Volumes of distribution of sodium and albumin in the sheep placenta.

Abstract

SUMMARY1. The volumes of distribution of 22Na and 24Na, 51Cr-EDTA and albumin labelled with 1311 and 1251 were measured in cotyledons and skeletal muscle taken from pregnant sheep and the fetuses near term. Radiolabelled substances were injected through chronically implanted catheters 0-5--32 hr before removal of tissues in 'short-term' experiments and 24-26 hr before in 'long-term' experiments. Volumes of distribution were expressed as percentages of tissue weights.2. Cotyledonary water content was 81 5 + 06 % (S.E. ofmean) ofwet weight (n = 10 sheep). Sodium content was 76-0 + 4-0 m-mole kg-' wet weight (n = 4) and potassium content was 58-8 + 2-4 m-mole kg-' wet weight (n = 4).3. In short-term studies cotyledonary spaces following injection into the fetal circulation were: Na, 20-5 + 0 9 % (n = 10 sheep); Cr-EDTA, 19-5 + 1-2 % (n = 3); and albumin, 2-8 + 0 3 % (n = 11). The equivalent maternal cotyledonary spaces were: Na, 23-5 + 1-6 %, or 22-1 + 1-6 % when 'corrected' for fetal radioactivity (n = 8); and albumin, 4-7 + 0 8 % (n = 8). Maternal and fetal cotyledonary sodium spaces were additive. Larger cotyledons had larger fetal sodium spaces. 4. In long-term studies, the value for maternal albumin spaces was considerably higher 12-3 + 3-2 % (n = 3), whereas that for fetal albumin spaces was only 3 9 + 0 4 % (n = 4). The higher fetal sodium space of 24-9 +1±0 % (n = 4) in the long-term experiments was attributable to rising sodium counts in the maternal circulation. 5. In the short-term experiments maternal skeletal muscle spaces were Na, 135+ 1-3% (n = 7) and albumin, 1-7+0-3% (n = 7), with higher values of 220% (n = 2) and 38 + 0-2 % (n = 3) respectively in long-term experiments.6. In the short-term experiments fetal skeletal muscle spaces were Na, 32-8 + 3-2 % (n = 7) and albumin, 2-5 + 0 3 % (n = 10), with higher values of 39-7 + 7-4 % (n = 4) and 16-2 + 0-6 % (n = 4) respectively in long-term experiments.

Extracellular Volume of the Human Placenta in vitro

“…It is, therefore, concluded, in agreement with the earlier studies [6,21], that the ECV of the human placenta is unusually large, even by comparison to other placentae such as the sheep where the total sodium space was 45.1 ± 0.7% [4],…”

“…The reasons for this discrep ancy are unclear, although it could be that there is steric hindrance to access of the larger 14C-inulin molecule to the placental ECV as compared to 5lCr-EDTA. In the sheep placenta, the fetal-side 5lCr-EDTA space (19.5 ± 1.2%) was found to be almost identical with the sodium space (20.5 ± 0.9%), but there was no further comparison to an inulin space [4], Inulin and 51Cr-EDTA also give similar values in other situations where they have been used essentially as extracellular tracers, e.g., in measurement of glomerular filtration rate [8,10], so the dis crepancies between the two found here may be peculiar to the human placenta.…”

“…The discrepancy between values obtained using 5lCr-EDTA and 14C-inulin requires further investigation, but if 5lCr-EDTA gives as good a measure of placental ECV as it appears to do in the sheep [4], it can be calculated, using the value of 20% for the combined maternal and fetal blood volumes obtained for placentae treated similarly [1] to those here, that the interstitial volume of the human placenta accessible from both cir culations is between 55 and 60%. The mech anisms controlling this large, rather constant interstitial volume remain obscure.…”

“…In a prelimi nary set of experiments the effect of perfu sion itself on placental water content and 51Cr-EDTA space, as measured by incuba tion alone, was determined. 5lCr-EDTA, unlike inulin, has not pre viously been used for ECV measurements in the human placenta, but has been used as a tracer confined to the extracellular space in permeability studies on human and other placentae [4,7,11 ] and has also been used as an extracellular tracer in permeability stud ies on endothelia [19], There is direct evi dence that 5lCr-EDTA is neither protein bound nor taken up by red cells [10], The molecular radius of 51Cr-EDTA as deter mined from its diffusion coefficient in water is 0.4 nm [2], there being no evidence of any inhomogeneity.…”

Permeability of the fetal villous microvasculature in the isolated perfused term human placenta.

“…It is concluded that the permeability of the fetal capillaries in the term human placenta resembles that of skeletal muscle both physiologically and ultrastructurally and that solute movement may be modulated by the presence of albumin in the perfusate. The human placenta lacks a lymph drainage system and in other species this has been associated with the fetal endothelium providing most of the resistance to the transplacental transfer of large molecules (Thornburg & Faber, 1976;Boyd, Canning, Stacey, Ward & Weedon, 1983). In vivo transfer data in human placenta (Bain, Copas, Landon & Stacey, 1988;Bain, Copas, Taylor, Landon & Stacey, 1990) would lend support to the conclusion that the endothelium exerts a major regulatory role in transplacental transfer.…”