Protein metabolism in rat tibialis anterior muscle after stimulated chronic eccentric exercise THEODORE

S. WONG

AND

FRANK

W. BOOTH

Department of Physiokgy and Cell Biology, University Medical School at Houston, Houston, Texas 77225

WONG, THEODORE

S., AND

W.

of I’exus

resistance-training paradigm on the concentrically contracted GAST. Among the results of that investigation eccentric exercise.J. Appl. Physiol. 69(5): 1718-1724, 1990.was the finding that the chronic muscle-training program In another study (J. Appt. Yhysiol. 69: 1709-1717, 1990) we failed to produce hypertrophy of the GAST muscle detabolism

in rat tibialis

FRANK

anterior

BOOTH.

Protein

muscle after stimulated

me-

chronic

reported that gastrocnemius(GAST) muscleenlargementfailed to occur after 10 wk of 192 contractions performed every 3rd or 4th day. This result was surprisingbecauseincreasedprotein synthesis rates were determined after an initial acute exercise bout with the same paradigms. In the same set of animals, tibialis anterior (TA) muscleswere enlarged 16-30% compared with sedentary control musclesafter the samechronic training regimen. This indicated that the regulation of protein expression may be different between the GAST and TA muscles.The present experiment attempted to define and explain these differencesby comparing changesin various indexes of protein metabolismin TA with the sameparametersdetermined in the accompanying study for the GAST. As in the GAST, results showedthat TA protein synthesis rates are increasedby acute exerciseand principally regulated by translational and possibly posttranslational mechanisms. The differential response in musclemassbetweenthe GAST and TA musclesafter training may be due, in part, to greater relative resistancesimposedon the TA than on the GAST that result in a more-prolonged effect on protein synthesis rates, with lower numbers of stimulated contractions required to stimulate increasesin protein synthesis.Data also revealedthat although as little as 1 min of total contractile duration (24 repetitions) increasedTA protein synthesisrate by 30%, $ min of total contractile duration (192 repetitions) further increased TA protein synthesis rates to only 45% above control.

spite the activation of protein synthesis mechanisms during acute exercise (increased protein synthesis rates) and chronic training (total RNA accumulation). In this paper, we report the finding that significant gains in muscle mass were induced in the eccentrically trained TA of the same animals and examine possible reasons for this occurrence. These rats underwent 192 contractions/bout with 2 or 3 days of rest, between bouts during the IO-wk (20-bout) chronic training period. Our initial hypothesis was that muscle protein regulatory mechanisms would respond differently between the concentric resistance-exercised GAST and the eccentric resistanceexercised TA muscles. Therefore, we examined some of

the factors that may contribute

to these differences by

determining protein synthesis rates and mRNA levels in TA muscles of rats employed in the companion study. MATERIALS AND METHODS Animal Care

The same set of rats was used as described in the other study on GAST (6). Procedures were approved by the Animal Care and Use Committee of the University of Texas Medical School at Houston.

skeletal muscle; enlargement,;anabolism; catabolism; muscle Electrically Stimulated Muscle Contraction contraction; cytochrome c; actin; muscleinjury Animals were exercised by use of a model of nonvolunINITIAL EXPERIMENTS employing a model of nonvoluntary resistance exercise to produce enlargement of rat gastrocnemius (GAST) muscles with concentric contrac-

tions have shown that hypertrophy is also produced in the tibialis anterior (TA) muscle after training (5). During that study, rats performed 24 electrically induced plantar flexions within a single acute exercise bout that was repeated every 4th day. It was later deduced that the TA was enlarged as a consequence of its activation by the electrical stimulation causing the muscle to be con-

tracted

(eccentrically)

during

the contraction

(concen-

tric) of the GAST. Although this occurrence was mentioned as a precautionary note for the use of the model, the significance of the phenomenon was unappreciated

at that time.

apparatus with the rat supported on a platform above a foot lever. In some experiments (chronic training), the contralateral muscles were used as a nonexercised internal control. The muscle contraction of the anesthetized rat was induced with a Grass S48 electrical stimulator with l-ms pulses at 100 Hz and 30 V with a 2.5-s train

duration.

