The development of fibre types in grafts of a slow tonic avian muscle, the dorsocutaneous latissimus dorsi

The dorsocutaneous (DLD) and anterior (ALD) latissimus dorsii are both homogeneous slow tonic muscles. Autografts of mature DLD were attached onto the ALD of chickens to study regeneration of slow tonic muscle fibres innervated exclusively by slow tonic nerves. Fifty-three grafts were examined from 3 to 231 days after implantation for myosin ATPase, and for heavy chains of fast myosin. New muscle fibres in grafts were initially type 1 (slow) or type 2 (fast twitch). Tonic type 3 fibres were slow to differentiate and were not seen within 59 days. From 105 days many fibres were type 3A and type 1 were no longer apparent. However, type 2 fibres persisted and appeared to be present instead of type 3B fibres even after 8 months.     

Tonic contractions allow metabolic recuperation of the adductor muscle during escape responses of giant scallop Placopecten magellanicus

To escape from starfish predators, giant scallops, Placopecten magellanicus, swim using series of strong phasic contractions interrupted by tonic contractions. To investigate whether these tonic contractions allow metabolic recuperation of the adductor muscle, we sampled scallops at rest (Control), after an initial series of phasic contractions (Phasic) and after 1 min of tonic contraction following their initial phasic contractions (Phasic + Tonic) and compared muscle levels of phosphoarginine, adenylate nucleotides (ATP, ADP and AMP) and adenylate energy charge (AEC). Scallops in the two active groups did not differ in the numbers of phasic contractions or the mean phasic force production. Phosphoarginine concentrations in the adductor muscle decreased with phasic activity and remained low after 1 min of tonic contraction. ATP and ADP and total adenylate levels did not differ between the three groups, but AMP levels were higher in the scallops sampled after phasic contractions than in control scallops. The AEC was reduced by phasic contractions but returned to control levels after 1 min of tonic contraction. A significant negative correlation between AEC and the number of claps in the Phasic group disappeared in the Phasic + Tonic group. Thus, tonic contractions following phasic contractions allow partial metabolic recovery of the adductor muscle by returning AEC to control levels. However, phosphoarginine levels did not recover during tonic contractions, and a negative correlation between the number of claps and phosphoarginine levels remained in the Phasic + Tonic group. By interspersing tonic contractions between series of phasic contractions, scallops improved muscle energetic status, which should help maintain phasic force production during the remainder of the escape response.     

Slow and fast fatigable frog muscle fibres: electrophysiological and histochemical characteristics

Continuous activity of isolated frog gastrocnemius muscle fibres provoked by repetitive stimulation of 5 Hz was used as an experimental model for fatigue development in different fibre types. Parameter changes of the elicited intracellular action potentials and mechanical twitches during the period of uninterrupted activity were used as criteria for fatigue evaluation. Slow fatigable muscle fibre (SMF) and fast fatigable muscle fibre (FMF) types were distinguished depending on the duration of their uninterrupted activity, which was significantly longer in SMFs than in FMFs. The normalized changes of action potential amplitude and duration were significantly smaller in FMFs than in SMFs. The average twitch force and velocity of contraction and relaxation were significantly higher in FMFs than in SMFs. Myosin ATPase (mATPase) and succinate dehydrogenase activity were studied by histochemical assessment in order to validate the fibre type classification based on their electrophysiological characteristics. Based on the relative mATPase reactivity, the fibres of the studied muscle were classified as one of five different types (1-2, 2, 2-3, 3 and tonic). Smaller sized fibres (tonic and type 3) expressed higher succinate dehydrogenase activity than larger sized fibres (type 1-2, 2), which is related to the fatigue resistance. The differences between fatigue development in SMFs and FMFs during continuous activity were associated with fibre-type specific mATPase and succinate dehydrogenase activity.     

Comparison of the sarcoplasmic and myofibrillar proteins of twitch and tonic fibres of frog muscle (Rana esculenta)

Sarcoplasmic and myofibrillar proteins of a frog mixed muscle (distal cruralis bundle) were investigated and compared to their fast twitch muscle homologues. Histochemical reactions revealed two populations of fibres in this muscle, differing from fast twitch fibres by the intensity of their myofibrillar ATPase reaction and by their mitochondrial NADH dehydrogenase activity. The distribution of parvalbumins and LDH isoenzymes in the whole muscle showed some features of tonic muscle type. Myosin light chains pattern of cruralis bundle fibres was characterized by the lower proportion of the LC3 subunit. These results confirmed the heterogeneity of this frog muscle and the presence of tonic or intermediate fibres with their typical sarcoplasmic and myofibrillar proteinic composition.     

