Responsiveness to norepinephrine of the vessels supplying the placenta of growth-retarded fetuses.

The effects of intravenous norepinephrine infusions on uteroplacental blood flow were studied in seven awake, chronically catheterized guinea pigs at 63 to 64 days' gestation after unilateral uterine artery ligation at days 31 to 37. An initial subpressor dose of norepinephrine (1.54 +/- 0.18 nmol/kg.min) (+/- SEM) was given, followed by a pressor dose (4.57 +/- 0.59 nmol/kg.min) that caused a 14% increase in mean arterial blood pressure. Placental blood flow as measured by the microsphere technique decreased by 24% in 11 placentas with normal-sized fetuses during the subpressor dose and by 46% during the pressor dose. In 11 placentas with growth-retarded fetuses the two norepinephrine doses reduced blood flow by 19% and 43%, respectively. These results indicate that there is no difference in adrenergic responsiveness between the vessels supplying the placenta of a normal-sized fetus and the arteries of the placenta of a growth-retarded guinea pig. It is suggested that an increase in circulating maternal catecholamine levels would affect a growth-retarded fetus more severely than the normal-sized fetus because the growth-retarded fetus even during normal conditions exists at, or even beyond, the border of imminent asphyxia and hypoglycemia.