Estimation of maternal fetal amino acid transport and alanine production in the fetus, with stable tracer L-[15N]alanine and L-[15N]valine

A mixture of 15N-labeled L-alanine and L-valine was infused at a constant rate into five rabbits with gestational age of 29–30 days. Steady state was achieved after 2 h of a prime dose-constant infusion. Maternal blood samples were taken during 3 h simultaneously with umbilical venous followed by arterial blood between 2–3 h. The 15N enrichment of the umbilical vein alanine was 69% lower than simultaneously obtained form maternal vein. A further decrease of 42% in the umbilical arterial samples compared to umbilical venous plasma was determined. While valine 15N enrichment showed umbilical venous blood 55% less than that of the maternal blood, no significant difference was found between umbilical venous and arterial plasma, although alanine and valine concentration in umbilical venous and arterial plasma were similar. A possible mechanism of alanine production in the fetus in utero is suggested.