Fetal and adult urine flow and ANF responses to vascular volume expansion

Isotonic saline (2% of body wt) warmed to body temperature was infused intravascularly over 10 min into chronically catheterized, near-term fetal sheep and into nonpregnant adult sheep. Although both groups of animals underwent the same initial increase in urine flow rate relative to body weight, the diuresis was significantly more prolonged in the adult compared with the fetus. The adult demonstrated a sustained increase in plasma atrial natriuretic factor (ANF) concentration with a suppression of plasma renin activity (PRA) and plasma arginine vasopressin (AVP). In the fetus, only a transient rise in plasma ANF with no change in PRA or AVP occurred. The increases in blood volume and arterial pressure were significantly greater in the adult compared with the fetus, but the venous pressure changes were the same. Thus it appears that fetal and adult kidneys have the same initial urine flow response to rapid vascular volume loading presumably due to a pressure diuresis and elevations in ANF. The more prolonged diuresis in the adult appears attributable to more extensive and prolonged changes in plasma hormone concentrations. The latter, in turn, may be due to the greater intravascular retention of the infused fluid in the adult than in the fetus.