Permeability of the near-term rat placenta to hydrophilic solutes

Trastuzumab in the Treatment of Pregnant Breast Cancer Patients - an Overview of the Literature

Breast cancer is one of the most common malignancies which appear during pregnancy. Since women are increasingly not giving birth until they are at a more advanced age, it can be assumed that the incidence of pregnancy-related breast cancers will continue to increase in the future. Because of pregnancy-induced changes and conservative diagnosis, these carcinomas are frequently not detected until they are at an advanced stage and thus generally require systemic adjuvant therapy. The available data on optimal chemotherapeutic management are limited. Particularly for the use of the target agent trastuzumab which could crucially contribute to improving the prognosis in the therapy of HER2-overexpressing breast cancer in non-pregnant women, there is a lack of definitive information regarding the profile of action and safety in pregnancy as well as with regard to any long-term effects on the child. Thirty-eight pregnancies on trastuzumab for the treatment of breast cancer were able to be analysed in the literature currently available. Information can be gained from this and conclusions can be drawn which can individualise and decisively improve therapeutic options in the future for the pregnant breast cancer patient.

Knowledge needed about the exchange physiology of the placenta

There is now a basic understanding of the driving forces and mechanisms underlying rates of solute exchange across the placenta but there are still major gaps in knowledge. Here we summarise this basic understanding, whilst highlighting gaps in knowledge. We then focus on two particular areas where more knowledge is needed: (1) the electrical potential difference (PD) across the placenta and (2) the paracellular permeability of the placenta to hydrophilic solutes. In many species a PD has been recorded between a catheter in a maternal blood vessel and one in a fetal vessel. However, the key question is whether this PD is the same as that across the placental exchange barrier. We addressed this in the human placenta using microelectrodes to measure the PD in isolated villi in vitro; the transtrophoblast PD so measured had a median value of -3 mV (range 0-15 mV). There have been no subsequent studies to validate this measurement. The syncytiotrophoblast of haemochorial placentas lacks any obvious extracellular water filled paracellular space between the syncytial nuclei. However, in mouse, rat, guinea pig and human there is an inverse relationship between the rate of diffusion of inert hydrophilic solutes across the placenta and their molecular size. The simplest explanation is that a paracellular route exists but its morphological identity is still uncertain. Areas of syncytial denudation could provide a paracellular route but this has not been proven. Answers to these and similar questions are required to fully understand the exchange physiology of the normal placenta and how this is affected in pathology.

Developmental programming of brain and behavior by perinatal diet: focus on inflammatory mechanisms

Obesity is now epidemic worldwide. Beyond associated diseases such as diabetes, obesity is linked to neuropsychiatric disorders such as depression. Alarmingly maternal obesity and high-fat diet consumption during gestation/lactation may “program” offspring longterm for increased obesity themselves, along with increased vulnerability to mood disorders. We review the evidence that programming of brain and behavior by perinatal diet is propagated by inflammatory mechanisms, as obesity and high-fat diets are independently associated with exaggerated systemic levels of inflammatory mediators. Due to the recognized dual role of these immune molecules (eg, interleukin [IL]-6, 11-1β) in placental function and brain development, any disruption of their delicate balance with growth factors or neurotransmitters (eg, serotonin) by inflammation early in life can permanently alter the trajectory of fetal brain development. Finally, epigenetic regulation of inflammatory pathways is a likely candidate for persistent changes in metabolic and brain function as a consequence of the perinatal environment.

Bone development and mineral homeostasis in the fetus and neonate: roles of the calciotropic and phosphotropic hormones

