Na(+)/K(+)-ATPase activity and expression in syncytiotrophoblast plasma membranes in pregnancies complicated by diabetes

Many of the transport processes across the syncytiotrophoblast (ST), such as amino acid transport, are Na(+)-coupled. The maintenance of a low intracellular Na(+) concentration by Na(+)/K(+)-ATPase is therefore crucial for placental transport of nutrients and consequently, foetal growth. In pregnancies complicated by diabetes foetal growth is often accelerated despite rigorous glycemic control of the mother, however the underlying mechanisms are not fully understood. We tested the hypothesis that Na(+)/K(+)-ATPase in ST plasma membranes is up-regulated in diabetic pregnancies associated with accelerated growth. ST microvillous (MVM) and basal (BM) plasma membranes were purified from term placentas of normal pregnancies (control, n=13) and pregnancies complicated by insulin-dependent diabetes mellitus (n=7) or gestational diabetes (n=6). All mothers with diabetes gave birth to large for gestational age babies. The Na(+)/K(+)-ATPase alpha(1)-subunit protein expression (Western blot) in MVM and BM was unaltered by diabetes. Na(+)/K(+)-ATPase activity (K(+)-stimulated, ouabain-sensitive phosphatase activity) in ST plasma membranes was not affected by diabetes. This is the first study of Na(+)/K(+)-ATPase in ST membranes of the human placenta in diabetes. Our data show that accelerated foetal growth in diabetic pregnancies is not associated with elevated ST Na(+)/K(+)-ATPase protein expression or activity.

Glucose transport and system A activity in syncytiotrophoblast microvillous and basal plasma membranes in intrauterine growth restriction

The mechanisms underlying the reduced fetal plasma concentrations of amino acids and glucose associated with intrauterine growth restriction (IUGR) remain to be fully established. The activity of the amino acid transporter system A has been shown to be reduced in the syncytiotrophoblast microvillous membrane (MVM) in IUGR, however the impact of these changes on transplacental transport is difficult to assess without information on system A activity in the basal plasma membrane (BM). In this study we measured system A activity and mediated D-glucose uptake using radiolabelled substrates and rapid filtration techniques, and glucose transporter isoform 1 (GLUT 1) protein expression using Western blots in MVM and BM isolated from human placentas. In term IUGR (n=11) MVM system A activity was unaltered compared to controls (n=9). In contrast, system A activity in MVM was reduced by 50 per cent (P< 0.05) in preterm IUGR (n=8, gestational age 28-36 weeks) as compared to controls (n=8, gestational age 28-35 weeks). BM system A activity was unaltered in both IUGR groups. Similarly, MVM and BM GLUT 1 expression and mediated D-glucose uptake was not affected by IUGR. In all preterm IUGR pregnancies signs of severe fetal compromise were present whereas term IUGR fetuses were less affected. These data support the view that MVM system A activity is related to the severity of compromise in IUGR. The markedly reduced system A activity in MVM in preterm IUGR together with the unaltered activity in BM is consistent with a decreased transplacental transport of neutral amino acids in this pregnancy complication. The hypoglycemia present in utero in some IUGR fetuses is not caused by a decreased glucose transport capacity across the syncytiotrophoblast plasma membranes.

Regulation of placental transfer: the Na(+)/H(+) exchanger--a review

This review article considers the purposes and mechanisms of regulation of placental transfer in general terms and then illustrates some key points with reference to the Na(+)/H(+) exchanger (NHE), a transport protein found in the syncytiotrophoblast. NHE probably has a role in the homeostasis of syncytiotrophoblast intracellular pH and may also be involved in syncytiotrophoblast cell volume regulation as well as H(+) loss from and Na(+) transfer to the fetus. The activity and expression of NHE in the microvillous plasma membrane of the syncytiotrophoblast is reduced in placentas from preterm, growth restricted babies as compared to their gestationally matched normally grown counterparts. There are differential effects of gestation in normal pregnancy on NHE mRNA, NHE protein and NHE activity. There is also evidence of acute modulation of NHE activity. Regulation of NHE in syncytiotrophoblast is therefore complex with control at transcription, post transcription and post translational loci.

