2004 Global Researcher Conference Proceeding
April 09 - 11, 2004
| Conference: | 2004 Global Researcher Conference |
|---|---|
| Title: | Antidiuretic Effect of Hydrochlorothiazide in Lithium-Induced Nephrogenic Diabetes Insipidus Is Associated with Upregulation of Aquaporin-2, Na-Cl Cotransporter and Epithelial Sodium Channel |
| Authors: | Kim, Gheun-Ho; Oh, Yun Kyu; Lee, Jay Wook; Joo, Kwon Wook; Na, Ki Young; Earm, Jae-Ho; Knepper, Mark; Han, Jin Suk |
| Institutions: | Hanyang University College of Medicine, Eulji Medical College, Seoul National University, Dr. Earm's Dialysis Clinic, National Institutes of Health, Seoul National University College of Medicine |
Hydrochlorothiazide (HCTZ) has been used in nephrogenic diabetes insipidus (NDI) patients to decrease urine volume, but the mechanism by which it produces the paradoxical antidiuretic effect remains unclear. Previous studies have reported that downregulation of aquaporin-2 (AQP2) is important for the development of lithium-induced polyuria, and that HCTZ increases renal papillary osmolality in Brattleboro rats. In order to elucidate the transport molecular basis of the antidiuretic action of HCTZ in diabetes insipidus, we investigated, using semiquantitative immunoblotting and immunohistochemistry, whether administration of HCTZ may affect the expression of AQP2 and major renal sodium transporters in lithium-induced NDI rats. NDI was induced in male Spraque-Dawley rats by feeding lithium carbonate-containing rat chow (40 mmol lithium/kg dry food) for 4 weeks. Normal controls were given food without lithium. During an additional week, HCTZ 3.75 mg/day or vehicle was subcutaneously infused to lithium-induced NDI rats via osmotic minipump. All rats were allowed free access to drinking fluid to prevent volume depletion. Daily urine flow measured at the end of the third week of experiment was remarkably high in lithium-administered rats (n=7, 110 + 7 mL/d), compared with controls (n=4, 10 + 3 mL/d). Urine osmolality was also lower in lithium-administered rats 89 + 10 mOsm/kgH2O) than in controls (1670 + 287 mOsm/kgH2O). After HCTZ treatment, daily urine flow was significantly decreased (n=3, 46 + 11 mL/d) whereas it did not show a significant change in vehicle-infused rats (n=4, 127 + 1 mL/d). Urine osmolality was also higher in HCTZ-treated rats (557 + 139 mOsm/kgH2O) than in vehicle-infused rats (207 + 9 mOsm/kgH2O). Semiquantitative immunoblotting using whole kidney homogenate revealed that HCTZ infusion caused a significant partial recovery in AQP2 abundance from lithium-induced down-regulation (39 ± 2% vs. 20 ± 9%, p<0.05, percent of normal controls). AQP2 immunohistochemistry showed compatible findings with the immunoblot results in different groups. The abundances of thiazide-sensitive Na-Cl cotransporter (292 ± 54% vs. 79 ± 25%, p<0.05, percent of normal controls) and a-ENaC (455 ± 60% vs. 191 ± 35%, p<0.05, percent of normal controls) were increased by HCTZ infusion in lithium-induced NDI rats, whereas the abundance of Na/H exchanger type 3 was not changed (94 ± 6% vs. 105 ± 15%, percent of normal controls). Notably, HCTZ infusion induced a shift in molecular weight of g-ENaC from 85 kDa to 70 kDa, consistent with previously demonstrated aldosterone stimulation. These upregulation of AQP2 and distal renal Na+ transporters in response to HCTZ treatment may account for the antidiuretic action of HCTZ in NDI.
When NDI patients are treated with hydrochlorothiazide (HCTZ), their urine volume decreases. Gheun-Ho Kim, et al., sought to determine how HCTZ works to do this. They experimented with two groups of rats. One group had lithium-induced NDI; one group was the control group. The NDI mice’s urine volume was greater than the control group’s, and it was less concentrated as well. The researchers took the NDI rats and split them into two groups, giving one HCTZ treatments, and one simulated treatments (i.e., they were vehicle infused). The HCTZ treated rats decreased their urine volume and had more concentrated urine. The vehicle infused rats showed no such change.
Studying the rats’ kidney cells revealed that HCTZ infusion increased the number of AQP2s. The thiazide sensitive transporter, Na-Cl co-transporter, also increased through HCTZ infusion, though the number of Na/H exchanger type 3 did not increase. The researchers concluded that HCTZ’s ability to increase the numbers of AQP2 and sodium transporters may account for its ability to reduce urine volume in rats with lithium-induced NDI. The number of alpha subunit of epithelial sodium channel was also increased by HCTZ treatment in lithium-induced NDI rats.