Presence Of Urea Concentrations Research Articles

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

The thick ascending limb of Henle's loop (TALH) is thought to be involved in the regulation of the renal urea gradient. We have characterized the uptake of urea (oil density centrifugation and 2-compartment-culture) and volume regulation (impedance measurement) in highly differentiated cells derived from rabbit outer medulla. 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 14C-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. 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.

Opposite effects of urea on hemoglobin-oxygen affinity in anemia of chronic renal failure

We studied the action of urea on the spin-spin relaxation rate of 2,3-diphosphoglycerate (2,3-DPG) phosphorus atoms in normal and uremic erythrocytes. At concentrations from 10 to 60 mM, urea increased the relaxation rates of 2,3-DPG P-3 phosphorus atoms. This evidenced a stronger binding of 2,3-DPG to hemoglobin (Hb), suggesting that the deoxyform of Hb was stabilized. This hypothesis was confirmed by measurements of the association constant of oxygen to hemoglobin (K) in normal erythrocytes in presence of urea concentrations in the range of those observed in uremic patients (30 mM). Indeed, the observed decrease in K suggests that the T structure of hemoglobin is stabilized. By contrast, with higher urea concentrations (120 mM), measurements of P50 showed an increase in the hemoglobin affinity for oxygen (decrease in P50). Moreover, the relaxation rates of 2,3-DPG P-3 phosphorus atoms were not modified, which is consistent with the simultaneous increase of K. This may be attributed to the formation of carbamylated hemoglobin in presence of urea. These results suggest two opposite effects of urea on Hb-O2 affinity: the first reinforces 2,3-DPG-Hb binding and leads to a decrease in O2 affinity; the second, mediated by carbamylation of Hb, hinders the binding of 2,3-DPG and increases the O2 affinity. These findings are consistent with the fact that, despite the presence of carbamylated hemoglobin, uremic patients do not present increased Hb-O2 affinity.

Unusual steady-state kinetic properties of a chilopod enzyme: l(+)lactate dehydrogenase purified from Scolopendra cingulata

1. 1. Lactate dehydrogenase was purified to homogeneity from the chilopod Scolopendra cingulata. 2. 2. The enzyme has the usual molecular weight of approximately 150,000 and exists as a tetramer. It is l(+)lactate specific and some lines of evidence ascribe the molecule to the A-class (muscle-type of vertebrates). 3. 3. Surprisingly it displays K cat values for both substrates pyruvate and lactate which are about two orders of magnitude lower than those known for other lactate dehydrogenases. 4. 4. Additionally, both K m for lactate and K cat/ K m for pyruvate are independent from assay temperature. 5. 5. The most striking feature of this enzyme, however, is its stability, as it still retains activity either after incubation at 90°C or in the presence of urea concentrations as high as 7.5 M.