Placental glucose transport in gestational diabetes mellitus

OBJECTIVE

We have previously reported that type 1 diabetes mellitus with hyperglycemia during the first trimester is associated with an up-regulation of placental glucose transport at term. We speculated that glucose concentrations regulate placental glucose transporters only during early pregnancy. To test this hypothesis we studied placental glucose transport in gestational diabetes mellitus, which is associated with hyperglycemia mainly during the second half of pregnancy.

STUDY DESIGN

Syncytiotrophoblast microvillous membrane vesicles and basal membrane vesicles were isolated from uneventful pregnancies (control group, n = 32) and pregnancies complicated by gestational diabetes mellitus (n = 18). Glucose uptake and glucose transporter 1 expression were studied by means of radiolabeled tracers and Western blotting, respectively.

RESULTS

Gestational diabetes mellitus was not associated with alterations in placental glucose transport. Separate analysis of 6 patients in the gestational diabetes mellitus group with large-for-gestational-age babies did not affect these results.

CONCLUSION

These findings are consistent with the hypothesis that the sensitivity of placental glucose transporters to regulation by nutrient availability is limited to early pregnancy.

Activity and expression of the Na(+)/H(+) exchanger in the microvillous plasma membrane of the syncytiotrophoblast in relation to gestation and small for gestational age birth

The effect of gestational age, low birth weight, and umbilical plasma pH on the activity and expression of the Na(+)/H(+) exchanger in the microvillous plasma membrane (MVM) of the placental syncytiotrophoblast was investigated. MVM were isolated from placentas of fetuses delivered in the first and second trimesters and from appropriately grown for gestational age (AGA) and small for gestational age (SGA) babies born at term. Na(+)/H(+) exchange activity (amiloride-sensitive Na(+) uptake) was higher (p<0.05) in second trimester and term AGA MVM versus first trimester MVM (median [range]: 1.80 [1.01-3.03], 1.72 [1.16-3.15] versus 1.48 [0.92-1.66] nmol/mg protein/30s, respectively, n = 6, 12, and 9). As regards exchanger isoforms, Western blotting showed that NHE1 expression did not change across gestation, but NHE2 and NHE3 expression were lower (p<0.01) in the first and second trimesters than in term AGA MVM. There were no differences in Na(+)/H(+) exchanger activity or in NHE1-3 expression in term AGA MVM versus SGA (n = 11) MVM. There was no correlation between exchanger activity and umbilical artery or vein plasma pH, although with a relatively small number of samples (n = 12 and 15, respectively). We conclude that there is differential regulation of the activity and expression of Na(+)/H(+) exchanger isoforms in the MVM over the course of gestation in normal pregnancy; this is not affected in pregnancies resulting in SGA babies at term.

Na(+)-K(+)-ATPase is distributed to microvillous and basal membrane of the syncytiotrophoblast in human placenta

Despite its importance for placental function, syncytiotrophoblast Na(+)-K(+)-ATPase has not been studied in detail. We purified syncytiotrophoblast microvillous (MVM) and basal (BM) membranes from full-term human placenta. Western blotting with isoform-specific antibodies demonstrated the presence of the alpha(1)-subunit, but not the alpha(2)- or alpha(3)-subunits, in MVM and BM. Relative density per unit membrane protein in BM was 48 +/- 1% (mean +/- SE, n = 4, P < 0.02) of that in the MVM. The activity of Na(+)-K(+)-ATPase was lower in BM (1.4 +/- 0.14 micromol. mg(-1). min(-1), n = 8, P < 0.02) than in MVM (3.9 +/- 0.25 micromol. mg(-1). min(-1)). Immunocytochemistry confirmed the distribution of Na(+)-K(+)-ATPase to MVM and BM. These findings suggest that the syncytiotrophoblast represents a type of transporting epithelium different from the classical epithelia found in the small intestine and kidney, where Na(+)-K(+)-ATPase is confined to the basolateral membrane only. This unique polarization of the Na(+) pump does not, however, preclude a net transcellular transport of Na(+) to the fetus.

