The influence of furosemide on free water clearance

Background. Laboratory measurements of osmolality and electrolyte concentrations are useful as a source of clinical and pathophysiologic information on kidney function. We obtained different conditions of baseline diuresis in 54 rats, which were then treated with furosemide 10 mg/kg. From measurements of plasma (P) and urine (U) osmolality and Na, we calculated the free water clearance (CH₂O) and the contributions of Na⁺ and non-Na⁺-solutes to its changes during diuretic administration. Results. The results show that furosemide abolishes both urine diluting and concentrating ability, by reducing the contribution of non-Na⁺-solutes to Uosm and the formation of CH₂O, while the contribution of Na⁺ to Posm remains unchanged. During furosemide the urines approach isosmoticity irrespective of the baseline Uosm with an asymptomatic function similar to that imposed by osmotic diuresis. It is suggested that the overflow to the concentrating sites of the nephron overwhelms their normal transport capacity independently of their baseline water permeability. The disruption of the transepithelial osmotic gradient caused by the drug impairs the transepithelial osmotic flow, leading to the excretion of isosmotic urines. Conclusions. The effect of non-Na⁺-solutes in the laboratory measurement of urine constituents can be reduced to that of plasma determination by diluting the urines according to empiric formulas derived from the present data.