Muscle

0161 -Z567/90 $1.50 Copyright

stimulation

results in contraction

of

both posterior and anterior compartment muscles, with a net plantar flexion causing upward excursion of a weight by the pulley system. Electrically Stimulated Muscle Tensions

The combined

In another study (6), we reported the effects of another l-28

tary electrically contracted skeletal muscle (5, 6). Subcutaneous platinum electrodes were positioned bilaterally along the lower leg muscles of an ether-anesthetized rat. The foot of the animal was secured to a pulley-bar

maximal

force output of the GAST(primary plantar flex-

plantaris-soleus muscle complex

li=: 1990 the American

Physiological

Society

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PROTEIN

EXPRESSION

DURING

ECCENTRIC

ors) has previously been determined to be 4,100 g, and the sum of the forces exerted by the tibialis anterior (TA) and extensor digitorum longus muscles (primary dorsiflexors) is -300 g. Because both compartments are stimulated to contract simultaneously in this model, these values represent our closest approximation of the antagonistic tensions imposed on each respective muscle

group (5). Exercise

ProtocoLT: Acute Exercise

Rout

Animals and experimental groups were the same as in the companion study (6). A control group of rats received no electrically stimulated muscle contractions. The remaining three groups performed defined regimens of exercise varying in the number of repetitions (muscle contractions) per bout, the resistance (amount of weight added to the pulley), or both. These groups were designated and exercised as follows. Sedentary control (0 repetitions/O g). Control animals received no exercise but were anesthetized, catheterized, and infused in parallel with exercised animals in acute studies. Moderate frequency and moderate resistance (24 repetitions/500 g). Rats performed 24 total repetitions during the bout, lifting a 500-g weight attached to the pulley during each contraction. The adjusted resistance for the lever arm advantage (1.6) is -300 g. Including the additional resistance (-300 g) imposed by the dorsiflexor muscles, the plantar flexor muscle group lifts >600 g during each contraction while the dorsiflexors are actively stretched due to the 1,100 g of direct resistance from the plantar flexors. Repetitions were done in sets of six with 5-min rest periods between each set and 20-s rests between each 2.5-s muscle contraction. The protocol was completed in -30 min with a total of 1 min of actual stimulated contractile activity. With this protocol, the rats isotonically plantar flex through an ~45” arc of the foot lever from the initial rest position. High frequency and low resistance (192 repetitions/O g). Animals completed 192 repetitions per bout in 32 sets and were allowed 10-s rests after each 2.5-s repetition with 1-min rests between each of the first 16 sets, 30 s between the last 16 sets, and an additional 2.5 min after each 4 sets of the bout. No weights were placed onto the pulley, although muscles were required to overcome the resistance of the dorsiflexors (-300 g). As during moderate-frequency moderate-resistance exercise, the dorsiflexors contract against the resistance of the plantar flexors, which is almost four times the estimated force of the TA and extensor digitorum longus muscles. This regimen required -80 min to complete, of which -8 min was actual stimulated contraction time. Rats performing this protocol completed isotonic contractions of -90” arcs, thus causing the greatest degree of stretch on the dorsiflexors of all protocols examined in the study. High frequency and high resistance (192 repetitions/ 800-1,100 g). A protocol identical to that described above for the high-frequency low-resistance group was performed, except weights were added to the pulley during contractions; 800- to 1,100-g weights were regressively placed on the apparatus such that l,lOO-, l,OOO-, 900-,

RESISTANCE

EXERCISE

1719

and 800-g weights were lifted in succession for one set of six repetitions. This cycle was completed eight times during the bout. We have calculated that the total effective resistance on the plantar flexors is -8004,000 g (6). Rats were capable of moving the lever through an -2O30” arc, which results in a significant but lesser degree of dorsiflexor muscle stretching than the high-frequency low-resistance paradigm. Exercise

Protocols:

Chronic

Training

Additional groups of animals were chronically trained by performing two acute bouts of exercise per week for 10 wk with 2 or 3 days of rest between bouts. Only the high-frequency paradigms (192 repetitions/O g and 192 repetitions/800-1,100 g) described above for acute exercise were examined in this part of the study because the chronic effects of the moderate-frequency moderate-resistance protocol had been evaluated previously (5). Sedentary control rats were maintained throughout the lowk training period but were not repeatedly anesthetized because ether anesthesia has no apparent effect on muscle mass or body weight in this model (5). Assays Protocols for the determination of protein synthesis rates, mRNA and rRNA subunit levels, and protein, RNA, and DNA concentrations were identical to those described in the companion paper (6). RESULTS

