Maternal-fetal Direction Research Articles

Fetal-maternal nitrite exchange in sheep: Experimental data, a computational model and an estimate of placental nitrite permeability

IntroductionNitrite conveys NO-bioactivity that may contribute to the high-flow, low-resistance character of the fetal circulation. Fetal blood nitrite concentrations depend partly on placental permeability which has not been determined experimentally. We aimed to extract the placental permeability-surface (PS) product for nitrite in sheep from a computational model. MethodsAn eight-compartment computational model of the fetal–maternal unit was constructed (Matlab® (R2013b (8.2.0.701), MathWorks Inc., Natick, MA). Taking into account fetal and maternal body weights, four variables (PS, the rate of nitrite metabolism within red cells, and two nitrite distribution volumes, one with and one without nitrite metabolism), were varied to obtain optimal fits to the experimental plasma nitrite profiles observed following the infusion of nitrite into either the fetus (n = 7) or the ewe (n = 8). ResultsThe model was able to replicate the average and individual nitrite–time profiles (r2 > 0.93) following both fetal and maternal nitrite infusions with reasonable variation of the four fitting parameters. Simulated transplacental nitrite fluxes were able to predict umbilical arterial-venous nitrite concentration differences that agreed with experimental values. The predicted PS values for a 3 kg sheep fetus were 0.024 ± 0.005 l∙min−1 in the fetal–maternal direction and 0.025 ± 0.003 l∙min−1 in the maternal–fetal direction (mean ± SEM). These values are many-fold higher than the reported PS product for chloride anions across the sheep placenta. ConclusionThe result suggests a transfer of nitrite across the sheep placenta that is not exclusively by simple diffusion through water-filled channels.

Transport kinetics of zinc, copper, selenium, and iron in perfused human placental lobule in vitro.

Transport characteristics of essential trace elements as zinc, copper, selenium and iron have been studied in maternal-fetal direction in normal pregnancies, using in vitro perfusion of human placental lobules. Solutions of trace elements corresponding to twice the physiological concentrations were injected (100 microl bolus) into the maternal arterial perfusate. Serial perfusate samples were collected every 30 sec from venous outflows for a study period of 5 min. Concentrations of these trace elements and their transport kinetics were determined. Transport fractions (TF) of zinc, copper, selenium and iron averaged 0.21, 0.49, 0.55 and 0.10% of maternal load respectively. Other parameters such as area under the curve, clearance, elimination constant, absorption and elimination rates showed some significant differences between the various elements. Copper and selenium appear to be transported passively in maternal-fetal direction, while for iron and zinc, role of active transport for transfer across the human placental membrane cannot be discounted. We speculate that alterations in copper: zinc TR50 (transport rate for 50% efflux) and TF ratios could serve as useful indicators for assessing placental transport status of these essential elements in complicated pregnancy states.

Effect of increased glucose load on maternal-fetal transport of alpha-aminoisobutyric acid in the perfused human placenta: in vitro study.

The role of hyperglycemia on modulation of maternal-fetal transport of amino acids in humans is little understood. Hence, we have explored the effect of increased glucose load on transport kinetics of a model non-metabolizable amino acid, alpha-aminoisobutyric acid (AIB), in the human placenta in vitro. Transport kinetics of AIB in maternal-fetal direction was studied using perfusion of isolated human placental lobules. NCTC (National Culture and Tissue Collection)-135 medium, diluted with Earle's buffered salt solution was used as the perfusate and tritiated water was used as the reference marker. Effect of increased glucose load on transport kinetics of study and reference substances was studied in normal term placentae (n=5; gestational age, 38.5 +/- 0.5 weeks) in succeeding experimental phases, after a control perfusion phase with physiological glucose concentration. AIB transport fraction (TF), relative to tritiated water TF, averaged 54.8% in control euglycemic phase while in hyperglycemic concentration phases of 27.8 and 55.6 mM, the AIB TF index averaged 42.4% and 38.2%, respectively. Analysis of variance revealed that the difference was statistically significant. Similarly, absorption rate index of the amino acid was also significantly lower in the hyperglycemic perfusion phases compared to control euglycemic phase. We conclude that hyperglycemia may play a deleterious role in limiting maternal-fetal transport of A-type amino acids in the in vivo state.

