Molecular transformations in sarcoplasmic reticulum of fast-twitch muscle by electro-stimulation

Age Related Changes in Motor Function (II). Decline in Motor Performance Outcomes

Age-related impairments in motor performance are caused by a deterioration in mechanical and neuromuscular functions, which have been investigated from the macro-level of muscle-tendon unit to the micro-level of the single muscle fiber. When compared to the healthy young skeletal muscle, aged skeletal muscle is: (1) weaker, slower and less powerful during the performance of voluntary contractions; (2) less steady during the performance of isometric contractions, particularly at low levels of force; and (3) less susceptible to fatigue during the performance of sustained isometric contractions, but more susceptible to fatigue during the performance of high-velocity dynamic contractions. These impairments have been discussed to be mainly the result of: a) loss of muscle mass and selective atrophy of type II muscle fibers; b) altered tendon mechanical properties (decreased tendon stiffness); c) reduced number and altered function of motor units; d) slower muscle fiber shortening velocity; e) increased oscillation in common synaptic input to motor neurons; and f) altered properties and activity of sarcoplasmic reticulum. In this second part of a two-part review we have detailed the age-related impairments in motor performance with a reference to the most important mechanical and neuromuscular contributing factors.

Sarcopenia: Aging-Related Loss of Muscle Mass and Function

Sarcopenia is a loss of muscle mass and function in the elderly that reduces mobility, diminishes quality of life, and can lead to fall-related injuries, which require costly hospitalization and extended rehabilitation. This review focuses on the aging-related structural changes and mechanisms at cellular and subcellular levels underlying changes in the individual motor unit: specifically, the perikaryon of the α-motoneuron, its neuromuscular junction(s), and the muscle fibers that it innervates. Loss of muscle mass with aging, which is largely due to the progressive loss of motoneurons, is associated with reduced muscle fiber number and size. Muscle function progressively declines because motoneuron loss is not adequately compensated by reinnervation of muscle fibers by the remaining motoneurons. At the intracellular level, key factors are qualitative changes in posttranslational modifications of muscle proteins and the loss of coordinated control between contractile, mitochondrial, and sarcoplasmic reticulum protein expression. Quantitative and qualitative changes in skeletal muscle during the process of aging also have been implicated in the pathogenesis of acquired and hereditary neuromuscular disorders. In experimental models, specific intervention strategies have shown encouraging results on limiting deterioration of motor unit structure and function under conditions of impaired innervation. Translated to the clinic, if these or similar interventions, by saving muscle and improving mobility, could help alleviate sarcopenia in the elderly, there would be both great humanitarian benefits and large cost savings for health care systems.

Review articles : Minimizing fatigue for functional electrical stimulation of muscle

Interest in the possibility of using electrically stimulated muscular contractions in rehabilitation medicine is increasing. Progress is impeded by the phenomenon of fatigue which impairs effectiveness and consistency of contractions. Various methods for minimizing fatigue have been proposed and are presently discussed. These include fibre type conversion as a result of chronic low frequency conditioning stimulation, sequential stimulation, optimization of stimulation parameters and the use of hybrid orthoses.

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.

Magnetic Stimulation of the Quadriceps Femoris Muscle

OBJECTIVE

The objective of this study was to compare pain induced by magnetic stimulation of the quadriceps femoris (QF) muscle with that induced by transcutaneous neuromuscular electrical stimulation (NMES).

DESIGN

Magnetic stimulation and transcutaneous NMES were applied to QF muscles of 17 normal volunteers. The intensity of each mode of stimulation was increased in a stepwise manner. Peak torque values of isometric contractions of QF muscles and visual analog scale (VAS) scores were recorded at each intensity level. The VAS scores of the two stimulating modalities were compared at the intensity-generating same peak torque values.

RESULTS

The median VAS scores for electrical and magnetic stimulation were 5.7 and 0.3, respectively. The median difference between the VAS scores for electrical and magnetic stimulation was 3.7 (range, 1.7-8.5). The mean of the maximum peak torque obtained from each subject was higher in magnetic stimulation than in electrical stimulation (9.5 +/- 4.8 vs. 4.4 +/- 2.9 Nm).

CONCLUSIONS

Magnetic stimulation of the QF muscle produced less pain at the same level of isometric peak torque than did transcutaneous NMES. Magnetic stimulation is a potential alternative to transcutaneous NMES, especially for persons with intact or residual sensory function.

Electrophysiological, histochemical, and hormonal adaptation of rat muscle after prolonged hindlimb suspension

The perspective of long-duration flights for future exploration, imply more research in the field of human adaptation. Previous studies in rat muscles hindlimb suspension (HLS), indicated muscle atrophy and a change of fibre composition from slow-to-fast twitch types. However, the contractile responses to long-term unloading is still unclear. Fifteen adult Wistar rats were studied in 45 and 70 days of muscle unweighting and soleus (SOL) muscle as well as extensor digitorum longus (EDL) were prepared for electrophysiological recordings (single, twitch, tetanic contraction and fatigue) and histochemical stainings. The loss of muscle mass observed was greater in the soleus muscle. The analysis of electrophysiological properties of both EDL and SOL showed significant main effects of group, of number of unweighting days and fatigue properties. Single contraction for soleus muscle remained unchanged but there was statistically significant difference for tetanic contraction and fatigue. Fatigue index showed a decrease for the control rats, but increase for the HLS rats. According to the histochemical findings there was a shift from oxidative to glycolytic metabolism during HLS. The data suggested that muscles atrophied, but they presented an adaptation pattern, while their endurance in fatigue was decreased.

