Urea handling by the medullary collecting duct of the rat kidney during hydropenia and urea infusion

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Abstract
Previous micropuncture studies of distal tubule fluid and ureteral urine have indicated a varying degree of urea reabsorption in the collecting duct. In the present experiments the microcatheterization technique was used to directly determine urea, Na, K, total solute and fluid reabsorption along the length of the medullary collecting duct in anaesthetized hydropenic rats and in rats given low dose urea infusion (Purea 18.9 mM/l). In hydropenic rats, the remaining fraction of filtered urea did not change significantly along the collecting duct, as indicated both by regression analysis of all samples and by comparison of paired samples from the corticomedullary junction and papillary tip. During low dose urea infusion, urine osmolality increased in proportion to the increase in urea concentration and again there was no net urea reabsorption between the beginning and end of the duct. However, during urea infusion, analysis of samples from the beginning, mid-zone, and end of the collecting duct indicated that urea entry occurred in the proximal portion of the duct (beginning to mid-zone,PP<0.01). The lack of significant net urea reabsorption along the duct despite the excretion of moderately concentrated urine, has implicatios for the concept of medullary urea recycling and for models of the urinary concentrating mechanism. The finding of functional heterogeneity with respect to urea handling in the collecting duct in vivo, with both reabsorption and secretion being demonstrated, raises the possibility that internal recycling of urea in the medullary collecting duct itself may contribute to maintenance of a high papillary interstitial urea concentration.