MATERNAL PLACENTAL BLOOD-FLOW IS REDUCED IN PROPORTION TO REDUCTION IN UTERINE DRIVING PRESSURE

The relationship between uterine driving pressure and maternal placent al blood flow was studied after inflation of an aortic occluder previo usly placed between the renal and ovarian arteries in 10 conscious pre gnant rabbits at 28 +/- (mean +/- SEM) d of a 30- to 31-d gestation to test the hypothesis that there is autoregulation of maternal placenta l blood flow. After control measurements, the femoral artery pressure was reduced 22 +/- 3% from 83 +/- 5 mm Hg and clamped at 65 +/- 4 mm H g (p < 0.001) for 54 +/- min by servo control. Carotid artery pressure increased from 86 +/- to 98 +/- 6 mm Hg (p <0.01). There was no chang e in cardiac output (839 +/- 78 vs 814 +/- 64 mL/min; NS), upper-body flow (651 +/- 62 vs 671 +/- 55 mL/min; NS), or renal flow (111 +/- 14 vs 104 +/- 8 mL/min; NS). Blood flow to tissues below the occluder dec reased from 188 +/- 18 to 143 +/- 14 mL/min for the lower body (p < 0. 05), 153 +/- 15 to 116 +/- 11 mL/min for the hindquarters (p < 0.05), and 17.7 +/- 1.9 to 12.9 +/- 1.4 mL/min for 13 pregnant uterine horns (p < 0.05). Placental flow to live fetuses per horn decreased from 13. 0 +/- 1.9 to 8.9 +/- 1.2 mL/min (p <0.01), whereas there was no signif icant change in myoendometrial flow (4.0 +/- 0.3 vs 3.5 +/- 0.5 mL/min ; NS). Uterine oxygen consumption was unchanged (1.15 +/- 0.16 vs 1.06 +/- 0.13 mL/min; NS). There was no change in the circulating arterial concentration of angiotensin I (2.95 +/- 1.07 vs 1.46 +/- 0.59 ng.mL( -1)) or active plasma renin activity (8.83 +/- 3.72 vs 10.70 +/- 5.56 ng.mL(-1).h(-1)), but trypsin-activated total plasma renin activity in creased from 20.24 +/- 2.39 to 27.34 +/- 5.33 ng.mL(-1).h(-1) (p < ).0 5). There was neither a uterine venoarterious difference nor a net ute rine release of angiotensin I or active plasma renin activity at any t ime, but the net uterine release of total plasma renin activity increa sed from 31.66 +/- 24.08 to 72.32 +/- 19.46 (ng.mL(-1).h(-1))-(mL.min( -1)) (p < 0.05). We conclude that there is no significant autoregulati on of placental blood flow in the first hour after reduction in uterin e driving pressure in the conscious pregnant rabbit. Uterine hypotensi on is associated with an increase in arterial pressure above the occlu der and with the release of an inactive form of renin from the uterus. The significance of the latter two findings remains conjectural.