Atrophy and Impaired Muscle Protein Synthesis during Prolonged Inactivity and Stress

Abstract

We recently demonstrated that 28-d bed rest in healthy volunteers results in a moderate loss of lean leg mass and strength. The objective of this study was to quantify changes in muscle protein kinetics, body composition, and strength during a clinical bed rest model reflecting both physical inactivity and the hormonal stress response to injury or illness. Muscle protein kinetics were calculated during a primed, continuous infusion (0.08 micromol/kg.min) of 13C6-phenylalanine on d 1 and 28 of bed rest. The setting for this study was the General Clinical Research Center at the University of Texas Medical Branch. Participants were healthy male volunteers (n = 6, 28 +/- 2 yr, 84 +/- 4 kg, 178 +/- 3 cm). During bed rest, hydrocortisone sodium succinate was administered iv (d 1 and 28) and orally (d 2-27) to reproduce plasma cortisol concentrations consistent with trauma or illness (approximately 22 microg/dl). We hypothesized that inactivity and hypercortisolemia would reduce lean muscle mass, leg extension strength, and muscle protein synthesis. Volunteers experienced a 28.4 +/- 4.4% loss of leg extension strength (P = 0.012) and a 3-fold greater loss of lean leg mass (1.4 +/- 0.1 kg) (P = 0.004) compared with our previous bed rest-only model. Net protein catabolism was primarily due to a reduction in muscle protein synthesis [fractional synthesis rate, 0.081 +/- 0.004 (d 1) vs. 0.054 +/- 0.007%/h (d 28); P = 0.023]. There was no change in muscle protein breakdown. Prolonged inactivity and hypercortisolemia represents a persistent catabolic stimulus that exacerbates strength and lean muscle loss via a chronic reduction in muscle protein synthesis.