Effect of endurance exercise on the Ca2+ pumps from transverse tubule and sarcoplasmic reticulum of rabbit skeletal muscle

The sarcoplasmic reticulum (SR) Ca(2+) pump is the main homeostatic regulatory mechanism in fast skeletal muscle that maintains intracellular Ca(2+) concentration ([Ca(2+)](i)) at the nanomolar level at rest. The transverse tubule (TT) Ca(2+) pump transports cytosolic Ca(2+) to the extracellular space. During prolonged muscular activity, [Ca(2+)](i) may increase. TT and SR isolated microsomal vesicles were highly purified, and the purity was checked by immunoblotting. The present study shows the effects of endurance exercise on the activities and structures of the TT and SR Ca(2+) pumps of fast skeletal muscle from rabbit at rest. The Ca(2+) pump activity increased manifolds in TT but did not change in SR. The protein denaturalization profiles obtained by differential scanning calorimetry showed 1) a shift in the transition temperature and an increase in the enthalpy of the TT Ca(2+) pump and 2) a significant change in the transition temperature of the SR Ca(2+) pump Ca(2+)-binding domain. We conclude that the TT Ca(2+) pump activity was upgraded in association with structural changes to handle the changes in [Ca(2+)](i) and TT lumen Ca(2+) concentration that occur during endurance exercise.

Inactivation of sarcoplasmic reticulum Ca(2+)-atpase in low-frequency stimulated rat muscle

Continuous low-frequency stimulation (CLFS) by implanted electrodes for 12-24 h led to a significant (approximately 30%) decrease in the activity of sarcoplasmic reticulum Ca(2+)-ATPase in fast-twitch extensor digitorum longus (EDL) and tibialis anterior (TA) muscles of intact rats. The decline in catalytic activity after 24 h of CLFS was accompanied by an approximately twofold increase in dinitrophenylhydrazine-reactive carbonyl groups of the enzyme. It also correlated with an immunochemically determined 30% decrease in Ca2(+)-ATPase protein. Recovery studies after 12 h of CLFS revealed a relatively slow (48-72 h) re-establishment of normal catalytic activity. These findings suggest that the 30% decline of Ca(2+)-ATPase activity in low-frequency stimulated rat muscle led to an irreversible modification by protein oxidation. The decrease in Ca(2+)-ATPase protein most likely resulted from the degradation of inactive Ca(2+)-ATPase molecules. The relatively slow recovery of Ca(2+)-ATPase activity suggests that de novo synthesis of the enzyme may be necessary to re-attain normal activity.

Dynamics of stimulation-induced muscle adaptation: insights from varying the duty cycle

We sought to gain insight into the dynamics of the signalling process that initiates adaptive change in mammalian skeletal muscles in response to chronic neuromuscular stimulation. Programmable miniature stimulators were implanted into rabbits and used to impose one of the following patterns on the dorsiflexors of one ankle: 10 Hz delivered in equal on/off periods of 30 s, 30 min, or 12 h (all equivalent in terms of aggregate impulse activity to continuous 5 Hz). Two further groups received continuous stimulation at 5 Hz or 10 Hz. In every case the stimulation pattern was maintained continuously for 6 weeks. Tibialis anterior muscles stimulated intermittently with equal on/off periods of 30 s, 30 min and 12 h had contractile characteristics that were significantly slower than the contralateral, unstimulated muscles but did not differ from those of muscles stimulated continuously at 5 Hz. Muscles stimulated continuously at 10 Hz were significantly slower than either contralateral muscles or muscles stimulated with any of the other patterns. Corresponding changes were seen in myosin heavy chain isoform composition. The fatigue index, defined as the fraction of tension remaining after 5 min of a standard fatigue test, was 0.4 for muscles in the contralateral group but equal to or greater than 0.85 for muscles of all the stimulated groups. These results were interpreted with the help of a simple model of the growth and decay of a putative signalling substance based on first order kinetics. The model suggests a rate constant for the accumulation of the signalling substance that is greater than 30 h(-1), and a rate constant for its removal that is greater than 50 h(-1).

Induction of a fatigue-resistant phenotype in rabbit fast muscle by small daily amounts of stimulation

We have shown that fatigue resistance can be induced in rabbit tibialis anterior (TA) muscles without excessive power loss by continuous stimulation at low frequencies, such as 5 Hz, and that the same result is obtained by delivering a 10-Hz pattern in equal on/off periods. Here we ask whether the same phenotype could be produced with daily amounts of stimulation that would be more appropriate for clinical use. We stimulated rabbit TA muscles for 6 wk, alternating fixed 30-min on periods of stimulation at 10 Hz with off periods of different duration. All patterns transformed fast-glycolytic fibers into fast-oxidative fibers. The muscles had fatigue-resistant properties but retained a higher contractile speed and power production than muscles transformed completely to the slow-oxidative type. We conclude that in the rabbit as little as one 30-min period of stimulation in 24 h can result in a substantial increase in the resistance of the muscle to fatigue.

