Effect of hyperinsulinemia on amino acid utilization in the ovine fetus

We studied the effect of an acute 4-h period of hyperinsulinemia (H) on net utilization rates (AAUR(net)) of 21 amino acids (AA) in 17 studies performed in 13 late-gestation fetal sheep by use of a novel fetal hyperinsulinemic-euglycemic-euaminoacidemic clamp. During H [84 +/- 12 (SE) microU/ml H, 15 +/- 2 microU/ml control (C), P < 0. 00001], euglycemia was maintained by glucose clamp (19 +/- 0.05 micromol/ml H, 1.19 +/- 0.04 micromol/ml C), and euaminoacidemia (mean 4.1 +/- 3.3% increase for all amino acid concentrations [AA], nonsignificantly different from zero) was maintained with a mixed amino acid solution adjusted to keep lysine concentration constant and other [AA] near C values. H produced a 63.7% increase in AAUR(net) (3.29 +/- 0.66 micromol. min(-1). kg(-1) H, 2.01 +/- 0.55 micromol. min(-1). kg(-1) C, P < 0.001), accounting for a 60.1% increase in fetal nitrogen uptake rate (2,064 +/- 108 mg. day(-1). kg(-1) H, 1,289 +/- 73 mg. day(-1). kg(-1) C, P < 0.001). Mean AA clearance rate (AAUR(net)/[AA]) increased by 64.5 +/- 18.9% (P < 0. 001). Thus acute physiological H increases net amino acid and nitrogen utilization rates in the ovine fetus independent of plasma glucose and [AA].

Regulation of uterine and umbilical amino acid uptakes by maternal amino acid concentrations

We tested the hypothesis that decreased fetal amino acid (AA) supply, produced by maternal hypoaminoacidemia (low AA) during hyperglycemia (HG), is reversible with maternal AA infusion and regulates fetal insulin concentration ([I]). We measured net uterine and umbilical AA uptakes during maternal HG/low AA concentration ([AA]) and after maternal intravenous infusion of a mixed AA solution. After 5 days HG, all maternal [AA] except glycine were decreased >50%, particularly essential [AA] (P < 0.00005). Most fetal [AA] also were decreased, especially branched-chain AA (P < 0.001). Maternal AA infusion increased net uterine uptakes of Val, Leu, Ile, Met, and Ser and net umbilical uptakes of Val, Leu, Ile, Met, Phe, and Arg but did not change net uteroplacental uptake of any AA. Fetal [I] increased 55 +/- 14%, P < 0.001, with correction of fetal [AA], despite the lack of change in fetal glucose concentration. Thus generalized maternal hypoaminoacidemia decreases uterine and umbilical uptakes of primarily the essential AA and decreases fetal branched-chain [AA]. These changes are reversed with correction of maternal [AA], which also increases fetal [I].

Response of skeletal muscle protein synthesis to insulin in suckling pigs decreases with development

The elevated rate of muscle protein deposition in the neonate is largely due to an enhanced stimulation of skeletal muscle protein synthesis by feeding. To examine the role of insulin in this response, hyperinsulinemic-euglycemic-amino acid clamps were performed in 7- and 26-day-old pigs. Pigs were infused with 0, 30, 100, or 1,000 ng . kg-0.66 . min-1 of insulin to mimic the plasma insulin levels observed under fasted, fed, refed, and supraphysiological conditions, respectively. Whole body amino acid disposal was determined from the rate of infusion of an amino acid mixture necessary to maintain plasma essential amino acid concentrations near their basal fasting levels. A flooding dose of L-[4-3H]phenylalanine was used to measure skeletal muscle protein synthesis. Whole body amino acid disposal increased progressively as the insulin infusion rate increased, and this response was greater in 7- than in 26-day-old pigs. Skeletal muscle protein synthesis was stimulated by insulin, and this response was maximal at a low insulin infusion rate (30 ng . kg-0.66 . min-1). The stimulation of muscle protein synthesis by insulin was also greater in 7- than in 26- day-old pigs. These data suggest that muscle protein synthesis is more sensitive to insulin than whole body amino acid disposal. The results further suggest that insulin is a central regulatory factor in the elevated rate of muscle protein deposition and the increased response of skeletal muscle protein synthesis to feeding in the neonate.

Roles of insulin and amino acids in the regulation of protein synthesis in the neonate

Neonates deposit protein at a very high rate and efficiently utilize dietary amino acids for protein deposition. This high efficiency is associated with an elevated stimulation of tissue protein synthesis by feeding. Our recent studies have focused on identification of the factors that mediate this response in the neonate. A positive curvilinear relationship between skeletal muscle protein synthesis and plasma insulin concentration was identified in fasted and fed suckling pigs; the relationship changes with development. To test the specific effects of insulin on protein metabolism in the neonate, a procedure to clamp amino acids, under hyperinsulinemic conditions, was developed. By using this technique, we showed that insulin-stimulated whole-body amino acid disposal is elevated in the neonate, and this response may account for the efficient use of dietary amino acids for protein accretion. More recent studies suggest that the enhanced stimulation of skeletal muscle protein synthesis by feeding in the neonate is primarily insulin mediated; however, the stimulation of liver protein synthesis by feeding seems to be largely a function of amino acid concentration.

Insulin-stimulated amino acid utilization during glucose and amino acid clamps decreases with development

Neonatal animals utilize their dietary amino acids for protein accretion with high efficiency, and this efficiency declines during early life. The factors responsible for this developmental change are unknown. Our objectives were to determine whether amino acid (AA) utilization is stimulated by insulin in the neonate and whether this response changes during the suckling period. Two hyperinsulinemic-euglycemic clamp infusion studies, using 10-2,000 ng insulin.kg-0.66.min-1, were performed in 7- and 26-day-old pigs. In study I, no AA were provided during the infusion, and the resultant decline in plasma AA levels was defined. In study II, plasma AA were clamped at near-fasting levels, and whole body utilization of exogenous AA was determined by measuring the rate of infusion of an AA mixture necessary to maintain basal plasma lysine concentrations. In study I, the half-maximal effective dose (ED50) for the fall in AA concentrations with increasing plasma insulin concentration was lower in 7- than in 26-day-old pigs, and the nadir in AA concentration was achieved by only 20 microU/ml insulin. In study II, the utilization of exogenous AA during hyperinsulinemic-euglycemic AA clamps exhibited a higher maximum response (Rmax) (49 vs. 26 mumol AAtotal.min-1.kg-1) and a lower ED50 (18 vs. 45 microU insulin/ml) in 7- than in 26-day-old pigs. Plasma urea nitrogen concentrations did not rise with increasing insulin and AA infusion rates. These results indicate that insulin stimulates the utilization of exogenous AA in neonatal pigs and that both the insulin sensitivity and responsiveness of AA utilization decline over the suckling period. The infused AA were likely utilized for protein accretion.