Fetal hepatic and umbilical uptakes of glucogenic substrates
during a glucagon-somatostatin infusion
Cecilia
Teng,
Frederick C.
Battaglia,
Giacomo
Meschia,
Michael R.
Narkewicz, and
Randall B.
Wilkening
Departments of Pediatrics and Physiology, University of
Colorado School of Medicine, Aurora, Colorado, 80045-0508
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ABSTRACT |
To
test the hypothesis that fetal hepatic glutamate output diverts the
products of hepatic amino acid metabolism from hepatic gluconeogenesis,
ovine fetal hepatic and umbilical uptakes of glucose and glucogenic
substrates were measured before and during fetal glucagon-somatostatin
(GS) infusion and during the combined infusion of GS, alanine,
glutamine, and arginine. Before the infusions, hepatic uptake of
lactate, alanine, glutamine, arginine, and other substrates was
accompanied by hepatic output of pyruvate, aspartate, serine,
glutamate, and ornithine. The GS infusion induced hepatic output of
1.00 ± 0.07 mol glucose carbon/mol O2 uptake, an
equivalent reduction in hepatic output of pyruvate and glutamate
carbon, a decrease in umbilical glucose uptake and placental uptake of fetal glutamate, an increase in hepatic alanine and arginine
clearances, and a decrease in umbilical alanine, glutamine, and
arginine uptakes. The latter result suggests that glucagon inhibits
umbilical amino acid uptake. We conclude that fetal hepatic pyruvate
and glutamate output is part of an adaptation to placental function
that requires the fetal liver to maintain both a high rate of
catabolism of glucogenic substrates and a low rate of gluconeogenesis.
fetal liver; placenta; glutamate; fetal gluconeogenesis
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INTRODUCTION |
UNDER NORMAL PHYSIOLOGICAL
CONDITIONS, the liver of the ovine fetus releases glutamate in
the fetal circulation (18, 23, 25). About one-third of the
glutamate released is produced from hydrolysis of plasma glutamine
(25), and the remainder is produced via hepatic oxidative
metabolism. The fetal infusion of uniformly labeled
[13C]lactate results in the appearance of hepatic venous
glutamate, which is labeled mostly with either one or two
13C carbons per molecule (T. S. Guyton, P. V. Fennessey, F. C. Battaglia, G. Meschia, and R. B. Wilkening, unpublished
observations). Glutamate output is an important aspect of fetal hepatic
metabolism, as demonstrated by the calculation that hepatic
O2 consumption would have to increase ~65% if the liver
were to retain and oxidize the glutamate that it normally excretes
(25).
Glutamate is only one of several glucogenic substrates that are
exchanged between the fetal liver and its circulation (3, 8,
18). The relative importance of each substrate in determining the balance of this exchange has not been evaluated, because different substrates have been studied at different times. Therefore, one aim of
the present study was to measure simultaneously and under normal
physiological conditions the fetal hepatic uptakes of glucose, lactate,
pyruvate, and all of the main glucogenic amino acids.
In fetal life, hepatic glutamate output may depend on a low rate of
hepatic gluconeogenesis (8) that prevents the utilization of the products of hepatic amino acid oxidation for glucose synthesis (19). Pharmacological doses of glucagon have been shown to
increase in the fetal rat the activity of hepatic
phosphoenolpyruvate carboxykinase (PEPCK; EC 4.1.1.32), a
rate-limiting enzyme of hepatic gluconeogenesis (7), and
to stimulate the appearance of fetal glucose in fetal lamb plasma
(6). This evidence suggests that the infusion of large
doses of glucagon in the fetus could be used as a test of the
hypothesis that stimulation of fetal hepatic gluconeogenesis reduces
fetal hepatic glutamate output. In preliminary experiments, we infused
glucagon for 3 days at a constant rate in the fetal circulation. In
apparent agreement with the predicted effect, we observed an increase
in fetal plasma glucose that was accompanied by a decline in fetal
plasma glutamate concentration. However, we also observed a decline in
the plasma concentrations of several other glucogenic amino acids,
notably glutamine, alanine, and arginine. Normally, these amino acids
are delivered by the placenta to the umbilical circulation and are
taken up by the fetal liver (18). All of these changes
persisted for the duration of the infusion and were reversed by its
termination. In addition, the glucagon infusion caused an increase of
fetal plasma insulin that could be suppressed by the coinfusion of somatostatin.