Calcium transients in single fibers of low-frequency stimulated fast-twitch muscle of rat

Ca(2+) transients were investigated in single fibers isolated from rat extensor digitorum longus muscles exposed to chronic low-frequency stimulation for different time periods up to 10 days. Approximately 2.5-fold increases in resting Ca(2+) concentration ([Ca(2+)]) were observed 2 h after stimulation onset and persisted throughout the stimulation period. The elevated [Ca(2+)] levels were in the range characteristic of slow-twitch fibers from soleus muscle. In addition, we noticed a transitory elevation of the integral [Ca(2+)] per pulse with a maximum ( approximately 5-fold) after 1 day. Steep decreases in rate constant of [Ca(2+)] decay could be explained by an immediate impairment of Ca(2+) uptake and, with longer stimulation periods, by an additional loss of cytosolic Ca(2+) binding capacity resulting from a decay in parvalbumin content. A partial recovery of the rate constant of [Ca(2+)] decay in 10-day stimulated muscle could be explained by an increasing mitochondrial contribution to Ca(2+) sequestration.

Effect of chronic electrostimulation of rabbit skeletal muscle on calmodulin level and protein kinase activity

(a) Chronic electrostimulation of fast-twitch skeletal muscles makes them resemble slow-twitch muscles. The involvement of second-messenger cascades in this muscle reprogramming is not well understood. The goal of this study was to examine protein kinase activities and calmodulin levels as a function of the duration of electrostimulation. (b) Fast-twitch rabbit muscle was subjected to continuous low-frequency electrostimulation for 2 weeks. The extensor digitorum longus was taken and examined for calmodulin concentration and cAMP-dependent (PKA). Ca(2+)-phospholipid-dependent (PKC) and Ca(2+)-calmodulin-dependent (CaM kinase or PKB) protein kinase activities. (c) Electrostimulation for 14 days led to a significant increase in total calmodulin level and PKB activity, both rising in the cytosolic fraction. Protein kinase C translocated to the membrane fraction, although total activity did not change. (d) These changes could be related with electrostimulation-induced changes in excitation-contraction coupling.

Exercise-induced decreases in sarcoplasmic reticulum Ca(2+)-ATPase activity attenuated by high-resistance training

Muscle biopsies were performed on the vastus lateralis muscle prior to and during a high-resistance training (HRT) programme in order to examine the effects of hypertrophy on sarcoplasmic reticulum Ca2+ ATPase activity at rest and during exercise. In six male untrained volunteers (peak aerobic power, Vo2 peak = 3.39 +/- 0.13 L min-1, mean +/- SE), the resting Ca2+ ATPase activity (mumol-min-1 g wet wt-1) at 0 (4.89 +/- 0.20), 4 (5.62 +/- 0.56), 7 (5.15 +/- 0.41) and 12 (4.82 +/- 0.11) weeks was unchanged by HRT. During cycle ergometer exercise, prior to training, Ca(2+)-ATPase was reduced (P < 0.05) by 14% during the initial 30 min at 58% Vo2 peak and (P < 0.05) a further 19% during 30 min at 72% Vo2 peak. Following 7 and 12 weeks of training, the decreases in SR Ca(2+)-ATPase were less pronounced (P < 0.05). These results indicate that muscle hypertrophy, although incapable of altering Ca(2+)-ATPase pump activity at rest, can attenuate the decrease observed in exercise by mechanism(s) as yet unknown.

Protein oxidation, tyrosine nitration, and inactivation of sarcoplasmic reticulum Ca2+-ATPase in low-frequency stimulated rabbit muscle

Sustained contractile activity by chronic low-frequency stimulation in rabbit fast-twitch muscle causes a partial (40-50%) inactivation of the sarcoplasmic reticulum (SR) Ca2+-ATPase and, with prolonged stimulation, a SERCA1a to SERCA2a transition. To investigate the underlying mechanism of the inactivation which precedes the isoform transition, we analyzed SR from 4-day stimulated muscles for Ca2+-ATPase activity, lipid peroxidation, SH and carbonyl groups, and nitrotyrosine. At unaltered SH group and malondialdehyde contents, carbonyl groups were elevated 50% in the SR from stimulated muscles. Immunoblotting with anti-dinitrophenyl and anti-nitrotyrosine antibodies revealed strong labeling of the Ca2+-ATPase, suggesting the inactivation of the enzyme to result from protein oxidation and peroxynitrite-mediated tyrosine nitration.

Cellular responses in exertion-induced skeletal muscle injury

Muscle injury is a common result of muscle exertion caused by overload and over-activity. In this presentation, an attempt was made to discuss models of muscle injury which involve exertion but not excessive strain, although most functional activities of the extremities require some eccentric muscle actions. Muscle injury is characterized by cellular and extracellular matrix responses which appear to be common to all types of muscle trauma -- even in the absence of bleeding. Using tenotomy and functional over-load of the rat hindlimb muscles as examples, illustrations of several of these responses are presented and discussed.

Stabilization of chronic remodeling by asynchronous cardiomyoplasty in dilated cardiomyopathy: effects of a conditioned muscle wrap

BACKGROUND

Dynamic cardiomyoplasty is a promising new therapy for dilated cardiomyopathy. The girdling effects of a conditioned muscle wrap alone have recently been postulated to partly explain its mechanism. We investigated this effect in a canine model of chronic dilated cardiomyopathy.

