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

The volumes of distribution of 22Na and 24Na, 51Cr-EDTA and albumin labelled with 131I and 125I were measured in cotyledons and skeletal muscle taken from pregnant sheep and the fetuses near term. Radiolabelled substances were injected through chronically implanted catheters 0.5-3.2 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. Cotyledonary water content was 81.5 +/- 0.6% (S.E. of mean) of wet weight (n = 10 sheep). Sodium content was 76.0 +/- 4.0 m-mole kg-1 wet weight (n = 4) and potassium content was 58.8 +/- 2.4 m-mole kg-1 wet weight (n = 4). 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. 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. In the short-term experiments maternal skeletal muscle spaces were Na, 13.5 +/- 1.3% (n = 7) and albumin, 1.7 +/- 0.3% (n = 7), with higher values of 22.0% (n = 2) and 3.8 +/- 0.2% (n = 3) respectively in long-term experiments. 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.