Growth of fetal guinea pig: physical and chemical characteristics.

The growth of the fetal guinea pig was studied in 32 fetuses in 12 litters, ranging from 39 days gestation to full term (67 days). The wet weight of the fetus was well approximated by an exponential function of gestational age (GA) in days [fetal weight (g) = 0.993 e(0.068 X GA), r = 0.94]. Dry weight increased more rapidly than wet weight [dry weight (g) = 0.039 e(0.102 X GA); r = 0.97], resulting in an increase in percent dry weight from approximately 10% at 40 days gestation to 30% at term. Fat content increased even more rapidly than dry weight [body fat (g) = 0.00123 e(0.136 X GA), r = 0.97], accounting for 33% of dry weight and 11.7% of wet weight at term. Using bomb calorimetric projections of caloric value of 9.3 kcal X g fat-1 X day-1 and 4.6 kcal X g nonfat dry wt-1 X day-1, we estimate that growth of the fetal guinea pig requires 220 kcal X kg fetal wt-1 X day-1 near term. Carbon and nitrogen contents of the fetus increased at different rates, reflecting the changes in fat and nonfat tissues. Amino acids contributed 80% of total body nitrogen and 41% of total body carbon near term. Cysteine concentrations increased and lysine concentrations decreased with gestational age; the concentrations of the other measured amino acids did not change with gestational age. These studies represent the first systematic study of the chemical growth of the fetus in a nonhuman species.

Development of gluconeogenesis in isolated hepatocytes from the rabbit.

Gluconeogenesis from 10 mM lactate has been studied in isolated hepatocytes from fetal, newborn, and 70-day-old rabbits. Gluconeogenesis proceeds to a very low rate in fetal rabbit hepatocytes despite substantial activities of all gluconeogenic enzymes including mitochondrial phosphoenolpyruvate carboxykinase. A tenfold increase in the rate of gluconeogenesis occurs in hepatocytes from 1- or 2-day-old fasting or suckling newborn rabbits. The emergence of gluconeogenic capacity in newborn rabbit hepatocytes is triggered by birth itself and not by a chronological factor, and it is primarily controlled by an increase in the activity of cytosolic phosphoenolpyruvate carboxykinase. Moreover, an active fatty acid oxidation is essential to support a high rate of gluconeogenesis in hepatocytes from newborn rabbits.

Relationship between ketogenesis and gluconeogenesis in isolated hepatocytes from newborn rats.

In hepatocytes from 1-day-old rats, active gluconeogenesis occurs in parallel with active ketogenesis, although the carbon atoms of non-esterified fatty acids do not participate in glucose synthesis. Once a significant ketogenesis is established, a further increase does not enhance gluconeogenesis. Indeed, octanoate is more ketogenic than oleate, but stimulates gluconeogenesis to a similar extent.

Fuel metabolism in fasted newborn rabbits.

Newborn rabbits delivered by Caesarean section at term were fasted for 72 h at 36 degrees C. Despite the abrupt interruption of maternal supply of energy substrates, glycaemia remains stable for 4 h after birth. This can be related to glucose production via rapid liver glycogenolysis; however, indirect evidence suggests that gluconeogenesis could also contribute to glucose production during this period. There is a selective decrease in the concentrations of gluconeogenic substrates and a suitable hormonal environment for gluconeogenesis as decreased insulin and increased glucagon concentration just after birth. The relative hypoglycaemia which develops after 6 h of life (2.6 mM at 72 h), despite high blood concentrations of non-esterified fatty acids and ketone bodies is not due to a deficient gluconeogenesis per se, as injection of gluconeogenic substrates to 72 h fasted newborns produces a three-fold increase in plasma glucose concentration. It is suggested that this relative hypoglycaemia is secondary to limited gluconeogenic substrate availability in the form of low circulting concentrations of gluconeogenic amino acids.

Perinatal development of glucoeneogenic enzymes in rabbit liver.

Activities of the 4 hepatic gluconeogenic enzymes: glucose-6-phosphatase, fructose-1,6-diphosphatase, pyruvate carboxylase, particulate and cytosolic phosphoenolpyruvate carboxykinase (PEPCK) have been measured in fetal rabbits (22, 25, 28, 30 and 31 days of gestation) and in fasted or suckling newborns (1 and 2 days after birth). Between days 25 and 31 of gestation, fructose 1,6-diphosphatase and particulate PEPCK activities represent 50% of adult (pregnant female) activities, while pyruvate carboxylase is present at adult values during the same period. Glucose-6-phosphatase is low and cytosolic PEPCK absent in fetal liver until 30 days of gestation and increase significantly during the day preceding birth. Al the enzymes show a further increase after birth independently of the nutritional status of the animals (starved or suckling).