The effect of ouabain on placental transport of 86Rb

Structure-based modelling in reproductive toxicology: (Q)SARs for the placental barrier

The replacement of animal testing for endpoints such as reproductive toxicity is a long-term goal. This study describes the possibilities of using simple (quantitative) structure-activity relationships ((Q)SARs) to predict whether a molecule may cross the placental membrane. The concept is straightforward, if a molecule is not able to cross the placental barrier, then it will not be a reproductive toxicant. Such a model could be placed at the start of any integrated testing strategy. To develop these models the literature was reviewed to obtain data relating to the transfer of molecules across the placenta. A reasonable number of data were obtained and are suitable for the modelling of the ability of a molecule to cross the placenta. Clearance or transfer indices data were sought due to their ability to eliminate inter-placental variation by standardising drug clearance to the reference compound antipyrine. Modelling of the permeability data indicates that (Q)SARs with reasonable statistical fit can be developed for the ability of molecules to cross the placental barrier membrane. Analysis of the models indicates that molecular size, hydrophobicity and hydrogen-bonding ability are molecular properties that may govern the ability of a molecule to cross the placental barrier.

Angiotensin II decreases system A amino acid transporter activity in human placental villous fragments through AT1 receptor activation

Reduced transport of amino acids from mother to fetus can lead to fetal intrauterine growth restriction (IUGR). The activities of several amino acid transport systems, including system A, are decreased in placental syncytiotrophoblast of IUGR pregnancies. Na(+)-K(+)-ATPase activity provides an essential driving force for Na(+)-coupled system A transport, is decreased in the placenta of IUGR pregnancies, and is decreased by angiotensin II in several tissues. Several reports have shown activation of the fetoplacental renin-angiotensin system (RAS) in IUGR. We investigated the effect of angiotensin II on placental system A transport and Na(+)-K(+)-ATPase activity in placental villi. Placental system A activity in single primary villous fragments was measured as the Na(+)-dependent uptake of alpha-(methylamino)isobutyric acid, and Na(+)/K(+) ATPase activity was measured as ouabain-sensitive uptake of (86)rubidium. Angiotensin II decreased system A activity in a concentration-dependent fashion (10-500 nmol/l). Angiotensin II type 1 receptor (AT1-R) antagonists losartan and AT1-R anti-peptide blocked the angiotensin II effect, but the angiotensin II type 2 receptor antagonist PD-123319 was without effect. System A activity was not altered by preincubation with AT1-R-independent vasoconstrictors, and antioxidants did not prevent the decrease in activity mediated by angiotensin II. Angiotensin II decreased Na(+)-K(+)-ATPase activity by an AT1-R dependent mechanism, and inhibition of Na(+)-K(+)-ATPase activity decreased system A activity in a dose-response fashion. These data suggest that angiotensin II, via AT1-R signaling, decreases system A activity by suppressing Na(+)-K(+)-ATPase in human placental villi, consistent with possible adverse effects of enhanced placental RAS on fetal growth.

Microvillous membrane potential (Em) in villi from first trimester human placenta: comparison to Em at term

The microvillous membrane (MVM) potential (Em) of first trimester human placental villi was measured and compared with that in villi from term human placentas. The median Em in first trimester villi (-28 mV) was significantly more negative than that at term (-21 mV; P < 0.001). The median Em measured in villi from early (weeks 6-11) first trimester (-32 mV) was significantly more negative than that in late (weeks 12 and 13) first trimester villi (-24 mV; P < 0.001). Elevating extracellular KCl concentration induced a significant depolarization of Em in both first trimester and term villi (P < 0.05 and P < 0.001, respectively). The magnitude of this depolarization was greater in first trimester than at term, indicating that the ion conductance of the MVM changes with gestation. Exposure to ouabain induced a significant depolarization of Em (3 mV: P < 0.05) in first trimester villi but had little effect at term. These results suggest that microvillous membrane electrophysiology changes with placental development. An alteration in the relative K+:Cl- conductance of the MVM is likely to be a major contributor to the change in the magnitude of Em.

Free vitamin B12 and transcobalamin II-vitamin B12 complex uptake by the visceral yolk sac of the Sprague-Dawley rat: effect of inhibitors

Exogenous free vitamin B12 or B12 bound to human transcobalamin II (TCII) accumulated in the near-term rat visceral yolk sac. The rates of their uptakes in vitro and in vivo increased rapidly with time then reached a plateau, which supports a saturable transport/binding process as the rate-limiting step for the uptake of free and TCII complexed B12. Both uptakes were significantly decreased by trypan blue, colchicine, and low temperature but not by ouabain. Such inhibition suggests that the absorption of free and bound B12 is via an endocytosis process dependent upon energy but not the magnesium-dependent sodium/potassium-activated ATPase. Thus, the role of the visceral yolk sac in vitamin transfer to the conceptus and the alterations in yolk sac function associated with birth defects and diminished growth can be integrally related.