Mineral and bone metabolism are regulated differently in utero compared with the adult. The fetal kidneys, intestines, and skeleton are not dominant sources of mineral supply for the fetus. Instead, the placenta meets the fetal need for mineral by actively transporting calcium, phosphorus, and magnesium from the maternal circulation. These minerals are maintained in the fetal circulation at higher concentrations than in the mother and normal adult, and such high levels appear necessary for the developing skeleton to accrete a normal amount of mineral by term. Parathyroid hormone (PTH) and calcitriol circulate at low concentrations in the fetal circulation. Fetal bone development and the regulation of serum minerals are critically dependent on PTH and PTH-related protein, but not vitamin D/calcitriol, fibroblast growth factor-23, calcitonin, or the sex steroids. After birth, the serum calcium falls and phosphorus rises before gradually reaching adult values over the subsequent 24-48 h. The intestines are the main source of mineral for the neonate, while the kidneys reabsorb mineral, and bone turnover contributes mineral to the circulation. This switch in the regulation of mineral homeostasis is triggered by loss of the placenta and a postnatal fall in serum calcium, and is followed in sequence by a rise in PTH and then an increase in calcitriol. Intestinal calcium absorption is initially a passive process facilitated by lactose, but later becomes active and calcitriol-dependent. However, calcitriol's role can be bypassed by increasing the calcium content of the diet, or by parenteral administration of calcium.

Understanding placental nutrient transfer--why bother? New biomarkers of fetal growth

The placenta, in general and the physiology of maternofetal nutrient transfer is under-researched compared to other organs with epithelial transport function, as evidenced, for example, by publication numbers. This report provides reasons why more researchers should become involved in this topic. First, the syncytiotrophoblast, the transporting epithelium of the placenta, though having many basic cell physiology properties similar to those of other transporting epithelia, has several properties which are markedly different. Better information on these might help fundamental understanding of how epithelia in general function as well as improving knowledge of how the syncytiotrophoblast operates. Second, the synctiotrophoblast has a key role in controlling fetal growth, not only by transporting nutrients and waste products of metabolism but also because it increasingly appears to be one site, perhaps even the dominant site, in which integration of, sometimes conflicting, signals between mother and fetus takes place. Finally, better understanding of placental nutrient transfer and especially of how it is regulated by maternal and fetal signals could provide better information on the placental phenotype in fetal growth disorders--information which might contribute to providing better biomarkers which the obstetrician could use to improve early diagnosis of these disorders.

Interleukin-6 in the maternal circulation reaches the rat fetus in mid-gestation

Maternal systemic infection during pregnancy may expose the fetus to infectious agents and high levels of mediators of the resulting inflammatory response, such as IL-6 (IL-6). Increased fetal and maternal levels of IL-6 have been associated with adverse neonatal outcome but might also stress the fetus and contribute to cardiovascular and neuroendocrine dysfunction in adulthood. It is unclear whether interleukins cross the placental barrier, although this matter has been little studied. The aim of this study was therefore to investigate if IL-6 administered to pregnant rats in vivo is transferred to the fetus. We injected 125I IL-6 i.v. to pregnant dams at gestation day 11-13 (mid-gestation) or 17-19 (late gestation). We found 125I-IL-6 in the exposed fetuses as well as in amniotic fluids. Fetal 125I-IL-6 levels were markedly higher in animals injected in mid-gestation compared with late pregnancy (p < 0.01). This difference was mirrored in a 15-fold higher unidirectional materno-fetal clearance for 125I-IL-6 in mid-gestation (p < 0.01). We conclude that the permeability of the rat placental barrier to IL-6 is much higher in mid-gestation than in late pregnancy. Maternally derived IL-6 may directly induce fetal injury but also stimulate the release of fetal stress hormones resulting in stimuli or insults in neuroendocrine structures and hormonal axes which might lead to disease at adult age.

Endothelia of term human placentae display diminished expression of tight junction proteins during preeclampsia

This study explores the molecular composition of the tight junction (TJ) in human term placenta from normal women and from patients with preeclampsia, a hypertensive disorder of pregnancy. Maternal endothelial dysfunction is a critical characteristic of preeclampsia; hence, we have analyzed its impact on placental vessels. The study concentrates on the TJ because this structure regulates the sealing of the paracellular route. We have found that, in placental endothelial vessels, TJ components include the peripheral protein ZO-1 and the integral proteins occludin and claudins 1, 3, and 5. During preeclampsia, the amounts of occludin and ZO-1 exhibit no significant variation, whereas those of claudins 1, 3, and 5 diminish, suggesting the presence of leakier TJs in the endothelia of the preeclamptic placenta, possibly in response to the decreased perfusion of this organ during preeclampsia. We have unexpectedly found that, in normal placentae, the multinucleated syncytiotrophoblast layer displays claudin 4 at the basal surface of the plasma membrane, and claudin 16 along the apical and basolateral surfaces. The presence of membrane-lined channels that cross the syncytiotrophoblast constituting a paracellular pathway has been determined by transmission electron microscopy and by the co-immunolocalization of claudin 16 with the plasma membrane proteins Na+K+-ATPase and GP135. Since claudin 16 functions as a paracellular channel for Mg2+, its diffuse pattern in preeclamptic placentae suggests the altered paracellular transport of Mg2+ between the maternal blood and the placental tissue.