On the basis of these preliminary observations, we designed the present
study to measure fetal hepatic and umbilical uptakes of amino acids,
glucose, lactate, and pyruvate under the following three conditions:
control, during a fetal glucagon-somatostatin infusion, and during the
combined infusion of glucagon, somatostatin, alanine, glutamine, and
arginine. The latter condition was meant to provide information about
the effect of glucagon on fetal hepatic glutamate output when there is
no restriction of the supply of glucogenic amino acids to the liver.
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MATERIALS AND METHODS |
All animal experimentation was performed according to the
Helsinki convention for the care and use of animals and was approved by
the local Institutional Animal Care and Use Committee.
Columbia-Rambouillet ewes, each carrying a single fetus, were studied.
Three animals were used to collect preliminary data and to develop the
following procedure, which was then applied to five sheep.
Animal preparation.
At gestational ages ranging between 118 and 123 days, the ewes were
placed under general anesthesia with pentobarbital sodium (20 mg/kg
initial dose) and spinal anesthesia with tetracaine hydrochloride (20 mg). Under sterile surgical conditions, the uterus was exposed via a
midline laparotomy incision. Through an 8-cm uterine incision, 20-gauge
polyvinyl catheters were introduced into the fetal abdominal aorta via
a hindlimb pedal artery, the fetal inferior vena cava via a hindlimb
pedal vein, the two fetal axillary veins via brachial veins of the
forelimbs, and the common umbilical vein via one of the two umbilical
veins. Next, the fetal left hepatic vein was catheterized via a right
thoracotomy, as previously described (25). After a
catheter was placed in the amniotic cavity and the uterine and
abdominal incisions were closed, a catheter was inserted into a
maternal femoral artery. All catheters were secured at their point of
insertion with cyanoacrylate adhesive and tunneled subcutaneously to a
pouch in the flank of the ewe.
Analgesics were given to the ewe in the first two postoperative days.
Ampicillin (500 mg) was administered via the amniotic catheter at the
end of surgery and for the first three postoperative days. After
surgery, the ewe was kept in a rectangular 1.5 × 1.0-m cart at
18 ± 2°C room temperature, with ad libitum access to alfalfa pellets, water, and mineral block. Two ewes were kept in adjacent carts
at all times to create a nonstressful environment. Each day, the cart
was cleaned, food intake was recorded, and all catheters were flushed
with a solution of heparin in isotonic saline (30 IU/ml). The
experimental protocol was carried out 5-11 days (mean 8 days)
after surgery. At the end of the study, both mother and fetus were
injected intravenously with an overdose of pentobarbital sodium.
Samples of fetal liver were obtained within 5 min after the
pentobarbital sodium injection and were analyzed for PEPCK activity.
Experimental protocol.
The experiment required the collection of fetal blood during three
separate sampling periods: the control period, during which the fetus
was infused with 3H2O, the first experimental
period (E1), during which the fetus was infused with
3H2O, glucagon, and somatostatin, and the
second experimental period (E2), during which the fetus was
infused with 3H2O, glucagon, somatostatin,
alanine, glutamine, and arginine. The 3H2O
infusion was via one of the two fetal axillary veins. Its purpose was
to estimate umbilical blood flow by means of the steady-state transplacental diffusion method (24) and to estimate the
contribution of umbilical venous blood to blood flow of the left
hepatic lobe (25). The glucagon-somatostatin infusion was
via the fetal inferior vena cava. It started 7 h after completion
of the control sampling and lasted 20 h. The amino acid infusion
was via one of the two axillary veins. It started immediately after
collection of the last set of E1 samples and lasted 3 h. The timing of infusions and sampling is summarized in Fig.