Functiunal Effects of Resistance Exercise and Training The functional responses and adaptations (maximal force and fatigue patterns) of the plantar-flexor muscle group to electrically induced resistance exercise and training have been described previously (6). Because of the nature of the model, it was not possible to obtain similar measurements for the dorsiflexor muscle group. Protein Synthesis Rate TA mixed and myofibril protein synthesis rates were measured 12-17 and 36-41 h after acute exercise (Table 1) and compared with values previously determined in GAST. All determinations were made in the the same set of animals from which GAST measurements were made such that exercise protocols and time points were identical. There were no significant differences in synthesis rate between left and right muscles of sedentary control (0 repetitions/O g) animals or between 12-17 and 36-41 h postexercise synthesis rates of exercised rats. The control (0 repetitions/O g) group synthesis rates were similar between TA and GAST muscles. 12-l 7 h after acute exercise. Mixed and myofibril protein synthesis rates in the TA were increased in all exercised groups compared with control. Surprisingly, this included the moderate-frequency moderate-resistance protocol (24 repetitions/500 g), which was increased by 32 and 45% for mixed and myofibril protein, respectively. After high-frequency low-resistance (192 repeti-

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1720 TABLE

PROTEIN

1. Fractional

EXPRESSION

DURING

ECCENTRIC

con-t ml Exercised 192

EXERCISE

protein synthesis rates in TA muscle after acute eccentric resistance exercise Total

24 rep/500

RESISTANCE

g

rep/0 g

192 rep/BOO-1,100 g

n

Time Postexercise,

9

12-17 36-41

3.3kO.6 3.2H.6

6

12-17 36-41 12-17 36-4 1

4.4H.6” 3.9*0.8* 4.7*0.9* 4.92 1.3* 4.x0.5* 4.7iO.8*

7 8

h

1247 36-41

%/day

Mixed

Myofibril

%Change from control

%/day

%Change from control

2.5kO.4 2.4k0.7 +32 +25 t41 +32 +42 +47

3.6+-0.5* 2.9kO.7 3.8kO.F 3.9k1.2’ 3.8kO.9’ 3.8&0.6*

+45

NS +56 +65 +54 +58

Values are means k SD; n, no. of muscles. Total mixed and myofibril protein synthesis rates were measured in TA muscles from nonexercised control rats and from animals after 1 acuk eccentric resistance exercise bout using various paradigms of a nonvoluntary resistance exercise model. * P < 0.05 compared with control.

tions/O g) or high-frequency high-resistance (192 repetitions/800-1,100 g) exercise, the percent increases (4151%) in synthesis rates in both groups were similar for mixed and myofibril proteins. Synthesis in the TA was not significantly different between any of the exercise paradigms. Compared with GAST, however, myofibril SYn thesis rates tended to i ncrease to a greater extent in the eccentrically exercised TA than in the concentrically exercised GAST in the high-frequency groups (54-56% in TA and 38% in GAST). It is not clear why mixed protein synthesis rates do not also show this trend. One explanation may be that protein synthesis in nonmuscle or inflammatory cells accounts for a larger proportion of the increase in mixed protein synthesis in the GAST than in the TA (see below). 36-41 h after acute exercise. Although mixed protein synthesis rates in the TA remained above the control rate at 36 h postexercise in all exercised groups, myofibril synthesis rate was not significantly different from control in the moderate-frequency moderate-resistance group and was lower than in the other two exercised groups. This suggests that changes in myofibril protein synthesis rate are more transient after 24 eccentric resistance contractions by the TA than its own mixed protein synthesis rate and in comparison to the highfrequency eccentric resistance exercise regimens. Contrary to findings in the concentric resistance-exercised GAST, there was no indication that 36-h synthesis rates were receding in the high-frequency high-resistance group muscles; in fact, they appeared to be rising, although not significantly, in this and the high-frequency low-resistance group. Similar to I2- to 17-h values, results suggested no obvious effect of resistance. In contrast to GAST, a lesser percent increase in synthesis rates for total mixed than for myofibril protein tended to occur in TA after eccentric exercise. Body Weight, Muscle Weight, and Protein Small but significant differences in preexercise body weight were evident between acute exercise groups despite random sampling but not between chronically trained animals (Table 2). As a consequence, absolute muscle wet weights were different, but when normalized to body weight they were unchanged after an acute

