Characteristics of urea transport of cells derived from rabbit thick ascending limb of Henle's loop

Physicochemical evolution and molecular adaptation of the cetacean osmoregulation-related gene UT-A2 and implications for functional studies

Cetaceans have an enigmatic evolutionary history of re-invading aquatic habitats. One of their essential adaptabilities that has enabled this process is their homeostatic strategy adjustment. Here, we investigated the physicochemical evolution and molecular adaptation of the cetacean urea transporter UT-A2, which plays an important role in urine concentration and water homeostasis. First, we cloned UT-A2 from the freshwater Yangtze finless porpoise, after which bioinformatics analyses were conducted based on available datasets (including freshwater baiji and marine toothed and baleen whales) using MEGA, PAML, DataMonkey, TreeSAAP and Consurf. Our findings suggest that the UT-A2 protein shows folding similar to that of dvUT and UT-B, whereas some variations occurred in the functional So and Si regions of the selectivity filter. Additionally, several regions of the cetacean UT-A2 protein have experienced molecular adaptations. We suggest that positive-destabilizing selection could contribute to adaptations by influencing its biochemical and conformational character. The conservation of amino acid residues within the selectivity filter of the urea conduction pore is likely to be necessary for urea conduction, whereas the non-conserved amino acid replacements around the entrance and exit of the conduction pore could potentially affect the activity, which could be interesting target sites for future mutagenesis studies.

Concept and computational design for a bioartificial nephron-on-a-chip

A MEMS-based, (Micro Electro Mechanical System) bioartificial device is proposed for replicating the function of a single nephron. Consistent with the anatomy and physiology of humans, our device has 3 distinct sections, replicating the function of the glomerulus, the proximal tubule, and the loop of Henle. Construction of a bioartificial loop of Henle in particular requires control of diffusion-scale features. The proposed device can be built using existing microfabrication technologies and populated with various renal cell types. A computational model is also developed to analyze the coupled, multiphase mass transport in this system. Using the model, a design is generated with flow and solute transport properties matching those of the human nephron.

A high-salt diet stimulates thick ascending limb eNOS expression by raising medullary osmolality and increasing release of endothelin-1

A high-salt diet increases renal endothelin (ET) production and thick ascending limb (THAL) endothelial nitric oxide synthase (eNOS) expression. ET stimulates THAL eNOS expression via ET(B) receptors. The tonicity of the renal medulla is highly variable, and hyperosmolality stimulates ET-1 synthesis by endothelial cells. We hypothesized that a high-salt diet raises medullary osmolality, increases ET release by the THAL, and thus enhances eNOS expression. Seven days of high salt (1% NaCl in drinking water) increased eNOS expression in THALs by 125 +/- 31%. High salt increased outer medullary osmolality from 362 +/- 13 to 423 +/- 6 mosmol/kg H(2)O (P < 0.05). Bosentan, a dual-ET receptor antagonist, blocked the increase in THAL eNOS expression caused by high salt (2.66 +/- 0.44 absorbance units with bosentan vs. 5.15 +/- 0.67 for vehicle; P < 0.05). Conscious systolic blood pressure did not differ between the two groups. In primary cultures of medullary THALs, raising osmolality from 300 to 350 and 400 mosmol/kg H(2)O using NaCl increased eNOS expression by 39 +/- 11% (P < 0.05) and 71 +/- 16%, respectively (P < 0.05). In primary cultures of THALs, raising osmolality from 300 to 400 mosmol/kg H(2)O for 1 h increased ET-1 release from 62 +/- 7 to 113 +/- 2 pg/mg protein (P < 0.05). BQ-788, an ET(B) receptor antagonist (1 muM), blocked the stimulatory effect of 400 mosmol/kg H(2)O on eNOS expression (70 +/- 13% vs. -5 +/- 10%; paired difference, 74 +/- 15%; P < 0.05). BQ-788 alone had no significant effect. We concluded that high salt stimulates THAL eNOS expression by increasing outer medullary osmolality, ET-1 release by the THAL and ET(B) receptor activation. This may be an important regulatory mechanism of THAL NaCl absorption when dietary salt intake is increased.

Polarized function of thick ascending limbs of Henle cells in osmoregulation

BACKGROUND

Organic osmolytes are necessary for osmoregulation in mammalian kidney. Since renal epithelial cells in many cases possess specific mechanisms both for uptake and osmotically regulated release, we investigated their localization in polarized cells.

METHODS

An immortalized epithelial cell line derived from the thick ascending limb of Henle's loop (TALH) was used to examine the transport characteristics of the apical and basolateral plasma membranes for osmotic regulation of organic osmolytes. Cells were cultured on filters in a two-compartment chamber.

RESULTS

In culture under hypertonic conditions the TALH cells accumulated in the following balance: sorbitoverline> betaine = myo-inositoverline> glycerophosphoryl choline (GPC). When extracellular osmolarity was decreased, then sorbitol was released on the apical side, whereas betaine and myo-inositol efflux occurred on the basolateral side. GPC release showed no preference of either side. Taurine did not seem to be necessary for osmoregulation under these conditions. Osmotically regulated myo-inositol and betaine uptake was located on the apical side, and choline uptake took place on both sides equally.

CONCLUSION

These results show that in renal epithelial cells, both osmotically induced release and the uptake of organic osmolytes are divided between the apical and the basolateral sides. This might be important for volume regulation.

Effects of osmolarity on taste receptor cell size and function

Osmotic effects on salt taste were studied by recording from the rat chorda tympani (CT) nerve and by measuring changes in cell volume of isolated rat fungiform taste receptor cells (TRCs). Mannitol, cellobiose, urea, or DMSO did not induce CT responses. However, the steady-state CT responses to 150 mM NaCl were significantly increased when the stimulus solutions also contained 300 mM mannitol or cellobiose, but not 600 mM urea or DMSO. The enhanced CT responses to NaCl were reversed when the saccharides were removed and were completely blocked by addition of 100 microM amiloride to the stimulus solution. Exposure of TRCs to hyperosmotic solutions of mannitol or cellobiose induced a rapid and sustained decrease in cell volume that was completely reversible, whereas exposure to hypertonic urea or DMSO did not induce sustained reductions in cell volume. These data suggest that the osmolyte-induced increase in the CT response to NaCl involves a sustained decrease in TRC volume and the activation of amiloride-sensitive apical Na(+) channels.