1. It was planned so all of the blood
samplings could be completed in the morning hours. An additional
purpose of the overnight glucagon-somatostatin infusion was to deplete
the liver of glycogen so that, in interpreting the subsequent
measurements of hepatic uptake, we could rely on the assumption that
the net hepatic glucose output included glucose produced by
gluconeogenesis. Information about the infusates is given in Table
1. In each sampling period, four sets of
simultaneously drawn blood samples were obtained at 20-min intervals
from the maternal artery, the fetal artery, the umbilical vein, and the left hepatic vein. Each fetal sample was analyzed for hematocrit, hemoglobin content, O2 saturation,
3H2O, glucose, lactate, pyruvate, and amino
acids. In addition, fetal arterial blood was analyzed for plasma
insulin and glucagon. The total amount of fetal blood drawn in each
sampling period was 28 ml. To compensate for blood loss, the fetus was
transfused two times with 14 ml of maternal blood at the beginning and
at the end of each sampling period. Given an estimated fetal blood volume of 250 ml, the transfusions were aimed at maintaining fetal blood volume within ±6% of normal. Maternal arterial blood was analyzed for plasma glucose and amino acid concentrations.

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Fig. 1.
Summary of experimental protocol, showing time of blood
sampling and fetal infusions. E1, first experimental
period; E2, second experimental period.
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Analytical methods.
Hematocrit was measured after a 3-min centrifugation in a
microcapillary centrifuge (model MB; International Equipment, Needham Heights, MA). Hemoglobin and O2 content were measured
spectrophotometrically (OSM3; Radiometer, Copenhagen, Denmark). Blood
O2 content was calculated as the product of hemoglobin
content, expressed as O2 capacity, times O2
saturation. Plasma glucose and lactate concentrations were measured in
duplicate with a glucose-lactate analyzer (YSI model 2700 Select and
Dual Standard). Blood pyruvate was measured in duplicate by an assay
using the oxidation of NADH to NAD+ by pyruvate in the
presence of lactate dehydrogenase (Sigma Diagnostics, St. Louis, MO).
Plasma 3H2O concentration was measured in
triplicate aliquots in a scintillation counter.
Plasma samples for amino acid analysis were stored at
70°C until
the day of measurement. At that time, the samples were thawed quickly
and deproteinized with 15% sulfosalicylic acid containing 0.3 µmol/ml norleucine as an internal standard. The pH was brought to 2.2 by titration with 1.5 N LiOH. After centrifugation, the supernatant was
analyzed with a Dionex HPLC amino acid analyzer (Dionex, Sunnyvale,
CA). Each set of simultaneously drawn samples was loaded on the
analyzer to run within 12 h, and the same column was used for all
samples from an individual animal. Reproducibility within the same
column had a mean value of ±2%. Amino acid absorbances were measured
after reaction with ninhydrin at 570 nm, except proline, which was
measured at 440 nm wavelength. Plasmal insulin and glucagon were
measured in duplicate using a rat insulin RIA kit (catalog no.
R1-13K) and a Glucagon RIA kit (catalog no. GL-32K) from Linco
Research (St. Charles, MO). Cross-reactivity of the rat insulin test
with sheep insulin was virtually 100%. According to manufacturer
specifications, the glucagon kit was specific for pancreatic glucagon.
Glucagon was measured in blood samples protected from proteolysis by
the addition of aprotinin.
For the PEPCK assay, cytosolic fractions were prepared from fresh liver
according to the method of Chang and Lane (4). PEPCK was
assayed by the [14C]bicarbonate fixation assay, as
previously reported (20). Results were expressed as
nanomoles of product fixed per minute per milligram of cytosolic
protein. Protein was determined by the method of Lowry as modified by
Hartree (11). The cytosolic fraction was assayed routinely
for mitochondrial contamination by measuring citrate synthase
(21) and was found to be <1%.
Calculations.
Umbilical blood flow was calculated by the steady-state transplacental
diffusion method, using tritiated water as the blood flow indicator
(24).
