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

Background. The thick ascending limb of Henle's loop (TALH) is thought to be involved in the regulation of the renal urea gradient. Methods. We have characterized the uptake of urea toil density centrifugation and 2-compartment-culture) and volume regulation (impedance measurement) in highly differentiated cells derived from rabbit outer medulla. Results. TALH cells exposed to 600 mOsm/liter (300 mM urea) shrunk to 72 +/- 5% of the isoosmotic volume. Due to a regulatory volume increase (RVI), the cell volume was almost completely regained at 92 +/- 6% after five minutes. The uptake of C-14-urea in the presence of urea concentrations up to 600 mM did not show any saturation. In the presence of phloretin the urea uptake decreased to 69 +/- 14%. The transport was sodium and chloride independent. Changing the membrane potential caused an increase of regulatory volume increase and urea uptake. Hyperosmolarity induced by sucrose (300 mM) and NaCl (150 mM) caused a decrease of urea uptake to 70 +/- 14% and 53 +/- 11%, respectively. The permeability coefficient (P) in a two compartment culture was P = 1.7 10(-6) +/- 0.39 . 10(-6) cm/second, suggesting a relatively low permeability. Conclusion. Due to the low permeability: it seems impossible to achieve a physiologically significant participation of the TALH in the urea circulation within the nephron. However, the results of this study provides significant hints about the existence of a specific urea transport mechanism that enables the cell to adapt rapidly to different osmolarities.