Effects of inorganic fluoride on renal excretion of fluid and sodium in diabetic rats

The hypothesis that the diabetic rat is more susceptible to the adverse renal effects (i.e., urine concentrating defect) of inorganic fluoride was tested by comparing fluid and sodium excretion rates in nondiabetic and diabetic rats before and after fluoride administration. Male Fischer 344 rats were injected with either vehicle (0.1 M citrate buffer, i.v.) or streptozotocin (55 mg/kg body weight). Renal excretions of fluid and sodium were determined in conscious animals following administration (i.v.) of a saline volume load containing or lacking inorganic fluoride (0.2 mg/ml). To examine the effect of fluoride without the contribution of a saline load, renal excretions of fluid and sodium were determined following a gradual, graded elevation in plasma fluoride concentration. The natriuretic and diuretic responses to an isotonic saline load were similar between the groups prior to and after fluoride administration. Graded elevations in the fluoride concentration of plasma mediated a slight increase in fluid and sodium excretion in the control rat while the treatment protocol led to a reduction in fluid excretion and an increase in the sodium excretion in the diabetic rat. As a result, the ratio of sodium/fluid excretion following fluoride administration was significantly higher in the diabetic compared to the control rat. These results suggest a differential effect of inorganic fluoride on the renal excretion of sodium and fluid between the nondiabetic and diabetic rat.