The role of cytokines in mediating effects of prenatal infection on the fetus: implications for schizophrenia

Maternal infections with bacterial or viral agents during pregnancy are associated with an increased incidence of schizophrenia in the offspring at adulthood although little is known about the mechanism by which maternal infection might affect fetal neurodevelopment. Exposure of pregnant rodents to the bacterial endotoxin, lipopolysaccharide (LPS), results in behavioral deficits in the adult offspring that are relevant to schizophrenia. It is however unknown whether these effects are due to the direct action of the inflammatory stimulus on the developing fetus, or due to secondary immune mediators (cytokines) activated at maternal/fetal sites. In this study we sought to elucidate the site of action of LPS, following a single intraperitoneal (i.p.) injection, in pregnant rats at gestation day 18. Animals received 5 muCi of iodinated LPS ((125)I-LPS) and its distribution was assessed in maternal/fetal tissues (1-8 h). In addition, induction of the inflammatory cytokines, TNF-alpha, IL-1beta and IL-6, was measured in maternal/fetal tissues following maternal LPS challenge (0.05 mg/kg, i.p.) (2-8 h). (125)I-LPS was detected in maternal tissues and placenta, but not the fetus. This distribution was accompanied by significant increases in TNF-alpha, IL-1beta and IL-6 in maternal plasma and placenta, but not in fetal liver or brain. A significant increase in IL-1beta was however detected in fetal plasma, possibly due to transfer from the maternal circulation or placenta. Collectively, these data suggest that effects of maternal LPS exposure on the developing fetal brain are not mediated by the direct action of LPS, but via indirect actions at the level of the maternal circulation or placenta.

Oral hypoglycemic agents and the pregnant diabetic: "from bench to bedside"

Today, the criteria for diagnosis and treatment have evolved into an evidence-based medicine approach. The need for evidence-based information is especially critical in the management of gestational diabetes, in general, and especially in the use of oral hypoglycemic agents. These agents have been categorically contraindicated for decades in the United States based on anecdotal and/or weak evidence for these recommendations. In this article, the similarities between gestational and type 2 diabetes are described and the rationale for the use of oral hypoglycemic agents for the treatment of both are discussed. The author will show how research from basic sciences (placental transfers) to clinical studies (perinatal outcome) can lead to significant evidence on which to base management recommendations.

Distensible transtrophoblastic channels in the rat placenta

Paracellular pathways in the haemotrichorial placenta of the rat were studied by electron microscopy using lanthanum hydroxide as an electron dense marker. Near term placentae were dually perfused in situ, adding lanthanum to the fetal perfusate. In some placentae outflow pressure on the fetal side was elevated (between 10 and 25 cm H(2)O) to promote filtration of fluid in a fetomaternal direction. Under normal pressure conditions lanthanum particles lined the subendothelial spaces and tubular structures in the inner, syncytial layer of trophoblast. Further penetration of lanthanum into the tubules was blocked by coarse lanthanum aggregates. Elevated fetal hydrostatic pressure resulted in a fluid shift across the placenta (filtration rate 50+/-16 per cent of fetal arterial inflow rate), distending the tubules in the inner trophoblast layer. Lanthanum particles gradually appeared in tubular structures in the middle (syncytial) layer and in the lateral intercellular spaces in the outer (cellular) layer. Finally lanthanum reached the maternal surface of the trophoblast. These pressure effects were only partially reversible. When the fetal pressure was returned to control values, some distension of the tubules persisted and the entire length of the paracellular pathways remained accessible to lanthanum. It is concluded that the placental barrier in the rat contains pressure dependent paracellular pathways connecting the maternal and fetal extracellular compartments.