The umbilical uptake of the two metabolic substrates that were measured
in whole blood (O2 and pyruvate) was calculated by means of
the equation
where Q is umbilical blood flow
(ml · min
· kg fetus
1) and
(
)b,x is the concentration
difference of substrate x per milliliter of blood (b)
between umbilical venous (
) and umbilical arterial (
) blood.
The umbilical uptake of substrates that were measured in plasma was
calculated by means of the equation
where (
)p,x is
the concentration difference of substrate x per milliliter
of plasma (p) across the umbilical circulation, Ht is the fractional
hematocrit, and
x is a coefficient whose
value is inversely related to the contribution of erythrocytes to
uptake. For lactate,
x was set to zero on the
assumption that there is rapid lactate exchange between plasma and
erythrocytes and because fetal plasma and blood lactate concentrations
are approximately equal (2). For each amino acid,
x was set to one, because in the sheep fetus
the amino acid exchange between plasma and erythrocytes is quite slow compared with the exchange between plasma and body organs
(5). For glucose,
x was set
equal to 0.24 on the basis of paired analysis of blood and plasma
glucose concentrations in the sheep fetus (unpublished observations).
The uptake of metabolic substrates by the left lobe of the fetal liver
was normalized for hepatic O2 uptake by calculating a
substrate-to-O2 uptake molar ratio. For pyruvate
where (hi
h)b represents the
concentration difference between the blood entering the circulation of
the left hepatic lobe (hi) and the blood exiting the liver
via the left hepatic vein (h).
For all of the other substrates, the substrate-to-O2 uptake
molar ratio was calculated according to the equation
where (hi
h)p,x
represents the input
output plasma concentration difference of
substrate x across the left hepatic lobe circulation.
The hi concentration of each substrate was calculated from
the measured values of umbilical venous and fetal arterial
concentration, according to the equation
where F represents the fractional contribution of umbilical
venous blood to the total blood flow of the left hepatic lobe (25). In each fetus, the F factor was calculated
(25) using the tritiated water concentration (t) in
,
, and h
Statistical analysis.
For each of the concentration and uptake measurements, the data were
subjected to two-way ANOVA (control, E1, and E2
studies in 5 sheep). Sheep were considered a random effect, and the
four repeated measurements during a study on a sheep were averaged before analysis. Comparison-specific P values were
calculated for the three paired studies (control vs. E1,
control vs. E2, and E1 vs. E2).
P values were considered significant at P < 0.05.
 |
RESULTS |
The experimental procedure had no significant effect on umbilical
blood flow and O2 uptake (Table
2). Umbilical venous blood contributed
most of the blood perfusing the left hepatic lobe (F = 0.92 ± 0.027), in agreement with previous studies (8, 13,
25).
Carbohydrate metabolism.
In the control period, the liver showed no glucose output and a
relatively large uptake of lactate. Hepatic pyruvate output accounted
for ~40% of the hepatic lactate uptake. The placenta delivered into
the umbilical circulation glucose and lactate and had a small uptake of
pyruvate (Table 3).
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Table 3.
Glucose, lactate, and pyruvate concentrations and uptakes and glucagon
and insulin concentrations in control and two experimental periods
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The glucagon-somatostatin infusion stimulated glucose output by the
fetal liver, increased fetal plasma glucose concentration, and
decreased umbilical glucose uptake. In addition, it decreased the
concentrations of both lactate and pyruvate in fetal plasma and
decreased the hepatic output of pyruvate. The
alanine-glutamine-arginine infusion was associated with an increase in
plasma lactate, pyruvate, and glucose concentrations and a decrease in
hepatic lactate uptake. Plasma insulin remained virtually constant
throughout the experiment.
Amino acid metabolism.
Most of the amino acids had positive hepatic and umbilical uptakes.
Aspartate, serine, glutamate, and ornithine had negative hepatic
uptakes (i.e., hepatic output). Glutamate also had a large negative
umbilical uptake (i.e., placental uptake of fetal glutamate), whereas
aspartate, serine, and ornithine had virtually no umbilical uptake
(Tables
4-6).