The so-called tonic muscle fibre type in cyprinid axial muscle: their morphology and response to endurance exercise training

The morphology and the effect of an endurance training programme on tonic muscle fibres were studied in chub Leuciscus cephalus , by means of histochemistry and immunohistochemistry, electronmicroscopy and morphometry/stereology. Location and distribution, SDH- and mATPase-activity, reaction to an anti-tonic myosin antibody and ultrastructural features of the fibre type were investigated. With regard to training conditions, fibre size was not significantly affected. However, an increase in the volume densities of mitochondria, lipid and myofibrils can be observed, suggesting a training influence on the aerobic capacity of the so-called tonic muscle fibres. Based on the quantitative findings, the fine structure and the response to training, similarities with intermediate muscle fibres and the functional role of these so-called tonic muscle fibres are discussed.     

Selectivity of sodium channels in denervated tonic muscle fibre membrane of the frog

Ionic selectivity of sodium channels was examined under voltage clamp conditions in normal and denervated twitch fibres and denervated tonic fibres isolated from m. ileofibularis of the frog (R. temporaria). Membrane currents were recorded by means of the Hille-Campbell vaseline-gap voltage clamp method from muscle fibre segments exposed to a potassium-free artificial internal solution. Permeability ratio (PS/PNa) were determined from changes in the reversal potential after replacing all Na ions in the solution bathing the voltage clamped external membrane area with sodium substituting ions (S). The permeability sequence was: Na+ greater than Li+ greater than NH4+ greater than K+. No inward currents were observed for Ca2+. The permeability ratios were as follows. Denervated tonic fibres: 1:0.88:0.23:0.012; control twitch fibres: 1:0.94:0.22:0.076; denervated twitch fibres: 1:0.91:0.14:0.082. The permeability to Li+ ions deviates from independence to a greater extent in tonic than in phasic fibres. Our results are consistent with the Hille model of sodium channel selectivity, and they support the hypothesis that sodium channels formed in denervated tonic muscle fibres of the frog are of the same genetic origin as Na channels expressed under physiological conditions.     

Twitchin of mollusc smooth muscles can induce “catch”-like properties in human skeletal muscle: support for the assumption that the “catch” state involves twitchin linkages between myofilaments

Molluscan catch muscles can maintain tension with low or even no energy utilization, and therefore, they represent ideal models for studying energy-saving holding states. For many decades it was assumed that catch is due to a simple slowing of the force-generating myosin head cross-bridge cycles. However, recently evidences increased suggesting that catch is rather caused by passive structures linking the myofilaments in a phosphorylation-dependent manner. One possible linkage structure is the titin-like thick filament protein twitchin, which could form bridges to the thin filaments. Twitchin is known to regulate the catch state depending on its phosphorylation state. Here, we found that twitchin induces a catch-like stiffness in skinned human skeletal muscle fibres, when these fibres are exposed to this protein. Subsequent phosphorylation of twitchin reduces the stiffness. These findings support the assumption that catch of molluscan smooth muscle involves twitchin linkages between thick and thin filaments.     

Properties of standard avian slow muscle grafts following long-term regeneration

The tonic anterior latissimus dorsi (ALD) of adult pigeons was orthotopically homografted and evaluated after 11 months of regeneration for histological, histochemical, electromyographic (EMG), and mechanical properties. The resting EMG activity of the grafts was lower in amplitude than that of the controls, but showed the tonic pattern typical for these tonic muscles. The control and grafted muscles had a histochemically homogeneous population of fibers with moderate myofibrillar adenosine triphosphatase activity. Succinic dehydrogenase activity was moderate for the control muscles, but low for the grafts. The regenerated muscles had fewer and smaller fibers and had much larger intersynaptic distances. Both the regenerated and the contralateral control muscles were slow contracting and maintained tetanic tension for prolonged periods with direct electrical stimulation. The relaxation was slower in the grafted muscle than in the control. The grafts produced 40% of the maximum tension of the control muscles, but the rate of tension development was similar between the two groups. The results indicate that the tonic properties were regenerated, but the innervation pattern was altered and the grafted muscles did not have normal mature fibers even after long-term regeneration.     

