The role of frequency in the effects of long-term intermittent stimulation of denervated slow-twitch muscle in the rat

Combination Treatment With Exogenous GDNF and Fetal Spinal Cord Cells Results in Better Motoneuron Survival and Functional Recovery After Avulsion Injury With Delayed Root Reimplantation

When spinal roots are torn off from the spinal cord, both the peripheral and central nervous system get damaged. As the motoneurons lose their axons, they start to die rapidly, whereas target muscles atrophy due to the denervation. In this kind of complicated injury, different processes need to be targeted in the search for the best treatment strategy. In this study, we tested glial cell-derived neurotrophic factor (GDNF) treatment and fetal lumbar cell transplantation for their effectiveness to prevent motoneuron death and muscle atrophy after the spinal root avulsion and delayed reimplantation. Application of exogenous GDNF to injured spinal cord greatly prevented the motoneuron death and enhanced the regeneration and axonal sprouting, whereas no effect was seen on the functional recovery. In contrast, cell transplantation into the distal nerve did not affect the host motoneurons but instead mitigated the muscle atrophy. The combination of GDNF and cell graft reunited the positive effects resulting in better functional recovery and could therefore be considered as a promising strategy for nerve and spinal cord injuries that involve the avulsion of spinal roots.

Adaptive muscle plasticity of a remaining agonist following denervation of its close synergists in a model of complete spinal cord injury

Complete spinal cord injury (SCI) alters the contractile properties of skeletal muscle, and although exercise can induce positive changes, it is unclear whether the remaining motor system can produce adaptive muscle plasticity in response to a subsequent peripheral nerve injury. To address this, the nerve supplying the lateral gastrocnemius (LG) and soleus muscles was sectioned unilaterally in four cats that had recovered hindlimb locomotion after spinal transection. In these spinal cats, kinematics and electromyography (EMG) were collected before and for 8 wk after denervation. Muscle histology was performed on LG and medial gastrocnemius (MG) bilaterally in four spinal and four intact cats. In spinal cats, cycle duration for the hindlimb ipsilateral or contralateral to the denervation could be significantly increased or decreased compared with predenervation values. Stance duration was generally increased and decreased for the contralateral and ipsilateral hindlimbs, respectively. The EMG amplitude of MG was significantly increased bilaterally after denervation and remained elevated 8 wk after denervation. In spinal cats the ipsilateral LG was significantly smaller than the contralateral LG, whereas the ipsilateral MG weighed significantly more than the contralateral MG. Histological characterizations revealed significantly larger fiber areas for type IIa fibers of the ipsilateral MG in three of four spinal cats. Microvascular density in the ipsilateral MG was significantly higher than in the contralateral MG. In intact cats, no differences were found for muscle weight, fiber area, or microvascular density between homologous muscles. Therefore, the remaining motor system after complete SCI retains the ability to produce adaptive muscle plasticity.

Contractile and metabolic properties of engineered skeletal muscle derived from slow and fast phenotype mouse muscle

Satellite cells derived from fast and slow muscles have been shown to adopt contractile and metabolic properties of their parent muscle. Mouse muscle shows less distinctive fiber-type profiles than rat or rabbit muscle. Therefore, in this study we sought to determine whether three-dimensional muscle constructs engineered from slow soleus (SOL) and fast tibialis anterior (TA) from mice would adopt the contractile and metabolic properties of their parent muscle. Time-to-peak tension (TPT) and half-relaxation time (1/2RT) was significantly slower in SOL constructs. In agreement with TPT, TA constructs contained significantly higher levels of fast myosin heavy chain (MHC) and fast troponin C, I, and T isoforms. Fast SERCA protein, both slow and fast calsequestrin isoforms and parvalbumin were found at higher levels in TA constructs. SOL constructs were more fatigue resistant and contained higher levels of the mitochondrial proteins SDH and ATP synthase and the fatty acid transporter CPT-1. SOL constructs contained lower levels of the glycolytic enzyme phosphofructokinase but higher levels of the β-oxidation enzymes LCAD and VLCAD suggesting greater fat oxidation. Despite no changes in PGC-1α protein, SOL constructs contained higher levels of SIRT1 and PRC. TA constructs contained higher levels of the slow-fiber program repressor SOX6 and the six transcriptional complex (STC) proteins Eya1 and Six4 which may underlie the higher in fast-fiber and lower slow-fiber program proteins. Overall, we have found that muscles engineered from predominantly slow and fast mouse muscle retain contractile and metabolic properties of their native muscle.