Controlling drug delivery across the placenta

A challenge in modern drug therapy is to develop strategies for safer and more selective targeting of drug delivery in pregnancy. Specifically, approaches are needed that would restrict unnecessary drug exposure to either mother or fetus. There is evidence emerging that indicates the placenta does express natural transport and metabolism processes that function to control drug and nutrient distribution between the mother and fetus. Further, in vitro techniques developed in the past 10 years now provide some of the tools necessary to elucidate transport and metabolism processes typical of the human placenta. As a consequence, pharmaceutical scientists are in a position to contribute significantly to the design and development of drugs for pregnancy.

Fetal excretion of the fluorescent bile acid derivative cholylglycylamido-fluorescein (FITC-GC) by the rat placenta-maternal liver tandem

Bile acid transfer from the fetus to maternal bile was studied using in situ perfused rat placenta on day 21 of gestation and a fluorescent derivative of glycocholate (GC): cholylglycylamido-fluorescein (FITC-GC). Single-pass perfusion of the placenta with 0.25 mumol FITC-GC via the umbilical artery over 5 min was followed by the output of 6 per cent of this amount in maternal bile collected over the ensuing 120 min. This amount was reduced (-35 per cent) by simultaneous administration of 2.5 mumol GC through the jugular vein of the mother. This inhibition was stronger (-73 per cent) when 2.5 mumol GC was co-infused with FITC-GC through the umbilical artery. These results suggested that FITC-GC was, at least in part, transported by bile acid carriers across both the liver and the placenta. Using isolated perfused rat livers obtained from female virgin or 21-day pregnant rats, a slight increase in the residence time of FITC-GC in the liver of pregnant rats was found. However, no change in the ability of the liver to take up FITC-GC was observed. By contrast, when FITC-GC was injected into the left jugular vein of anaesthetized pregnant rats, a delayed plasma disappearance of this compound was seen, which may have been due in part to the existence of a transient and reversible FITC-GC exchange with the placental-fetal compartment. The maximal rate of FITC-GC output into bile after FITC-GC administration (1 mumol/100 g body weight) to pregnant rats was approximately 0.2 mumol/min, while maximal FITC-GC bile output was approximately 1 nmol/min when this compound was given through the umbilical artery (2.5 mumol). Therefore, the rate of FITC-GC output into bile was considered to reflect the rate of transfer across the placenta. Using this approach no saturation but rather a linear regression (slope = 1.1 microliters/min, p < 0.05) was found between placental transfer and placental perfusate concentrations in the 10-1000-mumol/l FITC-GC range. In summary, the in situ perfused rat placenta is a useful model to study the fetal excretion of cholephilic compounds, and transfer across the trophoblast would be the limiting step in the excretion of fetal bile acids by the placenta-maternal liver tandem.

Permeability properties of monolayers of the human trophoblast cell line BeWo

The BeWo cell line (b30 clone) has been examined as a potential in vitro system to study transplacental transport. At the light and electron microscope level, the cells were observed to form confluent monolayers on polycarbonate filters in approximately 5 days and morphologically resembled the typical human trophoblast. BeWo monolayers developed a modest transepithelial electrical resistance and a molecular size-dependent permeability to hydrophilic passive diffusion markers, fluorescein, and selected fluorescein-labeled dextrans. Linoleic acid permeation across BeWo monolayers was asymmetric, saturable, and inhibited by low temperature and excess competing fatty acid. Forskolin and 8-bromoadenosine 3',5'-cyclic monophosphate treatments stimulated morphological changes in BeWo cultures and enhanced the asymmetric passage of linoleic acid across the BeWo monolayers while having minimal effects on passive permeability, affirming that the differentiation state of the cells can influence membrane transporters and transmonolayer permeability. The basic permeability properties of the BeWo monolayers suggest that the cells grown on permeable supports may be examined as a convenient in vitro system to evaluate some transplacental transport mechanisms.

