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PubMed1988

Recovery of muscle from tetrodotoxin-induced disuse and the influence of daily exercise. 2. Muscle enzymes and fatigue characteristics.

The fatigue characteristics and the activities of oxidative and glycolytic enzymes were determined in tetrodotoxin (TTX)-induced disuse muscles and in muscles of animals recovering from TTX-induced disuse (TTX-rec). In addition, the effects of additional daily exercise (grid-climbing and swimming) on the fatigue and metabolic profiles of muscles from TTX-rec and control animals were investigated. The activities of citrate synthase (CS), phosphofructokinase (PFK), and alpha-glycerophosphate dehydrogenase (alpha-GPD) were depressed following 28 days of inactivity produced by the chronic neural application of TTX. The response of these muscles to a pattern of stimulation that has been used to classify fast-twitch motor units according to their fatigability (6) (330 ms, 40 Hz, l/s, 4 min) was not affected to any great extent by inactivity, except for a loss in the ability to summate or maintain forces during each 330-ms burst, as fatigue developed. After 28 days of recovery, the concentration of CS had returned to normal, whereas the concentrations of PFK and alpha-GPD remained depressed. TTX-rec muscles, on the other hand, appeared more resistant to fatigue than control muscles, based on several indices of muscle fatigue. Control and TTX muscles responded similarly to daily training. Swimming but not climbing increased the activity of CS and the fatigue resistance of the muscle. Neither exercise influenced the activity of PFK and alpha-GPD. Although the activity of CS was influenced by the level of neuromuscular usage, the former did not appear to play a dominant role in determining the fatigue resistance of the muscle, emphasizing the need to consider other factors as primary determinants of muscle fatigue.

Animals↗
PubMed1993

Expression of Ca2+ binding proteins of the sarcoplasmic reticulum of striated muscle in the endoplasmic reticulum of pig smooth muscles.

The Ca2+ binding proteins in the lumen of intracellular Ca2+ stores differ between muscle and non-muscle cells, indicating a specific role of these proteins in intracellular Ca2+ regulation. Since smooth muscle cells possess both muscle and non-muscle characteristics, we have studied the presence and the differential expression of the muscle-type Ca2+ binding proteins--calsequestrin, sarcalumenin, and the histidine-rich Ca2+ binding protein (HCP)--in several smooth muscle tissues from the pig. Western blot analysis showed that among the smooth muscles studied, the cardiac isoform of calsequestrin is expressed at the highest levels in the stomach. Calsequestrin was present at lower levels in ileum and trachea, whereas this protein was undetectable in aorta and main pulmonary artery. The total amount of calsequestrin in the stomach was estimated to be 20-30-times lower than in the pig heart. Whereas calsequestrin from pig presented the same apparent M(r) in sodium dodecyl sulphate polyacrylamide gels as the well characterized protein from rabbit, the apparent M(r) of both sarcalumenin and HCP was lower in pig than in rabbit. The presence of HCP was demonstrated in pig stomach and ileum, while sarcalumenin was detected only in the stomach. These results demonstrate further biochemical differences between smooth muscle cells of large blood vessels and those of the digestive tract. The present findings on the differential distribution of muscle-type Ca2+ binding proteins are discussed in relation to biochemical and functional differences between these smooth muscle cells.

Animals↗
PubMed1994

Fusimotor-induced changes in muscle spindle outflow and responsiveness in muscle fatigue in decerebrate cats.

Changes in discharge rate and responsiveness of muscle spindle afferents from triceps surae muscles were studied during long-lasting fatigue isometric contractions of either medial gastrocnemius or lateral gastrocnemius and soleus muscles in decerebrate cats. The rest of the hind limb was either denervated or its innervation was preserved. In denervated preparations a long-lasting post-contraction increase in discharge rate developed in the majority of primary (15 of 18) and in all (20) secondary afferents. This increase was abolished, while the decrease during muscle contraction was enhanced after application of procaine to the corresponding muscle nerve, to block either the small-diameter afferents from the contracting muscle or the fusimotor axons to the spindle of origin of the afferent recorded. In innervated preparations the long-lasting increase was replaced in the majority of primary endings (14 of 22) by a sharp burst at the end of muscle contraction, while in secondary afferents it was either absent or shorter-lasting than in denervated preparations. Changes in responsiveness to sinusoidal muscle length changes, indicating influences of both static and dynamic fusimotor neurons, were, however, similar in innervated and denervated preparations. The results obtained provide evidence that changes in muscle spindle outflow and responsiveness are elicited by the reflex increase in fusimotor activity developing in response to the fatigue-induced afferent discharges from the contracting muscle. Concomitant afferent inflow of another origin to fusimotor neurons affects the changes in spindle outflow, but not in responsiveness. In this way an appropriate increase in support to skeletomotor activity as well as in information on the fatigued muscle of higher motor centres, initiated by the fatigue itself, could be achieved through the gamma loop.

