The dose-related response of rabbit fast muscle to long-term low-frequency stimulation

Long-term stimulation by implanted pacemaker enables non-atrophic treatment of bilateral vocal fold paresis in a human-like animal model

A wide variety of treatments have been developed to improve respiratory function and quality of life in patients with bilateral vocal fold paresis (BVFP). One experimental method is the electrical activation of the posterior cricoarytenoid (PCA) muscle with a laryngeal pacemaker (LP) to open the vocal folds. We used an ovine (sheep) model of unilateral VFP to study the long-term effects of functional electrical stimulation on the PCA muscles. The left recurrent laryngeal nerve was cryo-damaged in all animals and an LP was implanted except for the controls. After a reinnervation phase of six months, animals were pooled into groups that received either no treatment, implantation of an LP only, or implantation of an LP and six months of stimulation with different duty cycles. Automated image analysis of fluorescently stained PCA cross-sections was performed to assess relevant muscle characteristics. We observed a fast-to-slow fibre type shift in response to nerve damage and stimulation, but no complete conversion to a slow-twitch-muscle. Fibre size, proportion of hybrid fibres, and intramuscular collagen content were not substantially altered by the stimulation. These results demonstrate that 30 Hz burst stimulation with duty cycles of 40% and 70% did not induce PCA atrophy or fibrosis. Thus, long-term stimulation with an LP is a promising approach for treating BVFP in humans without compromising muscle conditions.

Effect of neuromuscular electrical stimulation frequency on muscles of the tongue

INTRODUCTION

Neuromuscular electrical stimulation (NMES) for the treatment of swallowing disorders is delivered at a variety of stimulation frequencies. We examined the effects of stimulation frequency on tongue muscle plasticity in an aging rat model.

METHODS

Eighty-six young, middle-aged, and old rats were assigned to either bilateral hypoglossal nerve stimulation at 10 or 100 Hz (5 days/week, 8 weeks), sham, or no-implantation conditions. Muscle contractile properties and myosin heavy chain (MyHC) composition were determined for hyoglossus (HG) and styloglossus (SG) muscles.

RESULTS

Eight weeks of 100-Hz stimulation resulted in the greatest changes in muscle contractile function with significantly longer contraction and half-decay times, the greatest reduction in fatigue, and a transition toward slowly contracting, fatigue-resistant MyHC isoforms.

DISCUSSION

NMES at 100-Hz induced considerable changes in contractile and phenotypic profiles of HG and SG muscles, suggesting higher frequency NMES may yield a greater therapeutic effect. Muscle Nerve, 2018.

The adaptive response of skeletal muscle: What is the evidence?

Adult skeletal muscle is capable of adapting its properties in response to changing functional demands. This now sounds like a statement of the obvious, and many people assume it has always been this way. A mere 40 years ago, however, the picture was entirely different. In this Review and personal memoir, I outline the scientific context in which the theory was generated, the objections to it from entrenched opinion, and the way those objections were progressively met. The material should be of some historical interest, but, more importantly, it collects together the full range of evidence on which the current paradigm is based. Muscle Nerve 57: 531-541, 2018.

In-situ measurements of tensile forces in the tibialis anterior tendon of the rat in concentric, isometric, and resisted co-contractions