ATP-dependent Ca2+ transport is up-regulated during third trimester in human syncytiotrophoblast basal membranes

In late gestation, Ca2+ transport across the human placenta must increase in response to the demands of accelerating bone mineralization of the fetus. This is an ATP-dependent transport against a concentration gradient across the basal or the fetal-facing plasma membrane of the syncytiotrophoblast. The aims of the present study were to determine the relationship between ATP-dependent Ca2+ transport and gestational age in the third trimester and to identify the specific isoforms of plasma membrane Ca2+ ATPase (PMCA) present in human syncytiotrophoblast. Basal membrane vesicles were isolated from normal placentas and from placentas obtained from preterm deliveries with no other complications (32-37 wk of gestation). We studied the uptake of 45Ca2+ into basal membrane vesicles in the absence and presence of ATP by using rapid filtration techniques. Western blot was used to assess the protein expression of the PMCA isoforms 1-4. Isoforms 1 and 4 of PMCA were identified in basal membrane of human placenta. The ATP-dependent Ca2+ transport increased linearly during the third trimester (r = 0.571, p = 0.0015, n = 28). However, PMCA protein expression was unaltered during the same period of gestation. Our results show that PMCA in the fetal-facing plasma membrane of the human syncytiotrophoblast is markedly activated toward the end of pregnancy. We suggest that these changes are critical in supplying the rapidly growing fetus with sufficient Ca2+ for bone mineralization.

Composition and permeability of syncytiotrophoblast plasma membranes in pregnancies complicated by intrauterine growth restriction

The objective of this study was to determine placental membrane permeabilities to water, urea and mannitol in intrauterine growth restriction (IUGR) and compare them to normal gestational age matched controls. Further, we wished to investigate whether potential changes in permeability were related to changes in membrane fluidity, cholesterol or phospholipid fatty acid content of the membranes. Syncytiotrophoblast microvillous (MVM) and basal membranes (BM) were isolated from normal and IUGR placentas at term. Passive permeability to water, urea, and mannitol showed no significant alterations in IUGR compared to controls. Cholesterol content in BM, but not in MVM, was lower in placentas from pregnancies complicated by IUGR. However, membrane fluidity did not change in these pregnancies. The phospholipid fatty acid composition of the plasma membranes isolated from all placentas showed a predominance of unsaturated fatty acid species in the BM and saturated species in the MVM. In the MVM from IUGR, mead acid (20:3), behenic acid (22:0) and nervonic acid (24:1) constituted higher percentages of the total when compared to normally grown controls. In the BM from IUGR, mead acid (20:3) was increased relative to the total phospholipid fatty acid content. In conclusion, the syncytiotrophoblast membranes exhibit only minor changes in passive permeability and composition when the pregnancy is complicated by IUGR.

Developmental pattern of phenylalanine hydroxylase activity in the chicken

Experiments were conducted to determine the conditions for assay of hepatic phenylalanine hydroxylase (PAH) activity in the chicken and to determine the developmental pattern of PAH activity in liver 25,000 x g supernatant. PAH activity was detected in liver supernatant and (postnuclear) 25,000 x g particulate fraction. Optimum assay conditions differed for the two cell fractions, the most notable difference being a broad pH optimum of 7.7 to 9.2 for the supernatant and 4.7 and 5.6 for the particulate fraction. The PAH activity in the supernatant increased to a maximum as L-phenylalanine concentration in the assay medium increased from 0.02 to 0.5 mM and 1.0 mM. Activity increased in the particulate fraction as the Phe concentration increased to 0.5 mM. Substrate inhibition of PAH activity occurred at Phe concentrations of 3 to 5 mM in the supernatant but not in the particulate fraction. Concentrations of the cofactor, 6(R)-5,6,7,8-tetrahydrobiopterin, ranging from 0.09 to 0.75 mM, resulted in maximal PAH activity. The developmental pattern of PAH in supernatant was determined using a modified assay in which substrate and cofactor concentrations and pH were optimum. The PAH activity in liver supernatant was present at a low level in 11 d chick embryos and increased several fold between Days 15 and 17 to a maximum at Days 17 to 21. Activity declined at hatching to levels that were present in 11 to 15 d embryos and remained at this level in male chicks through 4 wk of age. Mature males had higher PAH activity than mature laying females.

