The use of AIB transport to assess the suitability of a system of human placental perfusion for drug transport studies

Transfer of Heparin Across the Human Perfused Placental Lobule

A system of dual perfusion of an isolated lobule of term human placenta was used as a model to study the transfer of heparin from maternal to foetal circulation. The metabolic viability of the system was assessed by measuring β-HCG and alkaline phosphatase levels in both maternal and foetal perfusates. Creatinine and antipyrine were used as markers to determine juxtaposition of the maternal and foetal circulations. Results of this study indicate that following administration of a single bolus dose of heparin into the maternal circulation, its concentration declined slowly from 99·01 ±2·98 at 15 min to 97·23 ±4·12% and transfer of heparin in the foetal circulation was linear and increased from 0·10% ±0·05% at 15 min to 0·46 ±0·19% over a period of 120 min. The maternal (MAUC) and foetal (FAUC) concentration-time integrals were found to be 70160 ± 1332 and 340 ± 30 int. units min mL−1, respectively. Placental permeability of heparin and creatinine, calculated as the ratio of foetal concentration to the integral maternal–foetal concentration difference, was 8·65 × 10−5 ± 0·80 × 10−5 and 0·033 ±0006 mL min−1 g−1 of perfused placental weight, respectively. These data suggest that heparin was transferred from the maternal to the foetal circulation in small quantities.

Effect of insulin on transport kinetics of alpha-aminoisobutyric acid in the perfused human placental lobule in vitro

BACKGROUND

The limited data available on the role of insulin on maternal-fetal transport of amino acids prompted us to undertake this study.

METHODS

Transport kinetics of a model amino acid, alpha-aminoisobutyric acid (AIB) was investigated in perfusion of isolated human placental lobules in vitro in non steady-state conditions and the effect of therapeutic dose of insulin was assessed in parallel series of perfusions. National Culture and Tissue Collection medium diluted with Earle's buffered salt solution was used as the perfusate and tritiated water was used as the reference marker for comparison.

RESULTS

In five successful perfusions with insulin, differential transport rate indices of AIB for 10, 25, 50, 75 and 90% of efflux fractions in the fetal venous outflow averaged 0.76, 1.03, 1.02, 1.09, 1.04 and 1.03 times those of reference values, respectively. The indices differed significantly than in controls for 10, 25 and 50% efflux fractions, but not in the case of 75 and 90% efflux values. The AIB transport fraction (TF), expressed as percentage of injected maternal dose, averaged 29.4 +/- 5.4% and 38.7 +/- 6.2% of the corresponding reference marker value in control and insulin series, respectively. With regards to the pharmacokinetic transport parameters, the absorption and elimination rates of the amino acid were significantly higher in the study group than in the control.

CONCLUSION

We conclude that insulin, in physiological and therapeutic doses, stimulates maternal-fetal AIB transport in vitro, in the perfused human placental lobule.

Peptide transport and metabolism across the placenta

Advances made in the field of DNA and recombinant technology have led to the emergence of peptides and proteins as an important class of therapeutic compounds. While a significant amount of information exists regarding the transport and metabolism of peptides across different barriers (e.g. gastro-intestinal, nasal, and blood-brain barrier), limited attention has been paid towards their transport and metabolism across the placental barrier. The mechanism of placental transport of peptides is of importance in assessing the exposure of these drugs to the fetus, particularly when the drugs potentially may have adverse effects on the developing fetus.The absence of a well accepted, simple and convenient animal model may be a reason for the limited information available on the placental transport and metabolism of peptides. Although several in vivo models have been utilized to study the transport and metabolism of drugs across the placenta, species differences in the placental physiology and anatomy of the animal models with regard to the human placenta have prevented their widespread use. The in vitro human placental cell culture models are morphologically similar to the trophoblasts and often express the enzymes and carrier systems found in the human placenta. They can provide an easy and rapid method to determine the mechanisms of transport and metabolism of drugs across the placental barrier. These in vitro models have been utilized in the determination of transport mechanisms of drugs of abuse across the placenta.This article overviews the available literature on the placental transport of peptides and describes the application of an in vitro cell culture model (BeWo) to determine the mechanisms of transport of opioid peptides and their analogues across the placenta.