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Table 5.
Amino acid-to-O2 uptake molar ratios
(×103) across the fetal hepatic
circulation in the control period and two experimental periods
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The glucagon-somatostatin infusion significantly decreased the fetal
plasma concentrations of all amino acids, with the exception of
phenylalanine. For the glucogenic amino acids aspartate, threonine, glutamate, glutamine, proline, alanine, ornithine, and arginine, the
plasma concentration decreased to <50% of control values. The
alanine-glutamine-arginine infusion restored the plasma concentrations of the infused amino acids to normal values. The infusion also returned
plasma ornithine concentration to normal. The decrease in fetal plasma
glutamate was associated with a decrease in fetal hepatic glutamate
output and placental uptake of fetal glutamate to ~20% of control
values. Among the amino acids that are normally taken up by the fetal
liver, their decrease in plasma concentration during the
glucagon-somatostatin infusion was not associated with a significant
decrease in hepatic uptake, with the exception of glutamine. By
contrast, the increase in plasma alanine, glutamine, and arginine
concentrations caused by the alanine-glutamine-arginine infusion was
associated with a significant increase in the hepatic uptake of all
three amino acids (E2 vs. E1 comparison). The
relation between the hepatic uptake and hepatic input concentration for alanine, glutamine, and arginine is presented in Fig.
2. Figure 2 shows that hormonal
stimulation increased the hepatic uptake per unit plasma concentration
of alanine and arginine and may have had a similar but smaller effect
on glutamine uptake.

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Fig. 2.
Hepatic uptake vs. hepatic plasma input concentration for
alanine, glutamine, and arginine. E1 and E2
points, representing measurements during the glucagon-somatostatin
infusion, are joined by a solid straight line. Linear extrapolation
(dashed lines) is used to show that, at equal concentration values,
hepatic uptake was less in the control period (C). Mean uptake (Table
5) and concentration values for the 5 sheep were used to construct the
graph.
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The umbilical uptakes of alanine, glutamine, and arginine were
significantly less than control during the alanine-glutamine-arginine infusion. For each amino acid, the decrease in uptake was not associated with a significant increase in fetal plasma concentration above control and could not be accounted for by a decrease in maternal
concentration, which remained virtually constant throughout the
experiment (data not shown). Figure 3
presents a semilogarithmic plot of umbilical uptake vs. the
maternal-to-fetal plasma concentration ratio for the three amino acids.
It is apparent that, at comparable values of maternal and fetal
concentrations, the umbilical uptakes were less than normal during the
glucagon-somatostatin infusion.

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Fig. 3.
Umbilical uptake vs. the base 10 logarithm of the
maternal arterial (A)-to-fetal arterial ( ) plasma concentration
ratio for alanine, glutamine, and arginine. The E1 and
E2 points, representing measurements during the
glucagon-somatostatin infusion, are joined by a solid straight line.
Linear extrapolation (dashed lines) is used to show that, at equal
maternal-to-fetal concentration ratios, umbilical uptake was greater in
the control period. Mean uptake values (Table 6) and ratios of the mean
concentrations for the 5 sheep were used to construct the graph.
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At the end of the study, hepatic cytosolic PEPCK activity was
significantly higher than in seven fetuses of comparable age that had
undergone experiments that did not involve stimulation of fetal
gluconeogenesis (28.7 ± 7.1 vs. 7.5 ± 1.8 nmol · min
1 · mg protein
1,
P < 0.05).
 |
DISCUSSION |
The control data for hepatic and umbilical uptakes are in good
agreement with previous measurements (3, 5, 8, 12, 13, 18, 23,
25). However, this is the first study in which the fetal hepatic
uptakes of glucose and all major glucogenic substrates have been
measured in the same experimental preparation. The study demonstrates
that, under normal physiological conditions, the fetal liver has an
uptake of glucogenic substrates that is large compared with its rate of
oxidative metabolism. According to the data summarized in Fig.