Gestational development of water and non-electrolyte permeability of human syncytiotrophoblast plasma membranes

In order to establish a gestational profile for placental transcellular permeabilities to water, urea and mannitol, syncytiotrophoblast microvillous (MVM) and basal membrane (BM) vesicles were isolated from human placentae obtained from 16 weeks of gestation to term. Using stop-flow/light-scattering techniques the rate of change in vesicle volume in response to an osmotic challenge was measured and osmotic water permeabilities (Pf) and solute permeabilities (Ps) calculated. Membrane fluidity was assessed by steady-state DPH anisotropy. Permeability of MVM to water and solutes increased by 20-30 per cent in mid-pregnancy and declined again after the 36th week of gestation. In BM, this pattern was apparent only for water permeability; solute permeabilities were not significantly altered. MVM cholesterol content was approx two-fold higher and membrane fluidity lower compared to BM. Cholesterol content in BM, but not in MVM, increased during the late third trimester. Membrane fluidity did not change consistently during gestational development. We conclude that syncytiotrophoblast plasma membranes exhibit small but significant changes in passive permeability to water and non-electrolytes from 16 weeks of gestation to term. It is suggested that an increased water permeability of the syncytiotrophoblast plasma membranes might contribute substantially to the gestational increase in water exchange across the human placenta observed in vivo.

Placental glucose transport and GLUT 1 expression in insulin-dependent diabetes

OBJECTIVE

Altered transport functions in the placenta might contribute to adverse outcome of pregnancies in women with diabetes. Therefore we studied placental glucose transport in this pregnancy complication.

STUDY DESIGN

Syncytiotrophoblast microvillous membrane vesicles and basal membrane vesicles were isolated from women with uneventful pregnancies (control subjects, n = 21) and from women with pregnancies complicated by insulin-dependent diabetes mellitus, White class D (n = 7). Glucose uptake and GLUT 1 (glucose transporter 1) expression were studied by means of radiolabeled tracers and Western blot, respectively.

RESULTS

In the group with insulin-dependent diabetes mellitus, values for hemoglobin A1c were moderately elevated in the first trimester (6.61 +/- 0.35) but not later in pregnancy and 4 of the 7 neonates were large for gestational age. In the basal membrane vesicles, insulin-dependent diabetes mellitus was associated with a 40% increase in GLUT 1 expression and a 59% higher mediated uptake of d -glucose. No alterations could be demonstrated in microvillus membrane vesicles.

CONCLUSION

Placental glucose transport capacity appears to be increased in insulin-dependent diabetes mellitus. These alterations might explain the occurrence of macrosomia despite well-controlled diabetes.

Placental transport of leucine and lysine is reduced in intrauterine growth restriction

Intrauterine growth restriction (IUGR) is characterized by a reduction in fetal plasma concentrations of a number of essential amino acids. Whether this is caused by impaired placental transport is unknown. We studied transport of leucine and lysine in syncytiotrophoblast microvillous (MVM) and basal membrane (BM) vesicles isolated from uncomplicated (control) and IUGR pregnancies. In addition, we investigated the possibility that leucine uptake is stimulated by an outwardly directed glycine gradient. Uptake of 3H-L-lysine (0.1 microM) and 3H-L-leucine (0.25 microM) was studied at 37 degrees C using rapid filtration techniques. In IUGR, mediated uptake of lysine was reduced by 44% (p < 0.05) in BM and uptake of leucine was lower in both MVM (-46%, p < 0.05) and BM (-38%, p < 0.05) compared with control vesicles. Intravesicular glycine (2 mM) increased the uptake of leucine by 98% in MVM (p < 0.05). These data suggest that the activity of placental transporters for cationic and neutral amino acids is reduced in IUGR. We speculate that a reduced glycine gradient in the placenta in IUGR, due to reduction in system A activity, will impair leucine transport to the fetus, providing an additional mechanism for reduced placental transport of leucine in IUGR.

