Extensor Digitorum Longus Muscle Muscle Research Articles

Bariatric surgery induces morphological changes in the extensor digitorum longus muscle in the offspring of obese rats

IntroductionThe effects of the maternal nutritional environment on the growth and metabolism of the offspring, and its impacts on health in adult life are defined as metabolic programming. Thus, the objective of this study was to evaluate the effects of Roux-en-Y gastric bypass (RYGB) on the morphology of muscle fiber and neuromuscular junction (NMJ) of the offspring of rats submitted to RYGB. MethodsThree-week-old Wistar rats were separated into two groups: 1) CAF SHAM which received a cafeteria diet and was submitted to a sham operation and 2) CAF RYGB, which received a cafeteria diet and was submitted to RYGB. The first generation (F1) offspring (male) was named according to the treatment of mothers as CAF SHAM-F1 and CAF RYGB-F1 and received a standard diet after weaning. At 17 weeks, the animals were euthanized, and the extensor digitorum longus muscle (EDL) was collected and processed in light microscopy and transmission electron microscopy for morphological and morphometric analysis. ResultsThe CAF RYGB-F1 group showed a reduction in the weight of the EDL muscle and also a reduction in the area of type I, IIa and IIb fibers and a nucleus/fiber ratio. This same group also showed an increase in the capillary density and myofibrillar disorganization and in the Z-line, as well as a reduction in the area of the NMJs. ConclusionThe RYGB surgery in mothers produced morphological changes in the skeletal striated muscles of the offspring.

Glutamine supplementation versus functional overload in extensor digitorum longus muscle hypertrophy

• We compared glutamine supplementation and overload inducing EDL muscle hypertrophy. • Glutamine and overload raised muscle fiber cross-sectional area to a similar extent. • Glutamine potentiated the overload-induced increase in the resistance to fatigue. Abstract Glutamine levels directly associate with total protein content in cultured skeletal muscle cells, whereas glutamine supplementation enhances skeletal muscle mass in catabolic experimental conditions. We compared the effect of glutamine administration on Extensor Digitorum Longus muscle (EDL) weight, fiber cross-sectional area (CSA), contractile activity, and protein metabolism signaling with a functional overload-induced skeletal muscle hypertrophy protocol. Glutamine supplementation raised the predominance of EDL muscle fibers with CSA between 1001 and 2000 μm2 (49.7 %), the p-4E-BP1/total 4E-BP1 ratio, and the effect of overload on resistance to fatigue. The proportion of the EDL muscle fiber CSA distribution for the combination of both treatments was similar to that induced by overload or glutamine separately; 54.3 % muscle fibers with CSA between 1001 and 2000 μm². Glutamine supplementation did not markedly affect the changes induced by overload on protein synthesis signaling pathways, except for a further increase of the p-4E-BP1/total 4E-BP1 ratio. The effect of glutamine on EDL muscle fiber CSA distribution and protein synthesis signaling mimicked the response to overload. The association of glutamine and overload induced EDL muscle hypertrophy further increased the resistance to fatigue.

Changes in local capillarity of pure and hybrid MyHC muscle fiber types after nerve injury in rat extensor digitorum longus muscle (EDL).

Recently, we evaluated capillary indices without discrimination by fiber type in rat extensor digitorum longus muscle (EDL) 4weeks after nerve cut (NC), after double nerve crush (double NCR) and in two controls, from the start (CON-1) and the end (CON-2) of the experiment. In the present study, we determined the capillary indices related to specific myosin heavy chain (MyHC) fiber types. Fiber-type composition and local capillarity were assessed from a single, composite, multicolor image, where different MyHC-fiber types and capillaries were shown simultaneously. Applying local capillary indices [the number of capillaries around fiber (CAF) and the CAF scaled to fiber perimeter (CAF/FP)], to specific MyHC-fiber types, we found changes relevant to neuro-muscular studies. In the NC group, only type-2x fibers had a significantly lower CAF, and in the double NCR group, only type-2a fibers had a higher CAF in comparison with both controls. Both types of nerve injury elicited two responses: a coupled regulation of fiber size and capillarity in the oxidative, type 2a fibers and a capillarity independent regulation of fiber size in the glycolytic type-2b fibers. All subtypes of type-2 fibers had a better capillary supply (higher CAF/FP) in the NC and double NCR than in CON-2. The highest improvement was observed in type-2b fibers; this change was mirrored in an oxidative shift only in the double NCR group. Adopting fiber-type-specific capillary indices improves data analysis of rat EDL muscle samples.

