¹Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, Virginia

Correspondence: Dr. Ali A. Khraibi, Department of Physiological Sciences, Eastern Virginia Medical School, 700 Olney Road, Norfolk, VA 23507-1696 E-mail: khraibaa@evms.edu

^*This research was supported by a grant from the National Institute of Child Health and Human Development HD38240, and by Eastern Virginia Medical School.

Received 1 October 2004; Revised 24 November 2004; Accepted 3 December 2004.

Abstract

Background: 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.

Methods: 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.

Results: 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 (IHP) 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 (FE_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.

Conclusions: 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.