A New System and Paradigm for Chronic Stimulation of Denervated Rat Muscle

Traditionally, animal studies employing electrical stimulation for conditioning denervated muscle rely on 24-hour-based stimulation paradigms, most employing implantable stimulators. While these stimulators provide the necessary current to cause muscular contraction, they have problems with battery life, programmability, and long-term robustness. Continuous 24-hour stimulation, while shown to be effective in animals, is not easily translatable to a clinical setting. It is also difficult to evaluate animal comfort and muscular contraction throughout a 24-hour period. We have developed a system and stimulation paradigm that can stimulate up to five animals at one time for one hour per day. The constant current stimulator is a USB-powered device that can, under computer control, output trains of pulses with selectable shapes, widths, durations and repetition rates. It is an external device with no implantable parts in the animal except for the stimulating electrodes. We tested the system on two groups of rats with denervated gastrocnemius muscles. One group was stimulated using a one-hour-per-day, 5-days-per-week stimulation paradigm for one month, while the other group had electrodes implanted but received no stimulation. Muscle weight and twitch force were significantly larger in the stimulated group than the non-stimulated group. Presently, we are using the stimulator to investigate electrical stimulation coupled with other therapeutic interventions that can minimize functional deficits after peripheral nerve injuries.

Excitation-transcription coupling in skeletal muscle: the molecular pathways of exercise

Muscle fibres have different properties with respect to force, contraction speed, endurance, oxidative/glycolytic capacity etc. Although adult muscle fibres are normally post-mitotic with little turnover of cells, the physiological properties of the pre-existing fibres can be changed in the adult animal upon changes in usage such as after exercise. The signal to change is mainly conveyed by alterations in the patterns of nerve-evoked electrical activity, and is to a large extent due to switches in the expression of genes. Thus, an excitation-transcription coupling must exist. It is suggested that changes in nerve-evoked muscle activity lead to a variety of activity correlates such as increases in free intracellular Ca²⁺ levels caused by influx across the cell membrane and/or release from the sarcoplasmatic reticulum, concentrations of metabolites such as lipids and ADP, hypoxia and mechanical stress. Such correlates are detected by sensors such as protein kinase C (PKC), calmodulin, AMP-activated kinase (AMPK), peroxisome proliferator-activated receptor δ (PPARδ), and oxygen dependent prolyl hydroxylases that trigger intracellular signaling cascades. These complex cascades involve several transcription factors such as nuclear factor of activated T-cells (NFAT), myocyte enhancer factor 2 (MEF2), myogenic differentiation factor (myoD), myogenin, PPARδ, and sine oculis homeobox 1/eyes absent 1 (Six1/Eya1). These factors might act indirectly by inducing gene products that act back on the cascade, or as ultimate transcription factors binding to and transactivating/repressing genes for the fast and slow isoforms of various contractile proteins and of metabolic enzymes. The determination of size and force is even more complex as this involves not only intracellular signaling within the muscle fibres, but also muscle stem cells called satellite cells. Intercellular signaling substances such as myostatin and insulin-like growth factor 1 (IGF-1) seem to act in a paracrine fashion. Induction of hypertrophy is accompanied by the satellite cells fusing to myofibres and thereby increasing the capacity for protein synthesis. These extra nuclei seem to remain part of the fibre even during subsequent atrophy as a form of muscle memory facilitating retraining. In addition to changes in myonuclear number during hypertrophy, changes in muscle fibre size seem to be caused by alterations in transcription, translation (per nucleus) and protein degradation.