Transtrophoblast and microvillus membrane potential difference in mature intermediate human placental villi

Single mature intermediate villi from term placentas after normal gestation and vaginal or cesarian delivery were identified microscopically and mounted in a bath for conventional micro-electrode studies. With the application of strict selection criteria, the following observations on electrical potential difference (PD) were made. 1) With the tissue bathed in Earle's medium (37 degrees C) the PD across the syncytiotrophoblast microvillus membrane with respect to the bath was not normally distributed. The median PD was -22 mV (range -12 to -60 mV, n = 200). This fell to -6 mV after prior incubation for 4 h with cyanide (3 mM) or iodoacetate (2 mM) but was not altered by short-term application of these agents or by 0.1 mM ouabain. Substituting Na+ in Earle's medium with choline had no effect on PD, but replacing Cl- with gluconate caused a depolarization of 6 mV (P < 0.002). Increasing KCl in the bath fluid revealed an apparently low microvillus membrane K+ conductance. The low microvillus membrane PD may reflect a low Na(+)-K(+)-ATPase activity and/or a low membrane permeability to K+. 2) The transtrophoblast PD measured by insertion and withdrawal of the electrode was significantly different from zero [P < 0.003, median -3 mV (range 0 to -15 mV), n = 11]; and PD measured by insertion of the electrode into the villus core and beyond was -6 mV (significantly different from zero P < 0.003, range -2.5 to -10 mV, n = 6). If a similar PD were to exist in vivo, it could have a significant influence on ion transport across the placenta at term.

Anion transport by human placenta: a study of chloride and sulphate efflux from isolated placental tissue fragments

The efflux of radiolabelled sulphate and chloride from fragments prepared from normal human term placentae has been examined so that a comparison between the transport properties of whole tissue and isolated placental membranes may be made. Sulphate efflux was found via a temperature- and DIDS sensitive mechanism. External chloride and sulphate were able to trans-accelerate the egress of labelled sulphate from tissue fragments via a DIDS-inhibitable route. These results are consistent with sulphate transport being mediated via an anion-exchange process. Chloride efflux from fragments of placenta was via a system which could be trans-stimulated by external Cl-: this moiety of transport was inhibited by DIDS. Several differences between the characteristics of placental tissue anion transport and isolated membrane vesicles were noted and are discussed.

Inhibitor action on placental calcium transport

Human term placental lobules were dually perfused with Krebs Ringer solution at 37 degrees C under open circuit conditions. Provided that perfusate Ca2+ concentrations were between 2.33 and 2.55 mM, there was a steady release of Ca2+ into the fetal circulation and uptake of Ca2+ from the maternal circulation. There was no significant calcium (Ca) protein binding in the perfusates. Addition of dinitrophenol altered the release of Ca2+ to an uptake on the fetal circuit and enhanced Ca2+ uptake on the maternal circuit. It also produced a release of potassium (K)+ and an uptake of Na+ on both sides of the placenta. Ouabain had no significant effect on Ca movements although it produced a marked release of K+ into the fetal perfusate. The effect of cooling on the fetal circuit was similar to that of dinitrophenol (DNP), although it did not produce significant changes in either Ca2+ or K+ movements on the maternal side of the lobule. Both DNP and cooling reduced the Ca concentration ratio between fetal and maternal outflows to unity. Replacement of Na+ by choline Ringer had only transient effect on the extraction of 45Ca from fetal perfusate. These observations indicate that a Ca2+/Na (sodium)+ exchanger does not make a major contribution to the transplacental movement of Ca2+ from mother to fetus and that this process is more probably associated with membrane-bound ATPases.

Placental transport of riboflavin: differential rates of uptake at the maternal and fetal surfaces of the perfused human placenta

The kinetics of riboflavin transfer across the perfused human placenta has been studied. With both circulations open, the transfer index (clearance of riboflavin: clearance of antipyrine) toward the fetus was 3.3 +/- 0.08 (+/- SD) versus 0.87 +/- 0.02 in the reverse direction (p less than 0.01). Galactoflavin reduced the materno-fetal transfer rate. The uptake of riboflavin and L-glucose after bolus injection into the maternal perfusate was 35% +/- 9.6% versus 3% +/- 16%, p = 0.005, which indicates an uptake rate in excess of diffusion. There was no significant difference in uptake at the fetal surface. With recirculation of either the maternal or fetal perfusate, the transplacental gradient that was established was lower at high concentrations of riboflavin, which indicates the saturability of this function. Riboflavin was concentrated in the intracellular space and partly metabolized to flavin mononucleotide and flavin adenine dinucleotide. An equilibrium was approximated when both perfusates were recirculated, with the concentration lowest in the maternal perfusate, highest in the placenta, and intermediate in the fetal perfusate. It is concluded that the major factor directing riboflavin toward the fetus is the difference in rates of uptake at the two surfaces of the placenta.