Tensile-force transmitted by the tibialis anterior (TA) tendon of 11 anesthetized adult male Wistar rats (body-mass: 360.6 ± 66.3 g) was measured in-situ within the intact biomechanical system of the hind-limb using a novel miniature in-line load-cell. The aim was to demonstrate the dependence of the loading-profile experienced by the muscle, on stimulation-frequency and the resistance to shortening in a group of control-animals. Data from these acute-experiments shows the type of loading achievable by means of implantable electrical stimulators activating agonists or agonist/antagonist groups of muscles during programmed resistance-training in freely moving healthy subjects. Force-responses to electrical stimulation of the common peroneal nerve for single pulses and short bursts were measured in unloaded and isometric contractions. A less time-consuming approach to measure the force-frequency relationship was investigated by applying single bursts containing a series of escalating stimulus-frequencies. We also measured the range of loading attainable by programmed co-contraction of the TA-muscle with the plantar-flexor muscles for various combinations of stimulation-frequencies. The maximal average peak-force of single twitches was 179% higher for isometric than for unloaded twitches. Average maximal isometric tetanic-force per gramme muscle-mass was 16.5 ± 3.0 N g-1, which agrees well with other studies. The standard and time-saving approaches to measure the force-frequency relationship gave similar results. Plantar-flexor co-activation produced greatly increased tension in the TA-tendon, similar to isometric contractions. Our results suggest that unloaded contractions may not be adequate for studies of resistance-training. Plantar-flexor co-contractions produced considerably higher force-levels that may be better suited to investigate the physiology and cell-biology of resistance-training in rodents.

Enhanced exercise and regenerative capacity in a mouse model that violates size constraints of oxidative muscle fibres

A central tenet of skeletal muscle biology is the existence of an inverse relationship between the oxidative fibre capacity and its size. However, robustness of this relationship is unknown. We show that superimposition of Estrogen-related receptor gamma (Errγ) on the myostatin (Mtn) mouse null background (Mtn^-/-/Errγ^Tg/+) results in hypertrophic muscle with a high oxidative capacity thus violating the inverse relationship between fibre size and oxidative capacity. We also examined the canonical view that oxidative muscle phenotype positively correlate with Satellite cell number, the resident stem cells of skeletal muscle. Surprisingly, hypertrophic fibres from Mtn^-/-/Errγ^Tg/+ mouse showed satellite cell deficit which unexpectedly did not affect muscle regeneration. These observations 1) challenge the concept of a constraint between fibre size and oxidative capacity and 2) indicate the important role of the microcirculation in the regenerative capacity of a muscle even when satellite cell numbers are reduced.

DOI:http://dx.doi.org/10.7554/eLife.16940.001

Tongue muscle plasticity following hypoglossal nerve stimulation in aged rats

INTRODUCTION

Age-related decreases in tongue muscle mass and strength have been reported. It may be possible to prevent age-related tongue muscle changes using neuromuscular electrical stimulation (NMES). Our hypothesis was that alterations in muscle contractile properties and myosin heavy chain composition would be found after NMES.

METHODS

Fifty-four young, middle-aged, and old 344/Brown Norway rats were included in this study. Twenty-four rats underwent bilateral electrical stimulation of the hypoglossal nerves for 8 weeks and were compared with control or sham rats. Muscle contractile properties and myosin heavy chain (MHC) in the genioglossus (GG), styloglossus (SG), and hyoglossus (HG) muscles were examined.

RESULTS

Compared with unstimulated control rats, we found reduced muscle fatigue, increased contraction and half-decay times, and increased twitch and tetanic tension. Increased type I MHC was found, except for in GG in old and middle-aged rats.

CONCLUSION

Transitions in tongue muscle contractile properties and phenotype were found after NMES.

Practical and effective stomal sphincter creation: evaluation in pigs

PURPOSE

Stoma creation frequently presents complications for which there is no satisfactory surgical solution. We reexamined the feasibility of managing stoma continence with an artificial sphincter, addressing the outstanding issues of geometry, electrode disposition, and fatigue resistance.

METHODS

In 6 pigs, 1 rectus abdominis muscle was preconditioned with electric stimulation for 4 weeks by an implanted stimulator. A sphincter was then constructed and tested for its ability to provide continence against saline at a typical intestinal pressure. The result was compared with a sphincter fashioned from the unconditioned contralateral (control) muscle. In each case, stimulation was applied alternately to longitudinal segments.

RESULTS

A 2-layered wrap was required to achieve continence. Sphincters created from the preconditioned muscles could sustain continence continuously for at least 90 minutes.