METHODS AND RESULTS

Twenty dogs underwent rapid ventricular pacing (RVP) for 4 weeks to create a model of dilated cardiomyopathy. Seven dogs were then randomly selected to undergo subsequent cardiomyoplasty, and all dogs had 6 weeks of additional RVP. The cardiomyoplasty group also received 6 weeks of concurrent skeletal muscle stimulation consisting of single twitches delivered asynchronously at 2 Hz to transform the wrap without active assistance. All dogs were studied by pressure-volume analysis and echocardiography at baseline and after 4 and 10 weeks of pacing. Systolic indices, including ejection fraction (EF), end-systolic elastance (Ees), and preload-recruitable stroke work (PRSW) were all increased at 10 weeks in the wrap versus controls (EF, 34.0 versus 27.1, P=.008; Ees, 1.65 versus 1.26, P=.09; PRSW, 35.9 versus 25.5, P=.001). Ventricular volumes, diastolic relaxation, and left ventricular end-diastolic pressures stabilized in the cardiomyoplasty group but continued to deteriorate in controls. Both the end-systolic and end-diastolic pressure-volume relationships shifted farther rightward in controls but remained stable in the cardiomyoplasty group.

CONCLUSIONS

In addition to potential benefits from active systolic assistance, benefits from dynamic cardiomyoplasty appear to be partially accounted for by the presence of a conditioned muscle wrap alone. This conditioned wrap stabilizes the remodeling process of heart failure, arresting progressive deterioration of systolic and diastolic function.

Induction of molecular and mechanical transformations in canine skeletal muscle by chronic neuromuscular stimulation

The canine latissimus dorsi was stimulated at 1 Hz via the thoracodorsal nerve for 70 days. Seven days of muscle stimulation caused muscle mass, fibre cross-sectional areas, and tetanic tensions to decrease. Fourteen days of stimulation produced marked decreases in Ca(2+)-uptake rates in a membrane fraction containing sarcoplasmic reticulum. At this time there was a decline in fusion frequency, but no statistically significant changes in time-to-peak tension, total contraction times, or half-relaxation times. With 42 days of stimulation a switch from the fast-twitch to the slow-twitch phenotype was indicated by elevations in the levels of expression of the slow-twitch isoforms of sarco(endo)plasmic reticulum Ca(2+)-ATPase and myosin heavy chain-I, and increases in half-relaxation times, total contraction times and time-to-peak tensions. Decreases in muscle shortening velocity correlated negatively with increases in myosin heavy chain-I levels. Up-regulation of the slow-twitch isoforms of sarco(endo)plasmic reticulum Ca(2+)-ATPase increases in half-relaxation times. The changes in the slow-twitch isoform of sarco(endo)plasmic reticulum Ca(2+)-ATPase and myosin heavy chain-I levels indicated coordinate expression of these two proteins in chronically stimulated muscles.

Mammalian skeletal muscle fiber type transitions

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.

Influence of electrical stimulation of the tibialis anterior muscle in paraplegic subjects. 2. Morphological and histochemical properties

In adult paraplegic subjects one tibialis anterior muscle received daily electrical stimulation for 4 weeks at twice the motor threshold to determine changes of morphological and histochemical profiles (this paper) and of contractile properties (preceding paper). Bilateral biopsies, obtained 4 weeks before, and immediately after, electrical stimulation, were studied for fibre type proportions, fibre diameters, oxidative capacity, microvasculature and histopathology. Before stimulation the biopsies showed disuse with increased type 2 fibre proportions and decreased oxidative capacity (succinate dehydrogenase (SDH) activity). The effects of two stimulus patterns were compared. Following stimulation SDH activity increased significantly in all stimulated muscles. Inconsistent changes occurred in fibre type proportions, fibre diameters, capillary density and capillary/fibre ratios. Both stimulus patterns evoked similar results. In five/seven subjects subsarcolemmal vacuolation was observed. Electrical stimulation for 4 weeks at only twice motor threshold improves oxidative capacity, but different stimulus parameters are probably needed for significant fibre type conversion.

The input-output relations of skeletal muscle

We used three approaches to determine the stimulation patterns that maximize the isometric force-time integral per impulse (FTIpP) available from tibialis anterior muscles of the rabbit. Initially the interval between two pulses was fixed at the value that gave the maximum force-time integral, and successive pulses were added at intervals that maximized the FTIpP. We checked this iterative approach by a second method, in which a computer-generated protocol was used to deliver randomized bursts to the muscles. These experiments confirmed that optimal stimulation patterns for fast muscles consisted of an initial high-frequency portion followed by a train of impulses at a lower frequency. However, for muscles that had been stimulated chronically at a constant low frequency, an initial high-frequency portion conferred no advantage. In a third set of experiments we used constant-frequency bursts to generate contour surfaces that represented the dependence of FTIpP on the frequency and number of impulses. The results agreed with those from the earlier methods. We conclude that optimized patterns have potential for clinical use, but their value will depend strongly on the activation characteristics of the stimulated muscle.

