Cellular adaptation to repeated eccentric exercise-induced muscle damage

Response of the ubiquitin-proteasome pathway to changes in muscle activity

The ubiquitin-proteasome pathway plays a critical role in the adaptation of skeletal muscle to persistent decreases or increases in muscle activity. This article outlines the basics of pathway function and reviews what we know about pathway responses to altered muscle use. The ubiquitin-proteasome pathway regulates proteolysis in mammalian cells by attaching ubiquitin polymers to damaged proteins; this targets the protein for degradation via the 26S proteasome. The pathway is constitutively active in muscle and continually regulates protein turnover. Conditions of decreased muscle use, e.g., unloading, denervation, or immobilization, stimulate general pathway activity. This activity increase is caused by upregulation of regulatory components in the pathway and leads to accelerated proteolysis, resulting in net loss of muscle protein. Pathway activity is also increased in response to exercise, a two-phase response. An immediate increase in selective ubiquitin conjugation by constitutive pathway components contributes to exercise-stimulated signal transduction. Over hours-to-days, exercise also stimulates a delayed increase in general ubiquitin conjugating activity by inducing expression of key components in the pathway. This increase mediates a late-phase rise in protein degradation that is required for muscle adaptation to exercise. Thus the ubiquitin-proteasome pathway functions as an essential mediator of muscle remodeling, both in atrophic states and exercise training.

Short-term high- vs. low-velocity isokinetic lengthening training results in greater hypertrophy of the elbow flexors in young men

We performed two studies to determine the effect of a resistive training program comprised of fast vs. slow isokinetic lengthening contractions on muscle fiber hypertrophy. In study I, we investigated the effect of fast (3.66 rad/s; Fast) or slow (0.35 rad/s; Slow) isokinetic high-resistance muscle lengthening contractions on muscle fiber and whole muscle cross-sectional area (CSA) of the elbow flexors was investigated in young men. Twelve subjects (23.8 +/- 2.4 yr; means +/- SD) performed maximal resistive lengthening isokinetic exercise with both arms for 8 wk (3 days/wk), during which they trained one arm at a Fast velocity while the contralateral arm performed an equivalent number of contractions at a Slow velocity. Before (Pre) and after (Post) the training, percutaneous muscle biopsies were taken from the midbelly of the biceps brachii and analyzed for fiber type and CSA. Type I muscle fiber size increased Pre to Post (P < 0.05) in both Fast and Slow arms. Type IIa and IIx muscle fiber CSA increased in both arms, but the increases were greater in the Fast- vs. the Slow-trained arm (P < 0.05). Elbow flexor CSA increased in Fast and Slow arms, with the increase in the Fast arm showing a trend toward being greater (P = 0.06). Maximum torque-generating capacity also increased to a greater degree (P < 0.05) in the Fast arm, regardless of testing velocity. In study II, we attempted to provide some explanation of the greater hypertrophy observed in study I by examining an indicator of protein remodeling (Z-line streaming), which we hypothesized would be greater in the Fast condition. Nine men (21.7 +/- 2.4 yr) performed an acute bout (n = 30, 3 sets x 10 repetitions/set) of maximal lengthening contractions at Fast and Slow velocities used in the training study. Biopsies revealed that Fast lengthening contractions resulted in more (185 +/- 1 7%; P < 0.01) Z-band streaming per millimeter squared muscle vs. the Slow arm. In conclusion, training using Fast (3.66 rad/s) lengthening contractions leads to greater hypertrophy and strength gains than Slow (0.35 rad/s) lengthening contractions. The greater hypertrophy seen in the Fast-trained arm (study I) may be related to a greater amount of protein remodeling (Z-band streaming; study II).

Senescence of human skeletal muscle impairs the local inflammatory cytokine response to acute eccentric exercise

The impact of aging on the cytokine response of human skeletal muscle to exercise-induced injury remains poorly understood. We enrolled physically active, young (23-35 years old, n=15) and old (66-78 years old, n=15) men to perform 45 min of downhill running (16% descent) at 75% VO2max. Biopsies of vastus lateralis were obtained 24 h before and 72 h after acute eccentric exercise. Transcripts for inflammatory (TNF-alpha, IL-1beta) and anti-inflammatory cytokines (IL-6, TGF-beta1) were quantified by real-time PCR. Before exercise, cytokine transcripts did not differ with age. At old age, exercise induced a blunted accumulation of transcripts encoding the pan-leukocyte surface marker CD18 (young: 10.1-fold increase, P<0.005; old: 4.7-fold increase, P=0.02; young vs. old: P<0.05). In both age groups, CD18 transcript accumulation strongly correlated with TNF-alpha (young, r=0.87, P<0.001; old, r=0.72, P=0.002) and TGF-beta1 transcript accumulation (young, r=0.80, P<0.001; old, r=0.64, P=0.008). At old age, there was no correlation between IL-1beta and CD18 transcript accumulation. Furthermore, exercise induced IL-6 transcript accumulation in young (3.6-fold, P=0.057) but not in old men. Our results suggest that aging impairs the adaptive response of human skeletal muscle to eccentric exercise by differential modulation of a discrete set of inflammatory and anti-inflammatory cytokine genes.