CONCLUSION

This study establishes a practical approach to the creation of a sphincter from the rectus abdominis muscle in stoma patients. Continence can be achieved only with a double-layered wrap. Fatigue during long-term operation can be avoided by a combination of preconditioning and segmental stimulation of intramuscular nerve branches.

Adaptive change in electrically stimulated muscle: a framework for the design of clinical protocols

Adult mammalian skeletal muscles have a remarkable capacity for adapting to increased use. Although this behavior is familiar from the changes brought about by endurance exercise, it is seen to a much greater extent in the response to long-term neuromuscular stimulation. The associated phenomena include a markedly increased resistance to fatigue, and this is the key to several clinical applications. However, a more rational basis is needed for designing regimes of stimulation that are conducive to an optimal outcome. In this review I examine relevant factors, such as the amount, frequency, and duty cycle of stimulation, the influence of force generation, and the animal model. From these considerations a framework emerges for the design of protocols that yield an overall functional profile appropriate to the application. Three contrasting examples illustrate the issues that need to be addressed clinically.

Muscle and bone plasticity after spinal cord injury: review of adaptations to disuse and to electrical muscle stimulation

The paralyzed musculoskeletal system retains a remarkable degree of plasticity after spinal cord injury (SCI). In response to reduced activity, muscle atrophies and shifts toward a fast-fatigable phenotype arising from numerous changes in histochemistry and metabolic enzymes. The loss of routine gravitational and muscular loads removes a critical stimulus for maintenance of bone mineral density (BMD), precipitating neurogenic osteoporosis in paralyzed limbs. The primary adaptations of bone to reduced use are demineralization of epiphyses and thinning of the diaphyseal cortical wall. Electrical stimulation of paralyzed muscle markedly reduces deleterious post-SCI adaptations. Recent studies demonstrate that physiological levels of electrically induced muscular loading hold promise for preventing post-SCI BMD decline. Rehabilitation specialists will be challenged to develop strategies to prevent or reverse musculoskeletal deterioration in anticipation of a future cure for SCI. Quantifying the precise dose of stress needed to efficiently induce a therapeutic effect on bone will be paramount to the advancement of rehabilitation strategies.

Real-time polymerase chain reaction to follow the response of muscle to training

The adaptive response of muscle to changes in activity or loading can take many weeks. Changes in the levels of RNA within a muscle fiber can give an early indication of the nature of the response of that fiber to changes in activity or loading. We have designed a new primer set for quantitative polymerase chain reaction (PCR) that will allow us to follow these early transcriptional changes in rat muscle, and have shown that analysis can be performed by standard techniques on as little as 5 mg of muscle, an amount that can be obtained by needle biopsy.

Implantable stimulator featuring multiple programs, adjustable stimulation amplitude and bi-directional communication for implantation in mice

We describe an implantable stimulator with adjustable output amplitude and bi-directional communication at a size of approximately 1 cm(3). The user selects from preset patterns of stimulation and adjusts the stimulation amplitude by sending coded flashes of light, and receives active confirmation of the chosen settings via a powerful LED in the device. These characteristics allow selectivity of motor nerve stimulation and minimize unwanted excitation of adjacent structures. For example, stimulation of dorsiflexors can be achieved in mice without stimulation of plantarflexors. The device can deliver constant frequency stimulation as well as burst-like stimulation patterns with adjustable ON/OFF times. A lifetime of at least 4 weeks of stimulation at an average frequency of 40 Hz can be achieved. The device was built from standard surface mount components and encapsulated with biocompatible silicone rubber. The use of modern microelectronics allowed us to develop a versatile and highly customizable miniature stimulator.

