Changes in urea clearance after a single dose of diuretics during water diuresis

The action of various diuretics on the excretion fraction of urea (Curea/Ccr) was studied in 91 healthy volunteers under conditions of maintained maximal water diuresis. On the basis of previous data on transtubular transport or urea it should be expected that with an increase in the excretion fraction of water (V/Ccr) under the given experimental conditions we should find an increase in Curea/Ccr. After administration of polythiazide, which has a predominantly distal localisation of action in the nephron, Curea/Ccr did not change. After administration of chlorothiazide, furosemide and after i.v. administration of ethacrynic acid there was a marked increase in V/Ccr, ranging from 5.4 to 18%. Despite this there was no significant increase in Curea/Ccr. After i.v. administration of acetazolamide or oral administration of ethacrynic acid Curea/Ccr even showed a statistically significant decrease. These findings suggest that a decrease in proximal tubular reabsorption of water after administration of diuretics during maximal water diuresis is not a decisive factor for the renal excretion of urea. The data suggest that the diuretics affected the permeability of the distal segment of the nephron for urea in the sense of an increase of tubular reabsorption.