Diminished muscular activity is associated with alterations of protein
metabolism. The aim of this study was to evaluate the effect of short-term muscle
inactivity on regulation of whole-body protein deposition during amino acid
infusion to simulate an experimental postprandial state. We studied nine healthy
young volunteers at the end of 14 day periods of strict bed rest and of
controlled ambulation using a cross-over design. Subjects received a
weight-maintaining diet containing 1 g protein kg(-1) day(-1). l[1-(13)C]leucine
was used as a marker of whole-body protein kinetics in the postabsorptive state
and during a 3 h infusion of an amino acid mixture (0.13 g amino acid (kg lean
body mass)(-1) h(-1)). In the postabsorptive state, bed rest decreased (P < 0.05)
the rate of leucine disposal (R(d)) to protein synthesis and tended to decrease
leucine rate of appearance (R(a)) from proteolysis, whereas the rate of leucine
oxidation did not change significantly. Amino acid infusion increased leucine
R(d) to protein synthesis and oxidation and decreased leucine R(a) from
proteolysis in both the bed rest and ambulatory conditions. Changes from basal in
leucine R(d) to protein synthesis were lower (P < 0.05) during bed rest than
those in the ambulatory period, whereas changes in leucine R(a) from proteolysis
and oxidation were not significantly different. During amino acid infusion, net
leucine deposition into body protein was 8 +/- 3% lower during bed rest than
during the ambulatory phase. In conclusion, short-term bed rest leads to reduced
stimulation of whole-body protein synthesis by amino acid administration. Results
of this study were, in part, presented at the meeting, Experimental Biology,
2004, Washington DC.