4, the combined normal fetal hepatic uptake of lactate and the glucogenic amino acids glutamine, alanine, arginine, threonine, valine, methionine, glycine, proline, histidine, and asparagine represents a hepatic uptake of 2.47 ± 0.33 mol substrate carbon/mol O2 uptake. Contrary to what might be
predicted from knowledge of postnatal hepatic metabolism
(14), this uptake does not result in the hepatic output of
glucose. It results in the hepatic output of five other glucogenic
substrates, namely pyruvate, glutamate, serine, aspartate, and
ornithine for a total output of 1.53 ± 0.23 mol substrate
carbon/mol O2 uptake. The two major components of this
output are glutamate and pyruvate (1.30 ± 0.23 substrate
carbon-to-O2 uptake molar ratio).

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Fig. 4.
Fetal hepatic uptake and output of glucose carbon and
glucogenic substrate carbon under normal physiological conditions
(control) and during a glucagon-somatostatin infusion into the fetal
circulation. Each number represents a substrate
carbon-to-O2 uptake molar ratio.
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The hepatic and umbilical uptakes reported here are not directly
comparable because they are expressed in different units. However, a
rough estimate of hepatic uptake as micromoles per minute per kilogram
fetus can be obtained by using the glutamate data. Placental glutamate
uptake is ~60% of the fetal plasma glutamate disposal rate
(19). On the assumption that the fetal liver is virtually
the only net producer of fetal plasma glutamate, we can estimate normal
hepatic glutamate output as umbilical uptake divided by 0.6. Therefore,
we estimate that the control hepatic glutamate output was 8.8 µmol · min
1 · kg fetus
1
(i.e., 5.28/0.6) and that all of the hepatic uptakes in Tables 3 and 5
should be multiplied by 0.048 (i.e., 8.8/183) to estimate hepatic
uptakes in the same units as the umbilical uptakes. The 0.048 conversion factor represents hepatic O2 uptake in
millimoles per minute per kilogram fetus. Given the complexity of the
fetal hepatic circulation, the O2 consumption of the fetal
liver has been difficult to measure, and its true value is uncertain.
In a recent study of fetal hepatic blood flow (13), fetal
O2 consumption was estimated to be 0.058 mmol · min
1 · kg fetus
1.
Despite this uncertainty, the comparison of hepatic and umbilical uptakes allows two interesting conclusions. First, normal hepatic glucose uptake is virtually zero compared with umbilical uptake. Second, among the essential amino acids, hepatic uptake of the branched-chain amino acids (leucine, isoleucine, and valine) is a small
fraction of umbilical uptake, whereas the hepatic uptake of threonine,
methionine, phenylalanine, and histidine is approximately one-half the
umbilical uptake. The essential amino acids for which the fetal liver
is the main site of catabolism would be expected to show a relatively
high hepatic-to-umbilical uptake ratio.
The present study demonstrates that a glucagon-somatostatin infusion
markedly alters the metabolic exchange between the fetal liver and its
circulation. Approximately 16 h after the start of the infusion,
the fetal liver had switched from glucose uptake to glucose release.
Inspection of Table 3 shows that this switch amounted to a loss of
1.00 ± 0.07 mol glucose carbon/mol O2 uptake [i.e.,
6 × (0.132 + 0.035)]. Tables 3 and 5 show that this deficit was compensated for primarily by a decline in the hepatic release of
glucogenic substrates. Concomitant with the loss of glucose carbon,
there was a similar decrease in the combined output of pyruvate,
aspartate, serine, glutamate, and ornithine carbon from 1.53 ± 0.23 to 0.52 ± 0.13 mol substrate carbon/mol O2
uptake. The most important components of this decrease were glutamate and pyruvate (from 1.30 ± 0.23 to 0.34 ± 0.09 mol substrate
carbon/mol O2 uptake). By contrast, none of the glucogenic
substrates that are normally taken up by the fetal liver had a
significant increase in uptake. Their aggregate uptake showed a small,
statistically insignificant decrease (from 2.47 ± 0.33 to
2.34 ± 0.11 mol substrate carbon/mol O2 uptake). Note
that, in principle, a decrease in the substrate-to-O2
uptake molar ratio may be the result of an increase in O2
uptake. However, the relative changes in carbohydrate and amino acid
uptake would remain the same if each uptake were to be expressed per
gram of liver or any other parameter. The effect of the
glucagon-somatostatin infusion on the fetal hepatic exchange of glucose
and glucogenic substrates is summarized in Fig. 4.
