Maternal Fetal Osmolar Homeostasis: Fetal Posterior Pituitary Autonomy

Abstract

SummaryAfter the infusion of a bolus of 225 mEq NaCl (HS) to maternal ewes, we studied fetal plasma sodium, osmolality, total serum solids, plasma arginine vasopressin (AVP) and plasma renin activity (PRA) responses in 16 chronically catheterized, 112-139 days of gestation fetal lambs.To examine the degree to which this might have represented transplacental passage of AVP, we infused a large amount of synthetic AVP into the fetal circulation (protocol 3) and detected no change in maternal plasma AVP. The protocol was designed to allow multiple, frequent blood sampling not possible by infusing the synthetic AVP into the maternal circulation.In order to compare the fetal AVP response elicited by H S in the maternal ewe and that after direct H S into the newborn lamb and fetus, we calculated stimulus response ratio (SRR) as: Log (AVP)i -Log (AVP)z, divided by Aosmolality.The SRR of lamb fetuses after maternal H S was significantly greater (0.16 f 0.02) than that after direct fetal H S (0.04 + 0.01).In comparison, the newborn lamb has a SRR of 0.02 f 0.01, and the ewe has a SRR of 0.02 f 0.01 after HS. These data suggest that after maternal H S infusion, both a volume and, to a lesser extent, an osmolar stimulus for AVP secretion occurs after an induced water flux from the fetal to maternal compartments. Maternal plasma sodium concentration rose promptly from a base line of 146 f 2.2 (mean f SEM) to 157 f 2.8 mEq/liter by 1-20 min where it remained throughout the hour observation period. Fetal plasma sodium concentration rose more slowly from base line of 143 f 1.8 to 149 f 1.8 mEq/liter by 1 hr. When 85 m C i Z 2~a was additionally infused with the HS, fetal "Na constituted only 10% of maternal 22Na counts by the end of 1 hr.During the same period fetal PRA rose from a base line of 12.9 f 3.8 to 32.0 k 3.6 ng/ml/hr, while maternal renin remained unchanged. Maternal AVP rose modestly, 11 min after the HS, but promptly returned to base line. There was a rapid and sustained rise by fetal AVP from a base line of 0.7 pU/ml to a peak of 8.2 pU/ml by 22 min post HS.A fetal SRR, after maternal H S was greater than that after direct fetal H S or H S to the ewe or newborn lamb. In an additional experiment, using five chronically catheterized fetuses, 10 million pU AVP injected in the fetal circulation failed to produce an increase in maternal AVP. These results demonstrate that: 1) AVP does not cross the placenta; 2) the fetal sheep neurohypophysis is autonomous and responsive to both direct and indirect (maternal) osmolar stimulation; and 3) the relatively slow rate of maternal to fetal sodium transfer, the augmented SRR after maternal HS, and the elevated fetal PRA and AVP concentration suggest that there is a rapid fetal to maternal flow of water after maternal H S and a combined volume and osmolar stimulus to the fetus. SpeculationInfusions of hypertonic saline in pregnant ewes resulted in rapid increases of fetal plasma sodium due to transfer of water from fetus to mother. A marked rise in fetal renin activity and AVP w...

Changes in fetal renal function in response to infusions of a hyperosmotic solution of mannitol to the ewe.

“…The reduction in transplacental water flux was probably associated with a reduction in fetal blood volume which could be expected to stimulate release of renin from the fetal kidney (see Leake, Weitzman, Effros, Siegel & Fisher, 1979). However fetal p.r.a.…”

Gestation‐dependent Aspects of the Response of the Ovine Fetus to the Osmotic Stress Induced by Maternal Water Deprivation

“…Antidiuretic hormone (ADH) is involved in the maintenance of the normal volume and composition of body fluids in the adult animal. ADH is released by the fetal hypothalamic/pituitary system and does not cross the placenta (Leake, Weitzman, Effros, Siegel & Fisher, 1979). In the ovine fetus, plasma ADH concentration, [ADH] has been elevated by osmotic stimuliinfusion of hypertonic saline to the fetus (Weitzman, Fisher, Robillard, Erenberg, Kennedy & Smith, 1978) and volume stimulifetal hemorrhage (Robillard, Weitzman, Fisher & Smith, 1979;Rurak, 1979;Drummond, Rudolph, Keil, Gluckman, McDonald & Heymann, 1980).…”

Regulation of Urine Osmolality in Fetal Sheep

“…Such stimuli include hypoxia (Rurak, 1978;Stark, Daniel, Husain, James & Van de Wiele, 1979;Robillard, Weitzman, Burmeister & Smith, 1981), haemorrhage (Robillard, Weitzman, Fisher & Smith, 1979) and intermittent cord occlusion (Daniel, Husain, Milliez, Stark, Yeh & James, 1978). In addition the elevation of plasma osmolality by the infusion of hypertonic saline (Leake et al 1979;Weitzman, Fisher, Robillard, Erenberg, Kennedy & Smith, 1978) also acts as a stimulus to ADH secretion in the fetus. The effects of an increased fetal plasma ADH concentration include (1) an increase in blood pressure with redistribution of flow to placenta, heart and brain (Iwamoto, Rudolph, Keil & Heymann, 1978), (2) an increase in flux of sodium across the placenta (Thornburg, Binder & Faber, 1979) and (3) a decrease in free water clearance by the fetal kidney (Lingwood, Hardy, Horacek, McPhee, Scoggins & Wintour, 1978;Robillard & Weitzman, 1980).…”