Animals↗
PubMedDate not supplied

Vecuronium neuromuscular blockade at the cricothyroid and posterior cricoarytenoid muscles of the larynx and at the adductor pollicis muscle in humans.

STUDY OBJECTIVE: To compare the sensitivity to vecuronium in the cricothyroid (CT) and posterior cricoarytenoid (PCA) muscles of the larynx and the adductor pollicis muscles. DESIGN: Prospective, nonrandomized study. SETTING: Operating room at Sapporo Medical College and Hospital. PATIENTS: 9 ASA status I and II adult patients scheduled for total laryngectomy to resect laryngeal cancer of essentially unilateral involvement. INTERVENTIONS: During surgery, electromyographic (EMG) recording wire electrodes were inserted into the CT and PCA muscles with nitrous oxide, oxygen, and fentanyl anesthesia. The evoked compound EMG responses of the CT and PCA muscles were quantified simultaneously in an identical manner by supramaximally stimulating the superior laryngeal and recurrent nerves at 0.1 Hz and 0.2 millisecond. The ulnar nerve also was stimulated, and the adduction force of the thumb was measured. Vecuronium was infused continuously at a rate of 0.01 mg/kg/min. The evoked responses (percentage of control, means +/- SD) of the adductor pollicis, CT, and PCA muscles were 12.0 +/- 4.2, 22.0 +/- 5.3, and 32.9 +/- 7.8, respectively, 6 minutes after vecuronium administration. MEASUREMENTS AND MAIN RESULTS: Vecuronium produced significantly more intense neuromuscular blockade in the adductor pollicis than in the laryngeal muscles (p < 0.05). In comparing the laryngeal muscles, the depression of EMG at the CT was significantly greater than that at the PCA (p < 0.05). CONCLUSIONS: The laryngeal (CT and PCA) muscles are more resistant to vecuronium than is the adductor pollicis, and the PCA muscle is more resistant to vecuronium than is the CT muscle. This finding suggests that partially paralyzed patients are able to maintain laryngeal patency despite the apparent impaired neuromuscular transmission in the adductor pollicis muscle.

Aged↗
PubMed2004

Activity of peri-oral facial muscles and its coordination with jaw muscles during ingestive behavior in awake rabbits.

To study peri-oral facial muscle activity patterns and coordination with jaw muscles during ingestive behavior, electromyographic (EMG) activities in the peri-oral facial (buccinator: BUC, orbicularis oris: ORB) and jaw (masseter, digastric) muscles along with jaw movement trajectories were recorded in awake rabbits. A standardized amount of apple in a cylindrical shape was used as the test food. The period from food intake to just before swallowing (the masticatory sequence) was divided into three masticatory periods (preparatory period, rhythmic chewing period and preswallow period) based on the activity pattern of jaw muscles and jaw movement trajectories, and jaw movements and EMG activities in both the jaw and facial muscles during each masticatory period were assessed. Both the jaw and facial muscles were active throughout the masticatory sequence, and the activity patterns of facial muscles and the pattern of coordination between the facial and jaw muscles varied for each masticatory period. No consistent pattern was noted for the BUC activity during the preparatory period, whereas the ORB showed tonic activity throughout this period. During the rhythmic chewing and preswallow periods, both the ORB and BUC showed jaw-movement-related rhythmic bursts. However, significant differences were noted in the burst properties in both facial muscles and their temporal correlations with the jaw muscle activities between these two periods. Results suggest that the neural mechanisms regulating facial muscle activities may differ between the masticatory periods, and such mechanisms may contribute to the well-coordinated orofacial movements required for smooth masticatory sequence.

