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Urea secretion in medullary collecting duct of the rat kidney during water and mannitol diuresis.

During steady-state water or mannitol diuresis, the microcatheterization technique was used to study the handling of urea, fluid, sodium, potassium, and total solute along the length of the medullary collecting duct in anesthetized rats. During water diuresis, the remaining fraction of filtered urea increased along the collecting duct as indicated both by regression analysis of all samples and by comparison of paired data from the beginning and end of the duct [(TF/P)urea/In = 43.3 and 50.7%, respectively]. During mannitol diuresis, similar urea entry into the medullary collecting duct was observed, (TF/P)urea/In increasing from 60.7 to 66.5%. Comparison of collecting duct urea handling in proximal and distal segments (beginning to midzone and midzone to papillary tip) suggested that urea entry occurred to a greater extent in the distal portion of the medullary collecting duct. The results demonstrate urea secretion into the medullary collecting duct in diuretic states when urine flow is high and intratubular urea concentration low. Whether urea entry into the collecting duct is an active or passive process cannot be determined from this study, but comparison between urea concentrations in the papillary interstitial fluid and in the urine or tubular fluid raises the possibility of an active urea secretory mechanism in the collecting duct.