AIM:To quantify changes in urinary excretion of aquaporin2 water channels(u-AQP2),the sodiumpotassium-chloride co-transporter(u-NKCC2) and the epithelial sodium channels(u-ENa C) during treatment with bendroflumethiazide(BFTZ),amiloride and placebo.METHODS:In a randomized,double-blinded,placebocontrolled,3-way crossover study we examined 23 healthy subjects on a standardized diet and fluid intake.The subjects were treated with amiloride 5 mg,BFTZ 1.25 mg or placebo twice a day for 4.5 d before each examination day.On the examination day,glomerular filtration rate was measured by the constant infusion clearance technique with 51Cr-EDTA as reference substance.To estimate the changes in water transport via AQP2 and sodium transport via NKCC2 and ENa C,u-NKCC2,the gamma fraction of ENa C(u-ENa Cγ),andu-AQP2 were measured at baseline and after infusion with 3% hypertonic saline.U-NKCC2,u-ENa Cγ,u-AQP2 and plasma concentrations of vasopressin(p-AVP),renin(PRC),angiotensin Ⅱ(p-ANG Ⅱ) and aldosterone(p-Aldo) were measured,by radioimmunoassay.Central blood pressure was estimated by applanation tonometry and body fluid volumes were estimated by bio-impedance spectroscopy.General linear model with repeated measures or related samples Friedman’s two-way analysis was used to compare differences.Post hoc Bonferroni correction was used for multiple comparisons of post infusion periods to baseline within each treatment group.RESULTS:At baseline there were no differences in u-NKCC2,u-ENa Cγ and u-AQP2.PRC,p-Ang Ⅱ and p-Aldo were increased during active treatments(P < 0.001).After hypertonic saline,u-NKCC2 increased during amiloride(6% ± 34%;P = 0.081) and increased significantly during placebo(17% ± 24%;P = 0.010).U-AQP2 increased significantly during amiloride(31% ± 22%;P < 0.001) and placebo(34% ± 27%;P < 0.001),while u-NKCC2 and u-AQP2 did not change significantly during BFTZ(-7% ± 28%;P = 0.257 and 5% ± 16%;P = 0.261).U-ENa Cγ increased in all three groups(P < 0.050).PRC,AngⅡ and p-Aldo decreased to the same extent,while AVP increased,but to a smaller degree during BFTZ(P = 0.048).c DBP decreased significantly during BFTZ(P < 0.001),but not during amiloride or placebo.There were no significant differences in body fluid volumes.CONCLUSION:After hypertonic saline,u-NKCC2 and u-AQP2 increased during amiloride,but not during BFTZ.Lower p-AVP during BFTZ potentially caused less stimulation of NKCC2 and AQP2 and subsequent lower reabsorption of water and sodium. AIM: To quantify changes in urinary excretion of aquaporin2 water channels (u-AQP2), the sodiumpotassium-chloride co-transporter (u- NKCC2) and the epithelial sodium channels (u- ENa C) during treatment with bendroflumethiazide (BFTZ), amiloride and placebo.METHODS: In a randomized, double-blinded, placebocontrolled, 3-way crossover study we examined 23 healthy subjects on standardized diet and fluid intake. The subjects were treated with amiloride 5 mg, BFTZ 1.25 mg or placebo twice a day for 4.5 d before each examination day. On the examination day, glomerular filtration rate was was measured by the constant infusion clearance technique with 51 Cr-EDTA as reference substance. To estimate the changes in water transport via AQP2 and sodium transport via NKCC2 and ENa C, u-NKCC2, the gamma fraction of ENaC (u-ENaCγ), andu-AQP2 were measured at baseline and after infusion with 3% hypertonic saline.U-NKCC2, u-ENaCγ, u-AQP2 and plasma concentrations of vasopressin (p-AVP), renin (PRC), angiotensin II (p-ANG II) an Central blood pressure was estimated by applanation tonometry and body fluid volumes were estimated by bio-impedance spectroscopy. General linear model with repeated measures or related samples Friedman’s two-way analysis was used to compare differences. Post hoc Bonferroni correction was used for multiple comparisons of post infusion periods to baseline within each treatment group .RESULTS: At baseline there were no differences in u-NKCC2, u-ENa Cγ and u-AQP2.PRC, p-Ang Acute hypertonic saline, u-NKCC2 increased during amiloride (6% ± 34%; P = 0.081) and increased significantly during placebo (17% ± 24%; P = 0.010) .U-AQP2 increased significantly during amiloride (31% ± 22%; P <0.001) and placebo (34% ± 27%; P <0.001) while u-NKCC2 and u-AQP2 did not change significantly during BFTZ (-7% ± 28%; P = 0.257 and 5% ± 16%; P = 0.261) .U-ENa Cγ increased in all three groups (P < 050) .PRC, Ang Ⅱ andP-Aldo decreased to the same extent, while AVP increased, but to a smaller degree during BFTZ (P = 0.048) .c DBP decreased significantly during BFTZ (P <0.001), but not during amiloride or placebo.There were no significant differences in body fluid volume. CONCLUSION: After hypertonic saline, u-NKCC2 and u-AQP2 increased during amiloride, but not during BFTZ.Lower p-AVP potentially caused less stimulation of NKCC2 and AQP2 and subsequent lower reabsorption of water and sodium .