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 7.54 and 9.50 to 11.6 and 14.5 ml/100 ml of glomerular filtrate (GFR) Also the ratio of urea to inulin clearance (C(urea)/C(inulin)) increased from 0.59 to 0.64. The changes in the renal handling of water and urea suggest that fractional sodium reabsorption decreased at proximal nephron sites. The subjects then assumed a supine position, and the results were compared to those obtained during the lateral recumbent control periods. Filtered sodium decreased in 11 experiments, but in five studies it remained up to 2.6 mEq/min above control values. There was only one instance in which a significant increase in sodium excretion occurred. It was concluded that supine recumbency blunts natriuresis despite volume expansion or an increase in the filtered load of sodium.Finally, in the 12 hydrated subjects supine recumbency reduced C(H2O) and V from a mean of 11.6 and 14.5 to 6.2 and 8.2 ml/100 ml of GFR. In eight of these experiments urine osmolality fell or did not change. Simultaneously, C(urea)/C(inulin) fell from 0.64 to 0.57. These data suggest that the antinatriuresis, which occurred when the volume-expanded subjects were positioned in supine recumbency, was accompanied by a decrease in the fractional reabsorption of sodium at proximal nephron sites.