AJP - Endocrinology and Metabolism, Vol 273, Issue 1 E99-107, Copyright © 1997 by American Physiological Society
Mixed muscle protein synthesis and breakdown after resistance exercise in humans
S. M. Phillips, K. D. Tipton, A. Aarsland, S. E. Wolf and R. R. Wolfe
Metabolism Unit, Shriners Burns Institute, Galveston, Texas, USA.
Mixed muscle protein fractional synthesis rate (FSR) and fractional
breakdown rate (FBR) were examined after an isolated bout of either
concentric or eccentric resistance exercise. Subjects were eight untrained
volunteers (4 males, 4 females). Mixed muscle protein FSR and FBR were
determined using primed constant infusions of [2H5]phenylalanine and
15N-phenylalanine, respectively. Subjects were studied in the fasted state
on four occasions: at rest and 3, 24, and 48 h after a resistance exercise
bout. Exercise was eight sets of eight concentric or eccentric repetitions
at 80% of each subject's concentric 1 repetition maximum. There was no
significant difference between contraction types for either FSR, FBR, or
net balance (FSR minus FBR). Exercise resulted in significant increases
above rest in muscle FSR at all times: 3 h = 112%, 24 h = 65%, 48 h = 34%
(P < 0.01). Muscle FBR was also increased by exercise at 3 h (31%; P
< 0.05) and 24 h (18%; P < 0.05) postexercise but returned to resting
levels by 48 h. Muscle net balance was significantly increased after
exercise at all time points [(in %/h) rest = -0.0573 +/- 0.003 (SE), 3 h =
-0.0298 +/- 0.003, 24 h = -0.0413 +/- 0.004, and 48 h = -0.0440 +/- 0.005],
and was significantly different from zero at all time points (P < 0.05).
There was also a significant correlation between FSR and FBR (r = 0.88, P
< 0.001). We conclude that exercise resulted in an increase in muscle
net protein balance that persisted for up to 48 h after the exercise bout
and was unrelated to the type of muscle contraction performed.
