Natriuretic response to direct renal interstitial volume expansion (DRIVE) in pregnant rats

Basal renal interstitial hydrostatic pressure (RIHP) is lower in pregnant rats, suggesting an increase in renal interstitial compliance during pregnancy. The RIHP responses to increases in renal perfusion pressure (RPP) and acute saline volume expansion (VE) are attenuated in pregnant rats. Pressure natriuresis and diuresis responses are significantly attenuated during normal pregnancy, whereas the natriuretic and diuretic responses to VE remain intact.

The objectives of this study were to use direct renal interstitial volume expansion (DRIVE) to selectively increase RIHP and determine the renal interstitial compliance and the relationship between RIHP, natriuretic, and diuretic responses in nonpregnant (NP; n = 8), midterm pregnant (MP; n = 8), and late-term pregnant (LP; n = 8) Sprague-Dawley (SD) rats.

DRIVE resulted in significant increases in RIHP, fractional excretion of sodium (FE(Na)), and urine flow rate (V) in all groups of rats. The increase in RIHP (DeltaIHP) was greatest for NP (3.3 +/- 0.2 mm Hg) as compared to MP (1.3 +/- 0.1 mm Hg; P < .05 v NP) and LP (1.4 +/- 0.2 mm Hg; P < .05 v NP) in response to DRIVE showing that renal interstitial compliance was greater in MP and LP as compared to the NP group of rats. The increase in fractional excretion of sodium (DeltaFE(Na)) was similar in NP (3.1% +/- 0.3%) and LP (3.6% +/- 0.8%), but was significantly lower in MP (1.5% +/- 0.4%; P < .05 v NP and LP) in response to DRIVE.

These data suggest that the blunted increase in RIHP and natriuresis in response to DRIVE in midterm pregnant rats may play an important role in the gradual plasma volume expansion that is occurring during this stage of pregnancy.