A long-standing biological question regarding the mammalian kidney revolves around the role of the renal medulla in maintaining water balance on a minute-to-minute and day-to-day basis. The kidney's ability to excrete wastes while conserving water is especially important during periods when the body is losing water (e.g. exercise) or when water intake is low (e.g. sleeping overnight). The nephrons (kidney tubules) and blood vessels of the mammalian kidney are assembled into elaborate tubular networks. The nephrons and vessels interact with each other, exchanging water and solutes among many spatially-distinct compartments in a highly orchestrated manner in order to produce a urine that is concentrated in solutes. This research project will focus on the kidney of the kangaroo rat. This desert animal rarely drinks water and, therefore, it is anticipated that its kidney will be an ideal system to study the urinary concentrating mechanism. Physiological, molecular, and computational studies of protein expression and function in the various nephron segments of the kangaroo rat kidney will be carried out so as to more clearly define how water and solutes move within the medulla of the kidney. Models developed from these studies will reveal fundamental insights into how these fluid and solute exchanges play a role in maintaining proper water balance in the whole organism. Students will play a central role in this research project and by doing so they will gain research experience in cell biology, systems physiology, and computational approaches.
Our research focuses on a biological process in the kidney known as the "urine concentrating mechanism". Most of us are familiar with the dark yellow urine that is excreted after a full night’s sleep or after a long hike on a hot day. Urine is formed in the kidney in tubular structures known as nephrons. When the body is dehydrated, the fluid in the lumen of nephrons is "concentrated" by profuse diffusion (or reabsorption) of water across the wall of the nephron to the body and retention of salts and metabolic byproducts in a minimal volume of water within the tubule lumen, which is subsequently excreted as urine. Production of this "concentrated" urine is an important mechanism for excreting metabolic wastes while maintaining water balance in the body. The kangaroo rat is a desert species, which, remarkably, drinks no free water and can produce urine that is twice as concentrated as that of the common laboratory rat and about 5 times that of humans. Because of the profound concentrating ability of kangaroo rat kidney, we hypothesize that the anatomical and physiological characteristics most important for this process should be very apparent in these animals. Much of the concentrating effect occurs in a region of the kidney known as the medulla. The medulla is separated into two zones – the inner medulla and the outer medulla. We have shown that the arrangements (or architecture) of nephron and blood vessel segments of the inner medulla are distinctly different from their arrangements in the outer medulla. Nonetheless, the arrangements in the two zones are anatomically complementary and our work involves demonstrating how the two zones are also functionally complementary. The ability of mammals to produce concentrated urine requires that the solute concentration of the inner medulla substantially exceeds that of the outer medulla. This concentration difference between the two zones is known as the medullary solute gradient. Many underlying principles as to how the solute gradient is formed are understood to some extent; but how these principles are integrated into a unified process to form the gradient and produce concentrated urine has remained a key mystery of renal physiology for over five decades. Along with our colleagues, we propose that urine concentration may occur in two successive stages – stage 1 occurs in the outer medulla and stage 2 occurs in the inner medulla, where urine exits the kidney. In this scenario, the outer medulla acts as a "springboard" that serves as a catalyst to drive physiological processes taking place in the inner medulla. Our studies of kangaroo rat kidney support this idea in several ways. Most important, several proteins commonly associated with energized transport of sodium (chiefly Na-K-ATPase) are expressed at much greater levels in outer medullary tubular cells of the kangaroo rat than in outer medullary tubular cells of the laboratory rat. This protein directly contributes to the very high medullary solute gradient that is required for producing concentrated urine. However, energized transport occurs primarily in the outer medulla, not in the inner medulla where the greatest concentrating effect occurs prior to urine exiting the kidney. In some way that is not clear, energized transport that occurs in the outer medulla is translated into production of the high inner medullary solute concentration. A second way that this springboard effect could be accentuated in kangaroo rats relates to the amount of water that diffuses out of nephrons as they carry fluid from the outer medulla to the inner medulla. A higher outer medullary solute concentration directly leads to a higher rate of water diffusion out of the nephron. This means the urine is already substantially concentrated before it reaches the inner medulla and any additional concentrating effect could occur with lower energy input. As an aside, we found that the water channel aquaporin 1 is expressed at significantly higher levels in some nephrons of the kangaroo rat, and this itself could increase the rate of water diffusion compared to that of the laboratory rat. Although this project focuses on developing biological principles, individuals who work on the project also participate in teaching, partnering, and communicating science to the broader world at large. Drawing from a pool of more than 1800 University of Arizona undergraduate physiology majors and networking with NIH, NSF, American Physiological Society, University of Arizona underrepresented student organizations and other programs, the principal investigator has recruited an average of five undergrads each semester during the past four years. Students engage in individualized projects or team-based partnerships, and present research findings at local and national science conferences.