Effects of prolonged low frequency stimulation on skeletal muscle sarcoplasmic reticulum

The role of prolonged electrical stimulation on sarcoplasmic reticulum (SR) Ca2+ sequestration measured in vitro and muscle energy status in fast white and red skeletal muscle was investigated. Fatigue was induced by 90 min intermittent 10-Hz stimulation of rat gastrocnemius muscle, which led to reductions (p < 0.05) in ATP, creatine phosphate, and glycogen of 16, 55, and 49%, respectively, compared with non-stimulated muscle. Stimulation also resulted in increases (p < 0.05) in muscle lactate, creatine, Pi, total ADP, total AMP, IMP, and inosine. Calculated free ADP (ADPf) and free AMP (AMPf) were elevated 3- and 15-fold, respectively. No differences were found in the metabolic response between tissues obtained from the white (WG) and red (RG) regions of the gastrocnemius. No significant reductions is SR Ca2+ ATPase activity were observed in homogenate (HOM) or a crude SR fraction (CM) from WG or RG muscle following exercise. Maximum Ca2+ uptake in HOM and CM preparations was similar in control (C) and stimulated (St) muscles. However, Ca2+ uptake at 400 nM free Ca2+ was significantly reduced in CM from RG (0.108 +/- 0.04 to 0.076 +/- 0.02 mumol.mg-1 protein.min-1 in RG - C and RG - St, respectively). Collectively, these data suggest that reductions in muscle energy status are dissociated from changes in SR Ca2+ ATPase activity in vitro but are related to Ca2+ uptake at physiological free [Ca2+ bd in fractionated SR from highly oxidative muscle. Dissociation of SR Ca2+ ATPase activity from Ca2+ uptake may reflect differences in the mechanisms evaluated by these techniques.

Failure of short term stimulation to reduce sarcoplasmic reticulum Ca(2+)-ATPase function in homogenates of rat gastrocnemius

To examine the effect of short term intense activity on sarcoplasmic reticulum (SR) Ca2+ sequestering function, the gastrocnemius (G) muscles of 11 anaesthetized male rats (weight, 411 +/- 8 g, X +/- SE) were activated using supramaximal, intermittent stimulation (one train of 0.2 msec impulses per sec of 100 msec at 100 Hz). Homogenates were obtained from stimulated white (WG-S) and red (RG-S) tissues, assayed for Ca2+ uptake and maximal Ca2+ ATPase activity and compared to contralateral controls (WG-C, RG-C). Calcium uptake (nmoles/mg protein/min) determined using Indo-1 and at [Ca2+]i concentrations between 300-400 nM was unaffected (p > 0.05) by activity in both WG (6.14 + 0.43 vs 5.37 + 0.43) and RG (3.21 + 0.18 vs 3.07 + 0.20). Similarly, no effect (p > 0.05) of contractile activity was found for maximal Ca2+ ATPase activity (mumole/mg protein/min) determined spectrophotometrically in RG (0.276 + 0.03 vs 0.278 + 0.02). In WG, Ca2+ ATPase activity was 15% higher in WG-S compared to WG-C (0.412 + 0.03 vs 0.385 + 0.04). Repetitive stimulation resulted in a reduction in tetanic tension of 74% (p < 0.05) by 2 min in the G muscle. By the end of the stimulation period, ATP concentration was reduced (p < 0.05) by 57% in the WG and by 47% in the RG.(ABSTRACT TRUNCATED AT 250 WORDS)

Technical considerations for assessing alterations in skeletal muscle sarcoplasmic reticulum Ca(++)-sequestration function in vitro

A multiple measurement system for assessing sarcoplasmic reticulum (SR) Ca(++)-ATPase activity and Ca(++)-uptake was used to examine the effects of SR fractionation and quick freezing on rat white (WG) and red (RG) gastrocnemius muscle. In vitro measurements were performed on whole muscle homogenates (HOM) and crude microsomal fractions (CM) enriched in SR vesicles before and after quick freezing in liquid nitrogen. Isolation of the CM fraction resulted in protein yields of 0.96 +/- 0.1 and 0.99 +/- 0.1 mg/g in WG and RG, respectively. The percent Ca(++)-ATPase recovery for CM compared to HOM was 14.5% (WG) and 10.1% (RG). SR Ca(++)-activated Ca(++)-ATPase activity was not affected by quick freezing of HOM or CM, but basal ATPase was reduced (P < 0.05) in frozen HOM (5.12 +/- 0.18-3.98 +/- 0.20 mole/g tissue/min in WG and from 5.39 +/- 0.20-4.48 +/- 0.24 mumole/g tissue/min in RG). Ca(++)-uptake was measured at a range of physiological free [Ca++] using the Ca++ fluorescent dye Indo-1. Maximum Ca(++)-uptake rates when corrected for initial [Ca++]f were not altered in HOM or CM by quick freezing but uptake between 300 and 400nM free Ca++ was reduced (P < 0.05) in quick frozen HOM (1.30 +/- 0.1-0.66 +/- 0.1 mumole/g tissue/min in WG and 1.04 +/- 0.2-0.60 +/- 0.1 mumole/g tissue/min in RG). Linear correlations between Ca(++)-uptake and Ca(++)-ATPase activity measured in the presence of the Ca++ ionophore A23187 were r = +0.25, (P < 0.05) and r = +0.74 (P < 0.05) in HOM and CM preparations, respectively, and were not altered by freezing. The linear relationships between HOM and CM maximum Ca(++)-uptake (r = +0.44, P < 0.05) and between HOM and CM Ca(++)-ATPase activity (r = +0.34, P < 0.05) were also not altered by tissue freezing. These data suggest that alterations in maximal SR Ca(++)-uptake function and maximal Ca(++)-ATPase activity may be measured in both HOM and CM fractions following freezing and short term storage.

