Enhanced Glycogen Storage of a Subcellular Hot Spot in Human Skeletal Muscle during Early Recovery from Eccentric Contractions

Unaccustomed eccentric exercise is accompanied by muscle damage and impaired glucose uptake and glycogen synthesis during subsequent recovery. Recently, it was shown that the role and regulation of glycogen in skeletal muscle are dependent on its subcellular localization, and that glycogen synthesis, as described by the product of glycogen particle size and number, is dependent on the time course of recovery after exercise and carbohydrate availability. In the present study, we investigated the subcellular distribution of glycogen in fibers with high (type I) and low (type II) mitochondrial content during post-exercise recovery from eccentric contractions. Analysis was completed on five male subjects performing an exercise bout consisting of 15 x 10 maximal eccentric contractions. Carbohydrate-rich drinks were subsequently ingested throughout a 48 h recovery period and muscle biopsies for analysis included time points 3, 24 and 48 h post exercise from the exercising leg, whereas biopsies corresponding to prior to and at 48 h after the exercise bout were collected from the non-exercising, control leg. Quantitative imaging by transmission electron microscopy revealed an early (post 3 and 24 h) enhanced storage of intramyofibrillar glycogen (defined as glycogen particles located within the myofibrils) of type I fibers, which was associated with an increase in the number of particles. In contrast, late in recovery (post 48 h), intermyofibrillar, intramyofibrillar and subsarcolemmal glycogen in both type I and II fibers were lower in the exercise leg compared with the control leg, and this was associated with a smaller size of the glycogen particles. We conclude that in the carbohydrate-supplemented state, the effect of eccentric contractions on glycogen metabolism depends on the subcellular localization, muscle fiber’s oxidative capacity, and the time course of recovery. The early enhanced storage of intramyofibrillar glycogen after the eccentric contractions may entail important implications for muscle function and fatigue resistance.     

Effects of long-term immobilization and recovery on human triceps surae and collagen turnover in the Achilles tendon in patients with healing ankle fracture.

The aim of the present study was to analyze how human tendon connective tissue responds to an approximately 7-wk period of immobilization and a remobilization period of a similar length, in patients with unilateral ankle fracture, which is currently unknown. Calf muscle cross-sectional area (CSA) decreased by 15% (5,316 to 4,517 mm2) and strength by 54% (239 to 110 N.m) in the immobilized leg after 7 wk. During the 7-wk remobilization, the CSA increased by 9% (to 4,943 mm2) and strength by 37% (to 176 Nm). Achilles tendon CSA did not change significantly during either immobilization or remobilization. Local collagen turnover was measured as the peritendinous concentrations of NH2-terminal propeptide of type I collagen (PINP) and COOH-terminal telopeptide region of type I collagen (ICTP), markers thought to be indexes of type I collagen synthesis and degradation, respectively. Both markers were increased (PINP: 257 vs. 56 ng/ml; ICTP: 9.8 vs. 2.1 microg/l) in the immobilized leg compared with the control leg after the 7 wk of immobilization, and levels decreased again in the immobilized leg during the recovery period (PINP: 103 vs. 44 ng/ml; ICTP: 4.2 vs. 1.9 microg/l). A significant reduction in calf muscle CSA and strength was found in relation to 7 wk of immobilization. Immobilization increased both collagen synthesis and degradation in tendon near tissue. However, it cannot be excluded that the facture of the ankle in close proximity could have affected these data. Remobilization increased muscle size and strength and tendon synthesis and degradation decreased to baseline levels. These dynamic changes in tendon connective tissue turnover were not associated with macroscopic changes in tendon size.     

