Extracellular volume of the human placenta in vitro

The extracellular volume of fresh and perfused human placenta has been measured in two sets of experiments. In the first set fragments of placenta which had been dually perfused for either 45 or 110 min as well as fragments from unperfused areas of the same placenta were incubated for 4 h in Earle's bicarbonate buffer containing 51Cr-EDTA as an extracellular marker. The 51Cr-EDTA space, as a proportion of wet weight, was 76.5 +/- (SE) 1.4 (n = 11) and 79.4 +/- 1.8% (n = 6) for the short and long perfusions, respectively; these did not significantly differ from each other or from the values for the equivalent unperfused placenta: 75.2 +/- 1.0 (n = 11) and 77.7 +/- 0.9% (n = 6). In the second set of experiments 51Cr-EDTA and 14C-inulin were perfused through the maternal and fetal circulations of the placental lobules for 1, 2, or 4 h, and simultaneously unperfused fragments from the same placenta were incubated with the markers for 1, 2, or 4 h. The 51Cr-EDTA space was found to be significantly higher than the 14C-inulin space in both perfusion and incubation determinations. Irrespective of the marker, the incubation spaces were unaltered with time and were significantly higher than the perfusion spaces at all three time points. However, the difference decreased with the length of perfusion, so that by 4 h it was less than 10%. (At 4 h incubation the 51Cr-EDTA space was 76.2 +/- 0.9% and the perfusion space 67.4 +/- 1.1%, n = 6; the 14C-inulin incubation space was 60.9 +/- 2.2% and the perfusion space 52.5 +/- 2.6%, n = 5). As there was no evidence of increasing placental permeability with time, as judged by clearance of creatinine, it was concluded that the placental extracellular volume is large irrespective of the method of measurement.