Postprandial resynthesis of myofibrillar proteins is translationally rather than transcriptionally regulated in human skeletal muscle

Abstract

Feeding stimulates protein synthesis in skeletal muscles, although the regulatory mechanisms are incompletely understood. The aim of this study was to determine whether this could be detected at the gene transcription level for postprandial stimulation of the synthesis of muscle proteins. Healthy male volunteers were investigated after an overnight fast. Open muscle biopsies were performed in the starved state and 3 h after meal intake, consisting of 0.15 gN/kg, 12 kcal/kg. Blood samples were drawn every 15 to 30 min for 5 h. Myosin mRNA and insulin growth factor-I (IGF-I) mRNA were measured by solution hybridization assay in homogenized muscle specimens. After food intake, plasma glucose concentrations increased from 5.0 ± 0.1 to 7.3 ± 0.3 ( P ≤ 0.001), and insulin concentration rose from 3.8 ± 0.5 mU/L before to 75.3 ± 11.4 15 min after the meal ( P ≤ 0.001). Plasma concentration of free fatty acids declined after food intake ( P ≤ 0.001). Plasma concentrations of amino acids increased from basal values (2864 ± 128 μM) to 4419 ± 262 μM ( P ≤ 0.05) 90 min after meal ingestion. Myosin mRNA concentration in the biopsied muscle tissue was higher during starvation and was reduced by 20% after food intake: 10.8 ± 1.3 amol mRNA/μg DNA in the starved state and 8.5 ± 1.3 amol mRNA/μg DNA after food intake ( P ≤ 0.05). Feeding did not alter IGF-I mRNA concentrations in muscle: 0.51 ± 0.05 and 0.55 ± 0.06 amol/μg DNA in the starved and fed state, respectively ( P ≤ 0.48). Improved protein balance by stimulation of protein synthesis has been related to increased plasma amino acids. Interestingly, in the short term, this was not related to increases in gene transcription of either myofibrillar proteins (myosin) or muscle IGF-I. Thus, postprandial stimulation of protein synthesis appears not to be regulated by increased gene transcription but by increased translation using the increased concentrations of amino acids. In contrast, as far as the 2X myosin mRNA level is concerned, this is enhanced during starvation, which facilitates rapid recovery once the availability of substrate is resumed.