Excretion of urinary casts after the administration of diuretics

The administration of ethacrynic acid and frusemide to healthy volunteers was regularly followed by the excretion of hyaline casts, without any concomitant proteinuria. Hydrochlorothiazide and chlorthalidone did not themselves induce cylindruria but augmented that provoked by acidifying agents. It was shown by the indirect immunofluorescence method that the casts were composed of uromucoid (Tamm-Horsfall mucoprotein), which is always present in the urine, usually in solution, and originates predominantly from the tubule cells of the ascending limb of Henle's loop. The urinary excretion of Tamm-Horsfall mucoprotein was not increased after the administration of ethacrynic acid. This mucoprotein is precipitated and forms aggregates when the concentration of electrolytes increases and when the pH of the urine declines. The casts that appear in the urine after strenuous physical exertion are of essentially the same composition. Casts produced by patients with kidney diseases, on the other hand, contain various protein fractions derived from the blood as well as mucoprotein. Cylindruria occurring during diuretic therapy and physical exertion is of no pathological significance, and the diagnostic value of byaline casts is very much limited if their exact composition cannot be determined.