Optimizing skeletal muscle reinnervation with nerve transfer

Denervation as a consequence of nerve injury causes profound structural and functional changes within skeletal muscle and can lead to a marked impairment in function of the affected limb. Prompt reinnervation of a muscle with a sufficient number of motion-specific motor axons generally results in good structural and functional recovery, whereas long-term denervation or insufficient or improper axonal recruitment uniformly results in poor functional recovery. Only nerve transfer has been highly efficacious in changing the clinical outcomes of patients with skeletal muscle denervation, especially in the case of proximal limb nerve injuries. Rapid reinnervation with an abundant number of motor axons remains the only clinically effective means to restore function to denervated skeletal muscles.

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.

Plasticity from muscle to brain

Recognition that the entire central nervous system (CNS) is highly plastic, and that it changes continually throughout life, is a relatively new development. Until very recently, neuroscience has been dominated by the belief that the nervous system is hardwired and changes at only a few selected sites and by only a few mechanisms. Thus, it is particularly remarkable that Sir John Eccles, almost from the start of his long career nearly 80 years ago, focused repeatedly and productively on plasticity of many different kinds and in many different locations. He began with muscles, exploring their developmental plasticity and the functional effects of the level of motor unit activity and of cross-reinnervation. He moved into the spinal cord to study the effects of axotomy on motoneuron properties and the immediate and persistent functional effects of repetitive afferent stimulation. In work that combined these two areas, Eccles explored the influences of motoneurons and their muscle fibers on one another. He studied extensively simple spinal reflexes, especially stretch reflexes, exploring plasticity in these reflex pathways during development and in response to experimental manipulations of activity and innervation. In subsequent decades, Eccles focused on plasticity at central synapses in hippocampus, cerebellum, and neocortex. His endeavors extended from the plasticity associated with CNS lesions to the mechanisms responsible for the most complex and as yet mysterious products of neuronal plasticity, the substrates underlying learning and memory. At multiple levels, Eccles' work anticipated and helped shape present-day hypotheses and experiments. He provided novel observations that introduced new problems, and he produced insights that continue to be the foundation of ongoing basic and clinical research. This article reviews Eccles' experimental and theoretical contributions and their relationships to current endeavors and concepts. It emphasizes aspects of his contributions that are less well known at present and yet are directly relevant to contemporary issues.

Influence of knee joint angle on muscle properties of paralyzed and nonparalyzed human knee extensors

Muscles of individuals with a spinal cord injury (SCI) exhibit an unexpected leftward shift in the force (torque)-frequency relationship. We investigated whether differences in torque-angle relationships between SCI and able-bodied control muscles could explain this shift. Electrically stimulated knee-extensor contractions were obtained at knee flexion angles of between 30 degrees and 90 degrees. Torque-frequency relationships were obtained at 30 degrees, 90 degrees, and optimum angle. Optimum angle was not different between groups but SCI-normalized torques were lower at the extreme angles. At all angles, SCI muscles produced higher relative torques at low stimulation frequencies. Thus, there was no evidence of a consistent change in the length of paralyzed SCI muscles, and the anomalous leftward shift in the torque-frequency relationship was not the result of testing the muscle at a relatively long length. The results provide valuable information about muscle changes occurring in various neurological disorders.

Number of contractions to maintain mass and force of a denervated rat muscle

Within 5 weeks, denervated extensor digitorum longus (EDL) muscles of rats lose 66% of mass, 91% of force, and 76% of fiber cross-sectional area (CSA). We previously determined the parameters of electrical stimulation for denervated rat EDL muscles to generate tetanic contractions sufficient to maintain mass and force close to control values. Using these parameters, we tested the hypothesis that a range exists for number of contractions per day, below and above which values for mass, maximum force, and fiber CSA are lower than values for innervated control muscles. For 5 weeks, denervated EDL muscles were stimulated to generate between 25 and 5000 contractions daily with contractions separated by constant intervals of rest, repeated 24 h per day. Force was not maintained with fewer than 200 or more than 800 contractions daily, whereas mass and fiber CSA were not maintained with fewer than 50 contractions daily. Protocols of stimulation that maintain muscle mass and force during prolonged periods of denervation may minimize problems clinically associated with denervation atrophy.

