Gestation-dependent aspects of the response of the ovine fetus to the osmotic stress induced by maternal water deprivation.

Water deprivation was produced in pregnant sheep by withholding water for 24 h and then giving them 500 ml/d for the subsequent 3 d. All ewes bore chronically cannulated fetuses. Eleven experiments were performed in group I animals (gestation periods of 107-119 d at maximal stress) and eight experiments in group II (126-144 d of gestation). Measurements were made of maternal and fetal plasma osmolalities, fetal urine flow rate and osmolality, amniotic fluid osmolality and fetal plasma antidiuretic hormone (ADH) concentration. The experimental protocol produced a larger increment in maternal and fetal plasma osmolalities in group I than in group II (P less than 0.025). For group I fetuses the relationship between plasma ADH concentration, [ADH] and plasma osmolality was log [ADH] = -4.50 + 0.0174 (osmol) which was significantly different from that for group II fetuses, log [ADH] = -10.46 + 0.0373 (osmol) (P less than 0.02). Thus plasma [ADH] was significantly greater, at a given plasma osmolality, in older (group II) than younger (group I) fetuses. For a given value of fetal plasma [ADH] there was a greater reduction in fetal urine free water clearance in older than younger fetuses. Water deprivation of the ewe did disturb the composition of amniotic fluid and the trend in amniotic fluid osmolality followed the trend in fetal urinary osmolality.