Techniques to study human placental transport

Pregnancy in the 20th century involves women of many age groups from early teens to the fourth or fifth decade. Modern medicine and in vitro fertilization techniques have increased options for pregnancy and childbirth. Pregnancy is a dynamic state, and medical concerns may involve disorders of the fetus and mother requiring medications and special nutrients. Therefore, different techniques have been developed to evaluate the placental transfer of drugs and nutrients using tissues and cells derived from human placenta. These include (a) isolated tissues and cells to study placental transport, (b) primary and malignant trophoblast cell cultures and (c) biophysical methods for studying placental transport. Also, convenient study models have been developed to evaluate placental transfer of safe drugs in pregnant women. Some of the drugs studied by these techniques and models include (a) anesthetics and pain medications used during delivery, (b) antibiotics and anti-bacterials used to cure infections, (c) drugs abused by pregnant women and (d) nutrients required for proper fetal growth. Placental transfer and exchange mechanisms are complicated processes, and in vitro models reflect only partially the equilibria that exist among mother, placenta and fetus. The perfused cotyledon model is elegant and simple but gives only restricted information. Isolated placental tissues give useful information about the pharmacological effects of drugs. Metabolic studies using human placental models provide information on the metabolism of a drug during placental transfer and accumulation of the drug or its metabolite in the placenta or fetal circulation. Several studies on the transplacental passage of drugs exist but many questions regarding the transfer of drugs between the maternal and fetal circulations and clearance of drugs from fetal circulation have yet to be answered. This article reviews in vitro and in vivo methods for evaluation of transplacental transport of drugs and their current effectiveness to obtain clinically useful data.

Permeability of human placenta and fetal membranes to thyrotropin-stimulating hormone in vitro

We determined the placental transfer of TSH in an in vitro model of dually perfused isolated lobule in 28 human term placentas by adding varying concentrations (5-60 microIU mL(-1)) of TSH as a single bolus dose to the closed maternal circulation. Transmembrane transfer of TSH was also studied by adding 45 microIU mL(-1) to the maternal or fetal compartment of a dual chamber of fetal membranes in culture. Passage of freely diffusible markers creatinine and antipyrine were also studied in this model. TSH concentration was measured by third generation chemiluminescence assay with a sensitivity of 10 mIU mL(-1). In the perfusion experiments, at physiologic concentrations the slow decline of TSH in the maternal circulation was associated with a small linear increase in fetal levels to 0.11 +/- 0.04% of initial dose at 2 h. The placental transfer rate was 0.08 microIU min(-1). Increasing maternal concentrations of TSH were associated with proportional increases in transfer rate (y = 0.002x; R2 = 0.99) and placental uptake (y = 0.01x; R2 = 0.97). The placental permeability of TSH was 2.4 x 10(-4) mL min(-1) g(-1) and was proportional to its coefficients of diffusion in water and molecular size. The transmembrane transfer and permeability of TSH was comparable to those of the placenta. We conclude that TSH crosses the human term placenta and fetal membranes sparingly.

The effect of C5a and U46619 on the isolated, perfused human placental lobule: development of a method for the online estimation of tissue fluid accumulation

A method for the automatic and simultaneous determination of perfusion pressure and fluid accumulation in the isolated, perfused human placental lobule is described. We demonstrated that the inflammatory mediator, C5a, a C5a agonist analogue peptide, and the thromboxane mimetic U46619 caused increased fetal perfusion pressure and increased tissue weight when administered via the fetal arterial circulation. Occlusion of the fetal venous effluent tubing caused significantly greater increases in tissue weight than the pharmacological agents. Detectable increases in tissue weight occurred within 47 +/- 3 sec (n = 21) following pressure increases caused by the pharmacological agents. In each case, the increase in tissue weight was accompanied by an increased permeability of the materno-fetal barrier, shown by the transfer of Evans blue albumin from the fetal circulation to the maternal compartment.

Autacoids and control of human placental blood flow

1. Humans have a haemochorial, villous placenta. Uterine blood passes through maternal sinuses, bathing placental villi through which fetal blood circulates. Blood flow through each circulation is high and vascular resistance low. This haemodynamic situation is essential for efficient placental function. 2. The low placental vascular resistance is due to a lack of nervous influences together with pregnancy-induced changes promoting vasodilatation. Increases occur in output of the vasodilators prostacyclin and nitric oxide and also in membrane sodium pump activity. 3. Many autacoids are present in umbilical blood. Fetal vessels of the placenta develop intense vasoconstriction in the presence of some autacoids, such as thromboxane A2 and prostaglandins F2 alpha and E2, and respond weakly to others, such as angiotensin II and 5-hydroxytryptamine. Nevertheless, vasodilator influences predominate. 4. The diseases of pre-eclampsia and fetal growth retardation are associated with reduced output of nitric oxide and prostacyclin and with increased production of thromboxane A2 and endothelin-1. These changes promote vasoconstriction, increased vascular sensitivity to vasoconstrictor stimuli, platelet aggregation and intravascular coagulation, retarding blood flow and feto-placental growth. 5. Aspirin and glyceryl trinitrate have been investigated for possible therapeutic use in pre-eclampsia and fetal growth retardation. Improved drug therapy is likely as knowledge increases of the importance of autacoids in normal placental function and in the changes that occur during disease.

