DDAVP - INDUCED MATERNAL HYPOSMOLALITY INCREASES OVINE FETAL URINE FLOW

Fetal urine flow is influenced by fetal intravascular volume, glomerul ar filtration rate, tubular reabsorption, and fluid regulatory hormone s. As maternal-to-fetal fluid transfer is dependent on hydrostatic and osmotic gradients, we postulated that a chronic decrease in maternal plasma osmolality would promote transplacental fluid transfer and incr ease fetal urine flow. Six pregnant ewes and singleton fetuses (131 +/ - 2 days; term = 150 days) received bladder and hindlimb arterial and venous catheters. After 5 days, plasma and urine composition, urine fl ow rate (U-vol) and plasma arginine vasopressin (AVP) levels were meas ured during a 2-h control period. At 2 h, tap water (2 liter, 38 degre es C) was administered to the ewe. At 3 h, ewes received a 20-mu g bol us of 1-desamino-[D-Arg(8)]vasopressin (DDAVP), followed by continuous infusion (4 mu g/h). In response to water loading, maternal urine osm olality decreased (761 +/- 158 to 339 +/- 13 mosmol/kgH(2)O), and U-vo l increased. After DDAVP, maternal urine osmolality increased (1,270 /- 89 mosmol/kgH(2)O), and U-vol, hematocrit, plasma osmolality (304 /- 1 to 284 +/- 4 mosmol/kgH(2)O), and protein concentration decreased . Five hours after maternal DDAVP infusion, fetal plasma osmolality de creased (300 +/- 1 to 281 +/- 3 mosmol/kgH(2)O), and U-vol increased ( 0.4 +/- 0.1 to 1.3 +/- 0.2 ml/min) and remained elevated at 24 h. Ther e was no change in fetal plasma DDAVP (immunoreactive AVP) levels or f etal urine osmolality. Controlled changes in maternal plasma osmolalit y may prove useful in modulating fetal urine flow and, ultimately, amn iotic fluid volume.