Water Transport And Homeostasis: The Third Major Function of Erythrocytes

Aquaporin-1 (AQP1) was first identified in the red blood cell as an abundant transmembrane water channel protein in 1992. It is also expressed in the brain, lung, eye and kidney. However, since the initial discovery and subsequent characterization of molecular structure, the physiological role of AQP1 in the red blood cell has not been fully understood.We propose red blood cells with AQP1 act as regulators of local osmolarity and water homeostasis based on their capacity for volume change, rapid water transport, mobility and presence throughout the body. Firstly, use of a newly developed negative imaging technique along with confocal microscopy enabled large scale in vivo data collection and measurement necessary to explore red blood cell volume distributions in mouse kidneys. These results revealed the volume capacity of normal red blood cells showing a gradual decrease up to 40% in response to the presumed hyperosmotic gradient within the medulla. In contrast, AQP1 knockout (KO) cells displayed minimal reduction of volumes. Secondly, in continuation and expansion of these results, Kedem-Katchalsky equations of membrane transport were used to model normal and AQP1 KO systems beyond in vivo or in vitro experiments. The fast water transport coupled with cell volume changes enables erythrocytes to function as “micropumps” to facilitate osmolarity regulation. Simulations also uncovered the role red blood cells play in the osmotic gradient established by the countercurrent multiplier of the kidney. Thirdly, a microfluidic device was designed and constructed to measure the sensitive kinetics of normal and AQP1 KO red blood cells in vitro in conditions resembling capillary flow conditions. Testing this system against various hypotonic and hypertonic conditions with a fluorescent indicator present in the extracellular compartment revealed that exchanges between normal red blood cells and their surroundings were capable of reaching steady-state in 60 ms.Combining experimental results and theoretical analyses allows for greater insight in our understanding of the role red blood cells played in water balance. Thus, in addition to both O2 and CO2 exchanges, we propose water transport and homeostasis may be the third major function of red blood cells