AJP - Cell Physiology, Vol 236, Issue 1 58-C65, Copyright © 1979 by American Physiological Society
Distribution of ionic sulfate, lithium, and bromide across the sheep placenta
K. L. Thornburg, N. D. Binder and J. J. Faber
Salts of sulfate, lithium, and bromine were injected or infused
intravenously into ewes in the last trimester of gestation. Ewes and
fetuses had indwelling catheters; most fetuses were nephrectomized.
Concentrations were measured in paired maternal and fetal samples over
periods of 4--14 days. Maternal excretion of sulfate was too rapid to
permit near equilibration of fetal and maternal plasma concentrations; the
results, however, did not support the existence of a large potential
difference across the exchange barrier. The concentrations of Li+ (given by
continuous infusion) and 82Br- in maternal plasma did not change rapidly.
The concentrations of these tracers in fetal plasma rose until they were
nearly equal to the maternal plasma concentrations. Steady-state
transplacental potentials, calculated by use of the Nernst equation, were
5.2 +/- 2.0 (SEM) mV (n = 26) for Li+ and -2.2 +/- 0.8 (SEM) mV (n = 10)
for Br-. Nernst potentials calculated from previously measured maternal and
fetal plasma concentrations of Na+, K+, Mg2+, and Cl- were +0.4, +3.6,
+0.5, and -1.4 mV. We concluded that, of the total potential difference of
about -50 mV (fetus negative) between the fetal lamb and the ewe, only a
few mV are dropped across the placental exchange barrier.