Placental perfusion. An overview of the literature

Placental perfusion techniques are currently used to study not only the organ functions but also the transfer profile and metabolic pathway of different compounds. In view of the interest in the mechanism of transfer and potential adverse effects of compounds there are numerous publications on the topic, but no systematic picture is yet available. Thus an overview has been made of all studies published from 1966 to 1990 that use this experimental approach. Out of 359 computer-retrieved articles, 266 (74%) actually dealt with the target topic; 68 articles were added after a systematic hand search, so a total of 334 articles were analyzed. The distribution of papers per year was constant until 1980, and rose significantly thereafter. Animal studies using placental perfusion were performed either in situ or in vitro, whereas human investigations were mostly examined by in vitro perfusion techniques. Animal experiments were done on seven species, the guinea pig being the most widely used. The aims of all studies could be divided into five main categories: 132 studies researched the kinetics of compounds in the placenta; 100 studies investigated placental metabolism; methodology of perfusion was reported in 22 articles; and 49 studies examined physiological changes of placental variables. A clear increase in pharmacological studies was noted starting from 1986 (there were 31 such studies). Compounds studied were either endogenous or exogenous. Almost all endogenous compounds were investigated, some of them quite extensively (mainly hormones, angiotensin, glucose, and lactate). There seemed to be no preferential field for exogenous compounds (62 compounds could be assigned to 20 classes).(ABSTRACT TRUNCATED AT 250 WORDS)

Fine structure of the paracellular junctions of terminal villous capillaries in the perfused human placenta

Selected lobules of human term placentae were extracorporeally perfused for a recovery period of 20 min, fixed by perfusion and mordanted with ferrocyanide prior to processing for transmission electron microscopy. The lateral membranes of the endothelial cells of the terminal villous capillaries were found to be separated by paracellular clefts of mean width 15.6 nm. At tight junctional regions (1-4 sites per cleft) the two membranes approached each other more closely and frequently appeared to fuse. However, tilting of the sections in the electron microscope stage showed that the membranes were separated by a gap of mean width 4.1 nm in at least 94% of tight junctional profiles. When individual tight junctions were studied by a combination of serial sectioning and goniometric tilting, they were seen to widen abruptly within a distance of three to seven consecutive thin sections, indicating they were not continuous throughout the axial length of the capillaries. The wide regions of the clefts usually showed linkers, strands of glycocalyx-like material spanning the gap. Linkers may contribute to cell adhesion and possibly form part of a filter within the tortuous paracellular pathway provided by the discontinuous network of tight junctional strands. Human term placental capillaries appear to resemble closely other continuous non-brain capillaries.

Transfer of acipimox across the isolated perfused human placenta

The placental transfer of the new lipid-lowering agent, acipimox was investigated in the isolated perfused human placenta. Placentas obtained at caesarean section were perfused for 120 min, with both maternal and fetal circuits in closed recycling mode. Acipimox was added to either the maternal circuit alone (five experiments) or to both maternal and fetal circuits simultaneously (five experiments) to achieve initial concentrations of 5 micrograms/ml. Antipyrine (20 micrograms/ml) and l-(14C)-leucine (250 microM) were added in like fashion as reference compounds. Two hours after addition to the maternal circuit alone antipyrine was close to equilibrium across the placenta, but equilibration of acipimox was incomplete (fetal/maternal ratio = 0.58 +/- 0.11). Maternal to fetal placental clearance of acipimox (0.80 +/- 0.18 ml/min) was 25 per cent of antipyrine clearance. After simultaneous administration to both maternal and fetal circuits the l-(14C)-leucine fetal/maternal ratio was 1.44 +/- 0.13 at 120 min, whereas maternal and fetal concentrations of acipimox and antipyrine were at equilibrium for the duration of the experiment (fetal/maternal ratio of acipimox at 120 min = 1.10 +/- 0.06). This study shows that acipimox is transferred across the human placenta by diffusion at a slow rate. The low permeability of the placenta may afford some protection to the fetus from acipimox administered to the mother in vivo.