Analysis of Variance↗
PubMed2006

Increasing muscle strength as a treatment for strabismus: sustained release of insulin-like growth factor-1 in rabbit extraocular muscle.

PURPOSE: Currently, no drug treatment is available for strengthening underacting extraocular muscles (EOM) in strabismus. We showed previously that single injections of insulin-like growth factor (IGF-1) result in significant but short-term increases in muscle force generation. This study examined the effects of sustained release of IGF-1 on force generation in rabbit superior rectus muscles. METHODS: In adult rabbits, slow-release pellets containing IGF-1 were implanted on the global side of one superior rectus muscle. After 1 week, or 1, 2, 3, or 6 months, treated and control muscles were examined for force generation using an in vitro physiology apparatus. All muscles were prepared for histology and mean myofiber cross-sectional areas were determined. RESULTS: One and 3 months after pellet implantation, treated muscles generated significantly greater force than contralateral control muscles, whereas at 2 months, no significant difference was found. Force per cross-sectional area (mN/cm(2)) at 3 months also increased significantly in the treated muscles. Mean muscle cross-sectional area increased significantly after 1, 2, and 3 months of sustained exposure to IGF-1 compared with controls. After an additional 3 months without IGF-1 exposure, mean cross-sectional areas were significantly greater than controls but significantly reduced compared with areas at 1, 2, and 3 months. CONCLUSIONS: IGF-1 appears to be highly effective in increasing muscle force generation. Because slow release of IGF-1 results in sustained increases in EOM force generation, it may be a potentially useful alternative to surgical resection procedures because it avoids many of the potential long-term biomechanical hazards of resection surgery.

Animals↗
PubMed2006

Quantitative assessment of skeletal muscle activation using muscle functional MRI.

The purpose of the present study is to determine whether muscle functional MRI (mfMRI) can be used to obtain three-dimensional (3-D) images useful for evaluating muscle activity, and if so, to measure the distribution of muscle activity within a medial gastrocnemius (MG) muscle. Seven men performed 5 sets of 10 repetitions of a calf-raise exercise with additional 15% of body-weight load. Magnetic resonance images were obtained before and immediately after the exercise. To threshold images, only those pixels showing transverse relaxation time (T2) greater than the mean+1 S.D. of the entire regions of interest (ROIs) in the preexercise image and T2 lower than the mean+1 S.D. of the entire ROIs in the postexercise image were identified. The survived pixels showing T2 are defined as active muscle. Those thresholded images were 3-D reconstructed, and this was used to determine area of active muscle along transverse, longitudinal and vertical axes. At the exercise level used in the present study, the percentage volume of activated muscle in the MG was 62.8+/-4.5%. There was a significant correlation between percentage volume of activated muscle and integrated electromyography (r=.78, P<.05). Percentage areas of activated muscle were significantly larger in the medial than in the lateral region, in the anterior than in the posterior region and in the distal than in the proximal region (P<.05). These results suggest that mfMRI can be used to evaluate the muscle activity and to determine intramuscular variations of activity within skeletal muscle.

Adult↗
PubMed1995

Response of the urethral and intracorporeal pressures to cavernosus muscle stimulation: role of the muscles in erection and ejaculation.

OBJECTIVES: The role of the bulbocavernosus (BC) and ischiocavernosus (IC) muscles in erection and ejaculation was studied. METHODS: The response of the urethral and intracorporeal pressure to cavernosus muscle stimulation was evaluated in 18 male volunteers (mean age, 36.6 years). A two-channel microtip catheter was placed in the prostatic and bulbous urethra. Muscle stimulation was done by two needle electrodes inserted into the BC and IC muscles. RESULTS: BC muscle stimulation caused an increase in the pressure of the bulbous urethra (P < 0.001) and corpus spongiosum (P < 0.01) and an insignificant change in the prostatic and pendulous urethral and corpus cavernosal pressures (difference not significant). IC muscle stimulation effected an increase in the corpus cavernosal pressure (P < 0.001) without changing the urethral pressure (difference not significant). CONCLUSIONS: The BC muscle contracts rhythmically at orgasm and this might help to eject the semen from the posterior to the anterior urethra. It is apparent that the muscle has minimal or no role in erection. IC muscle may have a role in erection by increasing the intracavernosal pressure. It seems that it has no role in ejaculation. BC may be considered the "muscle of ejaculation," and IC the "muscle of erection."