Indices of skeletal muscle damage and connective tissue breakdown following eccentric muscle contractions

 Indirect indices of exercise-induced human skeletal muscle damage and connective tissue breakdown were studied following a single bout of voluntary eccentric muscle contractions. Subjects (six female, two male), mean (SD) age 22 (2) years performed a bout of 50 maximum voluntary eccentric contractions of the knee extensors of a single leg. The eccentric exercise protocol induced muscle soreness (P < 0.05 Wilcoxon test), chronic force loss, and a decline in the 20:100 Hz percutaneous electrical myostimulation force ratio [P < 0.01, repeated measures analysis of variance (ANOVA)]. Serum creatine kinase (CK) and lactate dehydrogenase (LDH) activities were elevated (P < 0.01, repeated measures ANOVA) following the bout. The mean (SD) CK and LDH levels recorded 3 days post-exercise were 2815 (4144) IU · l^–1 and 375 (198) IU · l^–1, respectively. Serum alkaline phosphatase activity showed no changes throughout the study, and a non-significant increase (P = 0.058, repeated measures ANOVA) in pyridinoline was recorded following the bout. Urinary hydroxyproline (HP) and hydroxylysine (HL) excretion, expressed in terms of creatinine (Cr) concentration, increased after exercise (P < 0.05 and P < 0.01, respectively, repeated measures ANOVA). An increased HP:Cr was recorded 2 days post-exercise and HL:Cr was increased above baseline on days 2, 5, and 9 post-exercise. This indirect evidence of exercise-induced muscle damage suggests that myofibre disruption was caused by the eccentric muscle contractions. Elevated urine concentrations of indirect indices of collagen breakdown following eccentric muscle contractions suggests an increased breakdown of connective tissue, possibly due to a localised inflammatory response. Accepted: 9 October 1996     

Effects of muscle damage on stretch-shortening cycle function and muscle stiffness control

This experiment examined the effect of eccentric contraction-induced muscle damage on the stretch-shortening cycle and vertical leg spring stiffness during jumping activities. Ten moderately active male and female adult volunteers participated in this study (aged 23 +/- 2.3 years). Temporary muscle damage to the knee extensors was administered by a bout of eccentric contractions on an isokinetic dynamometer. Measurements were obtained of maximum voluntary force and of take-off velocities for single-leg countermovement jumps (CMJs), squat jumps (SJs), and drop jumps (DJs), performed on a specially constructed sledge and force plate apparatus. These measurements were obtained before and after the damage intervention, and the undamaged leg was used as a control. The results indicated that eccentric muscle damage significantly affected stretch-shortening cycle performance by causing relatively greater reductions in SJ performance than CMJ or DJ. The muscle damage intervention also significantly increased leg-spring stiffness, which indicates that the changes in leg stiffness may be an important adaptation resulting from eccentric exercise.     

Cellular adaptation to repeated eccentric exercise-induced muscle damage.

Eccentrically biased exercise results in skeletal muscle damage and stimulates adaptations in muscle, whereby indexes of damage are attenuated when the exercise is repeated. We hypothesized that changes in ultrastructural damage, inflammatory cell infiltration, and markers of proteolysis in skeletal muscle would come about as a result of repeated eccentric exercise and that gender may affect this adaptive response. Untrained male (n = 8) and female (n = 8) subjects performed two bouts (bout 1 and bout 2), separated by 5.5 wk, of 36 repetitions of unilateral, eccentric leg press and 100 repetitions of unilateral, eccentric knee extension exercises (at 120% of their concentric single repetition maximum), the subjects' contralateral nonexercised leg served as a control (rest). Biopsies were taken from the vastus lateralis from each leg 24 h postexercise. After bout 2, the postexercise force deficit and the rise in serum creatine kinase (CK) activity were attenuated. Women had lower serum CK activity compared with men at all times (P < 0.05), but there were no gender differences in the relative magnitude of the force deficit. Muscle Z-disk streaming, quantified by using light microscopy, was elevated vs. rest only after bout 1 (P < 0.05), with no gender difference. Muscle neutrophil counts were significantly greater in women 24 h after bout 2 vs. rest and bout 1 (P < 0.05) but were unchanged in men. Muscle macrophages were elevated in men and women after bout 1 and bout 2 (P < 0.05). Muscle protein content of the regulatory calpain subunit remained unchanged whereas ubiquitin-conjugated protein content was increased after both bouts (P < 0.05), with a greater increase after bout 2. We conclude that adaptations to eccentric exercise are associated with attenuated serum CK activity and, potentially, an increase in the activity of the ubiquitin proteosome proteolytic pathway.     

