Human muscle protein synthesis is modulated by extracellular, not intramuscular amino acid availability: a dose-response study

Abstract

To test the hypothesis that muscle protein synthesis (MPS) is regulated by the concentration of extracellular amino acids, we investigated the dose-response relationship between the rate of human MPS and the concentrations of blood and intramuscular amino acids. We increased blood mixed amino acid concentrations by up to 240 % above basal levels by infusion of mixed amino acids (Aminosyn 15, 44-261 mg kg-1 h-1) in 21 healthy subjects, (11 men 10 women, aged 29 +/- 2 years) and measured the rate of incorporation of D5-phenylalanine or D3-leucine into muscle protein and blood and intramuscular amino acid concentrations. The relationship between the fold increase in MPS and blood essential amino acid concentration ([EAA], mM) was hyperbolic and fitted the equation MPS = (2.68 x [EAA])/(1.51 + [EAA]) (P < 0.01). The pattern of stimulation of myofibrillar, sarcoplasmic and mitochondrial protein was similar. There was no clear relationship between the rate of MPS and the concentration of intramuscular EAAs; indeed, when MPS was increasing most rapidly, the concentration of intramuscular EAAs was below basal levels. We conclude that the rates of synthesis of all classes of muscle proteins are acutely regulated by the blood [EAA] over their normal diurnal range, but become saturated at high concentrations. We propose that the stimulation of protein synthesis depends on the sensing of the concentration of extracellular, rather than intramuscular EAAs.

Figure 1. Time course of changes in plasma total amino acid concentrations during infusion of mixed amino acids at four different rates

Values are means ± s.e.m. for 4-6 subjects per point.

Figure 2. Changes in the concentrations of serum insulin and glucose during the infusion of mixed amino acids at four different rates

Serum insulin (top) and glucose (bottom) concentrations during infusion of mixed amino acids (AA infusion). Values are means ± s.e.m. for 4-6 subjects per point.

Figure 3. Relationship between extracellular and intramuscular concentrations of EAAs during infusion of mixed amino acids at different rates

In the basal state, extracellular EAAs are the mean concentration of EAAs of two blood samples taken 2.5 h apart before the beginning of the mixed amino acid infusion, and intramuscular EAAs the mean concentration of EAAs from the first two muscle biopsy samples. During the infusion of mixed amino acid solution (at 43.5, 87 and 261 mg kg⁻¹ h⁻¹), extracellular EAAs represents the mean concentration of EAAs of blood samples taken at 1, 1.5, 2.5 and 3.5 h during the infusion, and intramuscular EAAs the mean concentration of EAAs from the two last muscle biopsy samples. Values are means ± s.e.m. for 4-6 subjects per point (with the exception of the 11 subjects at basal). Infusion rates of mixed amino acids are shown near to the points. Asterisks indicate the values of intramuscular amino acids that were significantly different from basal values (P < 0.05, repeated-measures ANOVA).

Figure 4. Effect on mixed MPS of extracellular and intramuscular levels of EAAs

Upper panel, the effect on mixed MPS (measured as incorporation of D₅-Phe or D₃-leucine) in human skeletal muscle of extracellular EAA concentrations achieved by infusion of a mixed amino acid solution at different rates (shown near each point). Lower panel, the effect on mixed MPS (measured as incorporation of D₅-Phe) in human skeletal muscle of intramuscular EAA concentrations achieved by infusion of a mixed amino acid solution at different rates (shown near each point). Values are means ± s.e.m. for 4-6 subjects per point (with the exception for the 11 subjects at basal), each point being the mean value of the individual changes in MPS and the mean EAA concentration over the period of measurement. The upper curve fits the equation: MPS = 2.68 ×[EAA]/(1.51 +[EAA]). No curve fit has been attempted for the lower graph.

Figure 5. Hypothetical scheme to explain the effect of increased extracellular EAAs on MPS

Hypothetical scheme showing how increased extracellular EAAs (L) may modulate MPS (the last stage in the sequence L-Protein) and cause diminution in intramuscular EAAs. Stages M-O simply represent biochemical entities involved in the initiation and elongation of protein synthesis. The increased availability of the extracellular amino acids is signalled to the protein synthetic apparatus via a membrane-bound sensor labelled ‘?’, which is independent of the amino acid transporters. The sensor must transmit a signal through another unknown mechanism (X).