Adaptive conditioning of skeletal muscle in a large animal model (Sus domesticus)

Recognition of the adaptive capacity of mammalian skeletal muscle has opened the way to a number of clinical applications. For most of these, the fast, fatigue-susceptible fibres need to be transformed stably to fast, fatigue-resistant fibres that express the 2A myosin heavy chain isoform. The thresholds for activity-induced change are size-dependent, so although the requisite patterns of electrical stimulation are known for the rabbit, in humans these same patterns would produce type 1 fibre characteristics, with an undesirable loss of contractile speed and power. We have used histochemistry, immunohistochemistry and electrophoretic separations to evaluate a possible conditioning regime in a large animal model. Stimulation of the porcine latissimus dorsi muscle with a phasic 30-Hz pattern for up to 41 days converted all type 2X and 2A/2X fibres to 2A with only a small increase in the type 1 population, from 17% to 22%. Stimulation for longer periods increased the proportion of type 1 fibres to 52%. Based on this model, stimulation regimes designed to achieve a stable 2A phenotype in humans should deliver fewer stimulating impulses, possibly by a factor of 2, than the pattern assessed here. Any such pattern needs to be tested for at least 8 weeks.

Short-term electrical stimulation alters tongue muscle fibre type composition

OBJECTIVE

To examine whether short-term exogenous activation of a tongue muscle induced a phenotypic shift from a fast to a slow fibre-type, and thus assess a potential therapeutic avenue to protect against obstructive sleep apnoea (OSA).

METHODS

New Zealand White rabbit genioglossus (GG) muscle, characteristically a fast muscle, was continuously stimulated at a frequency attributed to slow muscle (10Hz, 3V DC pulses) using an implanted micro-circuit for 7 days. Changes in muscle fibre types and aerobic capacity were assessed between stimulated and un-stimulated (control) groups using immunohistochemistry and electrophoresis for myosin heavy chain (MHC) and assayed for citrate synthase.

RESULTS

Compared to the un-stimulated control group, stimulated GG muscles had more (approximately 13%) type I MHC (slow-twitch) content; a proportional decrease in type II MHC (fast-twitch) isoform also occurred in the stimulated GG muscle (P<0.05). Electrophoresis analysis on whole muscle and single fibre MHC showed an increased type I expression in the stimulated GG muscle (P<0.01). A commensurate rise in citrate synthase activity, indicating a change in aerobic capacity, was also observed in the stimulated GG muscles.

CONCLUSION

Together, these results demonstrate a successful alteration in tongue muscle characteristics using exogenous electrical stimulation and perhaps a potential therapeutic application for OSA.

[A method for analysis of muscle activity during electric stimulation]

The measurement of the surface-EMG during electrical stimulation requires the suppression of the stimulus pulse close to the source. This is necessary because of the discharge currents spreading within the human body caused by the stimulation pulse and the drift effects at the electrodes distorting the EMG-signal. A measurement-system will be presented, which splits the EMG in a detection and a processing path. A special converter keeps the base line at zero level. The detection path sets the gain of the recording amplifier and identifies the stimulation pulse to control its suppression. The processing of the EMG is conducted in the main path way. By these means the EMG including M-wave is undistorted and unbiased presented. The results will be discussed taking the physiology relevance into account.

Neuromuscular stimulation of the quadriceps muscle after hip fracture: a randomized controlled trial

OBJECTIVE

To study the feasibility and effect of neuromuscular stimulation on recovery of mobility after surgical fixation for hip fracture.

DESIGN

Double-blind study with stratified randomization.

SETTING

Home-based rehabilitation program.

PARTICIPANTS

Twenty-four women over the age of 75 years with hip fracture.

INTERVENTIONS

Neuromuscular or placebo stimulation of the quadriceps muscle of the fractured leg, applied for 3 hours a day, for 6 weeks, commencing 1 week after surgery.

MAIN OUTCOME MEASURES

Recovery of walking speed and ability, postural stability, lower-limb muscle power, and pain at 7 and 13 weeks after surgery.

RESULTS

Women in the neuromuscular stimulation group showed faster recovery of mobility. Of the women receiving stimulation, 9 of 12 recovered their prior levels of indoor mobility ability by 13 weeks compared with 3 of 12 in the placebo group (Fisher exact test, P=.046). There were no differences in recovery of walking speed in the first 7 weeks, but women in the stimulation group had greater recovery between 7 and 13 weeks (mean difference=-.13m/s; 95% confidence interval, -.23 to -.01).