Antioxidant enzyme activity is up-regulated after unilateral resistance exercise training in older adults

Cellular antioxidant capacity and oxidative stress are postulated to be critical factors in the aging process. The effects of resistance exercise training on the level of skeletal muscle oxidative stress and antioxidant capacity have not previously been examined in older adults. Muscle biopsies from both legs were obtained from the vastus lateralis muscle of 12 men 71 +/- 7 years of age. Subjects then engaged in a progressive resistance exercise-training program with only one leg for 12 weeks. After 12 weeks, the nontraining leg underwent an acute bout of exercise (exercise session identical to that of the trained leg at the same relative intensity) at the same time as the last bout of exercise in the training leg. Muscle biopsies were collected from the vastus lateralis of both legs 48 h after the final exercise bout. Electron transport chain enzyme activity was unaffected by resistance training and acute resistance exercise (p < 0.05). Training resulted in a significant increase in CuZnSOD (pre--7.2 +/- 4.2, post--12.6 +/- 5.6 U.mg protein(-1); p = 0.02) and catalase (pre--8.2 +/- 2.3, post--14.9 +/- 7.6 micromol.min(-1).mg protein(-1); p = 0.02) but not MnSOD activity, whereas acute exercise had no effect on the aforementioned antioxidant enzyme activities. Furthermore, basal muscle total protein carbonyl content did not change as a result of exercise training or acute exercise. In conclusion, unilateral resistance exercise training is effective in enhancing the skeletal muscle cellular antioxidant capacity in older adults. The potential long-term benefits of these adaptations remain to be evaluated.     

Influence of estrogen on markers of muscle tissue damage following eccentric exercise.

This study tested the hypothesis that estrogen levels of women influences the development of a muscle-tissue damage (creatine kinase, CK) marker and delayed onset muscle soreness (DOMS) following eccentric exercise. Seventeen oral contraceptive (OC) users and ten eumenorrheic (EU) subjects completed a 30-min downhill running bout at approximately 60% VO2max. The OC completed the exercise during the mid-luteal phase (day 22.9 +/- 1.5; high estrogen) while the EU did their exercise in the mid-follicular phase (day 9.6 +/- 4.4; low estrogen) of the menstrual cycle, respectively. The CK activity and DOMS were assessed pre-exercise, immediately post-, 24, 48 and 72 h post-exercise. ANOVA results indicated that there was a significant increase in CK activity in response to the downhill run (p < 0.001), and the interaction of group x time was significantly different (p < 0.01). The OC group had lower CK at 72 h post-exercise than did the EU group. Pre-exercise estrogen levels correlated with the overall mean CK (r = -0.43, p < 0.05) and 72 h (r = -0.38, p < 0.05) responses, respectively. Exercise caused an increase in DOMS in both groups (p < 0.001); but, no significant interaction was observed. These findings suggest that elevated estrogen levels have a protective effect on muscle tissue following eccentric exercise. The mechanism of this protective effect is unclear but may be related to the anti-oxidant characteristics and membrane stability properties associated with estrogen and its derivatives.     

Serum creatine kinase activity following repeated bouts of isometric exercise with different muscle groups

The purpose of this study was to examine the relationship between the muscle mass involved in exercise and post-exercise serum creatine kinase (CK) elevation. Twelve untrained college-aged men completed three isometric exercises: one arm flexion (OAF), two arm flexion (TAF) and one leg knee extension (OLE). These exercises were balanced over subjects and days and separated by two week intervals. Each exercise consisted of 40 maximal isometric concentrations lasting for 10 s with a 20 s rest between contractions. Relative increases in serum CK for OAF, TAF, and OLE were 181 +/- 70% (SD), 222 +/- 69% and 297 +/- 67%, respectively. An ANOVA using a latin square design for analysis of carry over effects showed that these CK increases were not significantly different (p greater than 0.05). However, the increase in serum CK following the first exercise (379 +/- 90%), regardless of what it was (OAF, TAF, or OLE), was significantly greater (p less than 0.05) than those following bouts 2 and 3 (155 +/- 29%; 167 +/- 54%). Regression analysis indicated that post-exercise serum CK elevation was not related to the amount of muscle mass involved in the exercise (r = 0.30, p greater than 0.05) nor to muscle tension developed (r = 0.28, p greater than 0.05). We conclude that post-exercise serum CK elevation is not necessarily related to the muscle mass involved in the exercise. Because each exercise involved the use of different muscle groups, factors outside the exercising muscle may contribute to post-exercise serum enzyme activity.     

