Some Effects of High and Low Sodium Intakes during Pregnancy in the Rat: II. Electrolyte Concentrations of Maternal Plasma, Muscle, Bone and Brain and of Placenta, Amniotic Fluid, Fetal Plasma and Total Fetus in Normal Pregnancy

Abstract

Maternal plasma sodium concentrations decreased slightly as a result of pregnancy, whereas potassium concentrations were unaltered. A significant decrease in sodium and an increase in potassium levels of plasma occurred in pregnant animals fed the low sodium diet. Muscle sodium concentrations were slightly lower in pregnant than nonpregnant groups but potassium concentrations were similar. With the low sodium intake, pregnant animals had an approximate 35% decrease in sodium and 4% increase in potassium concentrations of muscle. These animals also had significant increases in muscle moisture. Bone sodium was significantly decreased in the pregnant groups receiving the low sodium intake. However, potassium concentrations and moisture were not affected by the level of dietary sodium or by pregnancy. Brain sodium and potassium concentrations were significantly lower for pregnant groups receiving the low sodium intake than for any other experimental group. Small, but significant decreases in potassium concentrations and increases in moisture content of brain were observed as a result of pregnancy. Fetal plasma levels of sodium and potassium were not significantly influenced by the level of sodium in the maternal diet. Despite a marked decrease in sodium and an increase in potassium concentrations of plasma in pregnant animals fed the low sodium diet, the fetuses of these animals appeared to be able to maintain normal sodium levels. The mechanism by which the sodium ion can operate against a concentration gradient has not been defined. The concentrations of sodium and potassium in amniotic fluid did not appear to be statistically related to the maternal sodium intake. It is suggested that a reduction in fetal urinary excretion of sodium may be associated with a mechanism by which the young of animals fed low sodium diets are able to maintain normal plasma sodium levels. The sodium or potassium levels of fetuses were not signifcantly influenced by the maternal sodium intake. The slight increase in fetal moisture in animals receiving the low sodium diet may be associated with fetal immaturity. Placental sodium, potassium or moisture contents were not significantly affected by the maternal sodium intake. The data indicated that nonpregnant animals can handle wide ranges of dietary sodium without observable effects on sodium or potassium concentrations of tissues and fluids. Also, the levels of these electrolytes were not affected by feeding excessively high levels of dietary sodium during pregnancy. However, restriction of sodium intake during pregnancy led to marked electrolyte changes in plasma, muscle, bone and brain of the maternal organism in an attempt to maintain the normalcy of the fetus. In fact, pregnant animals receiving the restricted sodium intake exhibited changes in water and electrolyte metabolism that have been associated with complications of pregnancy in humans.