exercise bout. In contrast, after 10 wk (20 bouts) of chronic training, the high-frequency low-resistance group had increased muscle wet weight and protein content (milligrams/muscle) by 16 and 14%, respectively, compared with sedentary control rats. Furthermore, the high-frequency high-resistance group muscles were 30% greater in wet weight and 28% higher in protein content than muscles from nontrained rats. Muscle-to-body weight ratios between the two training protocols were not statistically different. Regardless, both eccentric resistance exercise regimens produced muscle enlargement in TA, whereas apparently similar concentric resistance paradigms failed to cause significant muscle growth in GAST. It should be noted that possible contralateral effects were evident after chronic training because contralateral nontrained muscles of trained rats were larger than sedentary control animal muscles despite similar body (Table 2), heart (control 861 t 87 g, high-frequency lowresistance 789 k 88 g, high-frequency high-resistance 834 2 65 g), and adrenal weights (control 56 t 8 mg, highfrequency low-resistance 67 t 7 mg, high-frequency highresistance 73 Tt 12 mg). TA protein concentration (mg/100 mg wet wt) did not differ after either acute exercise or chronic training. In contrast, GAST protein concentration was decreased lo11% 3 days after chronic concentric training, which supports the speculation above that inflammatory cells may influence the mixed protein synthesis response to acute exercise. RNA and DNA Seventeen hours after acute exercise, only the moderate-frequency moderate-resistance exercise regimen resulted in a significant rise in RNA concentration (milligrams/milligrams protein) in TA compared with control (Table 3). After 41 h postexercise, all exercise groups showed a significant increase in RNA concentration. Differences between exercise protocols 41 h postexercise were not significant. Because of differences in body and muscle weights, total RNA contents per whole TA muscle were statistically unchanged in all acute exercise groups compared with control rats. In contrast to GAST, no changes in DNA concentration or content were observed

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2. Body weights

and TA muscle wet weights

109 t15

18.9 to.9

mg/lOO

9 0

0.59

t6

(W

120 tl6

(W

101

W)

WS)

113 k14

W) 121 k13 W)

(W

WS) 114 t14 113 t16

W)

18.3 t1.5

19.0 t1.1 (NW 114 k18 WS)

19.0 kl.9 WS) 103 t13 WS)

18.5 tl.O

18.5 kc-o.7

18.4 H.5

0.20

17.8

0.18

0.20

(+W

0.61 to.05

(-8)

(+a

0.6O"f to.08

41

0.19

0.62'f zkO.08

8

g

(-6)

0.54-f* kO.03

41

1 17

192 rep/800-1,100

0.20

kO.8

0.20

0.55"r$ to.03

19.2

(+17) 0.21

(+lO)

0.65-f kO.04

0.68j-

rto.05

17

7 1

g 6 0

Control

k1.1 W) 110 kll

18.1

0.18

0.61 to.05

Total

bout:

h:

145t9 t20

(+N

(+12)

(NW

18.2 t2.4

(+30) 0.23

123ei t12

18.1 t1.6 (NW

0.20

3.2 to.2 0.37 to.04

3.3 to.4 0.36 ko.07

6.7

~1.3

4.8 to.2 0.55 to.07

to.09 6.8 t1.4

kO.3 0.54

4.9

9 0

Control

3.3 zkO.3 (NS) 0.40 to.03 (NS)

8.3-jtl.1 (+22)

5.3-F k0.3 (+8) 0.64 k0.05 (NS)

17

1

6

g

41

3.5 to.3 (NS) 0.42 kO.03 (NS)

5.4-f to.4 (+12) 0.65 rtO.06 (NS) 7.5 k1.5 (NS)

24 rep/500

17

3.3 to.5 (NS) 0.33 to.03 (NS)

9.2”r k1.9 (+34)

1

g

41

3.4 to.6 (NS) 0.35 -r-O.04 (NS)

8.8-fk2.0 (+30)

5.6tl k0.5 (+16) 0.57 to.04 (NS)