Permeability and metabolic properties of a trophoblast cell line (HRP-1) derived from normal rat placenta

The HRP-1 cell line is derived from normal rat placenta and appears morphologically similar to and retains characteristic expression of cellular markers of labyrinthine trophoblast cells. In this study, monolayers of HRP-1 cells grown on permeable supports were evaluated as a potential in vitro system to study trophoblast transport and metabolism. The cell line was shown to express and retain functional activity of the predominant placental cytochrome P450 isozyme, CYP1A1. Additionally, the HRP-1 cells retain functional activity of angiotensin I converting enzyme and carboxypeptidase N-like enzyme, peptidases characteristic of the trophoblast. The permeation of several hydrophilic, inert markers across the HRP-1 monolayers was observed to be dependent on effective molecular size and to be passive in nature. Functional asymmetry of the HRP-1 cells was illustrated by the predominant permeation of linoleic acid in the apical-to-basolateral direction across the monolayers. Transferrin passage across HRP-1 monolayers was concentration-dependent, was bidirectional, and could be inhibited by unlabeled transferrin, features typical of the trophoblast transport system for transferrin. Collectively, these properties suggest that the HRP-1 cell line may provide a useful tool for evaluating some of the permeability and metabolic properties of the trophoblast.

Bidirectional placental transfer ('leak') of L-glucose in control and diabetic rats

The placental transfer of D- and L-glucose was investigated in anaesthetised non-diabetic and streptozotocin-induced diabetic rats. Maternal to fetal transfer was determined by perfusing the fetal side of one placenta in situ whilst infusing a mixture of D-[3-3H]glucose and L-[1-14C]glucose into the maternal circulation. Back-transfer from the fetal to maternal circulation was assessed by determining the uptake of the radiolabelled glucoses from the perfusion fluid during a single passage through the placenta on the fetal side. Maternal diabetes resulted in a reduced utero-placental blood flow but an increased bidirectional transfer of D-glucose. Non-specific maternal to fetal placental transfer of L-glucose was greater in diabetic rats than in controls, and the loss of L-glucose during placental perfusion on the fetal side was, again, greater in diabetic than in control rats. This increased bidirectional 'leak' of glucose possibly reflects a functionally compromised placenta, caused by its formation in a diabetic milieu, and may explain the greater fetal-maternal glucose ratios found in diabetic rats relative to controls.

Mechanisms of the fetomaternal transfer of Na+ across the dually perfused placenta of the rat

In order to investigate mechanisms of fetal-maternal (F-M) transfer of Na+, clearance of 22Na+ and 51Cr-EDTA was measured simultaneously across the dually perfused placenta of the rat. In eight experiments clearance was measured successively in the F-M (Kfm) and in the maternal-fetal (M-F; Kmf) directions. Clearance of 22Na+ in the two directions was approximately equal (Kmf = 11.6 +/- 2.0 microliters/min; Kfm = 11.1 +/- 1.7 microliters/min: mean +/- s.d.) while Kfm of 51Cr-EDTA (4.4 +/- 0.7 microliters/min) was nearly double Kmf (2.4 +/- 0.8 microliters/min) for this tracer. Even greater asymmetry in the transfer of 51Cr-EDTA was found when measured across intact (non-perfused) placenta. It is suggested that this asymmetry is caused by volume flow in the F-M direction. In other experiments transfer was measured in the F-M direction only. Ouabain (0.1 mM) on the maternal side and reduced concentration of Na+ (25 mM) on the fetal side had no effect on the F-M transfer of the tracers. Reducing the temperature of the preparation by 5 degrees C significantly decreased transfer of 22Na+. The transfer of 22Na was inversely related to the concentration of K+ on the fetal side. These observations suggest that the F-M transfer of Na+ has three components: diffusion through paracellular routes; convective flow by filtration through wide placental pores, and transcellular transport by a mechanism which is uncertain at present.

Evidence for active maternal-fetal transport of Na+ across the placenta of the anaesthetized rat