Indices of free-radical-mediated damage following maximum voluntary eccentric and concentric muscular work

This study monitored plasma and skeletal muscle markers of free-radical-mediated damage following maximum eccentric and concentric exercise, to examine the potential role of free radicals in exercise-induced muscle damage. Fourteen male volunteers performed either (1) a bout of 70 maximum eccentric and a bout of 70 maximum concentric muscle actions of the forearm flexors (the bouts being separated by 4 weeks; n = 8) or (2) a bout of 80 maximum eccentric and a bout of 80 maximum concentric muscle actions of the knee extensors (the bouts being separated by 1 week; n=6). Plasma markers of lipid peroxidation, thiobarbituric acid-reactive substances (TBARS) and diene-conjugated compounds (DCC) were monitored in the arm protocol and skeletal muscle markers of oxidative lipid and protein damage, malondialdehyde (MDA) and protein carbonyl derivatives (PCD) respectively, were monitored in the leg protocol. In both protocols, the contralateral limb was used for the second bout and the order of the bouts was randomised between limbs. Repeated measures ANOVA indicated significant changes from baseline following eccentric arm work on the measures of serum creatine kinase activity (P < 0.05), maximum voluntary torque production (P < 0.01) and relaxed arm angle (P < 0.01). Subjective muscle soreness peaked 2 days after eccentric arm work (P < 0.05, Wilcoxon test). However, there were no changes in the plasma levels of TBARS or DCC following the eccentric or concentric arm exercise. Immediately after concentric leg exercise, skeletal muscle PCD concentrations was significantly higher than that observed immediately after eccentric work (P < 0.05). However, no significant difference between the eccentric and concentric knee extensor bouts was observed on the measure of skeletal muscle MDA concentration. The results of this study offer no support for the involvement of oxygen free radicals in exercise-induced muscle damage.     

Nbr1-regulated autophagy in Lactoferrin-induced osteoblastic differentiation

ABSTRACT

The molecular mechanism of autophagy in Lactoferrin (LF) induced osteoblast differentiation is not fully demonstrated. In this study, alkaline phosphatase (ALP) activity, alizarin red S staining and ELISA were used to study N-terminal propeptide of type I procollagen (PINP) expression. mRFP-GFP-LC3 adenoviruses, mono-dansylcadaverine (MDC) staining, scanning electron microscopy, and western blot analysis was employed to probe the LF induced autophagy. The interaction between autophagy receptor Neighbor of Brca1 gene (Nbr1) and pp38 was studied. 3-methyladenine (3-MA) and chloroquine (CQ) could inhibit the activity of ALP, PINP and the autophagy in LF group. LF treatment could up-regulate and down-regulate the expressions of pp38 and Nbr1with a dose-dependent manner, respectively. LF could inhibit the recognition of pp38 and Nbr1. In addition, LF can prompt Nbr1-medicated autophagy and prevent pp38 degradation by autophagy. LF can induce Nbr1-mediated autophagy and inhibit pp38 entering into autophagy flux in the physiological process of osteoblast differentiation.

Abbreviations: CQ:chloroquine;LF: Lactoferrin; 3-MA: 3-methyladenine; ALP: Alkaline phosphatase; ANOVA: Analysis of variance; CCK-8: Cell Counting Kit-8; LC3: Microtubule-associated protein light chain3; MDC: Monodansylcadaverine; Nbr1: neighbor of Brca1 gene; PINP: N-terminal propeptide of type I procollagen; PVDF: Polychlorotrifluoroethylene; pp38: phosphorylation p38; RAPA: Rapamycin; SDS: sodium dodecyl sulfate.     