Effect of muscle electrostimulation on afferent activities from tibialis anterior muscle after nerve repair by self-anastomosis

Numerous previous studies were devoted to the regeneration of motoneurons toward a denervated muscle after nerve repair by self-anastomosis but, to date, few investigations have evaluated the regeneration of sensory muscle endings. In a previous electrophysiological study (Decherchi et al., 2001) we showed that the functional characteristics of tibialis anterior muscle afferents are affected after self-anastomosis of the peroneal nerve even when the neuromuscular preparation was not chronically stimulated. The present study examines the regeneration of groups I-II (mechanosensitive) and groups III-IV (metabosensitive) muscle afferents by evaluating the recovery of their response to different test agents after self-anastomosis combined or not with chronic muscle stimulation for a 10-weeks period. We compared five groups of rats: C, control; L, nerve lesion without suture; LS, nerve lesion with suture; LSE(m): nerve lesion plus chronic muscle stimulation with a monophasic rectangular current; and LSE(b): nerve lesion plus chronic stimulation with a biphasic current with modulations of pulse duration and frequency, eliciting a pattern of activity resembling that delivered by the nerve to the muscle. Compared to the control group, (1) muscle kept only its original weight in the LSE(b) group, (2) in the LS group the response curve to tendon vibration was shifted toward the highest mechanical frequencies and the response of groups III-IV afferents after fatiguing muscle stimulation lowered, (3) in the LSE(m) group, the pattern of activation of mechanoreceptors by tendon vibrations was altered as in the LS group, and the response of metabosensitive afferents to KCl injections was markedly reduced, (4) in the LSE(b) group, the response to tendon vibration was not modified and the activation of metabosensitive units by increased extracellular potassium chloride concentration was conserved. Both LSE(b) and LSE(m) conditions were ineffective to maintain the post muscle stimulation activation of metabosensitive units as well as their activation by injected lactic acid solutions. Our data indicate that chronic muscle electrostimulation partially favors the recovery of mechano- and metabosensitivity in a denervated muscle and that biphasic modulated currents seem to provide better results.

Mechanical properties of the electrically silent adult rat soleus muscle

The isometric and isotonic in situ mechanical properties of the soleus muscle of adult female rats were determined after 60 days of inactivity induced by spinal cord isolation (SI). Compared to control, the absolute muscle mass, physiological cross-sectional area, and maximum tetanic tension of the soleus in SI rats were reduced by 69%, 66%, and 77%, respectively. Isometric twitch time-to-peak-tension and half-relaxation times were 41% and 60% shorter in SI than control rats. The maximum velocity of shortening (mm/s), as determined using the afterloaded technique, was 66% faster in SI than control rats, whereas unloaded shortening velocity was similar in the two groups (9% faster in SI rats). Peak power was 48% lower in SI than control rats. The SI soleus was 39% more fatigable than control. Thus, the soleus became a smaller, faster, and more fatigable muscle following 60 days of inactivity. In general, the results indicate that the adaptations are of a lesser magnitude than those reported previously following denervation for the same duration. These data provide a baseline for future efforts to experimentally define the mechanisms of neurally mediated, but activity-independent, regulation of the mechanical properties of the rat soleus muscle.

