Changes in renal function and blood volume in the newborn lamb delivered by cesarean section.

To determine the cause of the transient natriuresis in lambs within 1-2 h of birth, renal function and blood volume (BV) were measured in nine chronically catheterized fetal sheep aged 139-145 d before and after delivery by cesarean section. After delivery, sodium excretion increased 8-fold. This was due to a transient rise in glomerular filtration rate (by 39 +/- 21%, p < 0.02) and a fall in fractional reabsorption of sodium by the proximal tubule from 63.4 +/- 2.5% to 53.4 +/- 3.4% (p < 0.01). The distal tubule failed to compensate fully for this fall, because fractional reabsorption by the distal tubule rose from 35.5 +/- 2.4% to only 41.6 +/- 2.2% (p < 0.05). The extent of the natriuresis did not depend on the lamb's initial BV per kg at birth. However, the amount of fluid excreted and the clearance of sodium during a 45-min period within the first 1-1.25 h after birth were approximately equal to the fall in BV that occurred during this time. Thus, most of the fall in BV that occurs after delivery is due to renal salt and water losses. Because the natriuresis was greater if the lamb's arterial pressure rose after birth, it is possible that a high arterial pressure in the immediate newborn period could result in salt and volume depletion.