Henning Schneider

Transfer and metabolism of norepinephrine studied from maternal-to-fetal and fetal-to-maternal sides in the in vitro perfused human placental lobe

An in vitro system for the dual perfusion of an isolated lobe of human placenta was used to study transfer and metabolism of 3H-labeled norepinephrine at two different concentrations. At 39 mumol/L, the transfer of total radioactivity from the maternal to the fetal side was 11.60% +/- 0.60% and significantly higher (p less than 0.05) than that in the opposite direction (6.50% +/- 0.37%). In both directions almost 50% of tritium was transferred as norepinephrine while the rest was metabolized and appeared on the other side as normetanephrine, dihydroxymandelic acid, and vanillylmandelic acid. When a reduced concentration (870 nmol/L) was infused into either the maternal or the fetal compartment, transfer and metabolism were similar to those determined before. Analysis of the tissue showed 50% of total radioactivity as normetanephrine, dihydroxymandelic acid, and vanillylmandelic acid and 17% as conjugates. Inclusion of the monoamine oxidase inhibitor clorgyline in the perfusate significantly reduced fetal-to-maternal transfer but not transfer in the reverse direction. In addition, inhibition of placental monoamine oxidase resulted in a considerable reduction in dihydroxymandelic acid and vanillylmandelic acid while the normetanephrine fraction increased. Norepinephrine significantly increased glucose utilization and lactate production. A possible physiologic role of norepinephrine transferred from the fetal into the maternal circulation during labor is discussed.

Asymmetrical transfer of α-aminoisobutyric acid (AIB), leucine and lysine across the in vitro perfused human placenta

Abstract The mechanism for establishing transplacental gradients for leucine, lysine and α-aminoisobutyric acid (AIB) has been investigated in the perfused human placenta. Experiments were done with either the maternal or the fetal circulation closed and the donor circulation open. Transfer of the amino acids towards the fetal side was more rapid than it was in the reverse direction. When the maternal perfusate was recirculated, the amino acid concentrations were maintained at a considerably lower level in the maternal circulation than in the open fetal circuit. When the fetal circuit was closed, the concentrations approached or slightly exceeded those in the maternal perfusate over a period of three hours. Within the placenta, higher concentrations were established during the experiments with transfer towards the fetal side than in the reverse direction. of the three amino acids, leucine was transferred most rapidly across the placenta while AIB reached the highest concentrations in the placental tissue. The asymmetry of the transplacental amino acid flux is favoured by rapid uptake from the maternal circulation and transfer towards the fetus. Both rates exceed those observed in the reverse direction. The transfer rate of d -leucine was 1.7 times that of L -glucose. For in vitro studies of the transfer rate of physiological compounds a correction for diffusion is required. The results may differ considerably depending on which marker is used as the basis.

Placental transfer of β-adrenergic antagonists studied in an in vitro perfusion system of human placental tissue

Maternofetal transfer of five beta-blockers differing in molecular weight, solubility, and binding to albumin was studied using a dual in vitro perfusion system of an isolated cotyledon of human placenta. At steady state the diffusion rate of the lipid-soluble propranolol, timolol, and labetalol was three to four times higher than that of the hydrophilic atenolol and celiprolol. Albumin binding had no significant effect on diffusion when equal concentrations were used in the two perfusion circuits. With increased albumin concentration on the fetal side an acceleration of the diffusion of propranolol could be shown. Propranolol and labetalol showed considerable binding to placental tissue. After bolus injection transfer was clearly suppressed, as a result of tissue binding, and there was a delay until a steady state of diffusion was reached when constant concentrations were maintained in the maternal compartment. With recirculation of the fetal and maternal compartments propranolol rapidly equilibrated in the two perfusion circuits at 35% of the initial level in the maternal circuit. Atenolol after 4 hours of recirculation had not reached full equilibration between the two compartments, and the fetal concentration was at 55% of the initial level on the maternal side.

