Amino acids do not suppress proteolysis in premature neonates

To determine whether increased amino acid availability can reduce proteolys is in premature neonates and to assess the capacity of infants born prematu rely to acutely increase the irreversible catabolism of the essential amino acids leucine (via oxidation) and phenylalanine (via hydroxylation to form tyrosine), leucine and phenylalanine kinetics were measured under basal co nditions and in response to a graded infusion of intravenous amino acids (1 .2 and 2.4 g.kg(-1).day(-1)) in clinically stable premature (similar to 32 wk gestation) infants in the 1st wk of life. In contrast to the dose-depend ent suppression of proteolysis seen in healthy full-term neonates, the endo genous rates of appearance of leucine and phenylalanine (reflecting proteol ysis) were unchanged in response to amino acids (297 +/- 21, 283 +/- 19, an d 284 +/- 31 mu mol.kg(-1).h(-1) for leucine and 92 +/- 6, 92 +/- 4, and 84 +/- 7 mu mol.kg(-1).h(-)1 for phenylalanine). Similar to full-term neonate s, leucine oxidation (40 +/- 5, 65 +/- 6, and 99 +/- 7 mu mol.kg(-1).h(-1)) and phenylalanine hydroxylation (12 +/- 1, 16 +/- 1, and 20 +/- 2 mu mol.k g(-1).h(-1)) increased in a stepwise fashion in response to graded amino ac ids. This capacity to increase phenylalanine hydroxylation may be crucial t o meet tyrosine needs when exogenous supply is limited. Finally, to determi ne whether amino acids stimulate glucose production in premature neonates, glucose rate of appearance was measured during each study period. In respon se to amino acid infusion, rates of endogenous glucose production were unch anged (and near zero).