L-arginine Supplementation Protects Exercise Performance and Structural Integrity of Muscle Fibers after a Single Bout of Eccentric Exercise in Rats

Eccentric exercise is known to disrupt sarcolemmal integrity and induce damage of skeletal muscle fibers. We hypothesized that L-arginine (L-Arg; nitric oxide synthase (NOS) substrate) supplementation prior to a single bout of eccentric exercise would diminish exercise-induced damage. In addition, we used N-nitro-L-arginine methyl ester hydrochloride (L-NAME; NOS inhibitor) to clarify the role of native NOS activity in the development of exercise-induced muscle damage. Rats were divided into four groups: non-treated control (C), downhill running with (RA) or without (R) L-Arg supplementation and downhill running with L-NAME supplementation (RN). Twenty four hours following eccentric exercise seven rats in each group were sacrificed and soleus muscles were dissected and frozen for further analysis. The remaining seven rats in each group were subjected to the exercise performance test. Our experiments showed that L-Arg supplementation prior to a single bout of eccentric exercise improved subsequent exercise performance capacity tests in RA rats when compared with R, RN and C rats by 37%, 27% and 13%, respectively. This outcome is mediated by L-Arg protection against post-exercise damage of sarcolemma (2.26- and 0.87-fold less than R and RN groups, respectively), reduced numbers of damaged muscle fibers indicated by the reduced loss of desmin content in the muscle (15% and 25% less than R and RN groups, respectively), and diminished µ-calpain mRNA up-regulation (42% and 30% less than R and RN groups, respectively). In conclusion, our study indicates that L-Arg supplementation prior to a single bout of eccentric exercise alleviates muscle fiber damage and preserves exercise performance capacity.     

Sex-specific influence of aging on exercising leg blood flow.

Our previous work suggests that healthy human aging is associated with sex-specific differences in leg vascular responses during large muscle mass exercise (2-legged cycling) (Proctor DN, Parker BA. Microcirculation 13: 315-327, 2006). The present study determined whether age x sex interactions in exercising leg hemodynamics persist during small muscle mass exercise that is not limited by cardiac output. Thirty-one young (20-30 yr; 15 men/16 women) and 31 older (60-79 yr; 13 men/18 women) healthy, normally active adults performed graded single-leg knee extensions to maximal exertion. Femoral artery blood velocity and diameter (Doppler ultrasound), heart rate (ECG), and beat-to-beat arterial blood pressure (mean arterial pressure, radial artery tonometry) were measured during each 3-min work rate (4.8 and 8 W/stage for women and men, respectively). The results (means +/- SE) were as follows. Despite reduced resting leg blood flow and vascular conductance, older men exhibited relatively preserved exercising leg hemodynamic responses. Older women, by contrast, exhibited attenuated hyperemic (young: 52 +/- 3 ml.min(-1).W(-1); vs. older: 40 +/- 4 ml.min(-1).W(-1); P = 0.02) and vasodilatory responses (young: 0.56 +/- 0.06 ml.min(-1).mmHg(-1).W(-1) vs. older: 0.37 +/- 0.04 ml.min(-1).mmHg(-1) W(-1); P < 0.01) to exercise compared with young women. Relative (percentage of maximal) work rate comparisons of all groups combined also revealed attenuated vasodilator responses in older women (P < 0.01 for age x sex x work rate interaction). These sex-specific age differences were not abolished by consideration of hemoglobin, quadriceps muscle, muscle recruitment, and mechanical influences on muscle perfusion. Collectively, these findings suggest that local factors contribute to the sex-specific effects of aging on exercising leg hemodynamics in healthy adults.     