Effects of intensified endurance training on the concentration of Na,K-ATPase and Ca-ATPase in human skeletal muscle

Thirty-nine moderately endurance trained males increased their normal training programme of 2.2 h week-1 with an average training intensity of 65% of maximum heart rate (HRmax) to 2.7 h week-1 and a mean intensity of 78% of HRmax. Performance tests and measurements of the total concentrations of Na,K-ATPase (3H-ouabain binding) and Ca-ATPase, fibre type distribution and fibre area were performed in biopsies from the vastus lateralis muscle before and after increased training. The 6 weeks of training elevated VO2max from 54.9 +/- 3.1 to 58.3 +/- 3.0 ml O2 min-1 kg-1 (P < 0.0001). Exercise time to exhaustion at 86% of VO2max (pre-training) increased from 35 +/- 8 to 61 +/- 17 min (P < 0.0001). The concentration of Ca-ATPase was unaffected by the intensified training (6.74 +/- 1.03 vs. 6.68 +/- 1.07 nmol g wet wt-1), but the concentration of Na,K-ATPase increased from 307 +/- 43 to 354 +/- 59 pmol g wet wt-1 (P < 0.0001). The relative distribution of FT-fibres was correlated with the concentration of Ca-ATPase (r = 0.72, P < 0.0001). The data support the view that intensive training induces an upregulation of the concentration of skeletal muscle Na,K-ATPase, but no change in the total capacity for reaccumulation of Ca2+ into the SR. There was no correlation between the concentrations of Na,K-ATPase, Ca-ATPase and indices of endurance performance.

Control of myogenic factor genes by the membrane depolarization/protein kinase C cascade in chick skeletal muscle

Myogenic factor genes were found to respond differentially to electrical stimulation of denervated chick skeletal muscle. Myogenin gene activity declined rapidly (t1/2: approximately 2 min), comparable to the rate of acetylcholine receptor (AChR) gene inactivation, while other myogenic bHLH genes either lost activity more slowly (MyoD) or not at all (myf5, herculin). Protein kinase C (PKC) is known to couple membrane activity to AChR gene inactivation; myogenin gene transcription was also rapidly blocked by the PKC activator PMA, whereas electrostimulation remained without effect on myogenin gene activity in muscle that was either exposed to the kinase inhibitor staurosporine or chronically treated with PMA to deplete PKC. These results attest to a special role for myogenin in the activation of AChR genes in denervation supersensitivity.

Protein kinase C couples membrane excitation to acetylcholine receptor gene inactivation in chick skeletal muscle

The signaling pathway connecting membrane depolarization and gene activity in skeletal muscle remains largely unknown. Using transcription elongation (run-on) analysis we have found that electrical stimulation of denervated chick skeletal muscle in vivo rapidly and selectively results in inactivation of acetylcholine receptor (AChR) subunit genes. We have studied the possible involvement of protein kinase C (PKC) in this response and have observed that electrical stimulation increases the activity of PKC in the nucleus by over two orders of magnitude within 10 min; phorbol esters, within minutes after intramuscular application, block AChR subunit genes in the absence of electrical activity; and the activity-triggered gene inactivation is blocked by the protein kinase inhibitor staurosporine or by enzyme depletion resulting from chronic pretreatment of muscle with phorbol esters. We conclude that PKC is an integral component of the pathway coupling membrane excitation and AChR gene control.

Inactivation of sarcoplasmic-reticulum Ca(2+)-ATPase in low-frequency-stimulated muscle results from a modification of the active site

Molecular changes underlying the partial inactivation of the sarcoplasmic-reticulum (SR) Ca(2+-) ATPase in low-frequency-stimulated fast-twitch muscle were investigated in the present study. The specific Ca(2+)-ATPase activity, as well as the ATP- and acetyl phosphate-driven Ca2+ uptakes by the SR, were reduced by approx. 30% in 4-day-stimulated muscle. Phosphoprotein formation of the enzyme in the presence of ATP or Pi was also decreased to the same extent. Measurements of ATP binding revealed a 30% decrease in binding to the enzyme. These changes were accompanied by similar decreases in the ligand-induced (ATP, ADP, Pi) intrinsic tryptophan fluorescence. A decreased binding of fluorescein isothiocyanate (FITC) corresponded to the lower ATP binding and phosphorylation of the enzyme. Moreover, Pi-induced changes in fluorescence of the FITC-labelled enzyme did not differ between SR from stimulated and contralateral muscles, indicating that Ca(2+)- ATPase molecules which did not bind FITC were responsible for the decreased Pi-dependent phosphorylation, and therefore represented the inactive form of the enzyme. No differences existed between the Ca(2+)-induced changes in the intrinsic fluorescence of SR from stimulated and contralateral muscles which fit their similar Ca(2+)-binding characteristics. Taking the proposed architecture of the Ca2(+)-ATPase into consideration, our results suggest that the inactivation relates to a circumscribed structural alteration of the enzyme in sections of the active site consisting of the nucleotide-binding and phosphorylation domains.

