J. W. Sparks

Simultaneous measurements of lactate turnover rate and umbilical lactate uptake in the fetal lamb.

Lactic acid represents a major exogenous nutrient for the developing fetal lamb in utero. Our study was undertaken (a) to quantitate the net consumption of lactate by the fetus, (b) to quantitate the net lactate production and metabolism by the placenta, and (c) to compare the net fetal lactate consumption with fetal lactate use, measured simultaneously with radioactive tracers. 14 pregnant sheep were prepared with catheters in the maternal femoral artery and uterine vein and in the fetal aorta and umbilical vein. By simultaneous application of the Fick principle to the uterine and umbilical circulations, placental glucose consumption and placental lactate production were rapid, averaging 39.8 +/- 5.1 and 11.8 +/- 0.7 mg.min-1. Net lactate umbilical uptake averaged 1.95 +/- 0.16 mg-1.kg.min-1. During infusion of L-[14C(U)]lactate, fetal lactate turnover was much more rapid, averaging 6.5 +/- 0.8 mg.kg-1.min-1, and lactate utilization within the anatomic fetus was 5.9 +/- 0.7 mg.kg-1.min-1. During infusion of tracer glucose, endogenous fetal lactate production from glucose and nonglucose substrates averaged 3.0 and 1.5 mg.kg-1.min-1, respectively. The present studies have quantitated under well oxygenated, steady-state conditions, the rapid placental metabolism and production of lactate, the net fetal consumption of lactate, and the rapid endogenous fetal lactate production from glucose and nonglucose substrates.

Glucose and lactate oxidation rates in the fetal lamb

Abstract Both glucose and lactate are nutrients of the ovine fetus. Each may be used by the fetus as a fuel for oxidation or as a source of carbon for energy storage and net tissue accretion. The present report describes the oxidation rates of glucose and lactate in vivo for the fetal lamb over a relatively short time period. The fraction of fetal glucose or lactate oxidized was defined as the ratio of 14CO2 excretion across the umbilical circulation to the net entry of [14C]glucose or [14C]lactate into fetal tissues. The fraction of glucose oxidized over a 3-hr study averaged 61.2%, accounting for 2.55 mg · min-1 · kg-1 of glucose oxidized and for 28% of the simultaneous net oxygen uptake. The fraction of lactate oxidized averaged 71.5%, accounting for 4.12 mg·min-1 · kg-1 of lactate oxidized. Oxidation fractions and rates for both glucose and lactate increased with their concentrations in fetal blood suggesting sparing of other fuels for oxidation at higher glucose and lactate concentrations.

Partition of maternal nutrients to the placenta and fetus in the sheep

Utilizing the Fick Principle, the fluxes of oxygen and glucose leaving the uterine circulation and entering the fetal umbilical circulation were measured simultaneously in 35 chronically catheterized sheep. Additionally, the distribution of placentally produced lactate into the uterine and umbilical circulations was measured by the same techniques. Under unstressed conditions, placental oxygen consumption accounted for approximately half the oxygen exiting the uterine circulation. Placental glucose consumption averaged 75% of the glucose exiting the uterine circulation, and this proportion increased with decreasing glucose concentration in the maternal artery. Lactate was produced at a high rate by all placentae, and distributed disproportionately to the fetus, in spite of higher fetal lactate concentration. Fetal metabolism was aerobic, as demonstrated by a high rate of net oxygen consumption and a high rate of net lactate consumption. Fetal oxygen metabolism correlated well with fetal weight and with the sum of net fetal lactate and glucose consumption.

Effect of insulin on glucose uptake in near-term fetal lambs.

Abstract Glucose clamp experiments were performed in 27 chronically catheterized, late-gestation fetal lambs in order to measure the effect of fetal insulin concentration on fetal glucose uptake at a constant glucose concentration. Fetal arterial blood glucose concentration was measured over a 30-min control period and then maintained at the control value by a variable glucose infusion into the fetus while insulin was infused at a constant rate into the fetus. Plasma insulin concentration increased from 21 ± 10 (SD) to 294 ± 179 (SD) μU·ml−1. The exogenous glucose infusion rate necessary to maintain constant glycemia during the plateau hyperinsulinemia averaged 4.3 ± 1.6 (SD) mg·min−1 ·kg−1. In a subset of 13 animals, total fetal exogenous glucose uptake (FGU; sum of glucose uptake from the placenta via the umbilical circulation plus the steady-state exogenous glucose infusion rate) was measured during the control and hyperinsulinemia period. FGU was directly related to insulin concentration (y = 4.24 + 0.07x) at insulin levels < 100 μU/ml and increased 132% above control at insulin levels above 100 μU/ml. Hyperinsulinemia did not affect fetal glucose uptake from the placenta via the umbilical circulation. These studies demonstrate that insulin concentration is a major factor controlling glucose uptake in the near-term fetal lamb, and that an increase of fetal insulin does not affect the transport of glucose to the fetus from the placenta.

