Antidiuretic effect of hydrochlorothiazide in lithium-induced nephrogenic diabetes insipidus is associated with upregulation of aquaporin-2, Na-Cl co-transporter, and epithelial sodium channel.

Thiazides have been used in patients with nephrogenic diabetes insipidus (NDI) to decrease urine volume, but the mechanism by which it produces the paradoxic antidiuretic effect remains unclear. Previous studies have reported that downregulation of aquaporin-2 (AQP2) is important for the development of lithium-induced (Li-induced) polyuria and that hydrochlorothiazide (HCTZ) increases renal papillary osmolality and Na(+) concentration in Brattleboro rats. For elucidating the molecular basis of the antidiuretic action of HCTZ in diabetes insipidus, whether administration of HCTZ may affect the expression of AQP2 and major renal Na(+) transporters in Li-induced NDI rats was investigated, using semiquantitative immunoblotting and immunohistochemistry. After feeding male Sprague-Dawley rats Li chloride-containing rat diet for 4 wk, HCTZ or vehicle was infused subcutaneously via osmotic minipump. Urine output was significantly decreased by HCTZ treatment, whereas it was not changed in vehicle-treated rats. Urine osmolality was also higher in HCTZ-treated rats than in vehicle-treated rats. Semiquantitative immunoblotting using whole-kidney homogenates revealed that HCTZ treatment caused a significant partial recovery in AQP2 abundance from Li-induced downregulation. AQP2 immunohistochemistry showed compatible findings with the immunoblot results in both cortex and medulla. The abundances of thiazide-sensitive NaCl co-transporter and alpha-epithelial sodium channel were increased by HCTZ treatment. Notably, HCTZ treatment induced a shift in molecular weight of gamma-epithelial sodium channel from 85 to 70 kD, consistent with previously demonstrated aldosterone stimulation. The upregulation of AQP2 and distal renal Na(+) transporters in response to HCTZ treatment may account for the antidiuretic action of HCTZ in NDI.

Upregulation of Na+ transporter abundances in response to chronic thiazide or loop diuretic treatment in rats.

Furosemide and hydrochlorothiazide (HCTZ) exert their diuretic actions by binding to apical Na(+) transporters, viz., the Na(+)-K(+)-2Cl(-) cotransporter in the thick ascending limb and the Na(+)-Cl(-) cotransporter in the distal convoluted tubule, respectively. We carried out semiquantitative immunoblotting and immunohistochemistry of rat kidneys to investigate whether chronic administration of furosemide or HCTZ is associated with compensatory changes in the abundance of Na(+) transporters downstream from the primary site of action. Osmotic minipumps were implanted into Sprague-Dawley rats to deliver furosemide (12 mg/day) or HCTZ (3.75 mg/day) for 7 days. To prevent volume depletion, all animals were offered tap water and a solution containing 0.8% NaCl and 0.1% KCl as drinking fluid. The diuretic/natriuretic response was quantified in response to both agents by using quantitative urine collections. Semiquantitative immunoblotting revealed that the abundances of thick ascending limb Na(+)-K(+)-2Cl(-) cotransporter and all three subunits of the epithelial Na(+) channel (ENaC) were increased by furosemide infusion. HCTZ infusion increased the abundances of thiazide-sensitive Na(+)-Cl(-) cotransporter and beta-ENaC in the cortex and beta- and gamma-ENaC in the outer medulla. Consistent with these results, beta-ENaC immunohistochemistry showed a remarkable increase in immunoreactivity in the principal cells of collecting ducts with either diuretic treatment. These increases in the abundance of Na(+) transporters in response to chronic diuretic treatment may account for the generation of diuretic tolerance associated with long-term diuretic use.

Aquaporin-4 expression in adult and developing mouse and rat kidney.

Aquaporin-4 (AQP4) is a member of the aquaporin water-channel family. AQP4 is expressed primarily in the brain, but it is also present in the collecting duct of the kidney, where it is located in the basolateral plasma membrane of principal cells and inner medullary collecting duct (IMCD) cells. Recent studies in the mouse also have reported the presence of AQP4 in the basolateral membrane of the proximal tubule. The purpose of this study was to establish the pattern of AQP4 expression during kidney development and in the adult kidney of both the mouse and the rat. Kidneys of adult and 3-, 7-, and 15-d-old mice and rats were preserved for immunohistochemistry and processed using a peroxidase pre-embedding technique. In both the mouse and the rat, strong basolateral immunostaining was observed in IMCD cells and principal cells in the medullary collecting duct at all ages examined. Labeling was weaker in the cortical collecting duct and the connecting tubule, and there was no labeling of connecting tubule cells in the mouse. In adult mouse kidney, strong AQP4 immunoreactivity was observed in the S3 segment of the proximal tubule. However, there was little or no labeling in the cortex or around the corticomedullary junction in 3- and 7-d-old mice. Between 7 and 15 d of age, distinct AQP4 immunoreactivity appeared in the S3 segment of the mouse proximal tubule concomitant with the differentiation of this segment of the nephron. Labeling of proximal tubules was never observed in the rat kidney. These results suggest that there are differences in transepithelial water transport between mouse and rat or that additional, not yet identified water channels exist in the rat proximal tubule.