Renal interstitial hydrostatic pressure and natriuretic responses to volume expansion in pregnant rats.

During normal pregnancy, a gradual plasma volume expansion (VE) occurs and reaches a maximum level at late term. Pressure natriuresis and renal interstitial hydrostatic pressure (RIHP) responses are attenuated in pregnant rats. Also, basal RIHP is lower in pregnant rats, suggesting an increase in renal interstitial compliance during pregnancy. This adaptation may contribute to the increase in plasma volume that is required for a normal pregnancy, because increases in RIHP have been consistently shown to produce natriuresis and diuresis. Acute saline VE (5% body wt/30 min) has been shown to increase RIHP in normal nonpregnant rats. Therefore, the objective of this study was to determine RIHP, natriuretic, and diuretic responses to VE in nonpregnant (n = 7), midterm pregnant (n = 8), and late-term pregnant (n = 8) Sprague-Dawley rats. Although VE significantly increased RIHP, fractional excretion of sodium (FE(Na)), and urine flow rate (V) in all groups, DeltaRIHP was highest for nonpregnant (3.0 +/- 0.3 mmHg) compared with midterm pregnant (1.6 +/- 0.1 mmHg; P < 0.05 vs. nonpregnant) and late-term pregnant rats (1.2 +/- 0.1 mmHg; P < 0.05 vs. both midterm pregnant and nonpregnant rats). DeltaFE(Na) and DeltaV were similar in all groups: 5.8 +/- 1.0% and 231 +/- 27 microl/min for nonpregnant, 6.8 +/- 1.3% and 173 +/- 16 microl/min for midterm pregnant, and 7.6 +/- 1.2% and 203 +/- 10 microl/min for late-term pregnant rats, respectively. In conclusion, basal RIHP and the increase in RIHP during VE were attenuated during pregnancy; however, the natriuretic and diuretic responses to VE remain intact during the course of pregnancy.