Contractile properties of the quadriceps muscle in individuals with spinal cord injury

Selected contractile properties and fatigability of the quadriceps muscle were studied in seven spinal cord-injured (SCI) and 13 able-bodied control (control) individuals. The SCI muscles demonstrated faster rates of contraction and relaxation than did control muscles and extremely large force oscillation amplitudes in the 10-Hz signal (65 +/- 22% in SCI versus 23 +/- 8% in controls). In addition, force loss and slowing of relaxation following repeated fatiguing contractions were greater in SCI compared with controls. The faster contractile properties and greater fatigability of the SCI muscles are in agreement with a characteristic predominance of fast glycolytic muscle fibers. Unexpectedly, the SCI muscles exhibited a force-frequency relationship shifted to the left, most likely as the result of relatively large twitch amplitudes. The results indicate that the contractile properties of large human locomotory muscles can be characterized using the approach described and that the transformation to faster properties consequent upon changes in contractile protein expression following SCI can be assessed. These measurements may be useful to optimize stimulation characteristics for functional electrical stimulation and to monitor training effects induced by electrical stimulation during rehabilitation of paralyzed muscles.

What does chronic electrical stimulation teach us about muscle plasticity?

The model of chronic low-frequency stimulation for the study of muscle plasticity was developed over 30 years ago. This protocol leads to a transformation of fast, fatigable muscles toward slower, fatigue-resistant ones. It involves qualitative and quantitative changes of all elements of the muscle fiber studied so far. The multitude of stimulation-induced changes makes it possible to establish the full adaptive potential of skeletal muscle. Both functional and structural alterations are caused by orchestrated exchanges of fast protein isoforms with their slow counterparts, as well as by altered levels of expression. This remodeling of the muscle fiber encompasses the major, myofibrillar proteins, membrane-bound and soluble proteins involved in Ca2+ dynamics, and mitochondrial and cytosolic enzymes of energy metabolism. Most transitions occur in a coordinated, time-dependent manner and result from altered gene expression, including transcriptional and posttranscriptional processes. This review summarizes the advantages of chronic low-frequency stimulation for studying activity-induced changes in phenotype, and its potential for investigating regulatory mechanisms of gene expression. The potential clinical relevance or utility of the technique is also considered.

Postnatal development under conditions of simulated weightlessness and space flight

The adaptability of the developing nervous system to environmental influences and the mechanisms underlying this plasticity has recently become a subject of interest in space neuroscience. Ground studies on neonatal rats using the tail suspension model of weightlessness have shown that the force of gravity clearly influences the events underlying the postnatal development of motor function. These effects depend on the age of the animal, duration of the perturbation and the motor function studied. A nine-day flight study has shown that a dam and neonates can develop under conditions of space flight. The motor function of the flight animals after landing was consistent with that seen in the tail suspension studies, being marked by limb joint extension. However, there were expected differences due to: (1) the unloading of the vestibular system in flight, which did not occur in the ground-based experiments; (2) differences between flight and suspension durations; and (3) the inability to evaluate motor function during the flight. The next step is to conduct experiments in space with the flexibility and rigor that is now limited to ground studies: an opportunity offered by the International Space Station.

Determination of muscle contractile properties: the importance of the nerve

Contractile phenotype of muscle fibres is strongly influenced by hormones, stretch and influences from the motor neurones, although cell lineage probably also plays a role. Motor neurones can affect muscle fibres by releasing neurotrophic substances and by evoking electrical activity in the muscle. For regulating contractile properties such as speed, strength and endurance it has been demonstrated that electrical activity is crucial, while the role of putative neurotrophic substances remains unclear. The signal to change is coded in the pattern of electrical activity. Thus, high amounts of activity lead to slow shortening velocity and myosin heavy chains, while low amounts of activity lead to a fast phenotype. For regulation of twitch duration frequency also plays a role, and for preventing atrophy in denervated muscles high frequency seems to be beneficial, particularly in fast muscles. Little is known about the excitation-adaptation pathway linking action potentials to expression of genes that are relevant for contractile properties. Muscle specific transcription factors of the helix-loop-helix family such as myoD and myogenin could be important for regulating genes related to metabolic profile and fibre size/strength, while their role in determining myosin heavy chain expression and classical fibre type is more uncertain.