Eccentric strain at long muscle length evokes the repeated bout effect

The repeated bout effect (RBE) is a phenomenon characterized by less delayed onset muscle soreness (DOMS) and torque deficit after the second of 2 separate eccentric exercise bouts. Previous investigators have reported that shifting of optimum angle after an initial bout of eccentric exercise mediates the RBE. We hypothesized that an RBE for elbow extensor exercise occurs after an initial bout performed at long (starting position of 50 degrees to an end position of 130 degrees) but not short (starting position of 0 degrees to an end position of 80 degrees) muscle length because strain at long length evokes a shifting of the optimum angle to a longer length. Untrained women performed an initial bout at either long or short length (n = 9 per group) followed 1 week later by a repeated bout (RB) through the full ROM (0-130 degrees). Extensor torque and optimum angle was evaluated before, immediately after, and 2 days after each bout. A mechanical transducer depressed on the triceps brachii quantified DOMS. Torque deficits were 3% and 7% after exercise at short vs. long length, respectively. Two days after the RB, torque deficit was 8% and 1% for those previously exercising at short vs. long length (group x bout, p < 0.05). Greater DOMS (N) was observed after exercise at long (16 +/- 3) vs. short (23 +/- 2) length; whereas greater DOMS occurred for the short-length (17 +/- 2) vs. long (26 +/- 3) group after the RB (group x bout, p < 0.05). Optimum angle shifted to a longer length after exercise at long (+10 +/- 4 degrees) vs. short (+1 +/- 3 degrees) length (group x bout, p < 0.05). After the RB, those exercising previously at short length experienced a shift of +15 +/- 4 degrees (main effect, p < 0.05). The findings of this study indicate that the repetitive strain at long but not short muscle length evokes both immediate and sustained shifts in optimum angle to longer lengths, and that this shifting mediates (r(2) = 0.71) the RBE.     

No differential effects of divergent isocaloric supplements on signaling for muscle protein turnover during recovery from muscle-damaging eccentric exercise

Unaccustomed high-intensity eccentric exercise (ECC) can provoke muscle damage including several days of muscle force loss. Post-exercise dietary supplementation may provide a strategy to accelerate rate of force regain by affecting mechanisms related to muscle protein turnover. The aim of the current study was to investigate if protein signaling mechanisms involved in muscle protein turnover would be differentially affected by supplementation with either whey protein hydrolysate and carbohydrate (WPH+CHO) versus isocaloric carbohydrate (CHO) after muscle-damaging ECC. Twenty-four young healthy participants received either WPH+CHO (n = 12) or CHO supplements (n = 12) during post-exercise recovery from 150 maximal unilateral eccentric contractions. Prior to, at 3 h and at 24, 48, 96 and/or 168 h post-exercise, muscle strength, muscle soreness, and Akt-mTOR and FOXO signaling proteins, were measured in an ECC exercising leg and in the contralateral non-exercise control leg (CON). After ECC, muscle force decreased by 23–27 % at 24 h post-exercise, which was followed by gradual, although not full recovery at 168 h post-exercise, with no differences between supplement groups. Phosphorylation of mTOR, p70S6K and rpS6 increased and phosphorylation of FOXO1 and FOXO3 decreased in the ECC leg, with no differences between supplement groups. Phosphorylation changes were also observed for rpS6, FOXO1 and FOXO3a in the CON leg, suggesting occurrence of remote tissue effects. In conclusion, divergent dietary supplementation types did not produce differences in signaling for muscle turnover during recovery from muscle-damaging exercise.     

Regional hemodynamics during postexercise hypotension. I. Splanchnic and renal circulations.

Moderate exercise elicits a relative postexercise hypotension that is caused by an increase in systemic vascular conductance. Previous studies have shown that skeletal muscle vascular conductance is increased postexercise. It is unclear whether these hemodynamic changes are limited to skeletal muscle vascular beds. The aim of this study was to determine whether the splanchnic and/or renal vascular beds also contribute to the rise in systemic vascular conductance during postexercise hypotension. A companion study aims to determine whether the cutaneous vascular bed is involved in postexercise hypotension (Wilkins BW, Minson CT, and Halliwill JR. J Appl Physiol 97: 2071-2076, 2004). Heart rate, arterial pressure, cardiac output, leg blood flow, splanchnic blood flow, and renal blood flow were measured in 13 men and 3 women before and through 120 min after a 60-min bout of exercise at 60% of peak oxygen uptake. Vascular conductances of leg, splanchnic, and renal vascular beds were calculated. One hour postexercise, mean arterial pressure was reduced (79.1 +/- 1.7 vs. 83.4 +/- 1.8 mmHg; P < 0.05), systemic vascular conductance was increased by approximately 10%, leg vascular conductance was increased by approximately 65%, whereas splanchnic (16.0 +/- 1.8 vs. 18.5 +/- 2.4 ml.min(-1).mmHg(-1); P = 0.13) and renal (20.4 +/- 3.3 vs. 17.6 +/- 2.6 ml.min(-1).mmHg(-1); P = 0.14) vascular conductances were unchanged compared with preexercise. This suggests there is neither vasoconstriction nor vasodilation in the splanchnic and renal vasculature during postexercise hypotension. Thus the splanchnic and renal vascular beds neither directly contribute to nor attenuate postexercise hypotension.     

