The acute pressure natriuresis response blunted and the blood pressure response reset in the normal pregnant rat

Abstract

OBJECTIVES: Our purpose was to test the hypothesis that the acute pressure natriuresis curve was reset in pregnancy to facilitate the volume expansion. STUDY DESIGN: Studies were done with 14- to 16-day pregnant ( n = 8) and age-matched virgin female ( n = 6) Sprague-Dawley rats that were under general anesthesia. The left kidney was denervated, and mechanical clamps were placed on the aorta above and below the renal arteries for manipulation of renal perfusion pressure. Rats received intravenous 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 glomerular filtration rate, renal plasma flow, sodium excretion, and urine flow were measured in both kidneys at each renal perfusion pressure. Data were analyzed by unpaired t test and by homogeneity by slopes. RESULTS: The acute pressure natriuresis curve was blunted in pregnant rats versus virgins, and the renal nerves were not responsible. The blunted natriuretic response in pregnancy was due to loss of tubular epithelial responsiveness to increased blood pressure. CONCLUSION: The pressure 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 profound peripheral vasodilation. This suggests an alteration in the mechanism(s) normally linking blood pressure control to the acute pressure natriuresis relationship. (Am J Obstet Gynecol 1998;179:486-91.)