The effect of bilateral fetal adrenalectomy on fluid balance in the ovine fetus.

1. Bilaterally adrenalectomized adult sheep, taken off replacement therapy, exhibit decreased plasma sodium, and increased plasma potassium, urinary sodium excretion and urinary sodium:potassium ratio. The hypothesis tested was that these effects would not be seen in adrenalectomized ovine fetuses, due to the mother being the major regulator of fetal fluid balance. 2. The fetuses of seven Merino ewes were bilaterally adrenalectomized at 120 +/- 0.3 days of gestation (term is 150 days) and a bladder cannula inserted. Urine flow and composition were measured six times between 125 and 142 days of gestation, and compared with results in seven control fetuses at the same gestational ages. At between 142 and 145 days of gestation, blood samples were taken from the ewe and fetus, they were killed and the volume and composition of amniotic and allantoic fluids measured. 3. Adrenalectomized fetuses had significantly lower (P < 0.05) plasma aldosterone concentration than that observed in intact fetuses (adrenalectomized, 92 +/- 16 pmol l-1; intact, 224 +/- 46 pmol l-1; P < 0.001). Plasma cortisol level was also lower (adrenalectomized, 8.6 +/- 1.4 nmol l-1; intact, 110.3 +/- 24.8 nmol l-1). Maternal plasma steroids were similar in both groups. 4. There were no significant differences in fetal urine flow rate, osmolality or composition until 139-142 days of gestation. At this time urine sodium:potassium ratio was significantly greater in adrenalectomized (P < 0.05) than intact fetuses. 5. There were no significant differences in maternal or fetal plasma solutes in the two groups and only minor alterations in the volume and composition of amniotic and allantoic fluid. The conclusion is that the major determinant of fetal fluid and electrolyte balance is placental transfer of water and electrolytes from the mother.