7

192 rep/O

Exercised

5.1 kO.2 (NS) 0.52 to.04 (NS)

Acute

3.4 to.4 (NS) 0.38 kO.04 (NS)

9.2-f to.9 (+34)

5.2 k0.4 (NS) 0.58 -1-0.06 (NS)

17

1

8 41

g

kO.05

0.39

3.4 to.2

(NS)

(NS)

8.7jt1.6 (+28)

kO.5 (+14) 0.62 kO.07 (NS)

5.W$

192 rep/800-1,100

2.1 kO.4 0.23 -r-O.05

ND

4.9 k0.4 0.54 to.03

0

6

2.1 to.5 0.23 kO.06

ND

4.9 k0.4 0.54 &0.03

Control

ND

(+29)

kO.03

0.35Q (+50)

2.75 k0.2 (+30)

-0.05

0.70tg

WS k0.2 (+12)

72

20

TR (192 rep/O

g)

5 0

to.7 (NS) 0.24 kO.07 (NS)

1.9

ND

(NS)

(NS)

72

0.56t to.03

5

(+13)

17.7* t1.2 (NS) 124*j$ *19

0.20

(+W

to.06

0.71?§

72

0

UNT (0 reps)

kO.08

0.31tg

2.2 to.5 (NS)

(NS) 0.75”rs to.10 (+38) ND

5.2 to.8

7;

n

20

TR (192 rep/ 800-1,100 g)

5

2.1" k0.4 (NS) 0.26 to.04 (NS)

0.59t kO.06 (NS) ND

4.8* to.3 (NS)

72

UNT (0 rep)

(NS) values. * n = 4 observations. 17-h value. § P C 0.05 from

Trained UNT (0 rep)

4.6 to.4

Chronic

Values are means t SD; n, no. of observations unless otherwise denoted. Numbers in parentheses are directional percent changes from sedentary control TR and UNT, trained and untrained legs, respectively, of the same rat; ND, not determined. t P c 0.05 from sedentary control. $ P c 0.05 from intragroup contralateral control.

mg/muscle

RNA activity, mg protein synthesized. day-l mg RNA-l DNA, mg/protein

mg/muscle

RNA,

protein

postexercise,

n: daily

mg/mg

DNA,

18.3 k2.1 W) lW§ *11 (+W

0.22

(+N

0.807s to.06

72

20

TR (192 rep/ 800-1,100 g)

3. Nucleic acid levels in TA muscle from control, after a single acute bout, and after 10 wk of twice-weekly chronic eccentric resistance exercise

RNA,

Time

TABLE

0.67"fs kO.03

0.71-f§

(+W

72

72 to.11

0

UNT (0 rep)

Trained

20

5

Chronic TR (192 rep/O g)

from sedentary control values. * n = 4 observations. g group. 8 P < 0.05 from contralateral control.

levels after a single acute bout and after 10 wk

192 rep/O

Exercised

kO.8

0.20

20.06

41

g

17

6 1

24 rep/500

Acute

and protein

Values are means t SD; n, no. of observations unless otherwise denoted. Numbers in parentheses are directional percent changes TR and UNT, trained and untrained legs, respectively, of the same rat. t P c 0.05 from sedentary control. $ P < 0.05 from 24 rep/500

content, Protein mg/muscle

0.58

t0.08

0.20

h:

wt,

wet wt, g

Muscle wt/body g/l00 g Protein concn, mg wet wt

Muscle

n: Total daily bouts: Time postexercise,

Control

of twice- weekly chronic eccentric resistance training

TABLE

1722

PROTEIN

EXPRESSION

DURING

ECCENTRIC

after acute exercise. Chronic eccentric resistance training caused only a modest increase in RNA concentration (12%) after highfrequency low-resistance training, and high-frequency high-resistance group muscles were not significantly altered (Table 3). However, as a consequence of increased muscle size, RNA content was 29 and 38% greater in high-frequency low-resistance and high-frequency highresistance muscles, respectively. DNA concentration was significantly greater in the high-frequency low-resistance group but not in the high-frequency high-resistance animals, although DNA contents were similar between the two high-frequency trained groups. Overall, changes in RNA and DNA were comparable between TA and GAST after chronic training except that in the high-frequency high-resistance chronic training group RNA and DNA concentrations increased in the concentrically trained GAST (6) but not in the eccentrically trained TA. Messenger