\u03b2-arrestin 1 regulates \u03b22-adrenergic receptor-mediated skeletal muscle hypertrophy and contractility

Backgroundβ2-adrenergic receptors (β2ARs) are the target of catecholamines and play fundamental roles in cardiovascular, pulmonary, and skeletal muscle physiology. An important action of β2AR stimulation on skeletal muscle is anabolic growth, which has led to the use of agonists such as clenbuterol by athletes to enhance muscle performance. While previous work has demonstrated that β2ARs can engage distinct signaling and functional cascades mediated by either G proteins or the multifunctional adaptor protein, β-arrestin, the precise role of β-arrestin in skeletal muscle physiology is not known. Here, we tested the hypothesis that agonist activation of the β2AR by clenbuterol would engage β-arrestin as a key transducer of anabolic skeletal muscle growth.MethodsThe contractile force of isolated extensor digitorum longus muscle (EDL) and calcium signaling in isolated flexor digitorum brevis (FDB) fibers were examined from the wild-type (WT) and β-arrestin 1 knockout mice (βarr1KO) followed by chronic administration of clenbuterol (1 mg/kg/d). Hypertrophic responses including fiber composition and fiber size were examined by immunohistochemical imaging. We performed a targeted phosphoproteomic analysis on clenbuterol stimulated primary cultured myoblasts from WT and βarr1KO mice. Statistical significance was determined by using a two-way analysis with Sidak’s or Tukey’s multiple comparison test and the Student’s t test.ResultsChronic administration of clenbuterol to WT mice enhanced the contractile force of EDL muscle and calcium signaling in isolated FDB fibers. In contrast, when administered to βarr1KO mice, the effect of clenbuterol on contractile force and calcium influx was blunted. While clenbuterol-induced hypertrophic responses were observed in WT mice, this response was abrogated in mice lacking β-arrestin 1. In primary cultured myoblasts, clenbuterol-stimulated phosphorylation of multiple pro-hypertrophy proteins required the presence of β-arrestin 1.ConclusionsWe have identified a previously unappreciated role for β-arrestin 1 in mediating β2AR-stimulated skeletal muscle growth and strength. We propose these findings could have important implications in the design of future pharmacologic agents aimed at reversing pathological conditions associated with skeletal muscle wasting.

Heat stress promotes skeletal muscle regeneration after crush injury in rats

Influences of heat stress on skeletal muscle regeneration were examined in experimental rats. After crush injury to the Extensor digitorum longus muscle (EDL) of the left hindlimb, animals were randomly divided into non-heat and heat groups. In the latter, packs filled with hot water (42°C) were percutaneously applied to the injured EDL muscle for 20min to the front of the lower leg, soon after the injury. During the early stages of muscle regeneration, due to the heat stress, secondary degeneration at the injured site progressed faster, and migration of macrophages, proliferation and differentiation of satellite cells were facilitated. At 14 and 28 days after the injury, the ratio of regenerating muscle fibers exhibiting central nuclei in the heat treated group was significantly lower than that in the non-heat group, and cross sectional area in the heat group was evidently larger than that in the non-heat group. Moreover, in the heat group, the ratio of collagen fiber area at 14 and 28 days after the injury was smaller than in the non-heat group. Together, these findings suggest that acceleration of degeneration processes by heat stress soon after injury is likely to promote skeletal muscle regeneration and inhibit collagen deposition.

Expression of New Loci Associated With Obesity in Diet‐Induced Obese Rats: From Genetics to Physiology

