Ovine fetal swallowing and renal responses to oligohydramnios.

Amniotic fluid (AF) volume regulation is dependent on a balance between fluid production and fluid resorption. We examined the effects of reduced AF volume on AF production by fetal urine and resorption by fetal swallowing and the response of these parameters to AF volume replacement. Eight time-dated pregnant ewes (125 +/- 1 days gestation) were studied before (day 1) and after (day 3) AF and fetal urine drainage. Drainage resulted in a significant decrease in AF volume (415 +/- 89 to 157 +/- 36 ml). Fetal urine osmolality increased (139 +/- 10 to 286 +/- 33 mosmol/kgH2O), while urine flow did not change significantly (0.31 +/- 0.04 to 0.23 +/- 0.06 ml/min), resulting in nonsignificant increases in osmolar, sodium, and chloride excretions. Fetal electromyographic 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 (500 ml; 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 increased, though fetal swallowing activity and esophageal flow did not change. Thus the ovine fetus responded to reduced AF volume by maintaining AF production and decreasing AF resorption. In response to AF replacement, urine flow increased while fetal swallowing activity did not change, consistent with an intramembranous pathway for fetal AF resorption.

Maternal/fetal dehydration: prolonged effects and responses to oral rehydration.

Dehydration induces marked alterations in maternal-fetal fluid homeostasis and accompanying fetal endocrine responses. We sought to determine if the increase in fetal plasma arginine vasopressin (AVP) levels during maternal dehydration is mediated by fetal plasma hypovolemia in addition to hyperosmolality and to examine maternal and fetal plasma atrial natriuretic factor (ANF) responses to maternal dehydration and oral rehydration. Seven pregnant ewes (127 +/- 1 day) were water deprived for 72-96 h, and five of these were orally rehydrated. Dehydration induced significant increases in maternal plasma osmolality (pOSM) (300 +/- 2 to 325 +/- 8 mosmol/kg) and AVP (3.0 +/- 0.4 to 18.9 +/- 4.0 pg/ml), and decreases in plasma ANF levels (28.1 +/- 3.1 to 19.7 +/- 3.1 pg/ml). Fetal pOSM (293 +/- 3 to 314 +/- 4 mosmol/kg), AVP (2.5 +/- 0.6 to 8.1 +/- 4.8 pg/ml), and urinary fractional sodium excretion increased significantly, whereas plasma ANF and fetal blood volume did not change. After maternal water access maternal plasma AVP decreased rapidly in comparison to the gradual decrease in maternal pOSM. Fetal plasma AVP levels did not change significantly and fetal pOSM decreased more slowly than maternal pOSM. Fetal plasma ANF increased in association with increased urine flow and glomerular filtration rate after maternal rehydration. These data indicate marked differences in fetal and maternal plasma ANF and AVP responses with dehydration-induced increases in fetal plasma AVP being secondary to plasma hyperosmolality, rather than hypovolemia. Rapid suppression of maternal plasma AVP may contribute to the slower equilibration of fetal pOSM during oral, as compared with intravenous, maternal rehydration.

Amniotic fluid volume response to esophageal occlusion in fetal sheep.

Although ovine and human fetuses swallow considerable volumes of fluid, the impact of absence of fetal swallowing on amniotic fluid volume regulation is unclear. To study the role of fetal swallowing on urine production and amniotic fluid, seven ovine fetuses (126 +/- 1 days) were chronically prepared with fetal bladder and vascular catheters, an esophageal flow probe, an inflatable esophageal cuff, and amniotic fluid catheters. In the five fetuses that underwent esophageal ligation after the control period, fetal swallowing averaged 0.27 ml/min before occlusion. In response to esophageal occlusion, significant increases were noted in fetal plasma arginine vasopressin (6.9 +/- 2.6 to 16.6 +/- 4.4 pg/ml) and urine osmolality (159 +/- 1 to 324 +/- 30 mOsm/kg), whereas urine volume (0.25 ml/min) did not change. Amniotic fluid volume increased nearly threefold after 3 days of esophageal occlusion (582 +/- 180 to 1530 +/- 271 ml). Amniotic fluid volume remained normal (334 to 419 ml) in the one fetus in which the occluder did not inflate. In the one fetus in which the esophagus was occluded at surgery, amniotic fluid volume was increased after the surgical recovery period (1489 ml). These data indicate an important role of fetal swallowing in amniotic fluid homeostasis and the potential interaction of swallowing with fetal urine production.