and Ribosomal

RNA

The mRNA concentrations of skeletal ar-actin and cytochrome c were determined in purified RNA extracts from TA muscles after acute exercise and chronic training. Results indicated that the levels of these mRNAs per unit of extractable RNA in both acute exercise groups were not significantly changed from sedentary control levels, suggesting that mRNA accumulation was unaffected by acute eccentric resistance exercise (data not shown). These results were, in general, similar to those in GAST (6), although results in TA did not indicate an increase in 18s and 28s rRNA content per whole muscle after low-resistance acute exercise. Chronic eccentric resistance training of TA caused no significant changes in skeletal cr-actin or cytochrome c mRNA per unit of extractable RNA in either trained group compared with sedentary control levels. Unlike GAST, mRNA levels per unit of extractable RNA did not tend to decrease in response to the increased total RNA pool (increased total RNA content per whole muscle), suggesting that mRNA levels accumulate in proportion to rRNA in TA. This is supported by estimates of possible 34 and 67% increases of skeletal cu-actin mRNA per whole TA muscle after low- and high-resistance training, respectively (Table 4). Therefore, although increased mRNA levels may not be a major regulator of acute increases in protein synthesis rates after the initial exercise bout in TA or GAST, the mechanisms that determine mRNA accumulation may be active in this model during chronic training in TA. Although cytochrome c mRNA is unchanged per unit of extractable RNA, cytochrome c mRNA content per whole TA muscle may also be increased by chronic training (Table 4). However, it is unknown whether cytochrome c protein responds similarly. Interestingly, mitochondrial density is thought to be unaltered by resistance training in humans (4). In general, 18s and 28s subunits in TA were changed similarly to those in GAST. DISCUSSION

The model of resistance exercise utilized in these studies results in the simultaneous concentric contraction of

RESISTANCE

EXERCISE

rat GAST and eccentric contraction of TA in the same leg. The next paragraphs of this discussion summarize the differential responses between TA and GAST with regard to protein synthesis rates after an acute resistance exercise bout and muscle mass after chronic resistance training. Although protein synthesis rates after the first acute exercise bout were increased by 24 eccentric resistance contractions of TA, they were not increased by 24 concentric resistance contractions of GAST (6). The reason for this was not immediately apparent to us, because we had previously found that after 16 wk of 24-repetitions/ bout chronic training, TA and GAST were hypertrophied similarly (5). Other data revealed that when animals underwent high-frequency protocols consisting of 192 contractions/bout, protein synthesis rates were increased after the initial acute bout of both concentric resistance (GAST) and eccentric resistance (TA) exercise. However, after 10 wk of 192-repetitions/bout chronic training, the concentrically resistance-trained GAST did not enlarge, whereas the eccentrically resistance-trained TA showed a significant gain in muscle mass. The above results together demonstrate that increased muscle mass after chronic resistance training does not solely reflect the increases in protein synthesis rates after an acute resistance exercise bout by untrained rats. It is also apparent from these results that differential activation of protein synthesis and possibly degradation mechanisms occur between TA and GAST with response specific to each chronic resistance training regimen. Because direct determination of protein degradation rates cannot be performed reliably, protein degradation must be estimated from the measured rates of protein synthesis and net changes in muscle growth. For example, we found that although substantial increases of 41-65% in mixed and myofibril protein synthesis rates exist after acute eccentric resistance exercise by TA, only 14-2876 increases in TA protein occurred after 10 wk of 19% repetitions/bout chronic eccentric resistance training. This comparison indirectly suggests that protein degradation rate may also be increased during chronic training in TA. However, the use of this extrapolation in the present study is rather tenuous because of the limited time points at which synthesis was measured and because of the possibility that the measured increases in protein synthesis may not be associated with muscle fibers but with connective tissue or inflammatory cells. The 5465% increases in myofibril protein synthesis rati, however, indicate that increases in synthesis occur predominantly within myofibers because contractile protein is virtually absent in nonmuscle cells. Nevertheless, other studies with a different model of eccentric muscle training demonstrate concurrent increases in protein synthesis and degradation, resulting in a net increase in muscle mass. Laurent et al. (3) reported that only 20% of the increased protein synthesized appeared as increased muscle protein during the first 58 days of chronic stretching of the latissimus dorsi muscle of fowls. The remaining 80% of the increased protein synthesis was offset by increased protein degradation rates. These results support our speculation in the present investigation that chronic eccentric resistance training by TA may stimu-

