We have examined the effects of PGI2, 50 ug/kg, on norepinephrine induced placental vasoconstriction in 6 chronically catheterized nearterm sheep. Regional blood flows were measured with radioactive microspheres. Control flows were measured. Norepinephrine was then infused at 50 ug/min throughout the experiment. After 15 min the blood flows were again measured and PGI2 was then added to the infusate at 50 ug/min. In 15 min regional blood flows were again measured and the PGI2 infusion was stopped. Regional blood flows were measured for the last time 15 min later. The renal andnon placental uterine vasculatures behaved in a predictable manner. There was constriction with norepinephrine but PGI2 opposed the effects of norepinephrine and decreased the resistance towards the normal levels. The placenta did not behave as did the other organs. Norepinephrine increased placental resistance by PGI2 did not decrease the resistance and severely depress the placental blood flows. PGI2 does not appear to oppose norepinephrine induced placental vasoconstriction.
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