Introduction/purpose: Eccentric exercise causes muscle proteolysis that may be attenuated with repeated exercise. Therefore, this study determined the effect of repeated bouts of eccentric exercise on ubiquitin (UBI), ubiquitin conjugating enzyme (E2), and 20S proteasome (20S) and glucocorticoid receptor (GR) mRNA and protein expression, myofibrillar protein content, DNA content, caspase-3 activity, serum skeletal muscle troponin-I (sTnI) and cortisol (CORT), and muscle strength.
Methods: Nine males underwent two identical eccentric exercise bouts (BT1 and BT2) 3 wk apart involving seven sets of 10 repetitions at 150% one-repetition maximum of the dominant knee extensors. Blood and muscle biopsy samples were obtained before and at 6 and 24 h postexercise whereas muscle strength was assessed before and at 24, 48, and 72 h postexercise. Data were analyzed with separate 2 x 3 and 2 x 4 factorial ANOVA (P < 0.05).
Results: Decrements in strength and increased soreness occurred at 24 and 48 h postexercise for both bouts (P < 0.05); however, the changes for BT1 were greater than BT2. Serum CORT and sTnI were greater immediately after and at 6, 24, and 48 h postexercise for both bouts; however, the differences in BT1 were greater than BT2 (P < 0.05). Caspase-3 activity and the mRNA and protein levels of UBI, E2, 20S, and GR were increased at 6 and 24 h postexercise, and these differences were greater for BT1 than BT2 (P < 0.05). For BT1, DNA and myofibrillar protein content decreases were apparent at 24 h postexercise (P < 0.05) but not in BT2.
Conclusion: These results indicate that muscle injury occurring from an initial bout of eccentric exercise seems to decrease muscle strength and myofibrillar protein, possibly due to apoptosis and up-regulation of glucocorticoid receptor mediated increases in UBI-proteolytic pathway activity, all of which appear to be tempered with a repeated eccentric exercise bout.