Maternal-fetal transport kinetics of copper, selenium, magnesium and iron in perfused human placental lobule: in vitro study.

Transport characteristics of certain inorganic elements such as copper, magnesium, selenium and iron have been studied in maternal-fetal direction in normal pregnancies, using in vitro perfusion of isolated placental lobules. Copper, selenium, magnesium and iron salts corresponding to twice physiological concentrations were injected as a 100 microl bolus, into the maternal arterial perfusate. Serial perfusate samples were collected from venous outflows for a study period of 5 min. Concentrations of various inorganic elements and their transport kinetics were determined. Transport fractions of copper, selenium, magnesium and iron averaged 0.14, 0.19, 0.06 and 0.23% of maternal load respectively. The pharmacokinetic parameters such as area under the curve, clearance, elimination constant, and time for maximum response showed some significant differences between the various elements. We speculate that copper and selenium share the same transport pathway along a concentration gradient in maternal-fetal direction, while for iron and magnesium, active transport plays a predominant role for element transfer across the human placental membrane.

Transport Kinetics of Alpha-Aminoisobutyric Acid and Water in Pre-Eclamptic Pregnancies: An in vitro Study

Objective: To investigate the transport kinetics of a model amino acid, alpha-aminoisobutyric acid (AIB), in the maternal-fetal direction in placentae from pre-eclamptic pregnancies. Methods: Transport kinetics of the amino acid was assessed in vitro using perfusion of human placental lobules. Control placental lobules were perfused for comparison. National Culture and Tissue Collection-135 medium diluted with Earle’s buffered salt solution was used as the perfusate, and tritiated water served as the internal reference marker. AIB along with reference marker was injected as a single bolus into the maternal arterial perfusate, and serial perfusate samples were collected from maternal and fetal venous circuits for a study period of 5 min. Results: The differential transport rate of the amino acid for various efflux fractions differed significantly between control and experimental groups when compared to that of the reference marker. The transport rate (corresponding to 50% of efflux into the fetal vein), transport fraction, absorption and elimination rates of the amino acid differed significantly compared to the reference marker values in study and control groups. Conclusion: The results indicate that amino acid transport function is compromised in placentae of pre-eclamptic pregnancies.

Assessment of D-glucose transport kinetics in the perfused human placenta: an in vitro study.

There have been no reports to date on glucose transport kinetics and the effect of graded hyperglycemia in the human placenta in non-steady-state conditions. The transport kinetics of D-glucose in the human placenta was investigated in non-steady state conditions in vitro using perfusion of isolated placental lobules. National Cancer Tissue Culture (NCTC) 135 culture medium, diluted with Earle's buffered salt solution was used as the perfusate. 14C-Labeled D-glucose and water as reference were injected as a single bolus into the maternal arterial perfusate. Perfusate samples were collected and analyzed from the maternal and fetal venous outflows. In four successful perfusions, differential transport rates of glucose in the maternal-fetal direction averaged 1.03, 1.02, 1.09, 1.04 and 1.03 times those of corresponding tritiated water transport rates for 10, 25, 50, 75 and 90% of efflux fractions, respectively. Glucose transport fraction, expressed as percentage of injected maternal dose averaged 84 +/- 3.1% of water transport fraction in the four perfusions. Glucose kinetic parameters expressed as area under the curve, elimination constant (Kel), clearance, time for maximum response, absorption rate and elimination rate averaged 0.25, 0.29, 3.96, 1.02, 0.25 and 0.18 times that of the corresponding tritiated water value, respectively. Neither the different kinetic parameters nor the transport fraction indices differed significantly when glucose concentrations in the same perfusions were raised successively from 5.56 to 27.80 and then to 55.6 mmol/L. We speculate that within physiological limits, hyperglycemia per se plays no significant part in altering glucose transport kinetics across the human placenta in the maternal-fetal direction.

