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

Abstract

In 6 pregnant ewes a reduction in transplacental water transfer was produced by increasing maternal osmolality (by infusion of 180 g of mannitol in 500 ml of 0.15 M-NaCl) and the fetal renal responses to this reduction in water transfer were studied. These responses were compared with the renal responses of 5 other chronically catheterized fetal lambs whose mothers received i.v. infusions of 500 ml of 0.15 M-NaCl. I.v. infusion of 500 ml of 0.15 M-NaCl to the ewe produced no changes in fetal plasma Na, K or plasma renin activity and had no effect on fetal renal function. After i.v. infusion of mannitol to the ewe, fetal urinary flow rate fell from control levels of 0.69 .+-. 0.12 ml/min to 0.32 .+-. 0.04 ml/min (SE of mean, P < 0.006). This fall in urinary flow rate was due to increased water reabsorption because there was no change in glomerular filtration rate and osmolar clearance. Fetal urinary Na excretion increased from 16.3 .+-. 2.0 .mu.mol/min, to 34.2 .+-. 6.9 .mu.mol/min (SE of mean, P < 0.04). This increase in fetal urinary Na excretion was due to a fall in the fractional reabsorption of Na which was related to the rise in fetal plasma Na levels that occurred following infusion of mannitol to the ewe. The increases in fetal plasma Na levels were also associated with reductions in fetal plasma renin activity. The fetal lamb is capable of limiting renal water excretion and modifying renal Na excretion in order to maintin the volume and composition of its body fluids.