Developmental changes of free amino acid concentrations in fetal fluids of pigs

Free amino acid concentrations were determined in amniotic and allantoic fluids and in fetal and maternal uterine plasma of pigs during gestation. Glycine and glutamine were the most abundant free amino acids in maternal uterine arterial and fetal umbilical venous plasma, respectively. Plasma fetal/maternal ratios were remarkably < 1 for glycine, > 2 for glutamine and = 1 for total free alpha-amino acids. Glutamine was the most abundant free amino acid in amniotic fluid during the first 3 mo of gestation. The concentrations of most amniotic fluid amino acids progressively decreased with increasing gestational age from 45 to 110 d. Glycine was the most predominant amino acid in allantoic fluid on d 30 of gestation but decreased on d 45 and 60 of gestation. This decrease in allantoic fluid glycine coincided with a marked increase in concentrations of both ornithine and arginine, which accounted for 34-36% of total free alpha-amino acids in allantoic fluid. Such an abundance of ornithine and arginine in allantoic fluid has not been reported for any other biological fluid. On d 90 and 110 of gestation, glycine again became the most abundant amino acid in allantoic fluid, or 47 and 29% of total free alpha-amino acids, respectively. Urea concentrations in amniotic and allantoic fluids progressively increased during the second half of pregnancy, indicating enhanced fetal protein metabolism. Our results demonstrate the abundance of glutamine in fetal plasma, the unusual abundance of ornithine, arginine and glycine in allantoic fluid and developmental changes in plasma fetal/maternal ratios of amino acids.(ABSTRACT TRUNCATED AT 250 WORDS)