Comparative physiology and architecture associated with the mammalian urine concentrating mechanism: role of inner medullary water and urea transport pathways in the rodent medulla

Abstract

Comparative studies of renal structure and function have potential to provide insights into the urine-concentrating mechanism of the mammalian kidney. This review focuses on the tubular transport pathways for water and urea that play key roles in fluid and solute movements between various compartments of the rodent renal inner medulla. Information on aquaporin water channel and urea transporter expression has increased our understanding of functional segmentation of medullary thin limbs of Henle's loops, collecting ducts, and vasa recta. A more complete understanding of membrane transporters and medullary architecture has identified new and potentially significant interactions between these structures and the interstitium. These interactions are now being introduced into our concept of how the inner medullary urine-concentrating mechanism works. A variety of regulatory pathways lead directly or indirectly to variable patterns of fluid and solute movements among the interstitial and tissue compartments. Animals with the ability to produce highly concentrated urine, such as desert species, are considered to exemplify tubular structure and function that optimize urine concentration. These species may provide unique insights into the urine-concentrating process.(1)

Morphological and functional characteristics of the kidney of cartilaginous fishes: with special reference to urea reabsorption

“…Water reabsorption is one of the most important functions in tetrapod kidneys. This is particularly true in birds and mammals, and their kidneys have developed the urine concentrating mechanism consisting of a juxtamedullary nephron and countercurrent multiplier and exchange systems (87,91). As already described in the previous sections, marine cartilaginous fishes maintain their body fluid slightly hyperosmotic to the surrounding SW.…”

“…The kidney is one of the most important organs for urea retention in cartilaginous fishes, where urea, as well as water and other small molecules, are freely filtered by the glomerulus. As mentioned in other review articles (87,91), mammals and birds have developed elaborate mechanisms for renal water reabsorption. Their kidneys contain juxtamedullary nephrons that are characterized by a countercurrent multiplier system.…”

Architecture of the human renal inner medulla and functional implications

“…Equilibration of tubule fluid by transcellular water efflux is therefore unlikely to occur in these segments, and if they have exceedingly high urea and NaCl permeabilities as in the rat and chinchilla, then they likely equilibrate by way of solute entry (9,27). Urea and NaCl entry into the AQP1-null DTL segment in human and rodent kidney are essential for producing a concentrated urine, although these entry pathways remain unstudied (11,35). The DVR in the innermost 50% of the rat and kangaroo rat inner medulla also express little or no UT-B (15,49).…”

Evolution of urea transporters in vertebrates: adaptation to urea's multiple roles and metabolic sources

“…The role of urea and the general anatomy of the mammalian kidney in urine-concentrating mechanisms has been under extensive investigation over the last few decades, and has been recently reviewed in great detail (see Pannabecker, 2013;Yang and Bankir, 2005). In brief, the mammalian kidney concentrates urine to an exceptional degree, primarily due to a countercurrent multiplication of NaCl, the capacity of which is greatly enhanced by urea gradients.…”