We have recently demonstrated dose dependent suppression of proteolysis with graded amino acid (aa) infusion in healthy term neonates (Am J Physiol, 1997). Rates of proteolysis are higher in preterm individuals, perhaps related to a greater need for aa supply for tissue remodeling and rapid growth. We hypothesized that preterm neonates would be resistant to suppression of proteolysis in response to aa infusion. To assess this, we measured the endogenous rates of appearance (Ra) of the essential aa's leucine (LEU) and phenylalanine (PHE) (reflecting proteolysis), LEU oxidation (LEU OX) and phenylalanine hydroxylation (PHE OH) (reflecting irreversible losses), and utilization of LEU and PHE for protein synthesis (PS) in 7 clinically stable preterm infants (32±0.5 wks gestation, 1.5±0.1 kg birth wt, 1.4±0.1 kg study wt, 6±1 days of age) during a basal glucose infusion (6mg/kg/min) and in response to a graded infusion of aa's (Aminosyn PF-1.2 & 2.5gm/kg/day).

RESULTS: (Mean±SE, kinetics in μmol/kg/hr, p<0.05*compared to basal state, **compared to basal and AA1)Table

Table 1
Full size table

CONCLUSIONS: 1) Unlike term neonates, preterm neonates are resistant to suppression of proteolysis in response to graded amino acid infusion. 2) Irreversible losses of LEU and PHE both increase. 3) Utilization of PHE for PS increases by ≈13% in response to amino acid infusion (LEU for PS increases but not significant). An inadequate supply of conditionally essential amino acids (such as tyrosine or cysteine) may limit suppression of proteolysis and LEU utilization for PS in these preterm neonates. Suboptimal energy intake during the study may also limit these processes.