THE ACUTE PRESSURE NATRIURESIS RESPONSE BLUNTED AND THE BLOOD-PRESSURE RESPONSE RESET IN THE NORMAL PREGNANT RAT

OBJECTIVES: Our purpose was to test the hypothesis that the acute pres sure natriuresis curve was reset in pregnancy to facilitate the volume expansion. STUDY DESIGN: Studies were done with 14- to 16-day pregnan t (n = 8) and age-matched virgin female (n = 6) Sprague-Dawley rats th at were under general anesthesia. The left kidney was denervated, and mechanical clamps were placed on the aorta above and below the renal a rteries for manipulation of renal perfusion pressure. Rats received in travenous 0.9% sodium chloride (1.5% body weight/h) and a cocktail of vasoactive factors to suppress variation in endogenous hormones. Renal perfusion pressure was varied acutely from 125 to 95 mm Hg, and glome rular filtration rate, renal plasma flow, sodium excretion, and urine flow were measured in both kidneys at each renal perfusion pressure. D ata were analyzed by unpaired t test and by homogeneity by slopes. RES ULTS: The acute pressure natriuresis curve was blunted in pregnant rat s versus virgins, and the renal nerves were not responsible. The blunt ed natriuretic response in pregnancy was due to loss of tubular epithe lial responsiveness to increased blood pressure. CONCLUSION: The press ure natriuretic response is markedly blunted in pregnancy, permitting the cumulative plasma volume expansion to occur. Contrary to nongravid states, blunting of the acute pressure natriuresis curve in pregnancy is not associated with increased blood pressure because of the profou nd peripheral vasodilation. This suggests an alteration in the mechani sm(s) normally linking blood pressure control to the acute pressure na triuresis relationship.