In most mammalian species the developing fetus utilizes large amounts of glutamine derived both from the maternal circulation and synthesized de novo in the placenta. The present study was designed to determine the role of the placenta in glutamine synthesis in the horse. The placentae from eight Standardbred mares were sampled immediately after parturition together with additional tissues obtained at necropsy from three Standbred mares during diestrous. Glutamine synthetase protein was detectable in the non-pregnant horn of the placenta in amounts similar to those seen in gluteus muscle, but the amount in the pregnant horn was two times greater than in the non-pregnant horn. Glutamine was the second most abundant amino acid in amniotic fluid at a concentration of 310 +/- 26 micromole/L with that of glycine being 535 +/- 48 micromole/L. The most abundant amino acids in placental tissue were glycine (3,732 +/- 194 micromole/Kg), glutamate (3,500 +/- 343 micromole/Kg) and glutamine (2,836 +/- 208 micromole/Kg). The results illustrate the importance of glutamine to the equine fetus and establish that the placenta, particularly the pregnant horn, has considerable capacity for glutamine synthesis.