1. In order to investigate mechanisms of Na+ transfer, the unidirectional maternal-fetal clearance (Kmf) of 22Na+ and of 51Cr-EDTA (a marker of paracellular diffusion) was measured across the intact or umbilically or dually perfused placenta of the anaesthetized rat. 2. The Kmf of 22Na+ in the intact preparation (18.5 +/- 2.7 microliters min-1, mean +/- S.D., n = 105 placentas) exceeded that of 51Cr-EDTA in the same experiments (1.4 +/- 0.3 microliters min-1) by more than ten times, whereas the difference in their diffusion coefficients in water was only 2-fold. In the perfused preparations the difference in the Kmf values was 6-fold. 3. Assuming that a simple model of paracellular diffusion through wide pores was one component of transfer, the Kmf of 51Cr-EDTA and the diffusion coefficients were used to calculate a component of 22Na+ clearance (Kmf,residual) and of Na+ flux (Jmf,residual) across the perfused placentas which could not be accounted for by transfer through the paracellular route. 4. Kmf,residual of 22Na+ across the dually perfused placenta was significantly lower when temperature was reduced, the temperature quotient (Q10) of the transfer being about 2. Kmf,residual was also significantly lower when 0.1 mM ouabain was perfused on the fetal side. Jmf,residual exhibited saturation kinetics characterized by an apparent Michaelis constant (Km) of 90 mM. Kmf,residual was not influenced by 0.5 mM frusemide, 0.5 mM amiloride or by 0.5 mM hydrochlorothiazide administered to the maternal side. It was significantly increased by 1 mM alanine on the maternal side suggesting that the coupled transfer of Na+ and amino acids may contribute significantly to the maternal-fetal flux of Na+. 5. These observations suggest that most (80%) of the maternal-fetal flux of Na+ across the rat placenta is effected by active transcellular transport. This transport involves passive entry of Na+ into the trophoblast from the maternal side by a largely unknown saturable mechanism and active extrusion of Na+ from trophoblast to the fetal side by Na(+)-K(+)-ATPase.

Effect of maternal hypokalaemia on unidirectional maternofetal and net potassium fluxes across the placenta of the anaesthetized rat

Potassium (K+) fluxes across the placenta of rats, at 21 days gestation, fed a low K+ diet or a control diet were studied. The rats on the low K+ diet had a significantly (P less than 0.001) lower arterial plasma K+ concentration compared to those on the control diet (1.95 +/- 0.12 and 2.93 +/- 0.06 mmol/l respectively; mean +/- s.e., n = 17). Fetal umbilical arterial plasma K+ concentration was unaltered in maternal hypokalaemia and was significantly (P less than 0.001) higher than that of maternal plasma (4.58 +/- 0.15 and 4.66 +/- 0.12 mmol/l in low K+ and control groups respectively). Net K+ flux across the placentas (as measured by fetal accretion between days 20 and 21 of gestation) of hypokalaemic mothers (0.106 +/- 0.02 mumol/min/g placenta, n = 6) was not different to that in controls (0.104 +/- 0.01 mumol/min/g placenta, n = 8). Unidirectional maternofetal flux (Jmf) across the placentas, measured as the accumulation of 42K in the fetuses after injection of the radioisotope into maternal blood, was also not significantly different between the hypokalaemic and control mothers (0.5 +/- 0.08 mumol/min/g placenta, n = 8 versus 0.63 +/- 0.06 mumol/min/g placenta, n = 7, respectively). However, measurement of Jmf by perfusion of the placentas through their fetal circulations yielded a higher value than the accumulation method and in this analysis was significantly (P less than 0.02) lower in the low K+ than in the control group (0.75 +/- 0.10 mumol/min/g placenta, n = 11, and 1.27 +/- 0.14 mumol/min/g placenta, n = 9, respectively). These results show that net placental K+ fluxes are unaltered during maternal hypokalaemia but suggest that unidirectional maternofetal fluxes may be reduced.

Effect of chronic maternal dietary magnesium deficiency on placental calcium transport

Metabolisms of calcium (Ca) and magnesium (Mg) are closely interrelated in the intestine, bone, and kidney. Interaction of Ca and Mg at the placental level, however, is not well defined. The occurrence of decreased bone mineral content and hypocalcemia in infants of hypomagnesemic mothers led us to test the hypothesis that chronic dietary maternal Mg deficiency decreases placental Ca transport. On day 10 of gestation, 20 Sprague-Dawley rats were randomized to a Mg-deficient diet (3.3 mg/day, n = 10) or to a control diet (70 mg/day, n = 10). On day 20 of gestation (term = 22 days), intact placentas were perfused in situ through the umbilical artery and perfusate was collected through the umbilical vein. Calcium 45 (45Ca) and chromium 51-EDTA (51Cr-EDTA) (a diffusional marker for placental membrane integrity) were injected to the dam and steady state maternofetal clearance (Kmf45Ca, microliter/min/g placenta) of both isotopes were calculated. There was no difference in the clearance of 45Ca and 51Cr in both groups (55 +/- 10 vs 57 +/- 16 and 3.2 +/- 0.4 vs 3.6 +/- 0.4, respectively, mean +/- SEM). We conclude that, in the rat, placental Ca transfer is unaffected by chronic maternal dietary Mg deficiency. We speculate that Ca and Mg cross the placenta by independent mechanisms.

