AJP - Regulatory, Integrative and Comparative Physiology, Vol 268, Issue 2 358-R365, Copyright © 1995 by American Physiological Society
DDAVP-induced maternal hyposmolality increases ovine fetal urine flow
M. J. Nijland, M. G. Ross, L. K. Kullama, K. Bradley and M. G. Ervin
Department of Obstetrics and Gynecology, University of California Los Angeles School of Medicine, Harbor-UCLA Medical Center, Torrance 90502.
Fetal urine flow is influenced by fetal intravascular volume, glomerular
filtration rate, tubular reabsorption, and fluid regulatory hormones. 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 increase 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 flow rate (Uvol), and
plasma arginine vasopressin (AVP) levels were measured during a 2-h control
period. At 2 h, tap water (2 liter, 38 degrees C) was administered to the
ewe. At 3 h, ewes received a 20-micrograms bolus of
1-desamino-[D-Arg8]vasopressin (DDAVP), followed by continuous infusion (4
micrograms/h). In response to water loading, maternal urine osmolality
decreased (761 +/- 158 to 339 +/- 13 mosmol/kgH2O), and Uvol increased.
After DDAVP, maternal urine osmolality increased (1,270 +/- 89
mosmol/kgH2O), and Uvol, hematocrit, plasma osmolality (304 +/- 1 to 284
+/- 4 mosmol/kgH2O), and protein concentration decreased. Five hours after
maternal DDAVP infusion, fetal plasma osmolality decreased (300 +/- 1 to
281 +/- 3 mosmol/kgH2O), and Uvol increased (0.4 +/- 0.1 to 1.3 +/- 0.2
ml/min) and remained elevated at 24 h. There was no change in fetal plasma
DDAVP (immunoreactive AVP) levels or fetal urine osmolality. Controlled
changes in maternal plasma osmolality may prove useful in modulating fetal
urine flow and, ultimately, amniotic fluid volume.
