Fate of labeled albumin and erythrocytes following injection into amniotic cavity of sheep.

We have previously described a method to measure the amniotic fluid (AF) volume and fetal swallowing rate in near-term sheep by use of 125I-labeled albumin (RISA) and 51Cr-labeled red blood cells (51Cr-RBC). However, when we measured these volumes on consecutive days, reentry of the radionuclides into the amniotic cavity from fetal urine affected the calculated values of AF volume and swallowing rates. In an attempt to clarify the recirculation problem, we injected RISA and Cr-RBC daily for 9 days into the AF of five chronically catheterized pregnant sheep (124 days gestation on the 1st experimental day). We calculated AF volume and fetal swallowing rate, comparing those values to the values corrected for fetal urine isotopic counts. The mean AF volume and fetal swallowing rate measured by RISA were 808 +/- 48 ml (mean +/- SE) and 559 +/- 29 ml/day, respectively. These values were only slightly different from the corrected volumes, 808 +/- 48 ml and 561 +/- 29 ml/day, respectively, because fetal urine 125I activity reached only 4.8% of AF activity even on the 9th day. In contrast, 51Cr-activity in fetal urine on the 9th day showed 47% of the activity of AF. The mean uncorrected AF volume (785 +/- 44 ml) and swallowing rate (561 +/- 31 ml/day) measured by Cr-RBC were different from the corrected values (790 ml and 570 ml/day, respectively). After the 4th day these differences were particularly conspicuous.(ABSTRACT TRUNCATED AT 250 WORDS)