Effects of long-term phasic electrical stimulation on denervated soleus muscle: guinea-pig contrasted with rat

Guinea-pig soleus muscles were denervated and electrically stimulated for periods of 43 to 66 days. Stimuli were in 1 s bursts of 40 Hz pulses, repeated every 5 min. Other guinea-pigs were denervated for 82 days without stimulation and, in a third group, the soleus muscle was necrotized and allowed to regenerate without reinnervation for 13-15 days. Isometric and isotonic recordings were made in vivo. Denervated guinea-pig muscles were embedded in epoxy resin for light and electron microscopy. Chronic stimulation of denervated guinea-pig soleus had no effects on the prolonged twitch or on reduced maximal shortening velocity, maximal rate of rise of tension and tetanic force. This contrasts with the slow-to-fast conversion produced by denervation and denervation-stimulation of rat soleus. Loss of force was much greater in rat than guinea-pig after denervation, and chronic stimulation increased force in rat to the same level as in guinea-pig after denervation (with or without stimulation). Eighty-day denervated guinea-pig soleus did not reveal those morphological signs of fibre breakdown and regeneration which are prominent in denervated rat soleus muscles. Those changes in rat resembled aneurally regenerated muscles in several aspects, especially the increased incidence of fibres with internal myo-nuclei which did not appear in guinea-pig soleus after denervation. Aneurally regenerated guinea-pig soleus became fast like aneurally regenerated rat muscle. Our data are compatible with the hypothesis that slow-to-fast transformation of denervated rat soleus is not directly brought about by chronic stimulation but by de-novo formation of fast-contracting regenerated fibres. The persistence of fibrillation in guinea-pig but not rat after denervation may account for the species difference.

Electrical stimulation of denervated muscle: is it worthwhile?

Research conducted over the past 25 years has demonstrated that muscle activity, not neurotrophic substances, is the most important factor in the regulation of specific physiological and biochemical properties of muscle fibers. Application of this knowledge has led to considerable experimentation with chronic electrical stimulation as a possible clinical tool for the treatment of denervated muscles. Evidence accumulated from animal studies has indicated that direct electrical stimulation of denervated muscles can to a large extent substitute for innervation and preserve or restore the normal properties of the muscles. Appropriate stimulation parameters were critical for a successful intervention, and the best results were obtained when the stimulation pattern resembled the firing pattern of the normal motoneuron. Thus, fast muscles required intermittent, brief, high frequency stimulation and slow muscles needed continuous, low frequency stimulation. For human denervated muscles, critical questions still remain to be resolved before electrical stimulation will yield the optimum benefit. Research must be performed in human subjects to define the appropriate stimulation parameters the stimulation current, and the type and placement of electrodes.

Conversion of the rabbit gracilis muscle for transposition as a neoanal sphincter by electrical stimulation

To re-establish anal function in fecally incontinent patients it may be feasible to transpose the gracilis muscle around the anal canal, using electrical stimulation to trigger contraction. However, because the fast-twitching gracilis muscle is incapable of prolonged contraction without fatigue, it is necessary to convert it to a slow-twitching, fatigue-resistant muscle. We demonstrated this conversion by longterm electrical stimulation at low frequencies using a rabbit model. The nerve to the gracilis muscle was continuously stimulated at 2 Hz, 5 Hz, and 10 Hz for 2, 4, or 6 weeks. In the 6-week conditioning group, the percentage of type I fibers, identified by ATPase staining, increased as the conditioning frequency became higher, but the twitch contraction speed reduced with conditioning at a frequency of more than 5 Hz. The fatigue resistance improved by conditioning at 10 Hz, and conversion occurred in 6 weeks. Thus, we concluded that conditioning at 10 Hz for 6 weeks can convert rabbit gracilis muscle to a slow-twitching, fatigue-resistant muscle suitable for use as a neoanal sphincter.