Two-sided culture of human placental trophoblast. Morphology, immunocytochemistry and permeability properties

We describe the culture of human term placental trophoblast cells on cell-free amniotic membrane, with medium on both sides. Over the course of 2 days, the isolated cells, initially simple, mononucleated and probably cytotrophoblast, form a confluent layer of multinucleated syncytial cells with morphological and immunocytochemical properties of syncytiotrophoblast. This layer becomes polarized with respect to morphology, alkaline phosphatase distribution and hCG secretion. Contamination with amnion cells, and with other cell types that are present in placental tissue, was less than 1 per cent. A preliminary investigation of the permeability properties of the preparation showed that the trophoblast cell layer, rather than the amniotic membrane, was rate-limiting to transtrophoblast transfer, but that possible effects of the supporting membrane should be considered. The transtrophoblast transfer of D-glucose and the non-metabolisable analogue, 3-O-methyl-D-glucose (3OMG), had saturable and non-saturable/leak components in both directions, indicating that carrier-mediated processes were involved. The non-metabolisable amino acid 2-aminoisobutyrate (AIB) was both accumulated within the trophoblast cells, and transferred by saturable and non-saturable processes from the microvillous side, but no saturable accumulation or transfer was observed from the basal side, at the concentrations tried. The results suggest that this model may prove suitable for studies of transtrophoblast transfer.

Uptake and intracellular routing of peroxidase-conjugated immunoglobulin-G by the perfused human placenta

Selected lobules of term human placenta were extracorporeally perfused and human immunoglobulin-G complexed to horseradish peroxidase (IgG-HRP) was added to the maternal perfusate. After different durations of perfusion IgG-HRP was visualised by use of diamino-benzidine cytochemistry. Within the first 10 min of perfusion IgG-HRP was found bound to microvilli and coated pits of the syncytiotrophoblast; internalisation into coated vesicles and tubulo-vesicular bodies was also observed. Subsequently, IgG-HRP was found in multivesicular bodies and by 30 min appeared in basal vesicles, the frequency of the latter event increasing with time. No routing of IgG-HRP into Golgi regions or lysosomes could be detected. by 60 min IgG-HRP was found in a few caveolae of fetal endothelium of both terminal and intermediate villi. IgG-HRP was not found in intercellular clefts of the endothelium. The pattern of uptake and routing observed suggests a receptor-mediated transcytosis of IgG-HRP across the syncytiotrophoblast and a transcellular pathway through the endothelium.

Effect of ethanol on human placental transport of model amino acids and glucose

Prior studies in rodents, sheep, and subhuman primates have shown that ethanol, especially after chronic exposure, inhibits the transport of amino acids by the placenta. A small decrease in glucose transport by rat placenta chronically exposed to ethanol has also been noted. With human placental slices, however, only pharmacological (high) concentrations of ethanol impaired uptake of amino acids, and there are no data on glucose transport. In the present study, the effect of brief exposure to ethanol on human placental transport of model amino acids and glucose was studied by two techniques not previously jointly employed for this--the perfused human placental cotyledon and human placental vesicle systems. The nonmetabolizable amino acids, alpha-aminoisobutyric (AIB) acid and cycloleucine (CLEU), as well as D-glucose, and nonmetabolized glucose (3-O-methyl-D-glucose), were used as probes. AIB and CLEU are transferred normally by active transport and D-glucose by facilitated transport from maternal to fetal compartments. The perfused placental system was exposed to ethanol (300-500 mg%) for 2-4 hr and the vesicles to 200-400 mg% ethanol for times varying from 10 min to 48 hr. There was no impairment of AIB, D-glucose, or 3-O-methyl-D-glucose transfer by ethanol using these techniques. Normally, about 60% of AIB transport by human placenta is sodium dependent. This component (using the vesicle system) was also not impaired by ethanol. Ethanol caused a very small decrease of CLEU clearance by the perfused human placenta (p = 0.05) but not using vesicles.(ABSTRACT TRUNCATED AT 250 WORDS)

Uptake of alpha 2-macroglobulin-trypsin complex by human placenta is mediated by a microvillous membrane receptor