Mitochondrial,sarcoplasmic membrane integrity and protein degradation in heart and skeletal muscle in exercised rats

Several different exercise regimens varied in the severity of tissue damage induced. Therefore, this study investigated the effects of a single bout of exercise versus endurance training in heart and skeletal muscles with different predominant fiber types on indices of mitochondrial, endoplasmic reticulum (ER) integrity and protein degradation. Male Wistar rats performed different treadmill exercise protocols: exhaustive, maximal exhaustive, eccentric, training and exhaustive exercise after training. The maximal and eccentric exercises resulted in a significant loss of integrity of the sarcoplasmic and ER muscle, while no changes were observed in cardiac muscle. Mitochondrial membrane fluidity measured by the fluorescence polarization method was significantly increased post-acute exercises in heart and oxidative muscles. Regular exercise can stabilize and preserve the viscoelastic nature of mitochondrial membranes in both tissues. The highest increase in carbonyl content was obtained in heart after exhaustive exercise protocol, from 1+/-0.1 to 3.6+/-0.14 nmol mg protein(-1), such increase were not found after regular exercise with values significantly decreased. Nitrate heart levels showed attenuated generation of nitric oxide after training. Muscle protein oxidation was produced in all exhaustive exercises including eccentric exercise.     

MR measurements of muscle damage and adaptation after eccentric exercise.

The purposes of this study were, first, to clarify the long-term pattern of T2 relaxation times and muscle volume changes in human skeletal muscle after intense eccentric exercise and, second, to determine whether the T2 response exhibits an adaptation to repeated bouts. Six young adult men performed two bouts of eccentric biceps curls (5 sets of 10 at 110% of the 1-repetition concentric maximum) separated by 8 wk. Blood samples, soreness ratings, and T2-weighted axial fast spin-echo magnetic resonance images of the upper arm were obtained immediately before and after each bout; at 1, 2, 4, 7, 14, 21, and 56 days after bout 1; and at 2, 4, 7 and 14 days after bout 2. Resting muscle T2 [27.6 +/- 0.2 (SE) ms] increased immediately postexercise by 8 +/- 1 ms after both bouts. T2 peaked 7 days after bout 1 at 47 +/- 4 ms and remained elevated by 2.5 ms at 56 days. T2 peaked lower (37 +/- 4 ms) and earlier (2-4 days) after bout 2, suggesting an adaptation of the T2 response. Peak serum creatine kinase values, pain ratings, and flexor muscle swelling were also significantly lower after the second bout (P < 0.05). Total volume of the imaged arm region increased transiently after bout 1 but returned to preexercise values within 2 wk. The exercised flexor compartment swelled by over 40%, but after 2 wk it reverted to a volume 10% smaller than that before exercise and maintained this volume loss through 8 wk, consistent with partial or total destruction of a small subpopulation of muscle fibers.     