Individual differences in working memory and reasoning-remembering relationships in solving class-inclusion problems

In the present experiment, we evaluated the effects of individual differences in reading span and variation in memory demands on class-inclusion performance. One hundred twenty college students whose reading spans ranged from low to medium to high (as indexed by a computerized version of the Daneman and Carpenter [1980] reading-span task) solved 48 class-inclusion problems. Half of the subjects had the solution information available when the problems were presented; the other half performed a detection task between solution information and problem presentation. The results from both standard statistical analyses and from a mathematical model indicated that differences in reading span and memory load had predictable, similar effects. Specifically, the sophistication of reasoning strategies declined when memory demands increased or when reading spans decreased. Surprisingly, these effects were primarily additive. The results were interpreted in terms of global resource models and findings from the developmental literature.

Chloride transport across syncytiotrophoblast microvillous membrane of first trimester human placenta

There are significant changes in the activity of some placental transporters between first trimester and term. However, chloride transport has previously been studied only in the term placenta. Therefore. in this study, we investigated chloride transport mechanisms in syncytiotrophoblast microvillous membrane (MVM) vesicles from first trimester human placentas and compared them with those in vesicles from term placentas. 36Cl- uptake into MVM vesicles was linear up to 45 s and had reached equilibrium by 1 h for both first trimester and term vesicles. In first trimester MVM at 0 mV, 0.1 mM diisothiocyano-2,2'-disulfonic stilbene (DIDS) blocked 25+/-3% (n=8) of 36Cl- uptake at 30 s (initial rate), which was similar to the 30+/-7% (n=6) inhibition by DIDS in term MVM. In the presence of a 25 mV inside-positive electrical potential difference, induced by imposition of a K+ gradient after preincubation with 200 microM valinomycin, 0.5 mM diphenylamine-2-carboxylate (DPC) significantly blocked 30+/-4% of 36Cl- uptake at 30 s by first trimester MVM (p < 0.01); 18+/-5% (n=8) of total uptake was inhibited by DPC but not by DIDS. There was a similar 15+/-3% (n=6) component of 36Cl- uptake by term MVM, which was inhibited by DPC but not by DIDS. Using Western blotting, it was shown that the anion exchanger-1 protein was expressed in first trimester MVM in quantitatively similar amounts to that in term MVM. This study suggests that there is both an anion exchanger and a DPC-sensitive conductance in MVM of first trimester placenta with activity similar to that of term human placenta.

Intrauterine growth restriction is associated with a reduced activity of placental taurine transporters

Taurine is an essential amino acid during fetal life and appears to be vital for the growth of the fetus and for the development of the CNS. In intrauterine growth restriction (IUGR), fetal plasma concentrations of taurine are reduced, and we tested the hypothesis that this is caused by altered placental transport of taurine. Syncytiotrophoblast microvillous membrane (MVM) and basal membrane (BM) vesicles were isolated from control (fetal weight, 3068+/-191 g; gestational age, 37.0+/-0.7 wk; n=13) and IUGR pregnancies (fetal weight, 1724+/-118 g; gestational age, 35.8+/-0.7 wk; n=11). Uptake of [3H]taurine (0.5 microM) was studied at 22 degrees C using rapid filtration techniques. Sodium stimulated taurine uptake 35-fold in MVM, confirming Na+-dependent transport in this membrane. A Na+-dependent taurine transport could also be demonstrated in BM; however, the activity was only 6% of that in MVM. Na+-independent transport activities were similar in MVM and BM. In IUGR, MVM Na+-dependent taurine transport was reduced by 34% (p < 0.05), whereas Na+-independent uptake was unaltered. In contrast to MVM, Na+-dependent taurine uptake in BM was unaffected by IUGR, whereas Na+-independent transport was decreased by 33% (p < 0.05). The highly polarized distribution of the Na+/taurine cotransporter to the MVM in conjunction with similar Na+-independent transport rates for taurine in MVM and BM provides the basis for net taurine flux from the mother to the fetus. These data suggest that the low plasma concentrations of taurine in IUGR fetuses are caused by a reduced activity of placental taurine transporters.

