AFLUID November 46/5

César, Kátia R., and Antonio J. Magaldi. Thiazide induces water absorption in the inner medullary collecting duct of normal and Brattleboro rats. Am. J. Physiol. 277 (Renal Physiol. 46): F756–F760, 1999.—The reduction of urinary volume after the use of thiazide in the treatment of diabetes insipidus (DI) is known as the ‘‘paradoxical effect.’’ Since enhanced proximal solute and water reabsorption only partially account for the reduction in urinary volume, an additional diuretic effect on nephron terminal segments was postulated. Thus the aim of our work was to investigate the effect of hydrochlorothiazide (HCTZ) on water transport in the inner medullary collecting duct (IMCD) of normal and Brattleboro rats. Osmotic water permeability (Pf) and diffusional water permeability (Pdw) were studied at 37°C and pH 7.4 by the in vitro microperfusion technique. In the absence of antidiuretic hormone (ADH), HCTZ (1026 M) added to the perfused fluid enhanced Pf from 6.36 6 0.56 to 19.08 6 1.70 μm/s (P , 0.01) and Pdw from 38.01 6 4.52 to 52.26 6 4.38 31025 cm/s (P , 0.01) in normal rats and also stimulated Pf in Brattleboro rats from 3.53 6 1.41 to 11.16 6 1.13 μm/s (P , 0.01). Prostaglandin E2 (PGE2) (1025 M) added to the bath fluid inhibited HCTZ-stimulated Pf (in μm/s) as follows: control, 16.93 6 2.64; HCTZ, 29.65 6 5.67; HCTZ1PGE2, 10.46 6 1.84 (P , 0.01); recovery, 16.77 6 4.07. These data indicate that thiazides enhance water absorption in IMCD from normal rats (in the absence of ADH) and from Brattleboro rats and that the HCTZ-stimulated Pf was partially blocked by PGE2. Thus we may conclude that the effect of thiazide in the treatment of DI occurs not only in the Na1-Cl2 cotransport in the distal tubule but also in the IMCD.