Transplacental transfer of beta-adrenergic drugs studied by an in vitro perfusion method of an isolated human placental lobule

A study was made of the transfer across an in vitro perfused human placenta of four beta-mimetics commonly used in obstetric practice. Between 2% and 3% of fenoterol, ritodrine, and salbutamol appeared on the fetal side after bolus injection into the maternal circulation. For hexoprenaline, the transfer (1%) was significantly lower than that observed with other beta-mimetics studied. With constant infusion of fenoterol into the maternal circulation and recycling of a small volume of perfusate on the fetal side, a fairly rapid rise in the concentration was seen. Some sulfate conjugation of fenoterol during its diffusion from the maternal to the fetal side of the placenta could be seen. In the in vitro perfused human placental lobule there is no apparent effect of synthetic beta-mimetics on the transfer of antipyrine or glucose and no stimulation of the production of lactate and pyruvate.

De Novo Synthesis of Pregnancy-Specific and Pregnancy-Associated Proteins by the In Vitro Perfused Human Term Placenta

Placental perfusion is one of several in vitro techniques to investigate the production of pregnancy proteins by the trophoblast as well as their control mechanisms. The preparation as described in the chapter by Schneider et al is a closed-circuit system — that is, the released proteins and hormones accumulate in the circulating perfusate. This has the advantage of consuming less medium and yielding smaller volumes of more concentrated solutions when compared with the open perfusion protocol. We are aware that the release kinetics may differ between open and closed circuit protocols. For this reason we have abandoned our first protocol which required a full medium change after two hours of perfusion1 and have followed a new one which required the removal of 5 ml aliquots from both circuits every 30 minutes during the first two hours and every 60 minutes thereafter. The removed aliquots were replaced with fresh medium injected into both circulations.

Effects of Elevated Umbilical Venous Pressure on Fluid and Solute Transport Across the Isolated Perfused Human Placental Cotelydon

Everyone who has perfused the placenta of the guinea pig, rat or rabbit, has observed that the venous outflow rate decreased when the umbilical outflow pressure was elevated. This observation implies that in the placentae of these species, elevation of the umbilical venous pressure promotes a significant volume flow from the perfused umbilical side of the placenta to the maternal circulation. The phenomenon has been described in more detail in the placenta of the guinea pig (Dancis et al., 1962), under conditions of in situ perfusion of the umbilical circuit and in the isolated dually perfused preparation (Leichtweiss and Schroder, 1977). Dancis and coworkers observed that water flux from the fetal to the maternal side of the placenta was greatly increased when the umbilical outflow pressure was elevated to about 20 cm H2O, and that labeled albumin and erythrocytes, which had been added to the umbilical perfusion fluid, could readily be detected in the maternal circulation. In the experiments of Leichtweiss and Schroder, increasing the umbilical venous pressure caused a net fluid movement from the fetal to the maternal side without any observable evidence for sieving off the albumin molecules. Electron microscopy of the perfused tissue, using horseradish peroxidase as a tracer, demonstrated that wide bag-shaped channels open in the placental trophoblast when the umbilical outflow pressure is increased (Kaufmann et al., 1982).

Transfer of Antipyrine, H2O, L-Glucose and α-Aminoisobutyric Acid Across the in Vitro Perfused Human Placental Lobe

The dual circuit in vitro perfusion of an isolated lobe of human placenta is a useful experimental technique to study transfer of material across the placenta and to identify factors influencing exchange between maternal and fetal circulation. It has repeatedly been noted that there is considerable variation in results among experiments which are not fully explained by differences in size of the perfused lobe (Schneider et al., 1972; Welsch, 1980). While flow rates in the two perfusion circuits can easily be controlled, the surface area of exchange, shunts, and degree of overlap of the two circulations remain major factors causing interexperimental variability. Antipyrine which rapidly equilibrates across most biological membranes has been proposed as a reference for normalization of experimental data on transfer (Dancis et al., 1973). After this correction parameters like membrane permeability or specific transport mechanisms can be explored. This concept should be valid for the study of test compounds which are similarly affected in transfer by the uncontrollable variables as is antipyrine.