Placental, fetal, and neonatal carbohydrate metabolism

In summary, glucose metabolism in the placenta and fetus is characterized by a fairly exact balance between exogenous glucose supply from the mother and placental and fetal glucose utilization (directly and as lactate produced from glucose). The rate of glucose utilization and its rate of oxidation are largely determined by the maternal glucose concentration and are mediated in part by insulin. Thus, glucose and insulin act together to substitute glucose oxidation for the oxidation of other energy substrates and to direct glucose carbon into glycogen, fat, and protein accretion. After birth, endogenous glucose production and dietary glucose intake (as glucose or as galactose) must account for the maintenance of glucose supply. In many cases, however, the balance among glucose intake, glucose production, and glucose utilization is inexact in the transition from intrauterine to extrauterine life, leading to both hypoglycemia and hyperglycemia. Basic measurements of the glucose production rate and the glucose utilization rate can be made now in the fetus as well as in the neonate, but the factors producing perturbations in glucose supply and utilization and in regulating the responses to these perturbations remain to be measured.

Patterns of Serum Glucose and Galactose Concentrations in Term Newborn Infants after Milk Feeding

Galactose and glucose concentrations were measured in peripheral blood in relation to ad libitum milk feeding in 11 healthy near-term infants. Galactose and glucose concentrations before feeding averaged 1.06 +/- 0.21 and 60.3 +/- 3.2 mg/dl, respectively. After feedings containing 0.76-2.68 g lactose/kg body weight, both galactose and glucose rose by approximately 35% Galactose concentrations rose by 0.72 +/- 0.10 mg/dl at 30 min after feeding, while glucose concentration rose by 23.3 +/- 2.5 mg/dl at 30 min after feeding. The 25-fold greater absolute increase in the concentration of glucose than galactose is consistent with efficient first-pass clearance of galactose by the neonatal liver.

Substrate Concentration Changes during Pregnancy in the Guinea Pig Studied under Unstressed Steady State Conditions

Summary: Weight gain and food intake were measured in unstressed nonpregnant and pregnant guinea pigs fed ad libitum. Nonpregnant females consumed 33.2 ± 0.5 g·day“−1 of pellet diet and did not demonstrate a consistent pattern of weight gain. The average daily food intake of pregnant animals increased linearly from 38 g·day−1 at 27 days to over 60 g·day−1 near term, and the average maternal weight gain was 13.3 g·day−1.Using sterile technique, polyvinyl catheters were inserted under anesthesia into the carotid and femoral arteries of these animals. Animals recovered spontaneously, and catheters remained patent for up to 4 wk. The effect of surgical and anesthetic stress was evaluated by measurement of food intake and metabolite levels after surgery. In the nonpregnant animals, blood glucose decreased immediately after surgery (4.76 ± 0.36 versus 5.65 ± 0.25; P < 0.05), whereas the pregnant animals responded with a substantial increase in blood glucose (7.57 ± 0.48 versus 5.87 ± 0.33; P < 0.05). Lactate was increased intraoperatively in both groups (1.76 ± 0.22 versus 1.11 ± 0.07, nonpregnant; 1.80 ± 0.17 versus 1.10 ± 0.08, pregnant). The pregnant animals differed from the nonpregnant animals with regard to the pattern of changes in blood ketones after surgery. The nonpregnant animals achieved steady state within 1 day after surgery, whereas the pregnant animals required 4 days for recovery. Food intake and maternal weight gain were markedly reduced during the recovery period for the pregnant animals.Blood metabolite levels were measured in well-fed, unstressed pregnant and nonpregnant guinea pigs. Using measurements made after the third postoperative day, the unstressed pregnant animals demonstrated a progressive decline in blood glucose levels, reaching levels significantly below those of nonpregnant animals at 55 to 60 and 60+ days. Arterial concentrations of lactate, pyruvate, β;-hydroxybutyrate, acetoacetate, and free fatty acids did not change consistently with gestation and were not significantly different from nonpregnant values.Speculation: In animal studies, it is frequently difficult to differentiate the effects of advancing gestation from the effects of acute sample collection. Extension of these techniques of chronic catheterization to other areas of guinea pig metabolism and to other small mammalian species may provide more uniform conditions for the study of the comparative physiology of gestation and fetal development.