Genome-wide association studies (GWAS) are a powerful tool for revealing genes associated with common human obesity. New loci associated with obesity have recently been reported, but their function and metabolic implications remain to be elucidated. In order to begin identifying the role of some of these obesity-related loci, the closest genes to the polymorphism of each locus were selected and their expression was compared in the hypothalamus, adipose tissue, liver, soleus muscle, and extensor digitorum longus muscle (EDL) of Long-Evans rats maintained on chow or a high-fat diet (HFD) for 6 weeks. From a total of 19 genes analyzed, seven genes (ETV5, FTO, GNPDA2, KCTD15, TMEM18, MC4R, and SH2B1) were down-regulated in the hypothalamus of HFD compared to chow-fed rats. In adipose tissue of rats fed on HFD, the mRNA levels of BCDIN3, KCTD15, and SULT1A1 were down-regulated, whereas those of MTCH2, PTER, and TUFM were up-regulated. In the liver, three genes were up-regulated (PTER, SULT1A1, and TUFM) in HFD relative to chow-fed rats, and TMEM18 was down-regulated. Finally, in soleus muscle of HFD-fed rats, BCDIN3, BDNF, and TMEM18 were down-regulated, and in the EDL muscle SH2B1 and TUFM were up-regulated. mRNA levels in the hypothalamus were compared between fed and fasted states, and only KCTD15 was down-regulated during fasting when fed a chow diet. In conclusion, novel genes found to be associated with obesity are regulated by a HFD and the mRNA levels of KCTD15 is dependent on the nutritional status. These results suggest a potential role of these genes in the regulation of energy balance.

Plasma amino acid concentrations and mRNA expression of components related to protein degradation in lambs with nutritionally induced metabolic acidosis

Aspects of amino acid metabolism and proteolysis were examined in lambs with metabolic acidosis. Lambs (n = 30; 42.8 ± 4.1 kg BW) were randomly assigned to treatment groups, receiving a topdress of either (1) dried distiller’s grain (DDG; anion load of 7 mEq 100 g-1 DM, (2) DDG + NH4Cl-treated DDG (anionic load of -29 mEq 100 g-1 DM), or (3) DDG + NH4Cl-treated DDG (anionic load of -42 mEq 100 g-1 DM). Lambs were maintained on DDG topdress for 9 d, after which increasing amounts of DDG were substituted with NH4Cl-treated DDG in the treatment group diets from day 10 to day 21. Blood gas, blood acid-base, urine pH and plasma amino acid concentrations were determined on day 9 and day 21. On day 22, lambs were slaughtered and kidney, liver, extensor digitorum longus muscle (EDL), and soleus muscle samples were collected. Blood pH, bicarbonate, base excess of blood and extracellular fluid, total CO2 and urinary pH were linearly decreased as dietary anionic load increased (P < 0.001). Plasma glutamine, glycine, histidine, arginine and asparagine increased linearly (P < 0.06) due to acidosis, while renal glutamate dehydrogenase (P = 0.02) and hepatic glutamine synthetase (P = 0.02) mRNA expression were linearly increased. Caspase-3 mRNA expression was linearly decreased in soleus muscle by acidosis (P = 0.01), but not in EDL muscle. No change in mRNA expression of components of the ubiquitin-mediated proteolytic pathway was observed in either muscle. These results suggest muscle-specific regulation of caspase-3 mRNA expression during metabolic acidosis in sheep.

Laminin-111: A Potential Therapeutic Agent for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) still needs effective treatments, and myoblast transplantation (MT) is considered as an approach to repair damaged skeletal muscles. DMD is due to the complete loss of dystrophin from muscles. The lack of link between the contracting apparatus and the extracellular matrix leads to frequent damage to the sarcolemma triggering muscle fiber necrosis. Laminins are major proteins in the extracellular matrix. Laminin-111 is normally present in skeletal and cardiac muscles in mice and humans but only during embryonic development. In this study, we showed that intramuscular injection of laminin-111 increased muscle strength and resistance in mdx mice. We also used laminin-111 as a coadjuvant in MT, and we showed this protein decreased considerably the repetitive cycles of degeneration, inflammatory reaction, and regeneration. Moreover, MT is significantly improved. To explain the improvement, we confirmed with the same myoblast cell batch that laminin-111 improves proliferation and drastically increases migration in vitro. These results are extremely important because DMD could be treated only by the injection of a recombinant protein, a simple and safe therapy to prevent loss of muscle function. Moreover, the improvement in MT would be significant to treat the muscles of DMD patients who are already weak.

