Eccentric But Mot Concentric Exercise Activates P70s6 Kinase In Human Skeletal Muscle

Introduction
Resistance exercise is known to increase the rate of protein synthesis in skeletal muscle, up to 48 h after one session of resistance exercise protein synthesis was found to be increased [1]. However, little information is available on protein synthesis in the early recovery period after exercise as well as after different modes of contraction. The rate-limiting step in protein synthesis is known to be the translation of protein, particularly the initiation of translation. Phosphorylation (activation) of the enzyme p70S6 kinase is an important enzyme in this process and has been used in several studies to indicate an activation of the translation process [2]. The aim of the present study was to compare concentric and eccentric exercise with regard to activation of p70S6 kinase in human muscle.

Methods
Four moderately trained male subjects, mean (SE) age 25 (1) yrs and maximal oxygen uptake 4.07 (0.28) l/min performed maximal concentric and eccentric contractions with alternate legs. For example, the subject performed 4 sets of 6 maximal eccentric contractions with the right leg, rested 60 min and thereafter performed 4 sets of 6 maximal concentric contractions with the left leg. Blood samples were taken from an indwelling catheter before, immediately after and repeatedly during the recovery period for measurements of plasma lactate. Muscle biopsies were taken from the vastus lateralis before, immediately after, 1 h and 2 h after exercise in both legs. The biopsy samples were freeze-dried, homogenised and analysed for p70S6 kinase phosphorylation on Thr389 by Western blot technique. Proteins were visualised by enhanced chemiluminescence detection and the optical density of the bands were quantified.

Results
The average force produced during the eccentric exercise was 40% higher than during the concentric exercise, 2548 (62) vs. 1825 (23) N. Plasma lactate concentration increased similarly during both types of exercise, the highest values were obtained directly after exercise and averaged 3.3-3.4 mmol/l after both types of exercise. One hour after the eccentric exercise a pronounced increase in phosphorylation of p70S6 kinase was found, whereas no significant changes were found after the concentric exercise (Graph.1).
ANEXO

Discussion/Conclusions
Only the eccentric exercise led to phosphorylation of p70S6 kinase on Thr389 and thus an activation of the enzyme. No effect of the concentric exercise was found. One explanation for the different response between eccentric and concentric exercise could be that the force developed during the eccentric exercise was higher, however, results from animal studies indicate that eccentric contractions activate p70S6 kinase to a greater extent than concentric contractions [3]. In conclusion, eccentric exercise might be more effective than concentric exercise in initiating the translation process and thus stimulating protein synthesis.

References
[1]. Biolo G. et al. (1999). Diabetes, 48, 949-957
[2] Kimball SR. et al. (2002). J Appl Physiol, 93, 1168-1180
[3] Nader GA & Esser KA.. (2001). J Appl Physiol, 90, 1936-1942

NOTA: O texto com a iconografia está no anexo.

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