EFFECT OF UREA AND INDOMETHACIN INTAKE ON SOLUTE EXCRETION IN THE SYNDROME OF INAPPROPRIATE SECRETION OF ANTIDIURETIC-HORMONE

Our purpose was to compare the effect of urea and indomethacin on solu te excretion in hyponatremic patients with inappropriate secretion of antidiuretic hormone (SIADH). In 6 patients (serum Na:126 +/- 3 mmol/l ), the intake of urea (0.1 g/kg) induced a decrease in sodium excretio n while urine osmolality, urine flow and osmotic clearance (C(osm)) di d not change. In the control group, the urinary flow and C(osm) were i ncreased as expected, while sodium excretion tended to increase. In th e SIADH group, the decrease in the fractional excretion (FE) of Na+ (F E.Na+) (or FE.Cl-) after urea intake was negatively correlated with ur inary urea concentration while the FE.K+ was positively correlated wit h FE.Na+ (or FE.Cl-), which suggests that the effect of urea on sodium excretion takes place proximally to the distal tubule, probably at th e thin ascending limb. After indomethacin intake, FE.Na+ (or FE.Cl-), FE.K+, Fe.osm and Fe.urea decreased in the normal and hyponatremic gro ups. The mean free water reabsorption relatively to osmolar delivery w as lower in SIADH (p < 0.05), and did not change significantly after i ndomethacin intake. The fact that the decrease of FE.Na+ (or FE.Cl-) a fter indomethacin was associated with a decrease in FE.K+ suggests tha t the increase in sodium (or chloride) reabsorption occurred more prox imally to the distal tubule (probably a the medullary segment of the t hick ascending limb of the loop of Henle).