Mechanisms of chloride transport across the syncytiotrophoblast basal membrane in the human placenta

Chloride transport mechanisms in isolated plasma membrane vesicles were studied to characterize pathways for transcellular transport of chloride. Microvillous membrane (MVM) and basal membranes (BM) vesicles were isolated from term placentae. Western blot analysis of the anion exchanger isoform 1 (AE1) demonstrated that the density of AE1 was 12-fold higher on the MVM compared to the BM. At 30 sec, the Cl- uptake in the absence of a potential difference (p.d.) was 457.3 +/- 69.7 and 111.0 +/- 29.1 pmol/mg protein in MVM and BM, respectively (mean +/- SEM, n=6). Chloride transport pathways were characterized using diisothiocyano-2'2-disulphonic stilbene. (DIDS, 0.1 mM) and diphenylamine-2-carboxylate (DPC, 0.5 mM) in the absence or presence of inside positive membrane potentials. Anion exchange (DIDS-sensitive uptake at zero mV) was found in the MVM only. Both MVM and BM showed increased chloride uptake in the presence of inside positive potentials, suggesting the presence of chloride conductance pathways. The chloride uptake with a 25-mV inside positive p.d. could be inhibited by both DIDS and DPC in MVM and BM. However greater potentials (50 mV) showed no significant inhibition by DIDS or DPC in BM. In conclusion, the anion exchanger is unlikely to contribute significantly to chloride fluxes across BM. The data also suggest the presence of Cl- conductance pathways in both the MVM and BM which are sensitive to both DIDS and DPC.

A novel technique for studying cellular function in human placenta: gestational changes in intracellular pH regulation

This paper presents a new method for the study of cell function in primary human placental syncytiotrophoblast cells. Chorionic villous tissue fragments from term and first trimester placenta were loaded with fluorescent pH sensitive indicator dye HPTS and made adherent to a microscope cover-slip. The fragments were superfused and intracellular pH (pHi) was studied by microfluorimetry. We used this new methodology to examine the role of the Na+/H+ antiporter in pHi regulation. Syncytial cells demonstrated homeostatic pHi regulation, recovering back to basal pHi after intracellular acidification. In the absence of HCO3-, the Na+/H+ antiporter was the primary means by which syncytiotrophoblast cells recovered from an intracellular acid load in both term and first trimester samples. The rate of recovery from intracellular acidification showed a strong correlation to degree of acidification, confirming allosteric modification of antiporter activity by intracellular protons. The transporter was regulated by phosphorylation mediated by protein kinase C (PKC) at both gestational ages. This methodology represents a powerful new technique for the study of syncytiotrophoblast cell ionic regulation.

Non-electrolyte solute permeabilities of human placental microvillous and basal membranes

1. Permeability to non-electrolytes of isolated microvillous and basal membranes from human term placenta was measured using stopped-flow light-scattering techniques. The studied solutes were urea, ethylene glycol, glycerol, creatinine, erythritol, arabitol and mannitol. 2. At 37 degrees C, permeability of the microvillous membrane to mannitol and urea was 0.30 +/- 0.02 x 10(-6) cm/s (mean +/- S.E.M.) and 3.2 +/- 0.2 x 10(-6) cm/s, respectively. The corresponding permeabilities for the basal membrane were 1.2 +/- 0.1 x 10(-6) cm/s (mannitol) and 4.4 +/- 0.3 x 10(-6) cm/s (urea). The basal membrane was substantially more permeable to hydrophilic solutes than the microvillous membrane. This is probably due to differences in lipid composition, as illustrated by membrane cholesterol content, which was found to be approximately 50% lower in the basal as compared to the microvillous membrane. 3. Similarities between permeabilities in placental membranes and lipid bilayers and the linear relationship noted between solute hydrophobicity and placental permeability suggested that solutes permeate both human syncytiotrophoblast membranes by a solubility/diffusion mechanism. In the microvillous membrane this was supported by data obtained for activation energies (> 10 kcal/mol) and reflection coefficients (close to 1). In the basal membrane, low activation energies for glycerol and urea and a low reflection coefficient for urea indicated that these solutes may, in part, share a common pathway with water. 4. It was estimated that the placental permeability to molecules with a molecular weight under 200 observed in vivo can, to a great extent, be accounted for by transcellular permeation.

