Renal hemodynamic response to atrial natriuretic factor in fetal and newborn sheep

We have previously demonstrated that systemic atrial natriuretic factor (ANF) infusion induced a renal vasoconstrictor response in fetal and newborn sheep. The present study was designed to test the hypothesis that the fetal and neonatal renal vasculatures do, in fact, vasodilate in response to ANF but that this effect is negated by vasoconstrictor compensatory mechanisms when ANF is infused systemically. To test this hypothesis, the renal hemodynamic response to intrarenal infusion of ANF was studied in chronically instrumented fetal (125-135 d of gestation; term 145 d) and newborn (8-15 d) sheep. Intrarenal infusion of ANF (0.125 to 4.0 micrograms/kg of body wt in fetuses and 0.25 to 8.0 micrograms/kg in newborns) had no significant effect on mean arterial blood pressure and heart rate. However, ANF produced a concentration-dependent increase in renal blood flow velocity (F = 40.9, p less than 0.001) and a decrease in renal vascular resistance (F = 38.3, p less than 0.001) in both groups. The magnitude of changes in renal blood flow velocity and renal vascular resistance expressed as percentage of changes (% delta) from control values, were similar (p greater than 0.05) in both fetal and newborn sheep during intrarenal infusion of ANF. These results demonstrate that ANF exerts direct vasodilator action on the fetal and neonatal renal vasculature and that the renal vasoconstriction previously observed during systemic infusion of ANF was probably secondary to activation of compensatory mechanisms.