (sarcoplasmic reticulum, transverse tubules and triads) involved with the excitation-contraction coupling in rat soleus and extensor digitorum longus muscle following two disuse protocols: denervation and immobilization. The immobilized positions were: maximum dorsal flexor (soleus were stretched and extensor digitorum longus were shortened), maximum plantar flexor (soleus were shortened and extensor digitorum longus were stretched), and midway between the dorsal flexor and plantar flexor. The arrangement of the membrane systems was disordered following both disuse conditions. Increases in transverse tubule network were apparent; there were clearly more triads than in normal fibres, and pentadic and heptadic structures (i.e., a close approximation of two or three transverse tubule elements with three or four elements of terminal cisternae of sarcoplasmic reticulum) were frequently appeared following both denervation and immobilization. The most notable difference between the influence of denervation and immobilization on the membrane systems is the time at which the pentads and heptads appeared. They appeared much earlier (1 week after denervation) in denervated than in immobilized (3 or 4 weeks after immobilization) muscle fibres. On the other hand, the frequency of pentads and heptads is clearly related to the fibre type (significantly higher in extensor digitorum longus) and to extent of atrophy. The different influences of immobilization in each leg position suggest that disuse, but with neurotrophic factor(s), influences on the membrane systems were affected by sarcomere length, and the neurotrophic factor(s) and muscle activity were not always necessary to form new membrane systems in disuse skeletal muscle fibres.

Sarco(endo)plasmic reticulum Ca2+ pump and metabolic enzyme expression in rabbit fast-type and slow-type denervated skeletal muscles. A time course study

Recent reports by d'Albis et al. have shown that denervation of 8-day-old rabbit fast-twitch muscle (gastrocnemius) leads to the transformation of the muscle towards a slow phenotype but the changes towards slow-type myosin isoforms and contractile properties of the muscle were temporally uncoordinated. We analyzed the time course of the effects of denervation of the gastrocnemius on the expression of the sarcoplasmic reticulum calcium pump isoforms (SERCA) and on the metabolic state of the muscle. Northern-blot analysis showed a rapid loss of the fast Ca2+ pump isoform (SERCA 1) mRNA from the denervated gastrocnemius which became of the oxidative type. The changes observed were complete as early as 35 days post-natal, i.e at the time when changes in contractile properties were previously observed. Denervation of the slow-twitch soleus led to a 50% decrease in the level of the slow Ca2+ pump isoform (SERCA 2) mRNA and was without effect on the metabolic state of the muscle. These findings extend previous results suggesting that in rabbit, continuous innervation is required for differentiation of fast-twitch muscles but is not an absolute requirement for differentiation of the slow-twitch muscle.

Activation of the sarcoplasmic reticulum Ca2+-ATPase induced by exercise

Prolonged exercise has been shown to cause disruption of intracellular calcium homeostasis in skeletal muscle, which is normally maintained by the sarcoplasmic reticulum (SR) Ca2+-ATPase. We have investigated the response of this enzyme to increased intracellular calcium levels by investigating the functional and physical characteristics of the SR Ca2+-ATPase and membrane lipids following 2 h of treadmill running and throughout a period of post-exercise recovery. The Ca2+-ATPase of SR membranes purified from exercised rats shows increases in enzymatic activity correlating with post-exercise recovery time. Corresponding increases in active Ca2+-ATPase pump units are observed, as measured by the concentration of phosphorylated enzyme intermediate formed from ATP. However, catalytic turnover rates of the Ca2+-ATPase are unchanged. Using spin-label electron paramagnetic resonance to assess both membrane fluidity and associations between individual Ca2+-ATPase polypeptide chains, we find no exercise-induced alterations in membrane dynamics which could explain the observed increases in Ca2+-ATPase activity. Nor do we find evidence for altered membrane purification as a result of exercise. We suggest that the cell responds to the challenge of increased cytosolic calcium levels by increasing the proportion of functional SR Ca2+-ATPase proteins in the membrane for the rapid restoration of calcium homeostasis.