Wilkening, R.B.

Induction of glutamate dehydrogenase in the ovine fetal liver by dexamethasone infusion during late gestation (Article)

2003-01-01T00:00:00Z

Glucocorticoids near term are known to upregulate many important enzyme systems prior to birth. Glutamate dehydrogenase (GDH) is a mitochondrial enzyme that catalyzes both the reversible conversion of ammonium nitrogen into organic nitrogen (glutamate production) and the oxidative deamination of glutamate resulting in 2-oxoglutarate. The activity of this enzyme is considered to be of major importance in the development of catabolic conditions leading to gluconeogenesis prior to birth. Ovine hepatic GDH mRNA expression and activity were determined in near-term (130 days of gestation, term 147 +/- 4 days) control and acutely dexamethasone-treated (0.07 mg(-1) hr(-1) for 26 hr) fetuses. Dexamethasone infusion had no effect on placental or fetal liver weights. Dexamethasone infusion for 26 hr significantly increased hepatic GDH mRNA expression. This increased GDH mRNA expression was accompanied by an increase in hepatic mitochondrial GDH activity, from 30.0 +/- 7.4 to 58.2 +/- 8.1 U GDH/U CS (citrate synthase), and there was a significant correlation between GDH mRNA expression and GDH activity. The generated ovine GDH sequence displayed significant similarity with published human, rat, and murine GDH sequence. These data are consistent with the in vivo studies that have shown a redirection of glutamine carbon away from net hepatic glutamate release and into the citric acid cycle through the forward reaction catalyzed by GDH, i.e., glutamate to oxoglutarate.

Effect of dexamethasone on fetal hepatic glutamine-glutamate exchange (Article)

2000-01-01T00:00:00Z

Intravenous infusion of dexamethasone (Dex) in the fetal lamb causes a two- to threefold increase in plasma glutamine and other glucogenic amino acids and a decrease of plasma glutamate to approximately one-third of normal. To explore the underlying mechanisms, hepatic amino acid uptake and conversion of L-[1-(13)C]glutamine to L-[1-(13)C]glutamate and (13)CO(2) were measured in six sheep fetuses before and in the last 2 h of a 26-h Dex infusion. Dex decreased hepatic glutamine and alanine uptakes (P < 0.01) and hepatic glutamate output (P < 0.001). Hepatic outputs of the glutamate (R(Glu,Gln)) and CO(2) formed from plasma glutamine decreased to 21 (P < 0.001) and 53% (P = 0.009) of control, respectively. R(Glu,Gln), expressed as a fraction of both outputs, decreased (P < 0.001) from 0.36 +/- 0.02 to 0.18 +/- 0.04. Hepatic glucose output remained virtually zero throughout the experiment. We conclude that Dex decreases fetal hepatic glutamate output by increasing the routing of glutamate carbon into the citric acid cycle and by decreasing the hepatic uptake of glucogenic amino acids.

Relationship of fetal alanine uptake and placental alanine metabolism to maternal plasma alanine concentration (Article)

1998-01-01T00:00:00Z

Uterine and umbilical uptakes of alanine (Ala) were measured in 10 ewes before (control) and during intravenous infusion of Ala, which increased maternal arterial Ala concentration from 115 +/- 14 to 629 +/- 78 microM (P < 0.001). In 8 of these ewes, placental Ala fluxes were traced by constant intravenous infusion of L-[3,3,3-2H3]Ala in the mother and L-[1-13C]Ala in the fetus. Rates are reported as micromoles per minute per kilogram fetus. Ala infusion increased uterine uptake (2.5 +/- 0.6 to 15.6 +/- 3.1, P < 0.001), umbilical uptake (3.1 +/- 0.5 to 6.9 +/- 0.8, P < 0.001), and net uteroplacental utilization (-0.7 +/- 0.8 to 8.6 +/- 2.7, P < 0.01) of Ala. Control Ala flux to fetus from mother (Rf,m) was much less than the Ala flux to fetus from placenta (Rf,p) (0.17 +/- 0.04 vs. 5. 0 +/- 0.6). Two additional studies utilizing L-[U-13C]Ala as the maternal tracer confirmed the small relative contribution of Rf,m to Rf,p. During maternal Ala infusion, Rf,m increased significantly (P < 0.02) but remained a small fraction of Rf,p (0.71 +/- 0.2 vs. 7.3 +/- 1.3). We conclude that maternal Ala entering the placenta is metabolized and exchanged for placental Ala, so that most of the Ala delivered to the fetus is produced within the placenta. An increase in maternal Ala concentration increases placental Ala utilization and the fetal uptake of both maternal and placental Ala.