Exercise‐induced TBC1D1 Ser237 phosphorylation and 14‐3‐3 protein binding capacity in human skeletal muscle

TBC1D1 is a Rab-GTPase activating protein involved in regulation of GLUT4 translocation in skeletal muscle. We here evaluated exercise-induced regulation of TBC1D1 Ser237 phosphorylation and 14-3-3 protein binding capacity in human skeletal muscle. In separate experiments healthy men performed all-out cycle exercise lasting either 30 s, 2 min or 20 min. After all exercise protocols, TBC1D1 Ser237 phosphorylation increased (∼70-230%, P < 0.005), with the greatest response observed after 20 min of cycling. Interestingly, capacity of TBC1D1 to bind 14-3-3 protein showed a similar pattern of regulation, increasing 60-250% (P < 0.001). Furthermore, recombinant 5AMP-activated protein kinase (AMPK) induced both Ser237 phosphorylation and 14-3-3 binding properties on human TBC1D1 when evaluated in vitro. To further characterize the role of AMPK as an upstream kinase regulating TBC1D1, extensor digitorum longus muscle (EDL) from whole body α1 or α2 AMPK knock-out and wild-type mice were stimulated to contract in vitro. In wild-type and α1 knock-out mice, contractions resulted in a similar ∼100% increase (P < 0.001) in Ser237 phosphorylation. Interestingly, muscle of α2 knock-out mice were characterized by reduced protein content of TBC1D1 (∼50%, P < 0.001) as well as in basal and contraction-stimulated (∼60%, P < 0.001) Ser237 phosphorylation, even after correction for the reduced TBC1D1 protein content. This study shows that TBC1D1 is Ser237 phosphorylated and 14-3-3 protein binding capacity is increased in response to exercise in human skeletal muscle. Furthermore, we show that the catalytic α2 AMPK subunit is the main (but probably not the only) donor of AMPK activity regulating TBC1D1 Ser237 phosphorylation in mouse EDL muscle.

Ability of a synthetic coumestan to antagonize Bothrops snake venom activities

We investigated a synthetic coumestan named LQB93 and similar compounds abilities to antagonize activities of Bothrops jararacussu and Bothrops jararaca crude venoms in different protocols. The antimyotoxic activity was evaluated in vitro by the rate of release of creatine kinase (CK) from isolated mouse extensor digitorum longus muscle (EDL) induced by B. jararacussu (25 g/ml). For in vivo studies, B. jararacussu venom (1.0 mg/kg) was preincubated with LQB93 (0.1–30 mg/kg), during 30 min, for later injection in mouse tight and evaluation of the antimyotoxic and anti-edematogenic effects. LQB93 antagonized in vitro, the increase of CK release from the EDL muscle (IC 50 = 0.0291 M). It also showed in vivo, antimyotoxic and anti-edematogenic effects that were dose-dependent with ID50 of 0.17 mg/kg and 0.14 mg/kg, respectively. The hemorrhage induced by B. jararaca (1.0 mg/kg) venom in the mouse skin, was abolished by LQB93 (10.0 mg/kg) preincubated with venom. Like wedelolactone, LQB93 protected rat isolated heart on a Langendorff preparation, from the cardiotoxicity of B. jararacussu venom. LQB93 inhibit the effects of Bothrops venoms like wedelolactone, a natural compound isolated from the plant Eclipta prostrata.

Cocaine Does Not Alter Microvascular Po2 On-kinetics In Rat Hindlimb Skeletal Muscle

Cocaine abuse in athletes and the associated morbidity and mortality has piqued interest in its mechanisms of action. Acute cocaine intoxication initially enhances muscle performance, but ultimately leads to premature muscle fatigue and may precipitate renal failure, and death. In rats, cocaine administration accelerates muscle glycogen utilization and fatigue during treadmill exercise. Blood flow to rat hindlimb muscles during exercise is not reduced after cocaine administration (Hagen et al., MSSE 29: S218, 1997), suggesting that cocaine may elevate muscle metabolism. PURPOSE: To determine whether acute cocaine administration increases skeletal muscle metabolism and oxygen uptake, thereby accelerating microvascular PO2 (PO2m) on-kinetics. METHODS: PO2m was measured by phosphorescence quenching in the extensor digitorum longus muscle (EDL) of anesthetized female Sprague-Dawley rats at rest and during the rest-to-contraction transition (1 Hz, 6V). In each rat, PO2m was measured during a control stimulation (STIM 1) and after administration of either cocaine (C, n=13; 5 mg/kg i.v.) or an equivalent volume of saline (S; n=10) (STIM 2). EDL twitch tension was measured during each contraction bout using a muscle tension analyzer. RESULTS: During STIM 1, resting PO2m (R-PO2m), the time delay (TD) between the onset of contractions and the fall in PO2m, the time constant (Tau) and the magnitude and of the fall in PO2m at the onset of contractions (D-PO2m) were the same in the S and C groups. Cocaine administration had no effect on R-PO2m during prior to STIM 2 (S, 23 ± 2; C, 26 ± 1 mmHg). Further, cocaine had no effect on D-PO2m (S, -12 ± 2 vs. C, -15 ± 1 mmHg), TD (S, 6 ± 1; C, 6 ± 1 s) or Tau (S, 8 ± 2; C, 9 ± 2 s). Neither twitch tension (S, 31 ± 7; C, 28 ± 7 N) nor any other indices of muscle contractile function were different during STIM 2. CONCLUSION: The results of this study indicate that acute cocaine administration does not alter resting PO2m, PO2m on-kinetics, or contractile function in the rat EDL muscle. Thus, these data do not support the notion that cocaine enhances muscle metabolism in contracting skeletal muscle.

