Renal Transport Proteins Involved in Urinary Concentrating Mechanism

Kim, Dong Un

Abstract

Renal tubule and vasa recta are arranged in complex but specific anatomic relationships and the production of a concentrated urine is achieved by countercurrent multiplication mechanism in the renal medulla. This model requires that the ascending thin limb is highly permeable to NaCl but impermeable to water, while the descending thin limb is impermeable to NaCl but highly permeable to water. The single effect in the outer medulla is active NaCl reabsorption by the thick ascending limb of Henle, which is the primary energy source of urine concentration. On the other hand, the single effect for inner medullary concentration is the passive efflux of NaCl from the thin ascending limb, which requires a high concentration of interstitial urea that is reabsorbed from the terminal inner medullary collecting duct. The hypothesis has been supported by micorpuncture studies or isolated perfused tubule studies until early 1990s. In recent 10 years, many transport proteins involved in the urine concentrating mechanism have been cloned, which enabled us to understand how a concentrated urine is produced, how this process is regulated and the specific transport process that are involved, proving the countercurrent multiplication hypothesis. In this review, the transport properties of outer and inner medullary nephron segments and the transport proteins involved in the transport of NaCl, water(aquaporins), and urea(urea transporters) will be reviewed.