Time course of capillary structure changes in rat skeletal muscle following strenuous eccentric exercise

AIM

We examined the time course of capillary structure changes in rat skeletal muscle at 1, 3 and 7 days after strenuous eccentric exercise.

METHODS

The right gastrocnemius muscles of anaesthetized male Wistar rats were subjected to 300 controlled eccentric contractions using electrical stimulation. The contralateral gastrocnemius muscle was used as control. All morphometric parameters were determined in in situ perfused gastrocnemius muscles in red (Gr, predominantly slow-twitch fibre) and white (Gw, predominantly fast-twitch fibre) portions.

RESULTS

Muscle fibre damage was evident on days 1, 3 and 7 in Gr (29.3-53.9% damaged fibres) and Gw (58.9-86.8% damaged fibres) of exercised legs. Electron micrographs of transverse sections did not display collapsed or obstructed capillaries in exercised legs, and capillary endothelial cells retained their normal structures. However, capillary luminal shapes and area were altered in exercised legs on days 1 and 3. The ratio between minimal and maximal capillary diameter in a transverse section (i.e. luminal ellipticity) significantly differed when comparing control (Gr, 0.75 +/- 0.02; Gw, 0.79 +/- 0.03) and exercised legs (Gr, 0.65 +/- 0.03; Gw, 0.66 +/- 0.04) at 1 day after exercise. The mean capillary luminal area was significantly increased in exercised legs after 1 day (Gw, +24.3%) and 3 days (Gr, +31.9%; Gw, +62.2%) compared with control.

CONCLUSION

We conclude that (1) capillary endothelial cell structure was maintained in damaged muscles, (2) changes in capillary lumen shapes and distensibility occur in the degenerated muscle up to 3 days after the eccentric contraction period.

Excitation-induced Ca2+ influx and muscle damage in the rat: loss of membrane integrity and impaired force recovery

Prolonged or unaccustomed exercise leads to loss of contractility and muscle cell damage. The possible role of an increased uptake of Ca(2+) in this was explored by examining how graded fatiguing stimulation, leading to a graded uptake of Ca(2+), results in progressive loss of force, impairment of force recovery, and loss of cellular integrity. The latter is indicated by increased [(14)C]sucrose space and lactic acid dehydrogenase (LDH) release. Isolated rat extensor digitorum longus (EDL) muscles were allowed to contract isometrically using a fatiguing protocol with intermittent stimulation at 40 Hz. Force declined rapidly, reaching 11% of the initial level after 10 min and stayed low for up to 60 min. During the initial phase (2 min) of stimulation (45)Ca uptake showed a 10-fold increase, followed by a 4- to 5-fold increase during the remaining period of stimulation. As the duration of stimulation increased, the muscles subsequently regained gradually less of their initial force. Following 30 or 60 min of stimulation, resting (45)Ca uptake, [(14)C]sucrose space, and LDH release were increased 4- to 7-fold, 1.4- to 1.7-fold and 3- to 9-fold, respectively (P < 0.001). The contents of Ca(2+) and Na(+) were also increased (P < 0.01), a further indication of loss of cellular integrity. When fatigued at low [Ca(2+)](o) (0.65 mm), force recovery was on average twofold higher than that of muscles fatigued at high [Ca(2+)](o) (2.54 mm). Muscles showing the best force recovery also had a 41% lower total cellular Ca(2+) content (P < 0.01). In conclusion, fatiguing stimulation leads to a progressive functional impairment and loss of plasma membrane integrity which seem to be related to an excitation-induced uptake of Ca(2+). Mechanical strain on the muscle fibres does not seem a likely mechanism since very little force was developed beyond 10 min of stimulation.