Effects of the membrane potential level on serotonin-induced contraction of the pulmonary artery smooth muscle in rabbits]

The effects of changes in membrane potential level on the electrical and contractile responses induced by serotonin (10(-6) mol/l) were investigated in muscle strips from rabbit main pulmonary artery using sucrose-gap technique. In spite of the fact that serotonin-induced depolarization did not exceed the threshold level for development of contraction, it was followed by a strong tonic contraction. Nearly a half of this contraction could be relaxed by an electrotonic hyperpolarization of the membrane. A week preliminary depolarization of the muscle cells resulted in an increase while a strong depolarization--in dramatic decrease of serotonin-induced contraction. Nifedipine effectively blocked potassium-induced, but not serotonin induced contraction. We suggest that in addition to voltage-operated and receptor operated Ca channels in vascular smooth muscle cell membrane there is a separate class of nifedipine-insensitive Ca channels operated by both serotonin receptor and membrane potential.     

Diameter changes of muscle fiber types of the inferior oblique muscle of the rabbit following denervation]

Changes in fibre diameters of extraocular muscles of the rabbit were studied at different times after denervation. The whole inferior oblique muscle hypertrophied, while some of the muscle fibres hypertrophied and others showed atrophy, depending on the fibre type. Fibre types have been determined by their histochemical enzyme profile. In the central layer of the muscle the phasic muscle fibres, which are rich in mitochondria, exhibited a transient hypertrophy being maximal 4-5 weeks after denervation and afterwards they atrophied; other phasic muscle fibres, which are poor in mitochondria, atrophied without having shown any sign of hypertrophy. Special, putatively slow tonic muscle fibres, which have low enzyme activities, underwent small long-lasting increases of their diameters. In the superficial layer of extraocular muscle there are two types of extremely thin muscle fibres rich in mitochondira. Both these fibre types hypertrophied to the greatest degree and for a very long time. Comparable changes in fibre diameters as described here for the muscle fibre types of an extraocular muscle are known from special muscle fibres in other vertebrate     

The role of intra- and extracellular Ca in the activation of contraction of pulmonary artery smooth muscles induced by serotonin

In Ca-free EGTA-containing solution serotonin induced a transient contraction of rabbit pulmonary artery smooth muscle which decayed to nearly steady-state level accounted for 17.7 +/- 1.6% of original contraction in Krebs solution. Both phasic and tonic components of this contraction were effectively inhibited by verapamil and Cd2+. Caffeine induced no contraction of muscle strips if it was applied after withdrawal of serotonin. But when the sequence of these drugs application was reversed, serotonin still evoked contraction with reduced phasic component. The results obtained in these experiments suggest, that serotonin-induced contraction of pulmonary artery smooth muscle is partly (less than 20%) due to mobilization of bound calcium from at least two stores located on the opposite sides of the cell membrane. Calcium released from external store site enters the cell via receptor-operated calcium channels.     

The myosin cross-bridge cycle and its control by twitchin phosphorylation in catch muscle

The anterior byssus retractor muscle of Mytilus edulis was used to characterize the myosin cross-bridge during catch, a state of tonic force maintenance with a very low rate of energy utilization. Addition of MgATP to permeabilized muscles in high force rigor at pCa > 8 results in a rapid loss of some force followed by a very slow rate of relaxation that is characteristic of catch. The fast component is slowed 3-4-fold in the presence of 1 mM MgADP, but the distribution between the fast and slow (catch) components is not dependent on [MgADP]. Phosphorylation of twitchin results in loss of the catch component. Fewer than 4% of the myosin heads have ADP bound in rigor, and the time course (0.2-10 s) of ADP formation following release of ATP from caged ATP is similar whether or not twitchin is phosphorylated. This suggests that MgATP binding to the cross-bridge and subsequent splitting are independent of twitchin phosphorylation, but detachment occurs only if twitchin is phosphorylated. A similar dependence of detachment on twitchin phosphorylation is seen with AMP-PNP and ATPgammaS. Single turnover experiments on bound ADP suggest an increase in the rate of release of ADP from the cross-bridge when catch is released by phosphorylation of twitchin. Low [Ca(2+)] and unphosphorylated twitchin appear to cause catch by 1) markedly slowing ADP release from attached cross-bridges and 2) preventing detachment following ATP binding to the rigor cross-bridge.     

