Persistence of Maternal Plasma Volume Expansion in Midterm Pregnant Rats Maintained on A Zero Sodium Intake: Evidence That Early Gestational Volume Expansion does not Require Renal Sodium Retention

Abstract

Female Munich-Wistar rats were maintained on either a zero or 0.99% NaCl content diet for 20 days. After 7 days on the diet, some rats were mated and plasma volumes were measured on day 12 of pregnancy and compared to virgins receiving the same diet. Pregnant rats receiving the normal sodium diet exhibited the usual, moderate plasma volume expansion of midterm pregnancy, plasma volume = 8.0±0.3 ml vs 7.0±0.2 ml in virgins, p<0.01. Remarkably, rats receiving the zero sodium content diet also showed a significant plasma volume expansion at day 12 of pregnancy, plasma volume = 7.9±0.6 ml vs 6.5±0.2 ml in virgins. The uterus was inspected and all pregnant rats were found to have multiple fetuses in situ with normal appearance irrespective of the diet. Thus, the early plasma volume expansion of pregnancy can occur in the absence of a dietary source of sodium, indicating that renal sodium retention is not a prerequisite. Presumably redistribution of volume occurs in early pregnancy, with fluid shifting into the vascular compartment.