Identification of a critical period for motor development in neonatal rats

Manipulation of the developing nervous system has provided valuable insights into nervous system function. One important concept to arise from this type of study has been the identification of specific "critical periods" for the development of various functions. A critical period has been most clearly shown for the visual system where monocular eye closure for a few weeks led to functionally significant changes in visually guided behaviors and the connectivity of the visual cortex. Critical periods have also been defined for other sensory systems. Although studies of the effect of manipulating sensory systems during development are sometimes difficult to interpret (e.g. Ref. 7), this difficulty is compounded in the case of the motor system. Problems arise because manipulations of the postnatal motor system are difficult to implement and usually require invasive procedures such as tenotomy, neurotomy, and nerve crush (for review, see Ref. 17). We have approached the problem of manipulating the motor environment by adapting a paradigm widely used to study the experimental effects of simulated weightlessness in adult rats: namely, tail suspension. This method has several advantages for manipulating the motor system: (i) because it is noninvasive, it is less discomforting than neurotomy, tenotomy or nerve crush; (ii) it does not immobilize the animals, they move about the cage and extend and flex their hindlimbs; and (iii) it specifically examines the importance of load-bearing on the development of antigravity muscles and their neuronal circuits.(ABSTRACT TRUNCATED AT 250 WORDS)

The importance of frequency and amount of electrical stimulation for contractile properties of denervated rat muscles

Soleus (SOL) and extensor digitorum longus (EDL) muscles were denervated and directly stimulated for 23-69 days through implanted electrodes employing three different patterns. The stimulation was delivered in impulse trains where the pulse frequency differed (20, 75, and 150 Hz), while the train duration (0.3 s) and train repetition rate (1 min-1) were identical. Consequently, the number of pulses varied such that higher frequency was combined with a higher amount of stimulation. In both SOL and EDL the high-frequency pattern resulted in shorter twitch time-to-peak, greater post-tetanic potentiation, and greater tetanic force than the low frequency. Isotonic shortening velocity was increased to the same extent by all the patterns in SOL whereas in EDL fast intrinsic shortening velocity was maintained by the low-frequency pattern while it was decreased by the high-frequency pattern. We attribute this unexpected effect on the EDL to the larger number of pulses in the high-frequency pattern. By combining the present findings with previous data on directly stimulated rat muscles we conclude: in SOL the twitch duration is influenced by both the frequency and the amount of impulse activity, higher frequencies and smaller amounts leading to faster twitches. The EDL twitch duration is similarly dependent on the amount of activity, but the role of frequency is more unclear. In both SOL and EDL the isotonic shortening velocity is reduced by increasing amounts of activity and there is no evidence that impulse frequency plays a role. In EDL force output is strongly influenced by the impulse frequency, low frequencies resulting in low force outputs irrespective of the amount of activity.

Effects of chronic electrical stimulation on contractile properties of long-term denervated rat skeletal muscle

1. The contractile properties of fast-twitch (extensor digitorum longus or EDL) and slow-twitch (soleus) muscles in the rat were followed for periods of between 4 and 10 months after denervation. The effects of chronic electrical stimulation during the last 3-8 weeks of denervation were investigated. 2. The fall in tetanic tension that follows axotomy ended after about 4 months' denervation. The equilibrium tension was about 0.75% of control tension in EDL and 0.2-0.3% in soleus. 3. The low tension in soleus was due partly to the small diameter of the muscle fibres (atrophy) and partly to their necrosis that resulted in an 8-fold fall in specific tension (the force per unit cross-sectional area). Similar but less extreme changes occurred in EDL. 4. It is speculated that the final level of tension reached by unstimulated denervated muscles is an equilibrium between decrease in force due to atrophy and necrosis and increase due to regeneration. Differences between the final tension levels in soleus and EDL cannot be accounted for quantitatively by known differences in atrophy alone. Therefore, the rate of necrosis in soleus and of regeneration in EDL may be higher. 5. Chronic stimulation of long-term denervated muscle increased force generation by about 7-fold in EDL and between 20 and 55 times in soleus. The final tension reached was between 4 and 5% of normal in both muscles. Specific tension of fibres was almost completely restored by stimulation and the number of fibres was normal. The failure to recover full tension was largely due to failure to reverse denervation atrophy completely. 6. Twitch contraction and relaxation times were identical in denervated-stimulated soleus and EDL. There was no evidence for dependence on duration of stimulation or tension of the muscle. The normalized maximum rate of rise of tetanic tension remained higher in EDL than soleus.

