Alanine and
glutamine constitute the two most important
nitrogen carriers released from the muscle. We studied the intracellular
amino acid transport kinetics and
protein turnover in nine
end-stage renal disease (
ESRD) patients and eight controls by use of stable
isotopes of
phenylalanine,
alanine, and
glutamine. The
amino acid transport kinetics and
protein turnover were calculated with a three-pool model from the
amino acid concentrations and enrichment in the artery, vein, and muscle compartments.
Muscle protein breakdown was more than synthesis (nmol.min(-1).100 ml leg(-1)) during
hemodialysis (HD) (169.8 +/- 20.0 vs. 125.9 +/- 21.8, P < 0.05) and in controls (126.9 +/- 6.9 vs. 98.4 +/- 7.5, P < 0.05), but synthesis and catabolism were comparable pre-HD (100.7 +/- 15.7 vs. 103.4 +/- 14.8). Whole body
protein catabolism decreased by 15% during HD. The intracellular appearance of
alanine (399.0 +/- 47.1 vs. 243.0 +/- 34.689) and
glutamine (369.7 +/- 40.6 vs. 235.6 +/- 27.5) from
muscle protein breakdown increased during dialysis (nmol.min(-1).100 ml leg(-1), P < 0.01). However, the de novo synthesis of
alanine (3,468.9 +/- 572.2 vs. 3,140.5 +/- 467.7) and
glutamine (1,751.4 +/- 82.6 vs. 1,782.2 +/- 86.4) did not change significantly intradialysis (nmol.min(-1).100 ml leg(-1)).
Branched-chain amino acid catabolism (191.8 +/- 63.4 vs. -59.1 +/- 42.9) and nonprotein
glutamate disposal (347.0 +/- 46.3 vs. 222.3 +/- 43.6) increased intradialysis compared with pre-HD (nmol.min(-1).100 ml leg(-1), P < 0.01). The
mRNA levels of
glutamine synthase (1.45 +/- 0.14 vs. 0.33 +/- 0.08, P < 0.001) and branched-chain keto
acid dehydrogenase-E2 (3.86 +/- 0.48 vs. 2.14 +/- 0.27, P < 0.05) in the muscle increased during HD. Thus intracellular concentrations of
alanine and
glutamine are maintained during HD by augmented release of the
amino acids from
muscle protein catabolism. Although
muscle protein breakdown increased intradialysis, the whole body
protein catabolism decreased, suggesting central utilization of
amino acids released from skeletal muscle.