Adult↗
PubMed1983

Synapse elimination in neonatal rat muscle is sensitive to pattern of muscle use.

The synaptic connections among the cells of the vertebrate nervous system undergo extensive rearrangements early in development. During their initial growth, neurones apparently form synaptic connections with an excessive number of targets, later retracting a portion of these synapses in establishing the adult neural circuits. Because of the profound effects which experience has upon the developing nervous system, a question of considerable interest has been the role which the functional use of these developing synapses might play in determining the final pattern of connectivity. At the neuromuscular junction the early changes in synaptic connections are well documented, and here questions about the importance of function can be relatively easily addressed. Mammalian skeletal muscle fibres experience a perinatal period of synapse elimination so that all but one of several synapses formed on each muscle fibre are lost. This synapse elimination is sensitive to alterations of neuromuscular use or activity. Reduction of muscle use by tenotomy or by paralysis of the muscle with drugs blocking nerve impulse conduction or neuromuscular transmission delays or even prevents synapse loss, while increased use produced by stimulation of the muscle nerve apparently accelerates the rate at which synapses are lost. I report here a further examination of the role of neuromuscular activity in synapse elimination. I show that chronic neuromuscular stimulation accelerates synapse elimination but that this acceleration is dependent on the temporal pattern in which the stimuli are presented: brief stimulus trains containing 100 Hz bursts of stimuli produce this acceleration whereas the same number of stimuli presented continuously at 1 Hz do not. Furthermore, the 100 Hz activity pattern which is effective in altering synapse elimination also alters two other muscle properties: the sensitivity of the muscle fibers to acetylcholine and the 'speed' of muscle contractions. These findings suggest that the ability of muscle fibres to maintain more than one nerve terminal, like other muscle properties, is sensitive to the pattern of muscle use rather than just the total amount of use.

Acetylcholine↗
PubMed1996

Length-specific impairment of skeletal muscle contractile function after eccentric muscle actions in man.

1. The effects of maximum voluntary eccentric muscle actions on forearm flexor strength were studied at three different elbow joint angles (0.87, 1.57 and 2.79 rad) to examine whether subsequent impairment of contractile function is muscle length-specific. 2. A bout of 70 maximum voluntary eccentric muscle actions performed with the forearm flexors elicited delayed muscle soreness, efflux of creatine kinase into the blood, loss of elbow joint range of motion and a prolonged decline in maximum voluntary isometric contraction torque at the three elbow angles monitored (P < 0.01) in 30 subjects. 3. The relative decline in maximum voluntary isometric contraction torque after eccentric exercise differed between elbow angles (P < 0.01). The greatest decline in strength relative to baseline was observed at the most acute elbow angle of 0.87 rad, followed by 1.57 and 2.79 rad respectively. Superimposed electrical stimulation in a subgroup of eight subjects did not increase the torque elicited by the forearm flexors during maximum voluntary contraction tests at the most acute elbow angle (0.87 rad) at any time-point after eccentric exercise, suggesting the disproportionate loss of strength at short muscle length was not due to the inhibitory effects of muscle soreness. 4. Measures of arm circumference and ability to fully flex the elbow (in the same subgroup of eight subjects) suggested the greater decline in strength at the acute elbow angle was not due to the possible confounding influences of muscle swelling and/or impaired muscle shortening ability on strength measurements at short muscle length. 5. These data suggest a shift in the forearm flexor length-tension relationship could account for the disparate decline in strength between elbow angles after eccentric exercise. The disproportionate loss of strength at short muscle length suggests that eccentric exercise evoked a subtle increase in muscle length, perhaps attributable to the cumulative effects of regions of lengthened sarcomeres and/or series elastic elements.

Adolescent↗
PubMed1990

Doxorubicin selectively inhibits muscle gene expression in cardiac muscle cells in vivo and in vitro.

