Effect of endogenous insulin release on fetal alanine concentration and uptake.

Insulin secretion in the fetus may be important in the modulation of selected fetal amino acid concentration and uptakes. To test this hypothesis we observed the changes in fetal alanine concentrations and uptake after tolbutamide-induced insulin release in the fetal lamb. The basal umbilical venous-arterial alanine difference was 29.1 +/- 4.2 mumoles/L. Fetal alanine uptake was 5.1 +/- 0.6 mumoles/kg/min. After tolbutamide infusion fetal insulin concentration rose fourfold by 30 minutes. Fetal glucose concentration fell to approximately 75% of control values. Both arterial and umbilical venous alanine contents fell significantly (p < 0.01), yet the fetal alanine venous-arterial difference also fell significantly (p < 0.05) by 30 minutes. Fetal alanine uptake from the pooled experiments did not change significantly after tolbutamide. When only insulin responses of less than 150 microU/ml were considered, however, a significant (p < 0.02) fall in alanine uptake was noted. Two injections in one animal caused peak insulin levels above 150 microU/ml. Although limited, data from these latter experiments suggested an increase in fetal alanine uptake at these higher insulin concentrations. Thus, endogenous fetal insulin release caused a significant hyperinsulinism and consequent hypoglycemia. The fetal hypoalaninemia produced may have been due to a number of factors, including an acute decrease in placental alanine transfer and decreased fetal hepatic gluconeogenesis. A differential effect of high versus low insulin responses upon fetal alanine uptake is also suggested.