Effects of inactivity on human muscle glutathione synthesis by a double-tracer and single-biopsy approach

Oxidative stress is often associated to inactivity-mediated skeletal muscle atrophy. Glutathione is one of the major antioxidant systems stimulated, both at muscular and systemic level, by activation of oxidative processes. We measured changes in glutathione availability, oxidative stress induction and the extent of atrophy mediated by 35 days of experimental bed rest in vastus lateralis muscle of healthy human volunteers. To assess muscle glutathione synthesis, we applied a novel single-biopsy and double-tracer ([2H(2)]glycine and [15N]glycine) approach based on evaluation of steady-state precursor incorporation in product. The correlations between the traditional (multiple-samples, one-tracer) and new (one-sample, double-tracer infusion) methods were analysed in erythrocytes by Passing-Bablok and Altman-Bland tests. Muscle glutathione absolute synthesis rate increased following bed rest from 5.5 +/- 1.1 to 11.0 +/- 1.5 mmol (kg wet tissue)-1 day-1 (mean +/- s.e.m.; n = 9; P = 0.02) while glutathione concentration failed to change significantly. Bed rest induced vastus lateralis muscle atrophy, as assessed by pennation angle changes measured by ultrasonography (from 18.6 +/- 1.0 to 15.3 +/- 0.9 deg; P = 0.01) and thickness changes (from 2.3 +/- 0.2 to 1.9 +/- 0.1 cm; P < 0.001). Moreover, bed rest increased protein oxidative stress, as measured by muscle protein carbonylation changes (from 0.6 +/- 0.1 to 1.00 +/- 0.1 Oxydized-to-total protein ratio; P < 0.04). In conclusion, we developed in erythrocytes a new minimally invasive method to determine peptide synthesis rate in human tissues. Application of the new method to skeletal muscle suggests that disuse atrophy is associated to oxidative stress induction as well as to compensatory activation of the glutathione system.