The anthracycline antibiotic doxorubicin produces a characteristic myopathy in cardiac muscle that limits its use in cancer therapy. We have shown in cultured neonatal rat cardiac muscle cells that doxorubicin treatment resulted in a rapid, selective decrease in the expression of muscle-specific genes, which preceded other changes characteristic of doxorubicin cardiomyopathy. Doxorubicin selectively and dramatically decreased the levels of mRNA for the sarcomeric genes, alpha-actin, troponin I, and myosin light chain 2, as well as the muscle-specific, but nonsarcomeric M isoform of creatine kinase. However, doxorubicin did not affect nonmuscle gene transcripts (pyruvate kinase, ferritin heavy chain, and beta-actin). Actinomycin D, an inhibitor of DNA-dependent RNA polymerase, did not show a similar selective decrease of muscle-specific mRNAs but, rather, produced a nonspecific, dose-dependent decrease of muscle and nonmuscle transcripts. The doxorubicin effect on muscle gene expression was limited to cardiac muscle; cultured skeletal myocytes were resistant to the effects of doxorubicin at 100-fold greater doses than those causing changes in mRNA levels in cardiac muscle cells. These effects of doxorubicin were reproduced in vivo; rats injected with doxorubicin showed a dose-dependent decrease in the levels of mRNAs for alpha-actin, troponin I, myosin light chain 2, and M isoform of creatine kinase in cardiac but not skeletal muscle. These selective changes in gene expression in cardiocyte cultures and cardiac muscle precede classical ultrastructural changes and may explain the myofibrillar loss that characterizes doxorubicin cardiac injury.

Actins↗
PubMed1979

Divalent cation binding properties of slow skeletal muscle troponin in comparison with those of cardiac and fast skeletal muscle troponins.

1. New methods of preparing troponins from slow skeletal and cardiac muscle of the chicken have been developed. The electrophoretic mobilities of slow skeletal muscle troponin subunits were different from those of the corresponding fast skeletal muscle subunits. 2. A new method for determining the amount of divalent cations bound to troponin was developed. The principle of the method is to immobilize troponin by conjugating it with Sepharose 4B resin, thus making it readily sedimentable. 3. The numbers of Sr and Ca ions bound to slow muscle troponin at concentrations sufficient to produce maximum contraction were 1.73 and 1.36 mol per mol, respectively, being nearly equal to those of cardiac troponin but half of those of fast muscle troponin. 4. The concentrations of Sr and Ca ions giving half-maximal ion binding to slow muscle troponin (K50%) were 5.5 X 10(-6) M and 4.6 X 10(-7) M, respectively. 5. K50% for Sr of cardiac troponin was significantly higher than that of slow muscle troponin. Although K50% for Sr of cardiac troponin was the same as that of fast muscle troponin, cardiac troponin bound more Sr ions than fast muscle troponin at lower Sr ion concentrations. The mechanism underlying the high sensitivity of cardiac muscle contraction to Sr ions is discussed in comparison with that of slow muscle.

Animals↗
PubMed1995

Muscle strength, voluntary activation and cross-sectional muscle area in patients with fibromyalgia.

The objectives were to determine whether the low muscle strength in fibromyalgia is due to lack of exertion and to determine the relation between strength and muscle area. Secondarily we examined the voluntary muscle strength of the different muscles of the leg. The twitch interpolation technique was used to estimate the degree of central activation and the 'true' quadriceps muscle strength. Muscle cross-sectional area was determined with magnetic resonance imaging (MRI). The estimated 'true' muscle strength was 91 Nm (S.D. = 34 Nm) in 15 fibromyalgia patients compared with 125 Nm (28 Nm) in 14 healthy controls (P < 0.02). The 'true' strength divided by the sum of the maximal areas of the four bellies of the quadriceps muscle was lower, being 1.56 Nm/cm2 (0.32 Nm/cm2) in fibromyalgia patients compared with 2.11 Nm/cm2 (0.39 Nm/cm2) in the controls (P < 0.001). The voluntary muscle strength of the flexor muscles of the knee and of the plantar flexors of the ankle was markedly reduced in patients, but no significant differences could be observed in the strength of the dorsal flexors of the ankle. In conclusion, a reduction of the estimated 'true' quadriceps muscle strength per unit area of about 35% was found in fibromyalgia patients.

Activities of Daily Living↗
PubMed1981

Autonomic innervation of receptors and muscle fibres in cat skeletal muscle.