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PROTEIN

TABLE

EXPRESSION

DURING

ECCENTRIC

RESISTANCE

4. Percent chunges in mRNA and rRNA subunit levels determined resistance training

of chronic eccentric

mKNA or rRNA/Unit Extractable RNA,

of

Total Whole

%

RNA/ TA, %

1723

EXERCISE

in TA muscles after 10 wk Estimated

mRNA or rRNA/ Whole TA, %

wActin mRNA 192 rep/O g 192 rep/800-1,100

g

+4 +21

g

-4 +7

+29 +38

+34 +67

+29 +38

+24

+29 +38

+24 +61

+29 +38

-7 +30

Cytochrome c mRiVA 192 rep/O g 192 rep/800-1,100

+48

1% rRNA 192 rep/o g 192 rep/MO-1,100

g

-6 +17 28s

192 rep/O g 192 rep/8UO-1,100 Values determined RNA are nonexercised mRNA or could not

g

28 -7

rRNA

were taken 3 days after final exercise bout. Total RNA/whole TA values are from Table 3. mRNA and rRNA concentrations were by analysis of RNA dot blots (see MATERIALS AND METHODS; rt = G/g~oup). Indicated changes for mRNA or rRNA/unit of extractable not statistically significant and were estimated from differences between mean slope values determined for each exercised group and control group. Changes in mRNA and rRNA contents per whole muscle were approximated from product of percent change in rRNA/unit of extractable RNA and percent change in total RNA/whole TA. Statistical analysis of mRNA or rRNA/whole GAST be performed because of nature of estimation.

late greater protein breakdown as well as synthesis. If protein synthesis and degradation rates are altered in both GAST and TA during high-frequency (192-repetitio ns/bout) chronic resistance training, then one or both of these protein regulatory mechanisms must respond differently between TA and GAST to account for the greater muscle enlargement in TA. Evidence for a more prolonged increase of protein synthesis in the eccentric resistance-exercised TA than in the concentric resistance-exercised GAST is provided by the finding that synthesis rates in TA but not in GAST remain significantly higher than control values at 36-41 h after the initial acute exercise bout. It can be speculated that because synthesis rates appear to be more transient, GAST may require more than two exercise bouts per week to produce hypertrophy. However, this idea does not appear to hold because we have shown that twiceweekly training, even with lower numbers of repetitions per bout, is sufficient to produce muscle enlargement and that more frequent training may, in fact, hinder muscle growth (28). Other results show that 24 eccentric resistance contractions of TA stimulate increases in synthesis rates but that the same number of concentric resistance contractions of GAST do not significantly affect synthesis. These data suggest that a lower number of contractions of TA than of GAST are required to activate synthesis pathways. The reason for this result is unknown but may be a greater relative resistance (specific tension) imposed on TA during contraction. In another study (6), we reported our estimates of the antagonistic resistances placed on the anterior (dorsiflexors) and posterior (plantar flexors) muscle groups by each of the exercise paradigms. During high-frequency low-resistance exercise, GAST contracts only against the -300 g of resistance exerted by the dorsiflexors, whereas TA must work against the 1,100-g force of the GAST-plantaris-soleus complex. Although the resistan .ce i.mposed on the GAST is substantially increased with the high-frequ .ency high-