Identification of a Unique Form of Protein C in the Ovine Fetus: Developmentally Linked Transition to the Adult Form

ABSTRACT: To investigate fetal development of protein C, a pregnant ovine model was used. Protein C was isolated from ovine plasma, and a polyclonal antibody was raised. Citrated plasma was obtained from undisturbed chronically catheterized fetal lambs. On Western blot, nonreduced adult ovine protein C had a molecular mass of 70 kD. Fetal ovine protein C was determined to have a molecular mass of 4 to 6 kD larger than the adult molecule. Crossed immunoelectrophoresis demonstrated slightly increased anodal migration of the fetal form. Isoelectric focusing demonstrated a decreased pI of the fetal molecule (4.45 versus 4.6). The ovine protein C molecules were deglycosylated with N-glycanase. Deglycosylated fetal protein C migrated more similarly to the adult form, although a portion of the fetal form persisted. These experiments demonstrate the first example of a unique fetal form of a vitamin K-dependent protein and are compatible with increased glycosylation of fetal ovine protein C. It is speculated that altered posttranslational processing may exist as a general process by which certain coagulation proteins are modified during fetal development. mRNA was isolated from maternal and fetal hepatic tissue and analyzed by Northern hybridization. Fetal plasma concentration and hepatic mRNA for protein C were both 40% of normal maternal values from midgestation onward. At term, protein C mRNA increased to adult range (p < 0.025), although plasma protein C concentration decreased slightly (p < 0.001). A transition from fetal to adult protein C form was found beginning 6 d before term birth, with a doubling time of 24 h. These data are compatible with a gestationally determined maturation of ovine protein C. There was no evidence for very low plasma concentrations of protein C during normal fetal and neonatal development. Decreased plasma protein C concentration after birth associated with increased hepatic mRNA suggests increased turnover of protein C in the perinatal period. Further investigation of the transition from fetal to adult forms of protein C and altered hepatic mRNA expression will be necessary to understand, treat, and prevent complications of protein C deficiency in the neonate.

Comparison of fetal and maternal hind limb metabolic quotients in sheep

This study compared substrate utilization by the fetal hind limb and the maternal hind limb in 26 sheep at 120 to 135 days of gestation. Catheters were placed in the mother and the fetus to sample femoral arterial and venous blood by use of a nonocclusive technique. Arterial and venous concentrations of oxygen content, glucose, lactate, acetate, and ketoacids were measured simultaneously and were used to calculate metabolic quotients. The fetal hind limb was perfused with arterial blood having a lower oxygen content than the maternal hind limb (3.03 +/- 0.17 versus 4.94 +/- 0.24 mmol/L, p less than 0.001) and had a smaller arteriovenous difference of oxygen content (0.97 +/- 0.05 versus 2.68 +/- 0.104 mmol/L, p less than 0.001). Despite a lower fetal arterial glucose concentration (0.81 +/- 0.05 versus 2.58 +/- 0.13 mmol/L, p less than 0.001), the glucose/oxygen quotient (0.82 +/- 0.05 versus 0.20 +/- 0.02, p less than 0.001) and the arteriovenous difference of glucose (0.13 +/- 0.01 versus 0.08 +/- 0.01 mmol/L, p less than 0.001) were higher in the fetal hind limb than in the maternal hind limb. Both limbs were net producers of lactate. The (glucose + lactate)/oxygen quotient was also higher in the fetal hind limb than in the maternal hind limb (0.68 +/- 0.05 versus 0.12 +/- 0.04, p less than 0.001). In the maternal hind limb, acetate and ketoacids uptake could account for 48% +/- 6% of total oxygen consumption whereas in the fetal hind limb it accounted for only 12% +/- 4% (p less than 0.001). The data demonstrate that, in relation to oxygen uptake, fetal hind limbs have approximately a 2.8% higher rate of perfusion and take up approximately four times as much glucose as the hind limbs of the mother in the resting state.

Galactose, glucose, and lactate concentrations in the portal venous and arterial circulations of newborn lambs after nursing.

ABSTRACT: We have used the newborn lamb prepared with chronic indwelling catheters to study carbohydrate metabolism in the unstressed, postprandial state. Lambs were fasted 5 h and then allowed to nurse ad libitum from their mothers for 20 min. Serial determinations of whole blood galactose, glucose, and lactate concentration were then made from the portal venous and arterial circulations. Portal venous galactose concentration increased significantly after milk ingestion, but arterial galactose concentration did not increase from baseline unless the portal venous galactose concentration exceeded 10–12 mg/dl suggesting a threshold effect for hepatic galactose clearance. Glucose concentration increased significantly in both circulations with portal venous galactose concentration > arterial galactose concentration in all cases. Galactose and glucose were absorbed from the intestine at approximately equal rates. Lactate was not absorbed into the portal venous circulation to any great extent after lactose ingestion.