Cat hindlimb muscles, deprived of their somatic innervation, have been examined with fluorescence and electron microscopy and in teased, silver preparations; normal diaphragm muscles have been examined with electron microscopy only. An autonomic innervation was found to be supplied to both intra- and extrafusal muscle fibres. It is not present in all muscle spindles and is not supplied at all to tendon organs. Fluorescence microscopy revealed a noradrenergic innervation distributed to extrafusal muscle fibres and some spindles. On the basis of the vesicle content of varicosities the extrafusal innervation was identified as noradrenergic (32 axons traced), and the spindle innervation as involving noradrenergic, cholinergic and non-adrenergic axons (14 traced). Some of the noradrenergic axons that innervate spindles and extrafusal muscle fibres are branches of axons that also innervate blood vessels. We cannot say whether there are any noradrenergic axons that are exclusively distributed to intra- or extrafusal muscle fibres. The varicosities themselves may be in neuroeffective association with striated muscle fibres only, or with both striated fibres and the smooth muscle cells in the walls of blood vessels. The functional implications of this direct autonomic innervation of muscle spindles and skeletal muscle fibres are discussed and past work on the subject is evaluated.

Animals↗
PubMed1983

Motor unit numbers, motor unit sizes and innervation of single muscle fibres in hyperinnervated adult mouse soleus muscle.

The sural and the lateral plantar nerves were implanted simultaneously into the denervated soleus muscle of adult mice. Each of these nerves contained approximately the normal number of soleus motor axons. This procedure therefore allowed a study of how an initial excessive number of motor axons provided by two different, foreign nerves and terminating into the soleus muscle affected the final pattern of muscle innervation. In muscles examined two months or more after the implantation of the foreign nerves all muscle fibres were innervated. The fraction of the muscle innervated by either nerve varied widely from one preparation to another. However, all the motor axons which were implanted into the muscle appeared to make permanent synapses. Moreover, the distribution of motor unit sizes of each foreign nerve relative to the total number of muscle fibres innervated by that nerve was similar to the distribution of motor unit sizes in muscles cross-innervated by that nerve alone, although the absolute motor unit sizes were reduced. Estimated by intracellular recording, 20-30% of the muscle fibres were polyneuronally innervated. A similar fraction of teased muscle fibres stained for acetylcholinesterase had more than one end-plate.

Acetylcholinesterase↗
PubMed1980

Electrical responses of muscle fibres in a small foot muscle of Xenopus laevis.

1. Extensor digitorum longus IV (EDL IV) of Xenopus laevis was isolated together with its nerve and the 8th and 9th spinal roots. Motor units were functionally isolated.2. Glass micro-electrodes were used to penetrate the muscle fibres and to record their electrical responses to stimulation of single motor axons, or the total motor supply to the muscle.3. Three types of electrical response were recorded from muscle fibres: an action potential in response to a single nerve stimulus (type 1), an action potential following repetitive nerve stimulation but not in response to single shocks (type 2) and end-plate potentials which summed to a plateau of depolarization during repetitive nerve stimulation without producing action potentials (type 3).4. In general muscle fibres giving type 1 responses belonged to fast motor units with high twitch: tetanus ratios; those giving type 3 responses, to small units with low twitch: tetanus ratios; and those giving type 2 responses, to units with intermediate properties.5. In many cases the several muscle fibres innervated by a single axon gave different responses to stimulation of that axon and a single muscle fibre might give different responses to stimulation of two single axons. Thus it was not possible to classify motor units or muscle fibres by electrical responses.6. Stimulation of the total motor supply to the muscle favoured the production of type 1 responses so that the proportion of muscle fibres giving type 1 responses was higher when the muscle nerve was stimulated than when single motor axons were stimulated. The proportion of fibres giving type 2 responses was lower, and the proportion of fibres giving type 3 responses was similar, in both cases. The implications of this are discussed.7. Low neuromuscular junctional efficacy is suggested as an explanation for fluctuating twitch tensions and low twitch: tetanus ratios in many motor units in this muscle, and may also affect the activity patterns imposed on the muscle fibres by the motor axons which supply them.

Action Potentials↗
PubMed1997

Effects of reduced muscle glycogen concentration on force, Ca2+ release and contractile protein function in intact mouse skeletal muscle.