The artificially perfused guinea-pig yolk sac placenta: transfer and uptake of water, glucose and amino acids

The development of a new technique to investigate maternal-fetal transfer across the near term guinea-pig yolk sac placenta by in-situ perfusion of the yolk sac vessels is described. The maternal-fetal transfer of labeled water, D- and L-glucose, O-methyl-D-glucose (oMDG), D- and L-alanine, D- and L-aspartate, L-lactate and alpha-amino-isobutyric acid (AIBA) was investigated after injection of these substances into the maternal circulation. After 15 min of perfusion at 0.5 ml/min the water clearance was 132 +/- 12 microliters/min (SEM, n = 30). The clearances for D- or L-glucose were less than 1.2 microliters/min. The activity of label in the venous yolk sac perfusate of all other substances was not different from background activity when 14C-label was used. The clearance of 3H-L-alanine approached the clearance value of water. The total uptake (as defined for single-injection double tracer dilution experiments) from the perfusate of D-glucose, oMDG, alanine and aspartate in comparison to L-glucose was also studied. Mean D-glucose uptake was 11.2 +/- 1.9 percent (n = 8), it was significantly reduced to 4.9 +/- 2 percent (n = 5) by cytochalasin B (1 X 10(-4) mmol/l), and by increasing concentrations of D-glucose (1 to 20 mmol/l, n = 4). The uptake of oMDG was 8.8 +/- 1.5 percent (n = 8). L-alanine uptake was 25 +/- 3.4 percent, D-alanine uptake was 8.3 +/- 1.5 percent (n = 12). Both uptake values were decreased significantly by 10 mmol/l L-alanine, but unaffected by [Na+] (less than 15 mequ/l). There was no uptake of AIBA. The uptakes of L-aspartate were 34.9 +/- 3.7 percent and of D-aspartate 40.4 +/- 4.8 percent (n = 11). Both uptake values were significantly and reversibly reduced by 1 mmol/l L-aspartate and D-aspartate, and by low [Na+] (less than 15 mequ/l). It is concluded that water can move by diffusion from maternal circulation into the yolk sac capillaries in considerable amounts whereas the contribution of the yolk sac placenta to fetal nutrition with D-glucose, L-alanine and L-aspartate is negligible. The membranes of yolk sac cells contain specific transport systems for D-glucose, D-/L-alanine and D-/L-aspartate transfer. The function of the vitelline placenta in the near-term guinea-pig is comparable more to the gut than to the chorio-allantoic placenta.

Preparation of plasma membrane vesicles from the rat placenta at term and measurement of Na+ uptake

Cytochemistry revealed that alkaline phosphatase was localized predominantly to the maternal facing plasma membrane of syncytiotrophoblast layer II in the haemotrichorial rat placenta at term. Plasma membrane vesicles prepared from term rat placenta by homogenization, treatment with MgCl2, and differential centrifugation were enriched 14-fold in alkaline phosphatase activity as compared to homogenate. These vesicles were mainly oriented right side out, as shown by a lack of effect of saponin treatment on alkaline phosphatase activity. Na+ uptake into the vesicles under equilibrium exchange conditions was significantly stimulated (P less than 0.01) nearly threefold in the presence of an outwardly directed H+ gradient as compared to no gradient. The stimulation by a H+ gradient was abolished in the presence of 0.5 mM amiloride. Measurements with the pH-sensitive dye BCECF in the absence of Na+ showed that 95.2 +/- 0.6 per cent of a 1.2 pH unit H+ gradient was dissipated from the vesicles in 2 min, but the remaining gradient was maintained for up to 15 min. These experiments therefore provide evidence that vesicles derived mainly from the maternal facing plasma membrane of syncytiotrophoblast layer II of the rat placenta possess a Na+/H+ exchanger.