Vascular interrelationships of near‐term mink placenta: Light microscopy combined with scanning electron microscopy of corrosion casts

The microvasculature of the near-term zonary placenta of the mink has been studied using light microscopy and scanning electron microscopy of corrosion casts, prepared from maternal and fetal vessel systems, respectively. The zonary area, most important for placental exchange, includes a characteristic labyrinth. The labyrinth is composed of lobules oriented in a maternal–fetal direction. One maternal stem artery serves as the central axis of a lobule, and about six pairs of fetal stem arteries and stem veins of the chorionic primary villi mark the periphery of the lobule. Viewed from the fetal side of the labyrinth, this lobular structure presents a roughly hexagonal pattern, with the central maternal stem artery and radially oriented arteriolar branches giving the lobule the shape of a star. These arterioles frequently form bridges to neighboring lobular systems; however, the majority continue into the feto-maternally oriented three-dimensional network of maternal capillary sinusoids, which converge on the outlets of the maternal stem veins on the maternal side of the labyrinth. Maternal main crypts are delimited by the rays of the star-shaped lobules containing chorionic primary villi. The latter penetrate into maternal crypts from the fetal side, and are characterized by their axial arterial and venous stem vessels. Fetal secondary villi are arranged at different levels from these stem vessels. The secondary villi are characterized by arterioles and venules branching in pairs from the stem vessels and supply the tributary capillary complexes of terminal villi. The lobular structure of the placental labyrinth provides a three-dimensional framework of vessels where maternal capillary sinusoids and fetal capillaries meet in a one-way cross-current arrangement. The blood flow conditions and the peculiarities of the mink placenta interhemal membrane are compared to those of other carnivores and discussed with respect to the efficiency of the endotheliochorial placenta. Microsc. Res. Tech. 38:125–136, 1997. © 1997 Wiley-Liss, Inc.

Vascular interrelationships of near-term mink placenta: light microscopy combined with scanning electron microscopy of corrosion casts.

The microvasculature of the near-term zonary placenta of the mink has been studied using light microscopy and scanning electron microscopy of corrosion casts, prepared from maternal and fetal vessel systems, respectively. The zonary area, most important for placental exchange, includes a characteristic labyrinth. The labyrinth is composed of lobules oriented in a maternal-fetal direction. One maternal stem artery serves as the central axis of a lobule, and about six pairs of fetal stem arteries and stem veins of the chorionic primary villi mark the periphery of the lobule. Viewed from the fetal side of the labyrinth, this lobular structure presents a roughly hexagonal pattern, with the central maternal stem artery and radially oriented arteriolar branches giving the lobule the shape of a star. These arterioles frequently form bridges to neighboring lobular systems; however, the majority continue into the feto-maternally oriented three-dimensional network of maternal capillary sinusoids, which converge on the outlets of the maternal stem veins on the maternal side of the labyrinth. Maternal main crypts are delimited by the rays of the star-shaped lobules containing chorionic primary villi. The latter penetrate into maternal crypts from the fetal side, and are characterized by their axial arterial and venous stem vessels. Fetal secondary villi are arranged at different levels from these stem vessels. The secondary villi are characterized by arterioles and venules branching in pairs from the stem vessels and supply the tributary capillary complexes of terminal villi. The lobular structure of the placental labyrinth provides a three-dimensional framework of vessels where maternal capillary sinusoids and fetal capillaries meet in a one-way cross-current arrangement. The blood flow conditions and the peculiarities of the mink placenta interhemal membrane are compared to those of other carnivores and discussed with respect to the efficiency of the endotheliochorial placenta.