A comparison of fibrillation in denervated skeletal muscle of the anaesthetized rat and guinea-pig

We have used intracellular recording in vivo to study fibrillation (spontaneous repetitive membrane activity) in extensor digitorum longus (fast twitch) and soleus (slow twitch) muscles of the anaesthetized rat and guinea-pig denervated for periods of about 10 to 60 days. The proportion of fibres fibrillating in the guinea-pig soleus was greater than 50% in most animals up to the longest period of denervation (65 days). Fibrillation was rarely found in rat soleus after three weeks of denervation. Its incidence in the extensor digitorum longus muscles of both species was intermediate. The mean frequency of fibrillation was higher in guinea-pig extensor digitorum longus (16 Hz) and soleus (8 Hz) than in the rat extensor digitorum longus (3 Hz) and soleus (2 Hz). The resting membrane potentials of the denervated muscles were less than normal and correlated inversely with the frequency of fibrillation but not with the incidence of fibrillation: in rat soleus, many fewer fibers were fibrillating at a given membrane potential than in the other three muscles. The incidence of fibrillation was compared with previously reported tensions of the four denervated muscles and was found to have the same rank order. We suggest that fibrillation may reduce atrophy (and hence tension loss) of denervated muscle, which may have implications for artificial stimulation. Fibrillation frequency was directly related to changes in twitch speed of the four muscles after denervation.

Slow-to-fast transformation of denervated soleus muscles by chronic high-frequency stimulation in the rat

1. Adult soleus muscles were denervated and stimulated directly for 2-130 days with 'fast' (short pulse trains at 100 Hz) or 'slow' (continuously at 10 Hz, or long pulse trains at 15 Hz) stimulus patterns. 2. At the end of the period of stimulation isometric twitches and tetani and isotonic shortening velocities were measured. Frozen cross-sections were later examined with antibodies against myosin heavy chains specific for adult fast, adult slow and fetal myosin. 3. Isometric twitch duration (twitch time-to-peak and half-relaxation time) decreased during intermittent 100 Hz stimulation to values that were almost as fast as in the normal extensor digitorum longus (EDL) (95 and 94% transformation). The major part of the decrease occurred between 2 and 21 days after the onset of stimulation, and was accompanied by post-tetanic potentiation of the twitch, 'sag' in tension during an unfused tetanus, lower twitch/tetanus ratio and marked shifts to the right (higher frequencies) of the tension-frequency curve of the muscle. In contrast, during 10 or 15 Hz stimulation the isometric twitch duration remained slow, the twitch continued to show post-tetanic depression and absence of 'sag', while the twitch/tetanus ratio increased. 4. Denervation per se led to a slight increase and, then, after about a month, to a moderate and gradual decrease in twitch duration. The twitch/tetanus ratio increased markedly and post-tetanic depression became less pronounced or disappeared. Muscle weight and particularly tetanic tension were markedly reduced and these reductions were to a large extent counteracted by electrical stimulation. 5. Implantation of sham electrodes had no effect on twitch duration of denervated or innervated control muscles, but reduced tetanic tension in the innervated control muscles. 6. Maximum isotonic shortening velocity of the whole muscle (mm/s) increased during intermittent 100 Hz stimulation to a value as fast as in the normal EDL (110% transformation). Since the muscle fibres also increased in length (35%) maximum intrinsic shortening velocity (fibre lengths/s) was only incompletely transformed (55%). The increase in Vmax occurred between 7 and 14 days after the onset of stimulation. 7. All the fibres stimulated intermittently at 100 Hz were strongly labelled with anti-fast myosin and more than 90% were in addition weakly labelled by anti-slow myosin. Weak and variable labelling with anti-fast myosin was first detected 7 days after the onset of stimulation. In contrast, essentially all the fibres stimulated at 10 or 15 Hz showed no binding of anti-fast but strong binding of anti-slow myosin.(ABSTRACT TRUNCATED AT 400 WORDS)