Involvement of the Renal Kallikrein–Kinin System in the Diuretic Action of a Thiazide Using a Crossover Design in Rats: An Exploratory Study

Background: We have reported that the renal kallikrein–kinin system is involved in the diuretic action of potassium or furosemide in rats. Objectives: The aim of this study was to investigate whether the renal kallikrein–kinin system is involved in the diuretic action of thiazides in rats. Materials and Methods: In the first study, indapamide (INDP) was administered at 0.1 or 0.3 mg/kg/day, or a solvent, and urine was collected using metabolic cages at 0–6, 6–12, and 12–24 h. Urine output, urinary excretion of sodium, chloride, and potassium, and urine kallikrein activity were measured. In the second study, a bradykinin B2 receptor antagonist, icatibant, or a solvent was administered before INDP administration in a crossover design with a washout period, which was conducted under conditions with and without saline loading. Urine output and urinary excretion of sodium, chloride, and potassium were measured at 0–6 h after INDP administration. Results: The results showed that the diuretic actions of INDP increased with high-dose INDP and reached a maximum at 0–6 h: urine outputs were 5.0 (median, interquartile range 4.7–5.9) versus 1.8 (1.4–2.1, p < .05) mL/100 g body weight (b.w.), and urinary excretions of sodium, chloride, and potassium were 0.21 (0.18–0.23) versus 0.06 (0.05–0.07, p < .01), 0.18 (0.16–0.19) versus 0.05 (0.05–0.05, p < .01), 0.11 (0.08–0.13) versus 0.04 (0.03–0.05, p < .05) mmol/mg urinary creatinine (Cr) in the INDP versus control groups. Urinary kallikrein activity did not change with either dose of INDP. Icatibant did not inhibit the diuretic action of INDP under either condition. Conclusion: The conclusions were that although the crossover design was used to evaluate the urine data collected by a metabolic cage in rats, the study could not determine whether the renal kallikrein–kinin system is involved in the diuretic actions of thiazides. Future studies should improve regarding fluid loading and long-term sodium loading.