Gerta Vrbová

The influence of activity on some contractile characteristics of mammalian fast and slow muscles

1. The time course of contraction and relaxation in the isometric twitch of a rabbit soleus muscle becomes more rapid following tenotomy and spinal cord section. This increase in speed could be prevented by long‐term electrical stimulation at frequencies of 5 or 10/sec. It was not prevented by stimulation at frequencies of 20 or 40/sec.

Effects of long-term electrical stimulation on some contractile and metabolic characteristics of fast rabbit muscles.

SummaryIntermittant long-term stimulation of fast rabbit muscles up to 28 days with a frequency pattern resembling that of a slow muscle (10 Imp/sec) led to a slowing of the time course of contraction already during the first week. There was an increase of tetanic tension as well. The observed rearrangement of activities of key enzymes of energy supplying metabolism was found to occur sequentially. Decreases of extramitochondrial enzymes of glycogenolysis (phosphorylase), glycolysis (triosephosphate dehydrogenase, lactate dehydrogenase) and energy rich phosphate transfer were found initially together with a decrease of mitochondrial glycerolphosphate dehydrogenase. The isozyme pattern of lactate dehydrogenase was changed. Large initial increases were found in enzymes involved in glucose phosphorylation (hexokinase) and fatty acid activation (palmitoyl-CoA synthetase). Later an increase of key enzymes of the citric acid cycle (citrate synthase) and fatty acid oxidation (3-hydroxyacyl-CoA dehydrogenase) as well as ketone body utilization (3-ketoacid-CoA transferase) could be shown.Histochemical staining and comparative activity determination of succinate dehydrogenase in single fibres revealed that the mosaic like fibre composition of the fast muscle was transformed into a more uniform population resembling that of a predominantly slow muscle.

Observations on the elimination of polyneuronal innervation in developing mammalian skeletal muscle.

1. The mechanism responsible for the elimination of polyneuronal innervation in developing rat soleus muscles was studied electrophysiologically and histologically. 2. Initially all the axons contacting a single end‐plate have simple bulbous terminals. As elimination proceeds one axon develops terminal branches while the other terminals remain bulbous and may be seen in contact with, or a short distance away from, the end‐plate. It is suggested that the branched terminal remains in contact with the muscle fibre while the other terminals withdraw. 3. At a time when polyneuronal innervation can no longer be detected electrophysiologically, the histological technique still shows the presence of end‐plates contacted by more than one nerve terminal. 4. The effect of activity on the disappearance of polyneuronal innervation was examined. Activity was increased by electrical stimulation of the right sciatic nerve. This procedure also produced reflex activity in the contralateral limb. In both cases polyneuronal innervation was eliminated more rapidly in the active muscles. 5. The finding that proteolytic enzymes are released from muscles treated with acetylcholine (ACh), and the observation of the more rapid elimination of supernumerary terminals at the end‐plates of active muscles, lead to the suggestion that superfluous nerve‐muscle contacts are removed by the proteolytic enzymes in response to neuromuscular activity. The selective stabilization of only one of the terminals is discussed in the light of these results.

Lack of myostatin results in excessive muscle growth but impaired force generation

The lack of myostatin promotes growth of skeletal muscle, and blockade of its activity has been proposed as a treatment for various muscle-wasting disorders. Here, we have examined two independent mouse lines that harbor mutations in the myostatin gene, constitutive null (Mstn−/−) and compact (Berlin High Line, BEHc/c). We report that, despite a larger muscle mass relative to age-matched wild types, there was no increase in maximum tetanic force generation, but that when expressed as a function of muscle size (specific force), muscles of myostatin-deficient mice were weaker than wild-type muscles. In addition, Mstn−/− muscle contracted and relaxed faster during a single twitch and had a marked increase in the number of type IIb fibers relative to wild-type controls. This change was also accompanied by a significant increase in type IIB fibers containing tubular aggregates. Moreover, the ratio of mitochondrial DNA to nuclear DNA and mitochondria number were decreased in myostatin-deficient muscle, suggesting a mitochondrial depletion. Overall, our results suggest that lack of myostatin compromises force production in association with loss of oxidative characteristics of skeletal muscle.

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.

Time dependent effects on contractile properties, fibre population, myosin light chains and enzymes of energy metabolism in intermittently and continuously stimulated fast twitch muscles of the rabbit

SummaryFast-twitch tibialis anterior and extensor digitorum longus rabbit muscles were subjected to long-term intermittent (8 h daily) or continuous (24 h daily) indirect stimulation with a frequency pattern resembling that of a slow motoneuron.Increases in time to peak of isometric twitch contraction were observed without parallel changes in the pattern of myosin light chains or alterations in the distribution of slow and fast fibres as discernible by the histochemical ATPase reaction. However, changes in the fibre population and in the myosin light chain pattern were observed after intermittent stimulation periods exceeding 40 days or continuous stimulation periods longer than 20 days. Under these conditions even higher increases were found in contraction time. In one animal a complete change in fibre population was observed. In this case myosin light chains of the slow (LCS1, LCS2) and of the fast type (LCf1) were obviously synthetized simultaneously within the same fibre. Early changes in the enzyme activity pattern of energy metabolism indicated a conversion of the fibres including their mitochondrial population. These changes and the earlier reported changes in the sarcoplasmic reticulum are probably responsible for the early changes in contractile properties which occur before the transformation of the myosin.