The fate of native alpha 2-macroglobulin (alpha 2M) or its trypsin complex (alpha 2M-T) was studied in the isolated dually-perfused lobule of term human placenta. [125I]-alpha 2M added to the maternal circuit was unchanged during the course of the perfusion with minimal activity becoming associated with the placental tissue. Transfer of radioactivity into the fetal circulation accounted for only 0.07 per cent of the initial dose after 2 h. In contrast, [125I]-alpha 2M-T was rapidly taken up into the placental tissue (nearly 28 per cent of the initial dose during the 2-h perfusion) and breakdown products were released into both maternal and fetal circulations. At the end of 2 h, radioactivity levels on the fetal side were 13 times higher than those found with the native protein. These indications of a classical receptor-mediated uptake and breakdown pathway were confirmed in experiments in which the acidotrophic agent chloroquine was added to the maternal circuit prior to the alpha 2M-T. In the presence of chloroquine, tissue uptake was inhibited and the subsequent release of radioactive degradation products into the fetal circuit was similar to the levels seen with alpha 2M. Incubation of term trophoblast cells at 37 degrees C with [125I]-alpha 2M-T revealed over three-fold greater cell-associated activity than was found with the native protein. In another series of experiments, a purified microvillous membrane fraction was prepared from term placentae using buffers containing 1 mM iodoacetate. In the presence of this proteolytic enzyme inhibitor, binding studies showed a single class of low affinity receptors for the alpha 2M-T complex capable of binding 4.8 +/- 1.3 (SEM) micrograms of complex per mg of membrane protein. There was no binding of the native protein.

Placental amino acid uptake in normal and complicated pregnancies

Amino acids are the essential substrates for fetal growth and catabolism. The fetus is dependent on the placenta for the provision of amino acids, the first step being concentration of amino acids within the syncytiotrophoblast for subsequent transfer to the fetus. A reliable technique for the isolation of human syncytiotrophoblast plasma membrane has been described, and the suitability of this preparation for the study of amino acid uptake and membrane transport has been well documented. Using this technique, the microvillous vesicle uptake of alpha-aminoisobutyric acid (AIB), a nonmetabolizable amino acid, has been determined over multiple time points for normal (NL) pregnancies and those complicated by pregnancy-induced hypertension (PIH), non-insulin-dependent diabetes mellitus (NIDDM) and those delivering small-for-gestational-age (SGA) neonates. There was no significant difference in AIB uptake between NL pregnancies and those complicated by PIH or NIDDM. Compared to each of the above, AIB uptake by the SGA group was significantly less at each time point. These results suggest that normal placental amino acid transport mechanisms may be altered in SGA pregnancies. If so, such alterations may interfere with the normal provision of nutrients to the fetus and ultimately contribute to impaired growth in utero.

Human placental transport of cimetidine

This study addresses the mechanism of transport of the H2-receptor antagonist, cimetidine, by the human placenta. A 4-h recycling perfusion of a single placental cotyledon of normal, term, human placenta was used. At a maternal concentration of 1 microgram/ml, cimetidine clearance from the maternal circulation was 0.58 +/- 0.16 ml/min per g placenta, a rate about one third that of antipyrine. There was no evidence of cimetidine metabolism by the placenta. Transfer of cimetidine from maternal to fetal compartments showed no saturation kinetics and was not inhibited by putative carrier competitors. Cimetidine did not accumulate against a drug concentration gradient. Fetal clearance of cimetidine was similar to maternal clearance. Studies with placental apical vesicles confirmed lack of saturability of cimetidine transport and of its concentration within vesicles. Thus, (a) cimetidine is transported across the human placenta bidirectionally at a rate about one third that of antipyrine, (b) the drug is not metabolized by the placenta, and (c) the transport is a passive one.

Role of transferrin in iron transport between maternal and fetal circulations of a perfused lobule of human placenta

The transfer of iron between the maternal and fetal circulations of an isolated perfused lobule of term human placenta was investigated using 125I-labelled or 59Fe-labelled diferric transferrin. There was negligible transplacental transfer of intact transferrin whereas nearly 4 per cent of the added 59Fe was transferred into the fetal circulation after 2 h, where it became associated with fetal transferrin. Over 20 per cent of the added 59Fe radioactivity was sequestered within the placental tissue during this period, associated with transferrin, ferritin and other uncharacterized molecules. This suggests an important role for an intracellular pool in regulating transfer. The presence of 10 mM chloroquine in the maternal circulation substantially reduced tissue accumulation of 59Fe and totally inhibited transfer to the fetus. It is concluded that the initial stages of iron transfer to the fetus involve the internalization of maternal iron-saturated transferrin bound to membrane receptors by receptor-mediated endocytosis, which can be inhibited by the drug chloroquine. Subsequently, the transplacental transfer of iron to the fetus does not involve the concomitant movement of transferrin.