Modification of motoneuron size after partial denervation in neonatal rats.

Our previous studies have shown that partial denervation of extensor digitorum longus muscle (EDL) in the rat at 3 days of age causes an increase in the activity of the intact motoneurons. The originally phasic pattern of activity of EDL became tonic after partial denervation. These modifications of motoneuron activity were associated with the change in the phenotype of the muscle from fast to slow contracting and with a conversion of the muscle fibres from a fast to a slow type. The present study investigates whether the size of the cell body of the active EDL motoneurons change in parallel with the altered muscular activity. The study involved partial denervation of rat EDL muscle by section of the L4 spinal nerve at 3 days of age. Then the remaining motoneurons from L5 spinal nerve supplying the EDL muscle were retrogradly labelled with horseradish peroxidase two months later. The results show a reduction in motoneuron size in parallel with an increase in activity of the motoneurons after partial denervation of EDL muscle.

Adaptation of muscle fibre types and capillary network to acute denervation and shortlasting reinnervation

We postulated that, in rat extensor digitorum longus muscle (EDL), the length of capillaries per fibre surface area (Lcap/Sfib) and per fibre volume (Lcap/Vfib) could reflect fibre-type transformations accompanied by changes in oxidative metabolic profile and selective fibre-type atrophy. We excised rat EDL muscle 2 weeks after the sciatic nerve was cut (acute denervation; DEDL) and 4 weeks after the nerve was crushed (early reinnervation; REDL) and characterised muscle fibre-type transformation by the expression of myosin heavy-chain isoforms and by succinate dehydrogenase (SDH) and nicotinoamide adenine dinucleotide-tetrazolium reductase (NADH-TR) reactions. The numerical percentage (N/N) and area percentage (A/A) of pure and hybrid fibres and their diameter were determined, as was the A/A of SDH- and NADH-TR-positive fibres. The length of capillaries per fibre length (Lcap/Lfib), Lcap/Sfib and Lcap/Vfib were estimated in REDL and Lcap/Vfib in DEDL. In DEDL, the type 2x and 2b fibres evidently atrophied, with the N/N of type 2x fibres being lower and that of hybrid fibres higher. In REDL, the N/N of hybrid fibres was even higher, consequent to a lower N/N of type 2b fibres; however, fibre diameters approached values of the control EDL. Compared with control EDL, denervated and reinnervated muscles exhibited a higher A/A of oxidative fibres. This is probably the result of fibre-type transformation and selective fibre atrophy. We conclude that capillary length does not change during acute denervation and early reinnervation. The obtained higher values of Lcap/Sfib and Lcap/Vfib are related to changes in muscle fibre cross-sectional area.

Fetal muscle-derived cells can repair dystrophic muscles in mdx mice

We have previously reported that CD34 + cells purified from mouse fetal muscles can differentiate into skeletal muscle in vitro and in vivo when injected into muscle tissue of dystrophic mdx mice. In this study, we investigate the ability of such donor cells to restore dystrophin expression, and to improve the functional muscle capacity of the extensor digitorum longus muscle (EDL) of mdx mice. For this purpose green fluorescent-positive fetal GFP +/CD34 + cells or desmin +/ −LacZ/CD34 + cells were transplanted into irradiated or non-irradiated mdx EDL muscle. Donor fetal muscle-derived cells predominantly fused with existing fibers. Indeed more than 50% of the myofibers of the host EDL contained donor nuclei delivering dystrophin along 80–90% of the length of their sarcolemma. The presence of significant amounts of dystrophin (about 60–70% of that found in a control wild-type mouse muscle) was confirmed by Western blot analyses. Dystrophin expression also outcompeted that of utrophin, as revealed by a spatial shift in the distribution of utrophin. At 1 month post-transplant, the recipient muscle appeared to have greater resistance to fatigue than control mdx EDL muscle during repeated maximal contractions.