Effects of reduced joint mobility and training on Na,K-ATPase and Ca-ATPase in skeletal muscle

In guinea pigs, the ankle joint was partly immobilized in a position reducing dorsiflection to 105 degrees (as compared to the normal value of 30 degrees). When compared with the contralateral unrestrained leg, this led to a significant atrophy and a decrease in contractile force (-23%) of the gastrocnemius muscle. This was associated with a significant decrease in the total concentration of [3H]ouabain binding sites in gastrocnemius and plantaris muscle reaching minimum (-19% and -23%) after 3 weeks, but no evidence of degenerative changes. Total contents of Ca and Ca-ATPase were increased by 27% and 22%, respectively. After 4 to 5 weeks of reduced mobility, the concentration of [3H]ouabain binding sites in gastrocnemius muscle returned to control level. The lowest concentration of [3H]ouabain binding sites reached during reduced mobility was 258 +/- 13 pmol/g wet wt., and the maximum value attained following 3 weeks of reduced mobility and 3 weeks of training by running was 498 +/- 25 pmol/g wet wt., i.e, 93% higher. In soleus, training produced an increase of 25%. Clinically, it is important to realize that movable braces cannot prevent the development of muscular atrophy. The observed spontaneous recovery of the Na,K-pump concentration may partly explain why patients using movable casts show a better capacity for physical performance than those treated with complete immobilization. In conclusion, the total concentration of Na,K-pumps in guinea pig skeletal muscle undergoes downregulation and upregulation as a function of contractile activity as well as muscle length under conditions mimicking the constraints on mobility frequently used in the clinical treatment of lesions.

Sarcoplasmic-reticulum biogenesis in contraction-inhibited skeletal-muscle cultures

We have previously shown that inhibition of the spontaneous contractile activity of cultured embryonic-chick skeletal-muscle fibres with tetrodotoxin (TTX) leads to decreased sarcoplasmic-reticulum Ca(2+)-transport rates and steady-state concentrations of the high-energy Ca(2+)-ATPase phosphoenzyme intermediate [Charuk & Holland (1983) Exp. Cell Res. 144, 143-157]. In the present study we used a monoclonal antibody to the Ca(2+)-ATPase to show that there is a decreased amount of enzyme accumulated by contraction-inhibited myotubes. Indirect immunofluorescence microscopy using the monoclonal antibody to the Ca(2+)-ATPase also revealed a disordered subcellular organization of the sarcotubular system in contraction-inhibited myotubes. The biogenesis of sarcoplasmic-reticulum proteins in TTX-paralysed myofibres was studied by labelling cells with [35S]methionine before isolation of the active Ca(2+)-pump membrane fraction. Protein turnover was selectively increased in that fraction from TTX-treated muscle cultures. Electrophoretic analysis and quantitative fluorography confirmed that decreased accumulation of the Ca(2+)-ATPase enzyme in contraction-inhibited myotubes was associated with increased turnover of this protein. The present results demonstrate that biogenesis of the sarcoplasmic-reticulum Ca(2+)-ATPase is regulated by the contractile activity of skeletal-muscle fibres.

Separation of active and inactive (nonphosphorylating) Ca(2+)-ATPase in sarcoplasmic reticulum subfractions from low-frequency-stimulated rabbit muscle

Chronic low-frequency stimulation elicits in rabbit fast-twitch muscle a partial inactivation of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase and Ca(2+)-uptake activities. Inactive Ca(2+)-ATPase was enriched in a light microsomal fraction by sucrose density gradient centrifugation after calcium oxalate loading in the presence of ATP. This fraction showed a reduced specific activity and phosphoprotein formation of the Ca(2+)-transport ATPase. These results suggest that the inactivation of the Ca(2+)-ATPase as induced by increased contractile activity, is confined to a specific SR vesicle population.

Prolonged exercise induces structural changes in SR Ca(2+)-ATPase of rat muscle

Sarcoplasmic reticulum (SR) isolated from the deep red portion of the gastrocnemius muscle of Sprague-Dawley rats after a single bout of prolonged exercise was shown to have depressed Ca(2+)-stimulated Mg(2+)-dependent ATPase activity over a temperature range of 15 to 42.5 degrees C when compared to SR obtained from control muscle. Inclusion of the calcium ionophore, A23187, failed to restore the depressed ATPase activity from SR of exercised muscle to control values, but it did normalize the stimulatory effect of temperature on ATPase activity. This depression was also manifested as an increased activation energy when the data were converted to an Arrhenius plot. SR vesicles from both groups showed no differences or discontinuities in plots of steady-state fluorescence anisotropy. When the binding characteristics of the fluorescent probe, fluorescein isothiocyanate (FITC), were analyzed, SR vesicles prepared from exercised muscle displayed a 40% reduction in binding capacity with no apparent change in Kd. These findings support the conclusion that a single bout of exercise induces a structural change in the Ca(2+)-ATPase protein of rat red gastrocnemius muscle that is not a direct result of gross lipid alterations or increased muscle temperature.

Intrathoracic skeletal muscle ventricles: a feasibility study

For skeletal muscle ventricles (SMVs) to be applied clinically, it is likely that they will have to be placed within the chest. Ease of subsequent connection to the circulation, and avoidance of significant lung compression, are factors that could influence SMV size and shape in a way that may prejudice their ability to pump effectively at physiological preloads. In five dogs, specially designed SMVs were constructed from the latissimus dorsi muscle, and placed in the apex of the left hemithorax. After a 3-week delay, the muscle was preconditioned electrically by 2-Hz continuous stimulation for 6 weeks. At a later thoracotomy, this positioning of SMVs permitted easy surgical access to the heart and great vessels. SMVs were then connected to a mock circulation device for functional evaluation. As right-sided pumps, at a preload of 10 mmHg, SMVs generated a stroke volume (SV) and stroke work (SW) exceeding that of the native right ventricle (SV = 8.9 +/- 0.8 vs 7.9 +/- 0.6 mL; SW = 0.44 +/- 0.03 vs 0.20 ergs x 10(6)). As left-sided pumps, also at a preload of 10 mmHg, SMV SV, and SW was roughly half that of the left ventricle (SV = 3.7 +/- 0.2 vs 7.9 +/- 0.6 mL; SW = 0.29 +/- 0.03 vs 0.57 +/- 0.05 ergs x 10(6)). SMVs may conveniently be positioned inside the chest, where they have the potential to function as left or right heart assist devices.