CONCLUSIONS

Neuromuscular stimulation at home is feasible and may be effective in speeding recovery of mobility after surgical fixation of hip fracture.

Mechanical properties of the latissimus dorsi muscle after cyclic training

Cardiomyoplasty is a procedure developed to improve heart performance in patients suffering from congestive heart failure. The latissimus dorsi (LD) muscle is surgically wrapped around the failing ventricles and stimulated to contract in synchrony with the heart. The LD muscle is easily fatigued and as a result is unsuitable for cardiomyoplasty. For useful operation as a cardiac-assist device, the fatigue resistance of the LD muscle must be improved while retaining a high power output. The LD muscle of rabbits was subjected to a training regime in which cyclic work was performed. Training transformed the fiber-type composition from approximately equal proportions of fast oxidative glycolytic (FOG) and fast glycolytic (FG) fibers to one composed of almost entirely of FOG with no FG, which increased fatigue resistance while retaining rapid contraction kinetics. Muscle mass and cross-sectional area increased but power output decreased, relative to control muscles. This training regime represents a significant improvement in terms of preserving muscle mass and power compared with other training regimes, while enhancing fatigue resistance, although some fiber damage occurred. The power output of the trained LD muscle was calculated to be sufficient to deliver a significant level of assistance to a failing heart during cardiomyoplasty.

Analysis of chronic morphologic changes of small bowel in electrically stimulated canine island-flap rectus abdominis muscle stomal sphincters

PURPOSE

Dynamic myoplasty to achieve fecal continence has been used in humans with varying results. A potential complication of the use of dynamic skeletal sphincters to attain fecal continence is the development of ischemic strictures within the bowel encircled by the functional sphincter. This study examines the histologic changes present in the bowel wall used to create a functional dynamic island-flap stomal sphincter in a chronic canine model.

METHODS

The rectus abdominis muscles of canines were used to create island-flap stomal sphincters. Eight dynamic island-flap stomal sphincters were created from the rectus abdominis muscles in mongrel dogs by wrapping them around a blind loop of distal ileum that was no longer in continuity with the terminal small bowel. Temporary pacing electrodes were secured intramuscularly near the intercostal nerve entry point and connected to a subcutaneously placed pulse stimulator. Two different training protocols resulting in different contractile properties were used: Program A (n = 4) and Program B (n = 4). The island-flap sphincters were trained over 3 months to generate stomal intraluminal pressures of more than 60 mmHg in all animals. The intact sphincters, normal bowel, and contralateral stomal bowel were obtained when the animals were killed. Specimens were processed with paraffin embedding, sectioned, and stained with trichrome and hematoxylin-and-eosin stains. Measurements of the different bowel layers were made with a micrometer. The muscular sphincters were biopsied before and after training. Fiber-type histochemistry was performed with a monoclonal antibody to the fast isoforms of myosin. Pretrained and posttrained skeletal muscle specimens were examined histologically.

RESULTS

The bowel wall within the functional dynamic stomal sphincter did not exhibit any significant architectural changes related to ischemic fibrosis or mucosal damage. A significant fiber-type conversion was achieved in both training groups with Programs A and B, with a >50 percent conversion from fatigue-prone (type II) muscle fibers to fatigue-resistant (type I) muscle fibers. Biopsy specimens revealed that fiber-type transformation was uniform throughout the sphincters. Skeletal muscle fibers within both groups demonstrated a reduction in their fiber diameter. There was no evidence of significant fibrosis or deposition of fat within the skeletal muscle of the sphincters.

