PURPOSE OF REVIEW:
This review aims to illustrate why urea recycling may play an important role in potassium (K⁺) excretion and to emphasize its potential clinical implications.
A quantitative analysis of the process of intrarenal urea recycling reveals that the amount of urea delivered to the distal convoluted tubule is about two-fold larger than the quantity of urea excreted in the urine. As the number of osmoles delivered to the late cortical distal nephron (CCD) determines its flow rate when aquaporin 2 water channels have been inserted in the luminal membrane of principal cells, urea recycling may play an important role in regulating the rate of excretion of K⁺when the distal delivery of electrolytes is not very high.
Urea recycling aids the excretion of K⁺; this is especially important in patients with disorders or those who are taking drugs that lead to a less lumen-negative voltage in the CCD. As a large quantity of urea is reabsorbed daily in the inner medullary collecting duct, the assumptionmade in the calculation of the transtubular K concentration gradient that there is no appreciable reabsorption of osmoles downstream CCD is not valid.
Keenan Research Centre in the Li Ka Shing Knowledge Institute, St. Michael's Hospital, Division of Nephrology, University of Toronto, Toronto, Ontario, Canada.