Placental transfer of phosphate in anaesthetized rats

Mechanisms of transfer of inorganic phosphate, Pi, across the placenta of rats at 21 days of gestation were studied using 32Pi. In one group of experiments the unidirectional transfer of Pi from mother to fetus was estimated from radioactivity in the fetus at various intervals after the tracer injection into the mother. At 15 min after tracer injection, the transfer rate was only slightly higher than the estimated rate of fetal accretion of Pi, and it decreased rapidly with the length of the experiment suggesting deterioration of the transfer mechanism under conditions of an acute experiment. In other experiments, transfer of 32Pi and 51Cr-EDTA (a marker of paracellular transfer) were measured across the dually-perfused placenta in the maternal-fetal direction. The transfer rate of 32Pi was an order of magnitude higher than the transfer of 51Cr-EDTA indicating that most of the Pi transfer is transcellular. The transfer of 32P decreased when the concentration of Na + in the maternal perfusate was reduced, it was related inversely to the concentration of Pi on the fetal side of the placenta, and it was related directly to the concentration of Ca 2+ on the fetal side. The maternal-fetal transfer of Pi exhibited saturation kinetics with a K m of about 0.4 mm suggesting that at a physiological concentration of Pi in maternal plasma the transfer mechanism is nearly saturated. The present observations are consistent with Pi being transferred in cotransport with Na +. The maternal-fetal transport of Pi appears to be stabilized by the high affinity of the transport system to Pi, and controlled by a negative feedback between fetal concentration of Pi and the Pi transfer rate. It may also be controlled, to some degree, by the fetal plasma Ca 2+.

Placental passage of angiotensin-converting enzyme inhibitors

OBJECTIVE: Placental passage of the angiotensin-converting enzyme inhibitors temocapril and enalapril was investigated in a placental perfusion model. STUDY DESIGN: In an open system a placental lobe was perfused on both the maternal and the fetal side with a blood-free medium containing the test substances plus a reference substance on the maternal side. Six placentas were perfused with temocapril and five with enalapril. The drugs were measured by gas chromatography – mass spectrometry. RESULTS: Both angiotensin-converting enzyme inhibitors crossed the human placenta in the maternal-fetal direction in similar quantities. Temocapril showed the same pharmacokinetic characteristics as enalapril. CONCLUSIONS: This was the first study to quantify the placental transfer of angiotensin-converting enzyme inhibitors. These antihypertensive agents should not be taken during pregnancy, to avoid any potential hazards to the fetus. (A M J O BSTET G YNECOL 1996;174:1450-5.)

Parallel mechanisms of Ca + + transfer across the perfused human placental cotyledon

OBJECTIVES: We dissected the paracellular and transcellular components of Ca++ transfer across the perfused human placental cotyledon and explored the nature of the transcellular component. STUDY DESIGN: Transfer of 45Ca++ and ethylenediaminetetraacetic acid labeled with chromium 51 was measured across the in vitro perfused cotyledon of the human placenta, and paracellular and transcellular components of the transfer of Ca++ were calculated from the transfer of the two tracers. RESULTS: The transcellular component of the Ca++ transfer in the maternal-fetal direction represented about one third of the total maternal-fetal transfer. It was saturable, sensitive to cyanide, and insensitive to verapamil. The transcellular component in the fetal-maternal direction was not different from zero. The in vitro transfer rates correlated well with the transfer rates estimated for the in vivo situation from data published in the literature. CONCLUSION: There is a significant active transport of Ca++ across the human placenta in the maternal-fetal direction. (AM J OBSTET GYNECOL 1994;170:162-7.)

Transport of calcium across the dually perfused placenta of the rat.