CONCLUSION

Results of our experiment suggest that our anterior abdominal wall dynamic island-flap stomal sphincter, which generates a contractile force over the bowel wall capable of producing enough stomal pressure to achieve fecal continence, is not intrinsically harmful to the bowel that it encircles. The transformation of skeletal muscle to fatigue-resistant (type I) fibers occurred uniformly throughout the skeletal muscle sphincters without evidence of muscle fiber damage or significant fibrosis.

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).

Functional and biological test of a 20 channel implantable stimulator in sheep in view of functional electrical stimulation walking for spinal cord injured persons

A newly developed implantable stimulator with 20 output channels, mainly intended for the stimulation of lower extremities in paraplegics, was implanted in 6 sheep over a time period of 26 weeks. Five epineural electrodes each were used to contact various nerves at different locations to elicit hip and knee extension and flexion and to make carrousel and selective stimulation possible. Different electrode application strategies in view of paraplegic standing and walking were investigated. Additional implanted electrodes allowed M-wave monitoring for selectivity investigations in 3 sheep. Stimulator, electrode leads, and electrodes proved to be reliable. Selective stimulation with electrodes placed on the trunk of the sciatic nerve could be demonstrated but with bad reproducibility. Histological investigation of the tissues surrounding electrodes and leads showed the expected stable foreign body response. Strong hip and knee extension could be gained in all cases while only weak flexion forces could be elicited in most cases. Muscle biopsies showed that daily stimulation for 8 h at threshold level caused an increase in muscle Type I fibers and a decrease in Type IIc fibers. Implants and electrodes fulfill the most important functional and biological criteria for their clinical application for paraplegic walking. The intention to provide selective flexion functions via epineural stimulation could not be demonstrated sufficiently in this animal model.

Rabbit supraspinatus tendon detachment: effects of size and time after tenotomy on morphometric changes in the muscle

We studied the effects of size and time after rabbit supraspinatus tenotomy (group A--small tenotomy, group B--large tenotomy) on muscle morphometric changes in 48 rabbits. Animals were killed 6 (subgroups A1 and B1), 12 (subgroups A2 and B2) and 24 weeks (subgroups A3 and B3) after tendon detachment. Statistically significantly greater increases in interstitium volume were noted in subgroups A1-A3 and B1-B3 than in controls. Reductions in type I and II fiber diameters were mainly due to the length of observation. However, statistically significant differences in comparison with controls appeared earlier after large tenotomy. The size of the tenotomy primarily affected muscle fiber composition. Our results suggest that these changes were caused by fiber transformation from type I to type II and vice versa. These findings indicate that the interdigitations between the supraspinatus and infraspinatus and between the supraspinatus and subscapularis tendons are important in dynamics and the degree of morphometric changes in the rabbit supraspinatus after tenotomy.

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.

Historical Perspectives: plasticity of mammalian skeletal muscle

More than 40 years ago, the nerve cross-union experiment of Buller, Eccles, and Eccles provided compelling evidence for the essential role of innervation in determining the properties of mammalian skeletal muscle fibers. Moreover, this experiment revealed that terminally differentiated muscle fibers are not inalterable but are highly versatile entities capable of changing their phenotype from fast to slow or slow to fast. With the use of various experimental models, numerous studies have since confirmed and extended the notion of muscle plasticity. Together, these studies demonstrated that motoneuron-specific impulse patterns, neuromuscular activity, and mechanical loading play important roles in both the maintenance and transition of muscle fiber phenotypes. Depending on the type, intensity, and duration of changes in any of these factors, muscle fibers adjust their phenotype to meet the altered functional demands. Fiber-type transitions resulting from multiple qualitative and quantitative changes in gene expression occur sequentially in a regular order within a spectrum of pure and hybrid fiber types.

Functional assessment of skeletal muscle ventricles after pumping for up to four years in circulation

BACKGROUND

The successful treatment of cardiac failure by heart transplantation is severely limited by the shortage of donor organs, and alternative surgical approaches are needed. An experimental approach that holds considerable promise is the skeletal muscle ventricle (SMV), an auxiliary blood pump formed from a pedicled graft of latissimus dorsi muscle and connected to the circulation in a cardiac assist configuration. Adaptive transformation, or conditioning, by electrical stimulation enables the skeletal muscle to perform a significant proportion of cardiac work indefinitely without fatigue.

