Effect of hypotonic expansion on sodium, water, and urea excretion in late pregnancy: the influence of posture on these results.

Mineralocorticoid-treated, normotensive third trimester subjects positioned in lateral recumbency were studied before and during the infusion of 300 mEq of hypotonic saline. Urinary sodium excretion increased in all subjects from a mean value of 199 to 416 muEq/min. In 12 maximally hydrated subjects free water clearance (C(H2O)) and urine flow (V) increased from means of 7.

Evidence that an acute increase in glomerular filtration has little effect on sodium excretion in the dog unless extracellular volume is expanded.

The concept that acute increases in glomerular filtration rate (GFR) will cause large concomitant increases in sodium excretion has been re-examined. In previous work, GFR was elevated by volume expansion, usually with saline infusions. Recent evidence shows that tubular reabsorption is depressed during saline loading; hence, the independent effect of increased GFR on sodium excretion cannot be assessed.