Duration of the Fluoride-Induced Urinary Concentrating Defect in Rats

The changes in rat renal function produced by acute exposed to graded doses of fluoride were measured over 2 days. During the 2-hr iv infusion, urine flow rate increased moderately in the fluoride groups, while urinary osmolality, inner medullary sodium and chloride concentrations, glomerular filtration rate, and the excretion rates of sodium, chloride, and potassium decreased in a dose-response manner, Four hours after terminating the fluoride infusion, plasma fluoride concentrations had fallen 75% (peak value, 211 μM). This decline was not accompanied by recovery in either urinary osmolality or medullary sodium and chloride concentrations, while at 24 and 40 hr, these values showed a gradual return toward control values. Solute excretion rates, especially of sodium and chloride, were depressed even at 40 hr. Glomerular filtration rate continued to fall throughout the first day after stopping the fluoride infusion. These results suggest that the delayed recovery in urinary osmolality and medullary solute concentrations, as well as the continued depression in solute excretion, are related to the prolonged fluoride-induced decrease in glomerular filtration rate.