Dependence of postnatal motoneurones on their targets: review and hypothesis

Motoneurones are known to die (1) during embryonic development (naturally occurring cell death), (2) early in postnatal development after axonal injury, and (3) as a consequence of disease, such as spinal muscular atrophy or (in later life) amyotrophic lateral sclerosis. Naturally occurring motoneurone death has been extensively investigated, and interaction with the target muscle has emerged as an important factor for survival of embryonic motoneurones. Evidence that this target dependence of motoneurones continues postnatally is discussed in this review, as is the possible nature of the retrograde signal from the muscle. An explanation for the role of the muscle in motoneurone survival is also proposed, which may be applicable in situations where motoneurone death occurs postnatally. This proposal takes into account the changing functional demands imposed on motoneurones as a result of the gradual maturation of the CNS, and suggests that during development the muscle induces the motoneurones to become competent to carry out these requirements.

Prolonged contraction-relaxation cycle of fast-twitch muscles in parvalbumin knockout mice

The calcium-binding protein parvalbumin (PV) occurs at high concentrations in fast-contracting vertebrate muscle fibers. Its putative role in facilitating the rapid relaxation of mammalian fast-twitch muscle fibers by acting as a temporary buffer for Ca2+ is still controversial. We generated knockout mice for PV (PV -/-) and compared the Ca2+ transients and the dynamics of contraction of their muscles with those from heterozygous (PV +/-) and wild-type (WT) mice. In the muscles of PV-deficient mice, the decay of intracellular Ca2+ concentration ([Ca2+]i) after 20-ms stimulation was slower compared with WT mice and led to a prolongation of the time required to attain peak twitch tension and to an extension of the half-relaxation time. The integral [Ca2+]i in muscle fibers of PV -/- mice was higher and consequently the force generated during a single twitch was approximately 40% greater than in PV +/- and WT animals. Acceleration of the contraction-relaxation cycle of fast-twitch muscle fibers by PV may confer an advantage in the performance of rapid, phasic movements.The calcium-binding protein parvalbumin (PV) occurs at high concentrations in fast-contracting vertebrate muscle fibers. Its putative role in facilitating the rapid relaxation of mammalian fast-twitch muscle fibers by acting as a temporary buffer for Ca2+ is still controversial. We generated knockout mice for PV (PV -/-) and compared the Ca2+ transients and the dynamics of contraction of their muscles with those from heterozygous (PV +/-) and wild-type (WT) mice. In the muscles of PV-deficient mice, the decay of intracellular Ca2+ concentration ([Ca2+]i) after 20-ms stimulation was slower compared with WT mice and led to a prolongation of the time required to attain peak twitch tension and to an extension of the half-relaxation time. The integral [Ca2+]iin muscle fibers of PV -/- mice was higher and consequently the force generated during a single twitch was ∼40% greater than in PV +/- and WT animals. Acceleration of the contraction-relaxation cycle of fast-twitch muscle fibers by PV may confer an advantage in the performance of rapid, phasic movements.

Changes of activity patterns in slow and fast muscles during postnatal development

Abstract The motor activity patterns of the slow soleus and the fast extensor digitorum longus (EDL) muscles of the rat were studied by electromyography (EMG) in the awake unrestrained animal during postnatal development. In the adult soleus, tonic, low frequency activity of motor units was recorded at rest and during movement. In animals less than 12 days old the muscle was activated phasically, the motor unit firing frequency was slightly lower than in the adult, and tonic discharges were absent at rest. After this time, tonic postural activity gradually developed and resembled the adult pattern by 3 weeks of age. The adult EDL muscle was silent at rest and was activated phasically at high frequencies during movement. In animals of less than 12 days of age the muscle was also activated phasically but at lower frequencies. The firing frequency increased to adult values during the second and third week of postnatal development. The discharge properties of motor units from soleus and EDL were compared in rats of different ages. There was a greater variability of interspike intervals during firing in motor units from very young animals than in adults. Whereas in adult animals it was possible to discern a graded recruitment of motor units, animals less than 10 days old showed no regular recruitment order and some motor units appeared to be activated synchronously.

Two factors responsible for the development of denervation hypersensitivity

1. Innervated adult skeletal muscle is sensitive to acetylcholine at the end‐plate region only. After denervation the entire muscle membrane becomes chemosensitive. The period of greatest increase in sensitivity in rat soleus muscles following section of the sciatic nerve in the thigh is between 48 and 72 hr post‐operatively.