METHODS

In 10 dogs, SMVs were constructed from the latissimus dorsi muscle, lined internally with pericardium, and conditioned by electrical stimulation to induce fatigue resistant properties. The SMVs were connected to the descending thoracic aorta via two 12-mm Gore-Tex conduits and the aorta was ligated between the two grafts. The SMV was stimulated to contract during the diastolic phase of alternate cardiac cycles. The animals were monitored at regular intervals.

RESULTS

At initial hemodynamic assessment, SMV contraction augmented mean diastolic blood pressure by 24.6% (from 61 +/- 7 to 76 +/- 9 mm Hg). Presystolic pressure was reduced by 15% (from 60 +/- 8 to 51 +/- 7 mm Hg) after an assisted beat. Four animals died early, 1 from a presumed arrhythmia, and 3 during propranolol-induced hypotension. The other 6 animals survived for 273, 596, 672, 779, 969, 1,081, and 1,510 days. Diastolic augmentation was 27.4% at 1 year (93 +/- 9 vs 73 +/- 6 mm Hg; n = 5), 34.7% at 2 years (85 +/- 6 vs 63 +/- 7 mm Hg; n = 3), 21.2% (89 +/- 10 vs 73 +/- 8 mm Hg; n = 2) at 3 years, and 34.5% (78 vs 58 mm Hg; n = 1) after 4 years in circulation. After 4 years, the isolated SMV was able to maintain a pressure of over 80 mm Hg while ejecting fluid at 20 mL/s. No animal showed evidence of SMV rupture or thromboembolism.

CONCLUSIONS

The SMVs in this study provided effective and stable hemodynamic assistance over an extended period of time. There was no evidence that the working pattern imposed on the muscular wall of the SMV compromised its viability. Areas of fibrofatty degeneration were suggestive of early damage that future protocols should seek to minimize.

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.

Forced MyHCIIB expression following targeted genetic manipulation of conditionally immortalized muscle precursor cells

The ability to carry out gene targeting in somatic stem cells while maintaining their stem cell characteristics would have important implications for gene therapy and for the analysis of gene function. Using mouse myoblasts, we have explored this possibility by attempting to alter the promoter of a myosin heavy chain gene (MyHCIIB) characteristic of physiologically "fast" muscle so as to force its unscheduled expression in physiologically "slow" muscle fibers. Conditionally immortalized muscle precursor cells were transfected with a gene targeting construct designed to replace the MyHCIIB promoter with that for the carbonic anhydrase III gene (CAIII), which is highly expressed in slow muscle. A potentially targeted clone was isolated and differentiated in culture to form myotubes which expressed MyHCIIB. Cells from the same clone were injected into both slow and fast muscle of host mice, where they contributed to fiber formation. In slow muscle, the fibers derived from this clone did not express MyHCIIB; this may reflect an instability of the targeted MyHCIIB locus and/or a failure of the hybrid promoter to function in slow fibers in vivo. Nonetheless, we have demonstrated that a "promoter knock-in" gene targeting procedure can be used to generate unique MyHCIIB-expressing myotubes in culture and that conditionally immortalized myoblasts can be subjected to extensive passaging and genetic manipulation without losing their ability to form fibers in culture and in vivo.

Permanent cardiac assistance from skeletal muscle: a prospect for the new millennium

This paper looks at the prospects for new surgical solutions to the problem of end-stage heart failure based on cardiac assistance from skeletal muscle. The current status of the main biological approaches, cardiomyoplasty, aortomyoplasty, and the skeletal muscle ventricle, are discussed, followed by a consideration of some of the important basic issues that need to be addressed if these techniques are to achieve their full potential. Although there is a review element to the paper, the main emphasis is on the work of our own research group and collaborating workers.