New Insights into the Urine Concentrating Mechanism
Sponsored by APS Renal Section
Channels and Transporters Track

 SEQ CHAPTER \h \r 1The ability to concentrate or dilute the urine allows animals and humans to maintain a nearly constant blood plasma osmolality by varying water excretion to match water intake.  This is accomplished in the renal medulla through the combined actions of several transport proteins in the loops of Henle, collecting ducts, vasculature, and the complex but unique anatomical relationship these structures have to each other.  There is general consensus regarding the mechanisms that contribute to urinary concentration in the outer medulla.  In contrast, controversy remains regarding mechanisms in the inner medulla, although it is generally accepted that urea plays a critical role in the inner medullary concentrating mechanism.  The most widely accepted mechanism remains the passive reabsorption of NaCl, in excess of solute secretion, from the thin ascending limb of the loop of Henle, although the “passive mechanism” hypothesis is not universally accepted.   SEQ CHAPTER \h \r 1 In this symposium, four speakers will review important new insights into the inner medullary concentrating mechanism.  Dr. Pannabecker will present recent studies into the structural relationships between tubules and vasculature in the inner medulla.  Dr. Klein will present recent studies of short- and long-term regulation of urea transporters by vasopressin in the inner medullary collecting duct.  Dr. Knepper will present recent studies of long-standing hypotheses regarding urea’s role in the urine concentrating mechanism using mice in which specific urea transporters were genetically knocked-out.  Dr. Layton will present recent computer modeling studies of the urine concentrating mechanism that will provide important insights into our understanding of this fascinating and long-standing question in renal physiology.

3:15 PM	Three-dimensional functional reconstructions of renal inner medullary architecture: structural insights into the urine concentrating process. Tom Pannabecker, Univ. of Arizona
3:45 PM	Vasopressin regulation of the UT-A1 urea transporter. Janet D. Klein, Emory Univ.
4:15 PM	The physiological role of urea transporters and aquaporins in the inner medulla. Mark A. Knepper, NIH
4:45 PM	The urine concentrating mechanism: insights from mathematical modeling. Harold E. Layton, Duke Univ.