OVINE FETAL SWALLOWING AND RENAL RESPONSES TO OLIGOHYDRAMNIOS

Amniotic fluid (AF) volume regulation is dependent on a balance betwee n fluid production and fluid resorption. We examined the effects of re duced AF volume on AF production by fetal urine and resorption by feta l swallowing and the response of these parameters to AF volume replace ment. Eight time-dated pregnant ewes (125 +/- 1 days gestation) mere s tudied before (day 1) and after (day 3) AF and fetal urine drainage. D rainage resulted in a significant decrease in AF volume (415 +/- 89 to 157 +/- 36 mi). Fetal urine osmolality increased (139 +/- 10 to 286 /- 33 mosmol/kgH(2)O), while urine flow did not change significantly ( 0.31 +/- 0.04 to 0.23 +/- 0.06 ml/min), resulting in nonsignificant in creases in osmolar, sodium, and chloride excretions. Fetal electromyog raphic swallowing activity decreased 30% (1.0 +/-: 0.1 to 0.7 +/- 0.1 swallows/min; P < 0.05), while net esophageal flow decreased 74% (0.31 +/- 0.12 to 0.07 +/- 0.04 ml/min; P < 0.05). On day 4, 0.15 M NaCl (5 00 mi; 37 degrees C) was administered into the AF over 30 min. During the 2 h after reinfusion, urine flow(0.29 +/- 0.07 to 0.40 +/- 0.09 ml /min) and osmolar sodium and chloride excretion significantly increase d, though fetal swallowing activity and esophageal flow did not change . Thus the ovine fetus responded to reduced AF volume by maintaining A F production and decreasing AF resorption. In response to AF replaceme nt, urine flow increased while fetal swallowing activity did not chang e, consistent with an intramembranous pathway for fetal AF resorption.