Renal response to hypotonic saline load in fetal and new-born pigs

Kidney function has been studied in pig fetuses (105-109 d of gestation) and neonates (5-7 d old). Urine was collected by catheterization of the ureter. Inulin clearance, the excretion of electrolytes and the osmolality of urine and plasma were measured. In addition the response of the fetal neurohypophysis and fetal and neonatal kidney to a reduction of plasma osmolality was studied. The results show that the inulin clearance increases rapidly in the perinatal period, at a rate greater than would be expected from the gain in body weight. The re-absorption of Na and K is well developed. The fractional sodium excretion in fetuses is 2% and is 0.1% in the new-borns. The urine osmolality is high, probably due to high plasma lysine vasopressin levels persisting throughout the experiment. The infusion of hypotonic saline results in a significant decrease of plasma osmolality but only three out of nineteen animals showed an increase in urine flow. Although lysine vasopressin concentrations fell in some animals the urine stayed hyperosmotic as compared with plasma. The results show that fetuses and neonates may react to volume load but in the conditions of these experiments that regulation of plasma osmolality was inadequate.