Background The burden of chronic kidney diseases (CKD) is continuously increasing in the globe. Environmental factors are one of the trigger factors for chronic kidney diseases of unknown etiology (CKDu). However, the current toxicological evidence on the renal effects induced by environmental high concentrations of multiple ions in drinking water and high temperature exposure is very limited.

Objective To preliminary investigate the renal effects of exposure to drinking water with environmental high concentrations of fluoride, calcium, sodium, and bromide ions alone or in combination with high temperature in mice.

Methods A mouse drinking water exposure model was established using ICR male mouse (8 weeks old) with exposure to 3 mg·L⁻¹ fluoride ions, 250 mg·L⁻¹ calcium ions, 400 mg·L⁻¹ sodium ions, and 1 mg·L⁻¹ bromide ions (to mimic the high concentration of ions in the groundwater in the areas with a high prevalence rate of CKDu in Sri Lanka) and high temperature of 32 ℃. ICR male mice were randomly divided into a mixed fluoride-calcium-sodium-bromide ion and high temperature exposure group, exposure groups of each ion and high temperature alone, a fluoride-calcium-sodium ion exposure group, and a fluoride-calcium-sodium-bromide ion exposure group. In the control group, the animals were given normal purified water at room temperature of (23±2) ℃. After 12 consecutive weeks of exposure, body weights and liver (kidney) organ coefficients were determined. Assessment of renal histopathologic damage was performed by hematoxylin-eosin staining and pathology scoring. At the end of the 12-week exposure period, 24 h urine samples were collected for the measurements of creatinine (UCr), albumin (ALB), neutrophil gelatinase-associated lipocalin (NGAL), and β2-microglobulin (β2-MG) levels. Cell apoptosis was assessed by TUNEL assay.

Results The mice in the mixed exposure group showed a significant decrease in body weight and marked increases in the scores of renal histopathological injuries and the urinary levels of β2-MG compared to those of the control mice (P<0.05). Compared with the control group, the differences in body weight and urinary renal injury indexes of the mice in the fluoride-calcium-sodium and the fluoride-calcium-sodium-bromide ion groups (except for the decrease of the β2-MG levels in urinary in the latter group) were not statistically significant (P>0.05), but the renal histopathological injury scores were significantly increased (P<0.05). By contrast, body weights, liver (kidney) organ coefficient, and renal histopathological injury scores were comparable in the control mice and the mice fed with drinking water containing high levels of a single ion alone or housed at high temperature alone (P>0.05). Furthermore, the renal histopathological injury score showed no significant differences between the fluoride-calcium-sodium ion exposure group and the fluoride-calcium-sodium-bromide ion exposure group (P>0.05). The interaction between bromide ions and fluoride-calcium-sodium ions on renal tissue pathological damage was not statistically significant (P>0.05). Results from the TUNEL assay showed a significant increase in renal cell apoptosis in the fluoride-calcium-sodium ion exposure group (P<0.05).

Conclusions Environmental high levels of mixed fluoride, calcium, and sodium ions in drinking water induce renal pathological damage in mice, which are exacerbated in combination with high temperature environment. High temperature exposure alone does not affect the pathological damage of renal tissue.