Muscular mechanical hyperalgesia revealed by behavioural pain test and c‐Fos expression in the spinal dorsal horn after eccentric contraction in rats

Delayed onset muscle soreness (DOMS) is quite common, but the mechanism for this phenomenon is still not understood; even the existence of muscle tenderness (mechanical hyperalgesia) has not been demonstrated in experimental models. We developed an animal model of DOMS by inducing eccentric contraction (lengthening contraction, ECC) to the extensor digitorum longus muscle (EDL), and investigated the existence of mechanical hyperalgesia in the EDL by means of behavioural pain tests (Randall-Selitto test and von Frey hair test, applied to/through the skin on the EDL muscle) and c-Fos expression in the spinal dorsal horn. We found that the mechanical withdrawal threshold measured with the Randall-Selitto apparatus decreased significantly between 1 and 3 days after ECC, while that measured by von Frey hairs did not. The group that underwent stretching of the muscle only (SHAM group) showed no change in mechanical pain threshold in either test. These results demonstrated that the pain threshold of deep tissues (possibly of the muscle) decreased after ECC. c-Fos immunoreactivity in the dorsal horn (examined 2 days after ECC/SHAM exercise) was not changed by either ECC or compression (1568 mN) to the EDL muscle by itself, but it was significantly increased by applying compression to the EDL muscle 2 days after ECC. This increase was observed in the superficial dorsal horn of the L4 segment of the ipsilateral side, and was clearly suppressed by morphine treatment (10 mg kg(-1), i.p.). These results demonstrated the existence of mechanical hyperalgesia in the muscle subjected to ECC. This model could be used for future study of the neural mechanism of muscle soreness.

Muscle force is determined also by muscle relative position: isolated effects

Effects on force of changes of the position of extensor digitorum longus muscle (EDL) relative to surrounding tissues were investigated in rat. Connective tissue at the muscle bellies of tibialis anterior (TA), extensor hallucis longus (EHL) and EDL was left intact, to allow myofascial force transmission. The position of EDL muscle was altered, without changing EDL muscle–tendon complex length, and force exerted at proximal and distal tendons of EDL as well as summed force exerted at the distal tendons of TA and EHL muscles (TA+EHL) were measured. Proximal and distal EDL forces as well as distal TA+EHL force changed significantly on repositioning EDL muscle. These muscle position–force characteristics were assessed at two EDL lengths and two TA+EHL lengths. It was shown that changes of muscle force with length changes of a muscle is the result of the length changes per se, as well as of changes of relative position of parts of the muscle. It is concluded that in addition to length, muscle position relative to its surroundings co-determines isometric muscle force.

Time course of changes in EMG activity of fast muscles after partial denervation.

After partial denervation, the remaining motor units (MUs) of adult fast extensor digitorum longus muscle (EDL) expand their peripheral field. The time course of this event was studied using tension measurement and recordings of electromyographic (EMG) activity. The results show that after section of the L4 spinal nerve, when only 5.3 +/- 0.63 of the 40 MUs normally supplying EDL muscle remain, the force of individual motor units starts to increase between the 1st and 2nd week after the operation and continues to do so for a further week. The drastic reduction of the number of motoneurones supplying the fast EDL leads to an increase in activity of the remaining MUs. In the 1st week after partial denervation, there was a sharp increase in the EMG activity of remaining motor units. During the next 12 days, this increase became less marked, but EMG activity remained nevertheless significantly higher than that of the unoperated EDL muscle. Many MUs became tonically active during posture. The EMG activity pattern during locomotion was also altered, so that the burst duration was positively correlated with the step cycle duration. Moreover, shortly after partial denervation, the interlimb coordination was disturbed but returned to its original symmetrical use 1-2 weeks later.

Increased force development rates of fatigued mouse skeletal muscle are graded to myosin light chain phosphate content.