Effects of cements and eugenol on properties of a visible light-cured composite

An in vitro study evaluated the effects of barrier materials on mechanical properties of a visible light-cured composite restorative. The composite was cured and aged over a widely used cavity liner (Dycal), various cements (Fleck's, Durelon, Vitrabond, ZOE B & T) and free eugenol and tested at seven and 28 days. The properties of interest were tensile stress (strength) at rupture, compressive strain (deformation at rupture), and stress-strain ratio at rupture. The experiment indicated that underlying barrier materials exerted neither a beneficial nor an adverse effect upon the restorative's ability to withstand compressive strain and tensile stress (P < 0.05).

Elevated fetal plasma lactate produces polyhydramnios in the sheep

In human fetuses with hemolytic diseases such as erythroblastosis fetalis, hydrops fetalis or polyhydramnios often develops. The mechanism(s) that produces these fluid imbalances is unknown, although lactate concentrations have been reported to be elevated in hydropic human fetuses with erythroblastosis. In this study we explored the role of lactate in producing fetal fluid imbalances. In seven near-term fetal sheep, we infused 5 mol/L sodium lactate at a rate of 10 mmol/hr for 3 days. Fetal plasma lactate rose by 6.0 +/- 1.0 (mean +/- SE) mmol/L above control. Fetal plasma osmolality and Na+ increased slightly, Cl- decreased, and bicarbonate rose in proportion to the Cl- decrease. Fetal renal lactate excretion was 1.1 +/- 0.3 mmol/hr while Na+ excretion was 10.6 +/- 1.9 mEq/hr. Fetal urine flow increased by 1.9 +/- 0.4 L/day and the urine remained hypotonic relative to fetal plasma throughout the infusion. Amniotic fluid lactate and Na+ rose during the infusion period and remained elevated during a 24-hour recovery period. Amniotic plus allantoic fluid volume at autopsy was 5.3 +/- 0.8 L compared with a normal of 0.5 to 1.0 L. There was little evidence of fetal edema. In summary, a moderate sustained elevation in fetal plasma lactate concentration appears to be a powerful osmotic agent for fetal accumulation of fluid from the maternal compartment over a period of days. This may be the primary mechanism whereby hydrops fetalis or polyhydramnios develops in severely anemic human fetuses.

Fetal fluid responses to long-term 5 M NaCl infusion: where does all the salt go?

The fetus must obtain Na and Cl ions in order to grow. However, the regulation of electrolyte acquisition by the fetus is not well understood. To explore fetal electrolyte balance, we intravenously infused 5 M NaCl at a rate equal to 80% of the total fetal body Na+ and Cl- content per day (240 mM/day) for 3 days into late-gestation fetal sheep. We hypothesized that the increase in fetal osmolality resulting from the infusion would cause a transplacental water movement into the fetal compartment, leading to hydrops fetalis and/or polyhydramnios. The fetal-to-maternal osmotic gradient was initially -2.8 +/- 0.9 (SE) mosmol/kgH2O and rose by 4.8 +/- 1.8 mosmol/kgH2O during the infusion. Fetal plasma [Na+] and [Cl-] increased (3.0 +/- 0.4 and 5.5 +/- 0.5 meq/l, respectively), but the normal maternal-to-fetal transplacental concentration gradients for these ions were not reversed. Most of the infused Na+ (92 +/- 14%) and Cl- (82 +/- 12%) was excreted by the fetus in large volumes of hypotonic urine. Amniotic fluid osmolality and [Na+] were unchanged, but amniotic [Cl-] increased 5.7 +/- 2.4 meq/l. The amniotic plus allantoic fluid volume, as estimated by ultrasonography, was increased (43.5 +/- 14.5%) at day 2 and returned to control by day 3 of infusion. There was no fetal edema during the study or at autopsy. In light of these results, we propose a novel and somewhat complex mechanism for transplacental fluid and electrolyte movement in which placental capillary permeability increases along the length of the capillary.(ABSTRACT TRUNCATED AT 250 WORDS)