The irreversible loss of alanine and of glycine in fetal and sucking lambs

Alanine umbilical uptake, disposal rate, and decarboxylation rate in the fetal lamb

Fetal plasma alanine disposal rate (DR) and decarboxylation rate were measured at 132 +/- 1 days gestation in nine fetal lambs infused with L-[1-14C]alanine via a brachial vein. In five experiments, L-[1-13C]alanine was added to the infusate. Using L-[1-14C]alanine, we found mean DR to be 15.5 +/- 1.8 mumol.min-1 . kg-1. DR was significantly correlated to both arterial plasma alanine and whole blood lactate concentrations. Placental uptake of fetal plasma alanine accounted for 19 +/- 4% of DR. Fetal and placental production of CO2 from the first carbon of alanine were 61 +/- 2 and 16 +/- 2% of DR, respectively, for a total uterine excretion of 77 +/- 3%. Net alanine flux from placenta to fetus was 5.2 +/- 0.5 mumol.min-1 x kg-1, which was less than fetal plasma alanine decarboxylation (9.4 +/- 1.2 mumol.min-1 x kg-1) plus fetal alanine accretion (2.4 mumol.min-1 x kg-1). Utilizing L-[1-13C]alanine, we found DR to be 14.2 +/- 0.8 mumol.min-1 x kg-1, not significantly different from the 14C data. We conclude that both the umbilical uptake of alanine from the placenta and fetal alanine synthesis contribute to fetal alanine supply and that oxidation is the main route of fetal plasma alanine disposal.

Glycine turnover and oxidation and hepatic serine synthesis from glycine in fetal lambs

[1-13C]- and [1-14C]glycine were infused into chronically catheterized fetal lambs via a brachial vein. At tracer glycine steady state, samples were collected from the fetal abdominal aorta, umbilical vein, and fetal hepatic vein and from the maternal femoral artery and uterine vein. The samples were analyzed for plasma glycine and serine, for glycine and serine 13C atom% excess (APE), and for whole blood 14CO2 and O2 concentrations. Fetal plasma glycine disposal rate (DR) was 12.4 +/- 0.8 mumol.min-1.kg fetus-1.CO2 production from decarboxylation of fetal plasma glycine was 1.63 +/- 0.16 mumol.min-1.kg fetus-1 and represented 12.3 +/- 0.7% of DR. Approximately 50% of infused tracer glycine was taken up by the fetal liver with the release of labeled serine and CO2 in the fetal circulation. There was no detectable efflux of tracer glycine from the placenta into the maternal circulation. The tracer production of serine and CO2 accounted for 23 and 17%, respectively, of the hepatic tracer glycine uptake. The labeled CO2 released by the liver was a large fraction (approximately 70%) of the labeled CO2 produced by the fetus. The serine-to-glycine APE ratio in fetal plasma was approximately 5%. These results indicate that the fetal liver is the major site of fetal plasma glycine decarboxylation and of serine synthesis from plasma glycine.

Glucose metabolism in shorn and unshorn pregnant sheep

1. Whole-body, hind-limb and uterine tissue metabolism of glucose was studied using a combination of isotopic and arterio-venous difference techniques in shorn and unshorn pregnant sheep over the final 4 weeks of pregnancy. This was combined with the measurement of the concentrations of oxygen and carbon dioxide in arterial blood and plasma concentrations of lactate, acetate, non-esterified fatty acids, 3-hydroxybutyrate, glycerol, growth hormone (GH), insulin, glucagon, cortisol, thyroxine and 3,5,3'-triiodothyronine (T3). 2. Glucose entry rate was 28% higher in shorn ewes compared with unshorn controls, even though there was no difference in the arterial plasma concentration of glucose. This effect may have been caused by a decrease in the molar rate, insulin: glucagon (I:G), which was 40% lower in shorn ewes as a result of a significant decrease in the plasma concentration of insulin. There was no difference in the plasma concentration of cortisol or GH. 3. Blood flow across the hind-limb or uterine tissues was not significantly different between shorn and unshorn groups, neither were the net glucose uptake, glucose oxidation rate or contribution of glucose to O2 consumption across these tissues. 4. Insulin-tolerance tests performed on a separate group of shorn and unshorn ewes showed an increased sensitivity to the hypoglycaemic effects of insulin in the shorn group. 5. There was no significant difference between shorn and unshorn animals in the contribution of glucose to CO2 output or in the proportion of glucose entry rate oxidized. CO2 entry rate was 18% higher in shorn ewes compared with unshorn controls which resulted in a 26% higher estimated value for heat production. There was a 47% increase in glucose oxidation rate in shorn ewes but there was no significant difference in the proportion of total heat production which was derived from glucose. The arterial concentrations of O2 and CO2 were significantly higher in shorn ewes, which may be an indication of the higher metabolic rate in these animals. This effect may be mediated via a significant rise in plasma T3 concentration in the shorn group. 6. It is concluded that as a result of long-term cold exposure there is a significant increase in whole-body glucose entry and oxidation rates in the shorn pregnant ewe. The increase in insulin sensitivity at the same time as a decrease in plasma insulin concentration may represent a mechanism to ensure continued glucose supply to insulin-sensitive tissues while the concomitant decrease in plasma I:G stimulates hepatic gluconeogenesis.

Leucine disposal and oxidation rates in the fetal lamb

To verify previous indirect evidence suggesting an important role of amino acids as substrates of fetal oxidative metabolism, leucine disposal and oxidation rates were measured in chronically catheterized fetal lambs during the last month of gestation. Under normal physiologic conditions the leucine oxidation rates were 6.43 +/- 1.02 mumol min-1 for fetuses with an average weight of 3.22 +/- 0.07 kg and comparable in magnitude to the fetal rate of leucine accretion. In seven animals studied before and during fasting, the fetal leucine oxidation rate increased with fasting from 5.8 +/- 1.0 to 10.8 +/- 1.3 mumol min-1. These data demonstrate that there is rapid oxidative degradation of leucine by the fetus and that the rate of this process increases in response to maternal fasting.