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Placental permeability in the sheep

T E Stacey, R D Boyd, R H Ward

Annales De Recherches Veterinaires. Annals of Veterinary Research

|January 1, 1977

Summary

The sheep placenta acts as a tight barrier, limiting passive transfer of molecules based on size. Specialized carriers facilitate glucose transfer, indicating complex placental transport mechanisms.

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Area of Science:

Obstetrics and Gynecology
Physiology
Pharmacology

Background:

Understanding transplacental transfer is crucial for fetal development and drug delivery.
The sheep model offers a valuable system for studying feto-maternal exchange dynamics.

Purpose of the Study:

To quantify transplacental flux and permeability coefficients for various substances in pregnant sheep.
To elucidate the mechanisms governing the transfer of inert molecules and glucose across the placenta.
To investigate the fetomaternal electrical potential difference and its relationship with placental transport.

Main Methods:

Utilized chronically catheterized pregnant sheep (124-143 days gestation).
Applied Fick's principle to measure radiolabeled substance flux and uterine blood flow.
Calculated net flux and permeability coefficients (K).
Assessed transfer competition for monosaccharides and measured electrical potential differences.

Main Results:

Permeability (K) for inert molecules (urea, erythritol, mannitol, Cr-EDTA) decreased sharply with increasing molecular size, suggesting a pore radius of ~0.45 nm.
High permeability for 3-methylglucose and 2-deoxyglucose indicated carrier-mediated transport, confirmed by D-glucose competition studies.
Fetomaternal electrical potential difference varied, influenced by gestation and fetal acid infusion, and appeared independent in twin pregnancies.

Conclusions:

The sheep placenta functions as a selective barrier with limited passive diffusion for larger molecules.
Stereospecific carrier systems are involved in monosaccharide transport across the placenta.
Electrical and chemical gradients across the placenta are maintained by its low passive permeability.

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