1. A rat placenta was dually perfused in situ with modified Krebs fluid. Perfusion was carried out through the femoral artery on the maternal side and through the umbilical artery on the fetal side. 2. Transfer of 45Ca2+ and [3H]L-glucose across the placenta was measured in the maternal-fetal direction. The transcellular component of the maternal-fetal transport of Ca2+, Jmf,tc, was estimated from transfer rates of the two tracers and from Ca2+ concentration in maternal perfusate, [Ca2+]m. 3. At [Ca2+]m of 1.1 mM (physiological concentration of Ca2+ in plasma) Jmf,tc was 92.4 +/- 13.7 nmol min-1 (mean +/- S.D.), which is about 90% of the transport expected in an intact placenta. The permeability-surface area product (PS) of the placenta to [3H]L-glucose was 13.8 +/- 3.9 microliters min-1, about 4 times higher than that expected in intact placenta. 4. Transport of 45Ca2+ changed rapidly when [Ca2+]m was varied. Kinetic constants of the transcellular transport of Ca2+ are the Michaelis constant, Km, = 0.45 mM and the maximum rate of transport, Vmax, = 116 nmol min-1. It follows from this that at physiological levels of Ca2+, transport of Ca2+ to the fetus is relatively independent of changes in [Ca2+]m. 5. Strontium and barium (SrCl2 and BaCl2, 1 mM) decreased Jmf,tc; the response was prompt and reversible. Magnesium (2 mM) had no effect. Maternal-fetal transport of 85Sr2+ and 133Ba2+ was decreased rapidly and reversibly by elevating [Ca2+]m from 0.35 to 2 mM. These observations suggest that Sr2+ and Ba2+ are transported across the placenta by the Ca2+ transport system. This means that the transport is not substrate specific. 6. Cadmium (1 mM-CdCl2) decreased Jmf,tc irreversibly with some latency. The slowness of the response suggests a non-competitive inhibition. Cadmium (0.02 mM-CdCl2) was without effect on Jmf,tc. 7. A Ca2+ channel blocker, nifedipine (10 microM), administered to the maternal side had no effect on Jmf,tc.

Uptake, transfer and metabolism of prostaglandin E 2 in the isolated perfused human placental cotyledon

( 3H) PGE 2 uptake and transfer in the isolated perfused human placental cotyledon was assessed by a ingle pass paired isotope dilution technique utilising ( 14C) sucrose as an extracellular marker. Metabolism of ( 3H) PGE 2 was measured by analysing maternal and fetal effluents from perfused human placental cotyledons after bolus injection of ( 3H) PGE 2 into ither the maternal or fetal sides. Maximal uptake of ( 3H) PGE 2 was greater on the maternal (81 +/- 8%) than the fetal sides (42 +/- 12%) and showed saturation with increasing concentrations of PGE 2 only on the fetal side with an apparent Km of 12 +/- 4.9 nmol/l and v max of 1.5 +/- 0.2 pmol/min/g. Total recoveries of ( 3H) PGE 2 were 84.6 +/- 11.8 % and 32.6 +/- 6.3 % of the injected dose after injection on the fetal and maternal sides respectively.Transferof ( 3H) PGE 2 was the same in both directions being 6.4 +/- 1.2 % of the injected dose in the fetal-maternal direction and 5.8 +/- 2.7 % of the injected dose in the maternal-fetal direction. Metabolism was greater on the maternal side (35% of injected ( 3H) PGE 2) thanthe fetalside(18% of injected ( 3H) PGE 2) and was principally to the 13,14-dihydro-15-keto-PGE 2 metabolite. Metabolism of ( 3H) PGE 2 after passage across the placenta was the same in both directions and was of the order of approximately 60%.

Transport of inorganic phosphate by the fetal border of the guinea-pig placenta perfused in situ

Unidirectional flux of inorganic phosphate, Pi, from the umbilical vascular bed perfused in situ into the cell compartment of the guinea-pig placenta, Jfc , was estimated using the indicator dilution method, with 32P-phosphate and 14C-sucrose as the tracers. The flux did not depend on fetal weight; its mean value was 0.25 +/- 0.02 mumol/min (s.e., n = 19). Jfc decreased when the placenta was perfused with CN (10(-3)M) or with Na-free fluid. Elevation of the Pi concentration in the perfusate caused an increase in Jfc which, however, was not proportional. These properties of Jfc were taken to be compatible with a Na-dependent active transport mechanism. The flux in the opposite direction, Jcf , correlated with the fetal weight. It decreased when the umbilical concentration of Pi was elevated. The efflux of 32P from placentae preloaded with the tracer changed inversely with the umbilical concentration of Pi. It is concluded that there is a bidirectional transcellular transfer of Pi between the two sides of the placenta, the flux in the maternal-fetal direction being the prevailing one. The unidirectional transfer to the fetus seems to be regulated directly by the umbilical concentration of Pi.