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Abstract
Short-term (3 to 4 hours) infusion of branched-chain amino acids (BCAA) has been shown
to suppress muscle protein breakdown. Whether these effects are sustained with chronic
elevations of BCAA is not known. In the present study, we examined the effect of an
overnight (16-hour) systemic BCAA infusion on whole-body and skeletal muscle amino
acid metabolism, as assessed by simultaneously measured 3H-phenylalanine and 14C-leucine kinetics in eight normal volunteers; 10 overnight-fasted subjects studied
during a 4-hour saline infusion served as controls. Overnight BCAA infusion increased
plasma BCAA concentrations by fivefold to eightfold, and this was associated with
a 20% to 60% decline in arterial concentrations of other amino acids. For Phe, this
decline was mediated by a reduction in the systemic rate of appearance ([Ra] 0.38 ± 0.03 v 0.60 ± 0.01 μmol/kg/min for BCAA and saline, respectively, P < .001). Endogenous Leu Ra, calculated more indirectly as the difference between the total Leu Ra and the unlabeled Leu infusion rate, did not differ between groups. In the forearm,
overnight BCAA infusion resulted in a diminished net release of Phe (−3 ± 2 v −18
± 4 [saline] nmol/min/100 mL, P < .02), and BCAA balance became markedly positive (751 ± 93 v −75 ± 30, P < .001). The diminished net forearm Phe release was accounted for by a decrease in
local Phe Ra (P < .02). As with the systemic endogenous Leu Ra, forearm Leu Ra was not reproducibly affected by infused BCAA. These findings suggest a need for
caution in the application of amino acid tracer kinetic methods when tracer and unlabeled
tracee are infused simultaneously. In conclusion, overnight BCAA infusion caused a
sustained decline in most plasma amino acids and in net forearm release of Phe, effects
attributable to a sustained suppression of whole-body and muscle proteolysis.
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Article info
Publication history
Accepted:
July 1,
1994
Received:
June 16,
1992
Footnotes
☆Supported by the Yale Clinical Research Center (RR-125) and by US Public Health Service Grant No. DK-38578. R.J.L. was the recipient of a Clinical Associate Physician Award from the Yale General Clinical Research Center.
Identification
Copyright
© 1995 Published by Elsevier Inc.