1. The purpose of this study was to examine the effects of reduced glycogen concentration on force, Ca2+ release and myofibrillar protein function during fatigue in skeletal muscle. Force and intracellular free Ca2+ concentration ([Ca2+]i) were measured in single mammalian skeletal muscle fibres during fatigue and recovery. Glycogen was measured in bundles of 20-40 fibres from the same muscle under the same conditions. 2. Fatigue was induced by repeated maximum tetani until force was reduced to 30% of initial. This was associated with a reduction in muscle glycogen to 27 +/- 6% of control values. In fibres allowed to recover for 60 min in the presence of 5.5 mM glucose (n = 6), tetanic (100 Hz) force recovered fully but tetanic [Ca2+]i remained at 82 +/- 8% of initial values. This prolonged depression in Ca2+ release was not associated with decreased muscle glycogen since glycogen had recovered to pre-fatigue levels (157 +/- 42%). 3. To examine the responses under conditions of reduced muscle glycogen concentration, fibres recovered from fatigue for 60 min in the absence of glucose (n = 6). After glucose-free recovery, the decreases in tetanic force and [Ca2+]i were only partially reversed (to 64 +/- 8% and 57 +/- 7% of initial values, respectively). These alterations were associated with a sustained reduction in muscle glycogen concentration (27 +/- 4% of initial values). 4. In another set of fibres, fatigue was followed by 50 Hz intermittent stimulation for 22.6 +/- 4 min. With this protocol, tetanic force and [Ca2+]i partially recovered to 76 +/- 9% and 55 +/- 6% of initial levels, respectively. These changes were associated with a recovery of muscle glycogen (to 85 +/- 10%). 5. During fatigue, Ca2+ sensitivity and maximum Ca(2+)-activated force (Fmax) were depressed but these alterations were fully reversed when muscle glycogen recovered. When glycogen did not recover, Ca2+ sensitivity remained depressed but Fmax partially recovered. The altered myofibrillar protein function is probably due to alterations in inorganic phosphate levels or other metabolites associated with reduced levels of muscle glycogen. 6. These data indicate that the reductions in force, Ca2+ release and contractile protein inhibition observed during fatigue are closely associated with reduced muscle glycogen concentration. These findings also suggest that the changes in Ca2+ release associated with fatigue and recovery have two components-one which is glycogen dependent and another which is independent of glycogen but depends on previous activity.

Animals↗
PubMed1991

Muscle-specific expression of the troponin I gene requires interactions between helix-loop-helix muscle regulatory factors and ubiquitous transcription factors.

The quail fast skeletal troponin I (TnI) gene is a member of the contractile protein gene set and is expressed exclusively in differentiated skeletal muscle cells. TnI gene transcription is controlled by an internal regulatory element (IRE), located within the first intron, that functions as a muscle-specific enhancer. Recent studies have shown that the TnI IRE may interact directly with the muscle regulatory factors MyoD, myogenin, and Myf-5 to produce a muscle-specific expression pattern, since these factors trans-activate cotransfected TnI gene constructs in C3H10T1/2 fibroblasts. In this study, we have examined the protein-IRE interactions that are responsible for transcriptionally activating the TnI gene during skeletal muscle development. We demonstrate that the helix-loop-helix muscle regulatory factors MyoD, myogenin, Myf-5, and MRF4, when complexed with the immunoglobulin enhancer-binding protein E12, interact with identical nucleotides within a muscle regulatory factor-binding site (MRF site) located in the TnI IRE. The nuclear proteins that bind to the MRF site are restricted to skeletal muscle cells, since protein extracts from HeLa, L, and C3H10T1/2 fibroblasts do not contain similar binding activities. Importantly, the TnI MRF site alone is not sufficient to elicit the full enhancer activity associated with the IRE. Instead, two additional regions (site I and site II) are required. The proteins that interact with site I and site II are expressed in both muscle and nonmuscle cell types and by themselves are ineffective in activating TnI gene expression. However, when the MRF site is positioned upstream or downstream of site I and site II, full enhancer activity is restored. We conclude that helix-loop-helix muscle regulatory factors must interact with ubiquitously expressed proteins to generate the active TnI transcription complex that is present in differentiated muscle fibers.

Animals↗