resistance paradigm (-800-1,000 g after correction for lever arm advantage and antagonistic dorsiflexor resistance), the absolute as well as the specific tensions required of the smaller TA are obviously much greater than those of GAST and could therefore be the reason for the differential protein synthesis rate response. On the other hand, the additional weights placed on the pulley do not likely incur greater loading on TA because a bar is placed behind the foot lever of the pulley such that TA does not experience a preload. This factor may explain, in part, why TA responded similarly to both high-frequency protocols. Another possibility that could explain the different results from GAST and TA is that differential protein regulatory responses are caused by the concentric and eccentric contractions performed by these muscles (1,2). Therefore, differential effects on protein degradation or other posttranslational mechanisms may occur as a result of the two types of contractile work. However, this possibility can only be tested by examining the direct response of GAST to eccentric contractions and of TA to concentric contractions under identical muscle loading conditions. As in GAST, RNA accumulation in TA after acute eccentric resistance exercise likely occurs concurrently with posttranscriptional or translational mechanisms to cause the acute increases in protein synthesis rate. However, increased RNA may not be the major means through which synthesis rates are acutely increased in either muscle. Evidence supporting this conclusion in the TA is provided by data showing that 1) the percent increases in RNA concentrations are not commensurate with the percent increases in synthesis rates after acute exercise, i.e., the amount of protein synthesized per unit of RNA is increased, and 2) skeletal cr-actin or cytochrome c mRNA levels do not change significantly after acute eccentric exercise in TA. However, the implied increases in total RNA and possibly skeletal cr-actin and cytochrome c mRNA content per whole TA after chronic

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1724

PROTEIN

EXPRESSION

DURING

ECCENTRIC

training suggest that transcriptional mechanisms are activated during eccentric resistance training of TA. Finally, the increased DNA contents in TA (and GAST) after chronic training must originate from satellite cell or connective tissue cell proliferation or possible postexercise inflammatory cell infiltration. In conclusion, the regulation of acute increases in protein synthesis rate in both the eccentric resistanceexercised TA and the concentric resistance-exercised GAST appears to occur through pretranslational, translational, and posttranslational mechanisms. This is supported indirectly by RNA and specific protein mRNA measurements. We found that the greater muscle enlargement of TA compared with GAST after 10 wk of chronic training (160 min of total stimulated contractile activity over the training period) was due, in part, to more prolonged acute increases in protein synthesis rates

in TA. In addition,

lower numbers

of contractile

repeti-

tions were required to stimulate synthesis pathways in TA. These differences in protein synthesis rate and muscle mass response are probably due to the greater specific tension imposed on TA during exercise and

training,

although

tions performed

the eccentric and concentric

by TA and GAST, respectively,

contracmay also

have a role. Collectively, these studies support the idea that muscle protein adaptations to resistance training are in part specified

by the number

of repetitions,

the

KESISTANCE

EXERCISE

relative resistance placed on the muscle, and/or the type (concentric or eccentric) of contractile work performed. The authors

thank

Marjorie

Tucker

for typing

the tables,

Dr. Ken-

neth Ro for technical assistance, and Chris Kirby for assistance in editing the manuscript. This study was supported by National Institute of Arthritis and Mu,sculoskeletal

and Skin

Diseases

Grant

This work was performed in partial tion

requirements (T. S. Wong). Address for reprint requests:

AR-19393

(F. W. Booth).

fulfillment of doctoral disserta-

F. W. Booth,

Dept.

of Physiology

and

Cell Biology, University of Texas Medical School at Houston, PO Box 20708, Houston, TX 77225. Received 2f>January 1990; accepted in final form 6 June 1990, REFERENCES 1. ARMSTRONG, exercise-induced

R. B., R. W. OGILVIE, injury

to rat skeletal

AND J. A. SCHWANE. muscle. J. Appl.

Eccentric Physiol. 54:

80-93,1983. 2. EBBELINC, C. B., AND P. M. CLARKSON. damage and adaptation. Sports Med. 3. LAURHNT, G. J., M. P. SPARROW, AND

Exercise-induced muscle 7: 207-234, 1989. D. J. MILLWARD.

Turnover

of muscle protein in the fow1. Hihem. J. 176: 407-417, 1978. 4. MAcDouGAu, 3. D., D. G. SALE, J. R. MOROZ, G. C. B. ELDER, J. R. SIJ’ITON, AND H. HOWALII. Mitochondrial volume density in human skeletal muscle following heavy resistance training. bfed. Sci. Sports Exercise 11: 164-166, 1979. 5. WONG, T. S., AND F. W. BOOTH. Skeletal muscle enlargement with

weight-lifting exercise in rats. J. A&. I’hysiol. 65: 950-954, 1988. T. S., AND F. W. HOOTH. Protein metabolism in rat gastrocnemius muscle after stimulated chronic concentric exercise. 1990. J. Appl. Physiol. 69: 1709-1717,

6. WONG,

Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (192.236.036.029) on December 25, 2018.