Growth hormone decreases muscle glutamine production and stimulates protein synthesis in hypercatabolic patients

We determined the effects of 24-h recombinant human growth hormone (rhGH) i nfusion into a femoral artery on leg muscle protein kinetics, amino acid tr ansport, and glutamine metabolism in eight adult hypercatabolic trauma pati ents. Metabolic pathways were assessed by leg arteriovenous catheterization and muscle biopsies with the use of stable amino acid isotopes. Muscle mRN A levels of selected enzymes were determined by competitive PCR. rhGH infus ion significantly accelerated the inward transport rates of phenylalanine a nd leucine and protein synthesis, whereas the muscle protein degradation ra te and cathepsin B and UbB polyubiquitin mRNA levels were not significantly modified by rhGH. rhGH infusion decreased the rate of glutamine de novo sy nthesis and glutamine precursor availability, total branched-chain amino ac id catabolism, and nonprotein glutamate utilization. Thus net glutamine rel ease from muscle into circulation significantly decreased after rhGH admini stration (similar to 50%), whereas glutamine synthetase mRNA levels increas ed after rhGH infusion, possibly to compensate for reduced glutamine precur sor availability. We conclude that, after trauma, the anticatabolic action of rhGH is associated with a potentially harmful decrease in muscle glutami ne production.