A rat model for the study of transplacental pharmacokinetics and its assessment with antipyrine and aminoisobutyric acid

The suitability of a pregnant rat model for the study of transplacental pharmacokinetics was assessed by two test agents, antipyrine, which freely equilibrates in the maternal and fetal blood, and aminoisobutyric acid, which is actively transported from mother to fetus. In accordance with an ideal protocol for a two-compartment model solved for bolus injections, unlabeled antipyrine was injected into the mother (day 20 of gestation), and labeled antipyrine was injected into its fetuses following exteriorization under ether anesthesia. Maternal and fetal blood samples (2-3 fetuses removed at each time period) were collected sequentially under brief periods (2-3 min) of ether anesthesia up to 9-12 hr. In separate studies it was found that sequential removal of fetuses did not significantly alter maternal or fetal serum drug concentrations. As expected, placental clearances (ml/h-1/kg-1) of antipyrine from mother to fetus (392 +/- 86) and from fetus to mother (448 +/- 107) did not significantly differ. As an alternate approach, (3H)aminoisobutyric acid was either injected into the mother or into the fetuses, and maternal and fetal blood samples were sequentially collected in each case; the clearance of this agent from fetus to mother was 53 ml/h-1/kg-1, and, as expected, was much lower than that from mother to fetus (168 ml/h-1/kg-1). It is suggested that the pregnant rat model can be used as a substitute for the chronically catheterized pregnant sheep model for the study of pharmacokinetics in the maternal-fetal unit.

Uptake of transferrin and iron by cultured rat placental cells

This paper describes a method for the culture of rat placental cells. The method involved separation of the basal layer from the labyrinth and sequential digestion of the cells. The cells were demonstrated not to be fibroblasts and are described in terms of their appearance under the light and electron microscopes. Transferrin and iron uptake by the cells was examined and compared with results achieved using other methods of study. The results showed that transferrin bound to receptors on the cell surface and that the transferrin, once bound, was taken into the cell. Only this internalized transferrin was capable of donating iron to the cells. The iron was accumulated within the cells and did not appear to be released to the incubation medium. The apparent dissociation constant (Ka) for transferrin was found to be 6.96 X 10(6) M-1, a value similar to that described by earlier workers. The placental cells had 3.4 X 10(11) binding sites/microgram DNA, equivalent to approximately 1 X 10(6) sites/cell. From these data, and from the rate of accumulation of iron by the cells, the receptor turnover time was estimated as being between 5 and 10 min.

Mechanical and metabolic viability of a placental perfusion system in vitro under oxygenated and anoxic conditions

In vitro dual circuit perfusion of the placenta with well-oxygenated medium results in the continuous and stable consumption of oxygen and glucose over a 2-h perfusion period. This is reflected in a stable production of lactate and an energy charge which is higher at the end of the perfusion period than that seen in fresh placental tissue immediately after vaginal delivery. Anoxic perfusion causes an increase in glucose consumption which is more than twofold higher than that seen in the oxygenated perfusion, resulting finally in placental uptake of glucose not only from the maternal but also from the fetal circulation. Lactate production is increased during the anoxic perfusion, while the final tissue energy charge value lies between the values observed for fresh tissue and for the oxygenated perfusion. The shift to anaerobic metabolism shown by placental tissue in anoxic conditions enables continued functioning of the tissue over the 2-h perfusion period but it appears that under anoxic conditions the tissue may incur an energy debt not observed in oxygenated perfusions.

Uptake and fate of exogenous immunoglobulin G in the perfused human placenta

The steady-state transfer of 125I-labelled human or bovine immunoglobulin G from the maternal circulation and its fate during transport to the fetal circulation were studied over a 2 h period in an intact perfused human placental lobule. The 125I activity in the closed-circuit fetal circulation rose linearly throughout this time. The transfer rate, expressed as a percentage of the total activity administered, was 0.11% per hour for human IgG and 0.04% per hour for bovine IgG. 97-99% of the 125I activity in the maternal circulation was associated with high molecular weight protein which was characterised as IgG. The corresponding figure for the fetal side was 10-30%. It is concluded that human IgG is taken up in preference to bovine IgG at the maternal surface of the syncytiotrophoblast but subsequently the majority of the internalised immunoglobulin is broken down intracellularly and the fragments released into the fetal circulation.