Short-term immobilization and recovery affect skeletal muscle but not collagen tissue turnover in humans

Not much is known about the effects of immobilization and subsequent recovery on tendon connective tissue. In the present study, healthy young men had their nondominant leg immobilized for a 2-wk period, followed by a recovery period of the same length. Immobilization resulted in a mean decrease of 6% (5,413 to 5,077 mm(2)) in cross-sectional area (CSA) of the triceps surae muscles and a mean decrease of 9% (261 to 238 N.m) in strength of the immobilized calf muscles. Two weeks of recovery resulted in a 6% increased in CSA (to 5,367 mm(2)), whereas strength remained suppressed (240 N.m). No difference in Achilles tendon CSA was detected between the two legs at any time point. Local tendon collagen synthesis, measured as the peritendinous concentrations of PINP (NH(2)-terminal propeptide of type I collagen; indirect marker for collagen synthesis), was unchanged after 2 wk of immobilization. However, peritendinous levels of PINP were significantly elevated in the immobilized leg (15 to 139 ng/ml) following 2 wk of remobilization compared with preimmobilization levels. In contradiction hereto, systemic concentrations of PINP remained unchanged throughout the study. Immobilization reduced muscle size and strength, while tendon size and collagen turnover were unchanged. While recovery resulted in an increase in muscle size, strength was unchanged. No significant difference in tendon size could be detected between the two legs after 2 wk of recovery, although collagen synthesis was increased in the previously immobilized leg. Thus 2 wk of immobilization are sufficient to induce significant changes in muscle tissue, whereas tendon tissue seems to be more resistant to short-term immobilization.     

Do P2X purinergic receptors regulate skeletal muscle blood flow during exercise?

Although there is evidence that sympathetic nerves release ATP as a neurotransmitter to produce vasoconstriction via P2X purinergic receptors, the role of these receptors in the regulation of blood flow to exercising skeletal muscle has yet to be determined. We hypothesized that there is tonic P2X receptor-mediated vasoconstriction in exercising skeletal muscle. To test this hypothesis, the effect of P2X receptor blockade on skeletal muscle blood flow was examined in six exercising mongrel dogs. P2X receptor antagonism was accomplished with pyridoxal-phosphate-6-azophenyl-2'4'-disulfonic acid (PPADs). Animals were instrumented chronically with flow probes on the external iliac arteries of both hindlimbs and a catheter in one femoral artery. PPADs (40 mg) was infused as a bolus into the femoral artery catheter during steady-state exercise at 6 miles/h. Intra-arterial infusion of PPADs increased iliac blood flow from 542 +/- 55 to 677 +/- 69 ml/min (P < 0.05) and iliac vascular conductance from 5.17 +/- 0.62 to 6.53 +/- 0.80 ml.min(-1).mmHg(-1). The PPADs infusion did not affect blood flow in the contralateral iliac artery. These data support the hypothesis that P2X purinergic receptors produce vasoconstriction in exercising skeletal muscle.     

Mechanical Load Enhances Procollagen Processing in Dermal Fibroblasts by Regulating Levels of Procollagen C-Proteinase

Mechanical forces are emerging as key regulators of cell function. We hypothesize that mechanical load may influence dermal fibroblast activity. We assessed the direct effects of mechanical load on human dermal fibroblast procollagen synthesis and processing in vitro. Cells were loaded in a biaxial loading system (Flexercell 3000). Hydroxyproline levels were measured in the medium and cell layer as an estimate of procollagen synthesis and processing to insoluble collagen. Mechanical load (in the presence of serum or TGF-beta) enhanced procollagen synthesis by 45 +/- 3% (P < 0.001), and 38 +/- 4% (P < 0.001), respectively, over unloaded growth factor controls after 48 h. Insoluble collagen deposition was enhanced in the same cultures by 115 +/- 8% (P < 0.01) and 72% +/- 9% (P < 0.01), respectively. This effect was inhibited using l-arginine suggesting that procollagen C-proteinase, the enzyme which directly cleaves the C-terminal propeptide of procollagen to form insoluble collagen, is required for the fiber formation observed. Procollagen mRNA levels in loaded samples increased by more than two-fold in both serum and TGF-beta-treated cultures at 48 h. Procollagen C-proteinase mRNA levels were also enhanced by a similar magnitude, although the increase was observed at 24 h. Procollagen C-proteinase protein levels were also increased at this time. Protein and mRNA levels of the procollagen C-proteinase enhancer protein, which binds the C-terminal propeptide of procollagen to enhance the rate of peptide cleavage, were unaffected by mechanical load. This study demonstrates that mechanical load promotes procollagen synthesis in dermal fibroblasts by enhancing gene expression and posttranslational processing of procollagen.