Evidence for myofibril remodeling as opposed to myofibril damage in human muscles with DOMS: an ultrastructural and immunoelectron microscopic study

The myofibrillar and cytoskeletal alterations observed in delayed onset muscle soreness (DOMS) caused by eccentric exercise are generally considered to represent damage. By contrast our recent immunohistochemical studies suggested that the alterations reflect myofibrillar remodeling (Yu and Thornell 2002; Yu et al. 2003). In the present study the same human muscle biopsies were further analyzed with transmission electron microscopy and immunoelectron microscopy. We show that the ultrastructural hallmarks of DOMS, Z-disc streaming, Z-disc smearing, and Z-disc disruption were present in the biopsies and were significantly more frequent in biopsies taken 2-3 days and 7-8 days after exercise than in those from controls and 1 h after exercise. Four main types of changes were observed: amorphous widened Z-discs, amorphous sarcomeres, double Z-discs, and supernumerary sarcomeres. We confirm by immunoelectron microscopy that the main Z-disc protein alpha-actinin is not present in Z-disc alterations or in the links of electron-dense material between Z-discs in longitudinal register. These alterations were related to an increase of F-actin and desmin, where F-actin was present within the strands of amorphous material. Desmin, on the other hand, was seen in less dense regions of the alterations. Our results strongly support that the myofibrillar and cytoskeletal alterations, considered to be the hallmarks of DOMS, reflect an adaptive remodeling of the myofibrils.

Ibuprofen and acetaminophen: effect on muscle inflammation after eccentric exercise

PURPOSE

We examined the influence of ibuprofen and acetaminophen on muscle neutrophil and macrophage concentrations after novel eccentric contractions.

METHODS

Twenty-four males (25 +/- 3 yr) were divided into three groups that received the maximal over-the-counter dose of either ibuprofen (1200 mg x d-1), acetaminophen (4000 mg x d-1), or a placebo after eccentric contractions of the knee extensors. Biopsies from the vastus lateralis were taken before and 24 h after exercise. Inflammatory cells were quantified in muscle cross-sections using immunohistochemistry.

RESULTS

Macrophage concentrations were elevated by 1.5- to 2.5-fold (P < 0.05) at 24 h postexercise relative to preexercise concentrations, whereas neutrophil concentrations were not significantly elevated. Muscle inflammatory cell concentrations were unaffected by treatment with ibuprofen or acetaminophen when compared with placebo.

CONCLUSIONS

Maximal over-the-counter doses of ibuprofen or acetaminophen, when administered therapeutically, do not affect muscle concentrations of neutrophils or macrophages 24 h after a novel bout of eccentric contractions.

Contraction-induced muscle damage in humans following calcium channel blocker administration

Following contraction-induced damage of skeletal muscle there is a loss of calcium homeostasis. Attenuating the damage-induced rise in myocellular calcium concentration may reduce proteolytic activation and attenuate other indices of damage; calcium channel blockers have been shown to be effective in this regard. The effect of administration of a calcium channel blocker (CCB), amlodipine, on indices of muscle damage following a unilateral 'damage protocol', during which subjects performed 300 maximal isokinetic (0.52 rad s(-1)) eccentric contractions with the knee extensors was investigated. The design was a randomized, double-blind crossover. On one occasion, prior to the damage protocol, subjects consumed CCB for 7 days prior to and for 7 days following the damage protocol. Biopsies were taken from the vastus lateralis prior to (baseline) and following the damage protocol at 4 h and 24 h post-damage. Isometric peak knee extensor torque was reduced (P < 0.05) immediately post-, 24 h post- and 48 h post-damage protocol compared to pre-exercise values with no effect of treatment. Desmin disruption was attenuated (P < 0.05) with CCB versus placebo at 4 h post-damage. Z-band streaming was significantly (P < 0.05) elevated compared to baseline at both times post-damage, but was lower with CCB at 4 h (P < 0.05). Damage resulted in increased inflammatory cell (macrophage) infiltration into skeletal muscle at both 4 h and 24 h post-damage, with no effect of CCB. Neutrophil number was elevated by the damage protocol, but was higher at 24 h post-damage in the CCB condition (P < 0.05). Creatine kinase (CK) activity was higher (P < 0.05) at 24 h and 48 h following the damage protocol compared to baseline, with no effect of treatment. In conclusion, the reduction in desmin disruption and Z-band streaming indicates that CCB attenuated, or delayed, the contraction-induced damage to sarcomeric proteins.