The most likely explanation for the decrease in hepatic pyruvate and
glutamate output is that glucagon stimulation increased the flux of
glucogenic amino acids and lactate carbon into hepatic glucose
production and decreased the availability of pyruvate and glutamate for
hepatic excretion. In favor of this explanation is the finding that the
glucagon-somatostatin infusion significantly increased the activity of
cytosolic PEPCK. The result that hepatic uptake of glucogenic amino
acids was not greater than normal is explained by the decrease in their
plasma concentrations. When the fetal infusion of alanine, glutamine,
and arginine restored the plasma concentrations of these amino acids to
the normal range, their hepatic uptakes increased significantly.
Concomitant with this increase, there was a decrease in lactate uptake,
suggesting competition for hepatic uptake between lactate and the
infused amino acids. A plot of the hepatic uptake vs. plasma
concentration (Fig. 2) shows that hormonal stimulation had actually
increased alanine and arginine uptake per unit plasma concentration;
i.e., it had increased their hepatic clearance. The increase in
clearance is likely to represent a change in the activity of hepatic
amino acid transporters. Alanine uptake by the liver is mediated in part by the sodium-dependent "system A" transporters, the activity of which can be increased by glucagon (16). In this
context, it is important to note that the fetal infusion of the
glucocorticoid dexamethasone decreases hepatic glutamate output
primarily via inhibition of the hepatic uptakes of glutamine and
alanine and does not result in the output of glucose (23).
Together, the glucagon-somatostatin and dexamethasone experiments point
to the conclusion that the high glutamate output of the fetal liver is sustained by combining a low rate of gluconeogenesis with a high rate
of catabolism of glucogenic amino acids taken up from the fetal circulation.
The decrease in umbilical glucose uptake associated with the
glucagon-somatostatin infusion is a predictable effect of the significant increase in fetal plasma glucose. The transport rate of
maternal glucose to the fetus is a function of the glucose concentration difference between maternal and fetal plasma
(12). We surmise that fetal hepatic glucose production was
the main cause of the increase in fetal plasma glucose, which in turn
caused a decrease in the transplacental glucose concentration gradient that drives maternal glucose into the umbilical circulation. Similarly, the decrease in placental uptake of fetal glutamate was a consequence of the decrease in hepatic glutamate output, which caused the decrease
in fetal arterial plasma glutamate and reduced the availability of
glutamate for placental uptake.
The data about alanine, glutamine, and arginine umbilical uptakes
require a different interpretation. The glucagon-somatostatin infusion
inhibited the umbilical uptake of these amino acids, and the inhibition
could not be accounted for by changes in maternal and fetal plasma
concentrations. Tracer studies have shown that, for any given amino
acid, umbilical uptake is the difference of two opposite fluxes, i.e.,
from placenta to fetus and from fetus to placenta (3, 9,
25). The flux of fetal alanine into the placenta is mediated by
system A transporters (22). This suggests that glucagon
may have acted similarly on hepatic and placental alanine transport by
increasing the alanine flux mediated by these transporters. An increase
of alanine flux into the placenta would result in a decrease of alanine
umbilical uptake. The significant decrease in proline umbilical uptake
may have also been the result of a change in system A activity. It is
uncertain whether the suggested mechanism for the decrease in alanine
uptake could be applied to glutamine. There is no information about the
role of system A transporters in mediating glutamine exchange between the fetus and placenta. Furthermore, a decrease in the placental uptake
of fetal glutamate could, by itself, decrease placental glutamine
output via a decrease in placental glutamine production. Concomitant
with the decrease in arginine umbilical uptake, there was a decrease in
lysine uptake. Because arginine and lysine are basic amino acids, this
finding suggests a change in the placental transport system that
mediates the fetoplacental exchange of these amino acids. The
unexpected finding of umbilical uptake inhibition suggests that fetal
glucagon plays a role in regulating placental amino acid transport.