Paramyosin and the catch mechanism

1. Catch is a mechanism found in many molluscan smooth muscles in which tension is maintained at relatively low energy cost. 2. Paramyosin forms the core of thick filaments. In catch muscle paramyosin concentrations are high and the thick filaments are relatively long. 3. The mechanism of catch is not understood, but the consensus is that tension during catch is borne by slowly-cycling cross-bridge attachments to actin. 4. Stimulation by acetylcholine increases intracellular Ca2+ and initiates a contraction characterized by a relatively rapid cross-bridge cycling. Reduction of Ca2+ can lead to relaxation or catch. Relaxation occurs only when a second neurotransmitter, serotonin, is present. 5. The catch state is released by serotonin, via activation of adenylate cyclase, increased levels of cAMP and phosphorylation of one or more contractile proteins, possibly paramyosin. Other targets for phosphorylation are discussed. 6. The contractile cycle of catch muscles, therefore, is controlled by both Ca2+ and cAMP.     

Histochemical differentiation of extrafusal muscle fibres of the anterior latissimus dorsi in the chick

Histochemical differentiation of the chick anterior latissimus dorsi (ALD) muscle was studied during embryonic development and after hatching. The two types of adult ALD tonic fibres (alpha' and beta') differentiate from a pool of acid and alkali-stable myofibrillar ATPase fibres. Intermediate stages of the transformation from beta' to alpha' were observed. At all developmental stages studied, a low percentage of formalin-resistant, alkali-stable and acid-labile ATPase fibres were observed. Such fibres have the histochemical properties of the alpha R or fast oxidative-glycolytic fibres and are assumed to be focally innervated.     

Experimental studies upon a bundle of tonic fibres in the locust extensor tibialis muscle

The bundle of tonic fibres situated at the proximal end of the locust metathoracic extensor tibialis muscle is innervated by the dorsal unpaired median neurone (DUMETi) as well as by the slow excitatory (SETi)) and common inhibitor (CI) neurones. It is not innervated by the fast excitatory neurone (FETi).These fibres contract spontaneously and rhythmically. The myogenic rhythm can be modified by neural stimulation.Spontaneous slow depolarizing potentials resembling the pacemaker potentials of insect cardiac muscle were demonstrated in these fibres.The actions of glutamate on the tonic muscle fibres are not compatible with its being a specific excitatory transmitter. Glutamate can stimulate weak contractions of the muscle, but this action is inhibited when chloride ions are removed from the saline.10^?6 M Octapamine hyperpolarizes the tonic fibre membrane. Octopamine, GABA and glutamate all inhibit the myogenic contractions and reduce the force of the neurally evoked contractions.The tonic muscle is very responsive to proctolin. At 5 × 10^?11 M proctolin enhances the force and increases the frequency of myogenic contractions. At 10^?9 M it depolarizes the muscle membrane potential, and at that and higher concentrations it causes the muscle to contract. At 2 × 10^?7 M proctolin induces contractures which resemble those evoked by sustained high-frequency neural stimulation. Iontophoretic experiments show that proctolin receptors occur at localized sites on the tonic fibre membrane.     

Adaptation of mouse skeletal muscle to a novel functional overload test: changes in myosin heavy chains and SERCA and physiological consequences

We have used a new approach to study the effects of overload on skeletal muscle phenotype in mice. The method used avoids any traumatising contact with muscles and the inflammatory reaction that this may provoke. Blocks of lead embedded in silicone were inserted under the skin of the lower part of the back. After 1 month, a 17% hypertrophy was found to have occurred in the tonic soleus muscle, but no change was observed in the fast-twitch extensor digitorum longus (EDL) muscle. The main effects on the contractile properties of the soleus muscle were a decrease in the tetanic relaxation rate and a reduction in the maximal velocity of shortening. Immunohistological analysis of the soleus muscles revealed an increase in the proportion of fibres that express myosin heavy chain (MHC) 1, from 54.2% to 73.9%, with a reduction in the proportion of MHC2a-positive fibres, from 45.8% to 30.2%. These changes were accompanied by an increase in the proportion of fibres that express the slow type of sarcoplasmic reticulum calcium pump (SERCA2a), from 61.8% to 84.7%. In EDL muscles, overload induced only minor changes. Thus, this method of overload affected the soleus muscle in particular. The observed changes in the control of muscle contraction were significantly larger than the changes in typical myofibrillar properties that were observed. These results indicate that there is a temporal dissociation between the relative expression of MHCs and SERCAs.     