Expression of Ca2(+)-ATPase isoforms in denervated, regenerating, and dystrophic chicken skeletal muscle

The expression of fast and slow isoforms of the sarcoplasmic reticulum Ca2(+)-ATPase was studied in denervated, regenerating, and dystrophic fast and slow avian skeletal muscles. We found that both fast and slow Ca2(+)-ATPase isoforms were expressed in most myofibers following denervation of adult fast-twitch muscle, but only the slow Ca2(+)-ATPase isoform was found in slow-tonic muscle which had been denervated. Regenerating myotubes in normally innervated and previously denervated adult fast-twitch or slow-tonic muscle expressed both Ca2(+)-ATPase isoforms. Expression of the slow Ca2(+)-ATPase isoform was found to persist in dystrophic fast-twitch muscle, long after it had disappeared from normal fast-twitch muscle. However, the fast Ca2(+)-ATPase isoform disappeared from slow-tonic muscle similarly in normal and dystrophic birds. These results demonstrate that the appearance of myosin heavy chain isoforms characteristic of developing muscle is correlated with similar changes in the expression of sarcoplasmic reticulum Ca2(+)-ATPases.

Relationship of primary and secondary myogenesis to fiber type development in embryonic chick muscle

The formation of fast and slow myotubes was investigated in embryonic chick muscle during primary and secondary myogenesis by immunocytochemistry for myosin heavy chain and Ca2(+)-ATPase. When antibodies to fast or slow isoforms of these two molecules were used to visualize myotubes in the posterior iliotibialis and iliofibularis muscles, one of the isoforms was observed in all primary and secondary myotubes until very late in development. In the case of myosin, the fast antibody stained virtually all myotubes until after stage 40, when fast myosin expression was lost in the slow myotubes of the iliofibularis. In the case of Ca2(+)-ATPase, the slow antibody also stained all myotubes until after stage 40, when staining was lost in secondary myotubes and in the fast primary myotubes of the posterior iliotibialis and the fast region of the iliofibularis. In contrast, the antibodies against slow muscle myosin heavy chain and fast muscle Ca2(+)-ATPase stained mutually exclusive populations of myotubes at all developmental stages investigated. During primary myogenesis, fast Ca2(+)-ATPase staining was restricted to the primary myotubes of the posterior iliotibialis and the fast region of the iliofibularis, whereas slow myosin heavy chain staining was confined to all of the primary myotubes of the slow region of the iliofibularis. During secondary myogenesis, the fast Ca2(+)-ATPase antibody stained nearly all secondary myotubes, while primaries in the slow region of the iliofibularis remained negative. Thus, in the slow region of the iliofibularis muscle, these two antibodies could be used in combination to distinguish primary and secondary myotubes. EM analysis of staining with the fast Ca2(+)-ATPase antibody confirmed that it recognizes only secondary myotubes in this region. This study establishes that antibodies to slow myosin heavy chain and fast Ca2(+)-ATPase are suitable markers for selective labeling of primary and secondary myotubes in the iliofibularis; these markers are used in the following article to describe and quantify the effects that chronic blockade of neuromuscular activity or denervation has on these populations of myotubes.

Chronic low-frequency stimulation of rabbit fast-twitch muscle induces partial inactivation of the sarcoplasmic reticulum Ca2(+)-ATPase and changes in its tryptic cleavage

Persistently increased contractile activity as induced by low-frequency stimulation in fast-twitch rabbit muscle elicits a partial inactivation of the sarcoplasmic reticulum Ca2(+)-ATPase function with regard to Ca2+ transport and ATP hydrolysis. Electron microscopy showed no differences in the frequency and structure of the two-dimensional Ca2(+)-ATPase crystals between microsomal fractions from normal and stimulated muscles. However, differences existed between the tryptic digestion of the Ca2(+)-ATPase in both the membrane-bound and solubilized enzyme at the first tryptic cleavage site, named T1 (Arg505). This followed from a delayed appearance of the A and B fragments of the Ca2(+)-ATPase in the electrostimulated muscle. No differences existed with regard to the second tryptic cleavage site, named T2 (Arg198). Confirming previous results, fluorescein isothiocyanate (FITC) binding to the enzyme of the chronically stimulated muscle was markedly reduced. The FITC-labeled fraction of the enzyme from both the normal and the stimulated muscle followed similar time courses of tryptic cleavage. The fraction of Ca2(+)-ATPase that did not bind TITC was identified by immunoblot analysis as the trypsin-resistant form. In view of the vicinity of T1, the FITC- and the ATP-binding sties, these results point to a modification of the enzyme in that region leading to an inactivation of about 50% of the sarcoplasmic reticulum Ca2(+)-ATPase molecules.