Phosphorylation of myosin regulatory light chain (R-LC) increases the Ca2+ sensitivity of cross-bridge transitions, which determine rate of force development in skinned skeletal muscle fibers. The purpose of this study was to determine whether phosphorylation of R-LC is the molecular basis for the increased force development rates (+dF/dtmax) observed in fatigued mouse extensor digitorum longus muscle (EDL) (stimulated in vitro at 25 degrees C). Parameters of twitch and tetanic force were obtained after the application of different-frequency conditioning stimuli (CS), which were used to vary R-LC phosphorylation and reduce peak tetanic force (Po). Without CS, R-LC phosphorylation (in moles phosphate per mole R-LC) was not elevated above rest (0.11 +/- 0.02 vs. 0.13 +/- 0.02, respectively), and no aspect of the twitch (Pt) Po was altered. Stimulating muscles at 2.5-20 Hz increased R-LC phosphorylation in a frequency-dependent manner, from 0.23 +/- 0.04 to 0.82 +/- 0.03, respectively. Moreover, stimulation at 2.5-20 Hz potentiated Pt (range: 4 +/- 2-28 +/- 2%), increased the +dF/dtmax of potentiated twitches (range: 5 +/- 1-28 +/- 2%), and reduced Po (range: 6 +/- 1-21 +/- 1%). Higher-frequency stimulation (40 or 100 Hz) did not phosphorylate R-LC or potentiate Pt or twitch +dF/dtmax further. Stimulation at 40 and 100 Hz did, however, have different effects on Po compared with 20-Hz data (Po reduced 27 +/- 2 and 11 +/- 2%, respectively). The increased +dF/dtmax of potentiated twitches observed after different CS procedures were graded to R-LC phosphorylation (r = 0.97, P < 0.001). It is concluded that phosphorylation of R-LC increases extent of twitch force development in mouse EDL muscle fatigued by CS.

Capillary Adrenoceptors in Rat Skeletal Muscle

The purpose of this study was to examine whether functional α- and β-adrenoceptors exist on capillaries of rat skeletal muscle, and further to determine which subtype of these receptors predominates on these capillaries. Using intravital video microscopy, we measured red blood cell velocity (VRBC) responses in capillaries of rat extensor digitorum longus muscle (EDL) following a local application of these agonists: norepinephrine (NE; α1, α2; 10−7to 3 × 10−3M), phenylephrine (PE; α1; 3 × 10−4to 10−2M), clonidine (CLO; α2; 3 × 10−3to 10−2M), UK14304 (α2; 3 × 10−4to 10−2M), and isoproterenol (IPR; β1, β2; 10−7to 3 × 10−3M). Responses to NE (10−5M) were also measured after a local pretreatment with prazosin (α1antagonist; 10−5to 10−3M) and rauwolscine (α2antagonist; 3 × 10−4to 3 × 10−2M), while responses to IPR (10−5M) were measured after local atenolol (ATE; β1antagonist; 10−3to 10−2M) and butoxamine (BUT; β2antagonist; 10−3to 10−2M) pretreatment. The overall control VRBCwas 226 μm/sec. NE, PE, CLO, and UK14304 resulted in concentration-dependent decreases of VRBC(from −12 to −89%) from the control level, while IPR caused concentration-dependent increases (17 to 174%). PE reduced VRBCto a larger degree than CLO and UK14304. NE-induced VRBCresponses tended to be attenuated more by prazosin than by rauwolscine. Both ATE (10−2M) and BUT (10−3and 10−2M) alone decreased VRBC. However, only ATE significantly attenuated the IPR-induced VRBCresponses. These results suggest that the capillary of rat EDL muscle has α- and β-adrenoceptors. From the two α-adrenoceptor subtypes, the capillary may be predominated by the α1-adrenoceptors.

Function of the Myogenin Gene Promoter in the Muscular Dystrophic Transgenic dy/dy Mouse.

The mouse myogenin gene carrying the LacZ gene coding for β-galactosidase was microinjected into fertilized eggs of dystrophic (dy/dy) mice. Concentrations of the LacZ gene varied between the fast and slow regions of skeletal muscle. The slow or gastrocnemius muscle region, exhibited unusually high levels of the LacZ gene compared to the levels found in the fast muscle region which is also known as the extensor digitorum longus muscle (EDL). In addition, morphological studies indicated that a maturational defect was present in the dystrophic dy/dy EDL muscle.