Effects of Na-octanoate on potassium contractures in normal and denervated frog tonic fibres

In frog twitch muscle fibres, Na-octanoate (NaC8) shifted the relation between potassium induced tension and membrane potential to the right. The present study has been carried out to investigate the effect of this fatty acid on frog tonic fibres. Potassium contractures measured on bundles of 30-40 fibres of ileofibularis muscles were less decreased by NaC8 (2.5-10 mmol/l) than those of twitch fibre bundles. In denervated muscles the sensitivity to NaC8 was increased, probably due to the development of sodium channels in the membranes. Experiments with mixed fibre bundles also showed a lower influence of NaC8 on potassium contracture of tonic fibres. On the other hand, tonic fibres showed a lower threshold of the potassium induced tension as well as a lower K+ concentration for maximal activation. This lower threshold was further lowered by NaC8, corresponding to a shift of the relation between potassium concentration and tension to the left. The membrane resting potentials were -58 +/- 9 mV in tonic fibres and -83 +/- 5 mV in twitch fibres. Five mmol/l NaC8 only induced depolarization of the membrane of tonic fibres. This depolarization (by about 20 mV) may be responsible for the threshold shift to lower K+ concentration in NaC8-exposed tonic fibres. In addition to the effects of NaC8 on sodium channels, interactions with Ca2+ binding sites are discussed.     

Effect of strength training on the relationship between magnetic resonance relaxation time and muscle fibre composition

The effect of muscle hypertrophy on the relationship between magnetic resonance (MR) relaxation time and muscle fibre composition was investigated. Relaxation time and muscle fibre composition were measured in five subjects before and after a 20-week period of strength training. Muscle fibre composition in all subjects exhibited a significant shift to a predominance of fast-twitch (FT) fibres as a result of 20-week strength training (% area FT fibres: mean values from 49.8%, SD 17.9% to 57%, SD 5.6%; P less than 0.05). Longitudinal relaxation time (T1) and transverse relaxation time (T2) were prolonged significantly after strength training (T1 mean values from 334.9 ms, SD 13.6 to 359.0 ms, SD 9.0, P less than 0.001; T2 from 27.5 ms, SD 0.9 to 30.8 ms, SD 2.3, P less than 0.05). A constant relationship was observed in changes caused by strength training in muscle fibre composition (% area FT) and relaxation time, with a high correlation obtained between both parameters. These results indicate that MR relaxation time can be used for non-invasive estimation of muscle fibre composition.     

Structure of muscle fibres and motor end plates in extraocular muscles of the grass snake, Natrix natrix L.

Six extraocular muscles of the grass snake, Natrix natrix L. together with their motor end plates were examined in the light and electron microscope, and the measurements of the diameter of muscle fibres and the area of their motor end plates were performed. Morphologically, two types of muscle fibres: tonic and red phase ones were distinguished. The former fibres, 2,3 to 14,5 mum in diameter possess single or multiple (up to five on a single fibre) "en grappe" motor end plates, without postsynaptic junctional folds. The latter fibres, 10...40 mum in diameter have single, "en plaque" motor end plates, with numerous postsynaptic junctional infoldings. The morphological features of muscle fibres and motor end plates as well as the correlation between the diameter of muscle fibres and the area of motor end plates are discussed.     

Effects of agonists and antagonists of rhyanodine receptors on potassium contractures in twitch and tonic frog skeletal muscle fibers

A comparative analysis of the effects of the concentrations of Ca2+ in external medium and the inhibitor (dantrolene) and activator (4-chloro-m-cresol) of rhyanodine-sensitive Ca2+ channels of carcoplasmic reticulum on the characteristics of potassium contracture in frog twitch and tonic skeletal muscles has been performed. It was shown that the duration of contracture in tonic muscles is not restricted by the presence of Ca2+, as distinct from twitch muscles. Dandrolene does not practically affect the contractile responses of tonic fibres, and the concentration of cresol eliciting the contracture for tonic fibres is substantially higher (1 mM) than for twitch fibers (0.25 mM). In twitch fibers, the potassium contracture activated in the presence of cresol is comparable in amplitude and dynamics with the contracture under control conditions, and in tonic fibers a summing of responses without relaxation after the washing of excessive potassium is observed. This suggests that, in twitch fibers, the influx of Ca2+ can directly create the concentration sufficient for the maintenance of contraction, and in tonic fibers its involvement is mediated through the Ca(2+)-dependent activation of the beta-isoform of rhyanodine-sensitive channels.