Variability in estimating eccentric contraction-induced muscle damage and inflammation in humans

We studied five young healthy volunteers who performed a "damage protocol" consisting of 240 (24 sets x 10 repetitions/set) maximal isokinetic eccentric muscle contractions (30 degrees/s) on each leg one week apart. Biopsies were taken from the vastus lateralis on two occasions. Two biopsies were taken from within the same muscle 24h following the damage protocol. On a second occasion a single biopsy was taken from the contralateral leg at 24h following the same damage protocol. Biopsies at all three sites showed Z-band disruption, much greater (i.e., approximately 14-fold) than is typically observed in resting biopsies, with no significant differences (ANOVA) according to site location (within legs or between legs). The within-leg coefficient of variation (CV) was, however, 41 +/- 30%, and the between-leg CVs were 57 +/- 36% and 68 +/- 36%. Macrophage cells were also detected within the muscle, and cell numbers were not statistically different between biopsy sites. However, the within-biopsy CV = 52 +/- 19% and the between-biopsy CVs of 34 +/- 24% and 48 +/- 27%. We conclude that eccentric contraction-induced Z-band streaming and inflammatory cell response, as detected in muscle biopsy samples from humans, is highly variable with a CV of 40-70%.

Interactions between muscle and the immune system during modified musculoskeletal loading

Interactions between the immune system and skeletal muscle may play a significant role in modulating the course of muscle injury and repair after modified musculoskeletal loading. Current evidence indicates that activation of the complement system is an early event during modified loading, which then leads to inflammatory cell invasion. However, the functions of those inflammatory cells are complex and they seem to be capable of promoting additional injury and repair. Recent findings implicate an early invading neutrophil population in increasing muscle damage that is detected by the presence of muscle membrane lesions. Macrophages that invade subsequently serve to remove cellular debris, and seem to promote repair. However, macrophages also have the ability to increase damage in muscle in which there is an impaired capacity to generate nitric oxide. In vivo and in vitro evidence indicates that muscle-derived nitric oxide can serve an important role in protecting muscle from membrane damage by invading inflammatory cells. Collectively, these findings indicate that the dynamic balance between inflammatory cells, the complement system, and muscle-derived free radicals can play important roles in the secondary damage of muscle during modified musculoskeletal loading.

IGF-I has no effect on postexercise suppression of the ubiquitin-proteasome system in rat skeletal muscle

Both exercise and insulin-like growth factor I (IGF-I) are known to have major hypertrophic effects in skeletal muscle; however, the interactive effect of exogenous IGF-I and exercise on muscle protein turnover or the ubiquitin-proteasome pathway has not been reported. In the present study, we have examined the interaction between endurance exercise training and IGF-I treatment on muscle protein turnover and the ubiquitin-proteasome pathway in the postexercise period. Adult male rats (270-280 g) were randomized to receive 5 consecutive days of progressive treadmill exercise and/or IGF-I treatment (1 mg. kg body wt(-1). day(-1)). Twenty-four hours after the last bout of exercise, the rate of protein breakdown in incubated muscles was significantly reduced compared with that in unexercised rats. This was associated with a significant reduction in the chymotrypsin-like activity of the proteasome and the rate of ubiquitin-proteasome-dependent casein hydrolysis in muscle extracts from exercised compared with unexercised rats. In contrast, the muscle expression of the 20S proteasome subunit beta-1, ubiquitin, and the 14-kDa E2 ubiquitin-conjugating enzyme was not altered by exercise or IGF-I treatment 24 h postexercise. Exercise had no effect on the rates of total mixed muscle protein synthesis in incubated muscles 24 h postexercise. IGF-I treatment had no effect on muscle weights or the rates of protein turnover 24 h after endurance exercise. These results suggest that a suppression of the ubiquitin-proteasome proteolytic pathway after endurance exercise may contribute to the acute postexercise net protein gain.

Lengthening contraction-induced inflammation is linked to secondary damage but devoid of neutrophil invasion

Inflammation triggered by exercise-induced muscle damage (EIMD) has been postulated to influence the extent of tissue destruction. We tested the hypotheses that 1) repressing inflammation decreases secondary damage production and 2) EIMD leads to a sequential appearance of inflammatory cells in which neutrophil accumulation precedes macrophage invasion. Rat ankle dorsiflexor muscles were submitted to in situ lengthening contractions. Measurement of in vitro contractile properties, inflammatory cell concentrations, and histological staining were performed postprotocol. Rats were treated with diclofenac, a nonsteroidal anti-inflammatory drug (NSAID group) to repress inflammation or with the vehicle solution (EIMD group). Muscles from the NSAID group had smaller force deficits on days 2 and 3 postexercise. This effect was associated with significantly smaller increases in the concentration of muscle macrophage ED1+ and ED2+. Surprisingly, neutrophils did not accumulate post-EIMD. These results suggest that inflammation-induced ED1+ macrophage accumulation is responsible for the secondary damage observed 2-3 days post-EIMD. We further conclude that an increase in ED1+ macrophage concentration can occur in absence of previous neutrophil invasion.