Prolonged bed rest decreases skeletal muscle and whole-body protein
synthesis.
Ferrando, Arny A., Helen W. Lane, Charles A. Stuart, and Robert R.
Wolfe.
NASA, Johnson Space Center, SD4, Houston TX 77058, Division of
Endocrinology, Department of Internal Medicine, UTMB, Galveston TX
77555-1060, Metabolism, Shriners Burns Institute, 815 Market St,
Galveston TX 77550
APStracts 2:0242E, 1995.
We sought to determine the extent to which the loss of lean body mass
and nitrogen during inactivity was due to alterations in skeletal
muscle protein metabolism. Six male subjects were studied during 7
days of diet stabilization and after 14 days of simulated
microgravity (-6_ bed rest). Nitrogen balance decreased toward
negative (p<0.03) during the second week of bed rest. Leg and
whole-body lean mass decreased after bed rest (p<0.05). Serum
cortisol, insulin, IGF-1 and testosterone values did not change.
Arterio-venous model calculations based on the infusion of ring-13C6
-phenylalanine in 5 subjects revealed a 50% decrease in muscle protein
synthesis (PS; P<0.03). Fractional PS by tracer incorporation
into muscle protein also decreased by 46% (P<0.05). The decrease
in PS was related to a corresponding decrease in the sum of
intracellular amino acid appearance from protein breakdown and inward
transport. Whole-body protein synthesis determined by 15N-alanine
ingestion on 6 subjects also revealed a 14% decrease (P<0.01).
Neither model-derived nor whole-body values for protein breakdown
changed significantly. These results indicate that the loss of body
protein with inactivity is predominantly due to a decrease in muscle
PS, and that this decrease is reflected in both whole-body and
skeletal muscle measures.
Received 31 August 1995; accepted in final form 16 November 1995.
APS Manuscript Number E423-5.
Article publication pending Am. J. Physiol. (Endocrinol. Metab.).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 8 December 95