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

The relationship between uterine driving pressure and maternal placental blood flow was studied after inflation of an aortic occluder previously placed between the renal and ovarian arteries in 10 conscious pregnant rabbits at 28 +/- (mean +/- SEM) d of a 30- to 31-d gestation to test the hypothesis that there is autoregulation of maternal placental blood flow. After control measurements, the femoral artery pressure was reduced 22 +/- 3% from 83 +/- 5 mm Hg and clamped at 65 +/- 4 mm Hg (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 change 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 decreased 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 significant 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 increased from 20.24 +/- 2.39 to 27.34 +/- 5.33 ng.mL(-1).h(-1) (p < ).05). There was neither a uterine venoarterious difference nor a net uterine release of angiotensin I or active plasma renin activity at any time, but the net uterine release of total plasma renin activity increased 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 autoregulation of placental blood flow in the first hour after reduction in uterine driving pressure in the conscious pregnant rabbit. Uterine hypotension is associated with an increase in arterial pressure above the occluder and with the release of an inactive form of renin from the uterus. The significance of the latter two findings remains conjectural.