However, the effect on umbilical uptake was produced by infusing the
high dose of glucagon that is required to stimulate fetal hepatic
glucose output. Experiments with much lower doses would be necessary to
verify this suggestion. An additional implication for which there is no
direct evidence is that glucagon receptors are present on the fetal
surface of the placenta.
It is apparent that one important function of the low rate of fetal
hepatic gluconeogenesis is to promote the uptake of maternal glucose by
the fetus. The absence of fetal hepatic glucose output maintains the
glucose concentration in fetal plasma at a much lower level than in
maternal plasma and determines the steep transplacental glucose
concentration gradient that drives glucose from mother to fetus.
Although, via this mechanism, maternal glucose represents a major
metabolic fuel for the fetus, the transport rate of maternal glucose
across the placenta is not sufficient, by itself, to sustain fetal
energy metabolism. In addition to glucose, fetal oxidative metabolism
uses lactate and amino acids supplied by the placenta (1).
The net flux of amino acids from placenta to fetus includes all of the
major neutral and basic glucogenic amino acids, with the exception of
the neutral amino acid serine (5). Adaptation of the liver
to prenatal life requires the ability to contribute to the rapid
disposal of the glucogenic substrates supplied by the placenta without
utilizing these metabolites for glucose synthesis. The present study
indicates that, within the fetal liver, the routing of glucogenic
substrates into gluconeogenesis is blocked at the formation of pyruvate
and glutamate and results in their hepatic output. Fetal hepatic
glutamate output is likely to depend on the activity of a specialized
glutamate transport system that is normally expressed much more in
fetal than in postnatal life. A candidate for this role is the
sodium-independent glutamate transporter described by Makowske and
Christensen (17). It is present in fetal hepatocytes and
hepatoma cells but is virtually absent in the adult hepatocyte.
Glutamate uptake by the placenta is mediated by a sodium-dependent
transport system (22). Glutamate is used by placental mitochondria in steroidogenesis (15). It is not yet clear
what other functions placental glutamate uptake and metabolism may serve. The lactate uptake-pyruvate output of the fetal liver is matched
by a reciprocal lactate output-pyruvate uptake by the fetal hindlimbs
(2).
Hepatic serine output is another interesting aspect of the integration
of placental and fetal hepatic function. Tracer studies have shown
large and opposite serine exchange fluxes between placenta and fetus,
which result in virtually zero uptake or a small net uptake of fetal
serine by the placenta (3). The placenta converts maternal
and fetal serine to fetal glycine, which is then used by the fetal
liver to produce some of the hepatic serine output (3). A
study of amino acid uptake by the fetal hindlimb has demonstrated a
significant serine uptake by this organ (26). It is
apparent that fetal hepatic serine output has the function of supplying
serine to other fetal organs and to the placenta. The
glucagon-somatostatin infusion significantly decreased fetal plasma
serine concentration but had no detectable effect on hepatic serine
output. Therefore, it is not clear from the present study whether a low
rate of fetal gluconeogenesis contributes to fetal hepatic serine output.
 |
ACKNOWLEDGEMENTS |
We are grateful to Professor Gary Zerbe for help in statistical
analysis and to Susan Anderson for technical assistance.
 |
FOOTNOTES |
This work was supported by National Institutes of Health Grants R29
HD-31648, PO1 HD-20761, and RO1 HD-34837.
Address for reprint requests and other correspondence: F. C. Battaglia, Fitzsimons, Bldg. 260, POB 6508, MS F441, 13243 E. 23rd
Ave., Aurora, CO 80045-0508.
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
10.1152/ajpendo.00248.2001
Received 7 July 2001; accepted in final form 7 November 2001.
 |
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