An NMR spectroscopic characterization of a new epithelial cell line, TALH-SVE, with properties of the renal medullary thick ascending limb of Henle's loop

Testing the ability of non-methylamine osmolytes present in kidney cells to counteract the deleterious effects of urea on structure, stability and function of proteins

Human kidney cells are under constant urea stress due to its urine concentrating mechanism. It is believed that the deleterious effect of urea is counteracted by methylamine osmolytes (glycine betaine and glycerophosphocholine) present in kidney cells. A question arises: Do the stabilizing osmolytes, non-methylamines (myo-inositol, sorbitol and taurine) present in the kidney cells also counteract the deleterious effects of urea? To answer this question, we have measured structure, thermodynamic stability (ΔG D (o)) and functional activity parameters (K m and k cat) of different model proteins in the presence of various concentrations of urea and each non-methylamine osmolyte alone and in combination. We observed that (i) for each protein myo-inositol provides perfect counteraction at 1∶2 ([myo-inositol]:[urea]) ratio, (ii) any concentration of sorbitol fails to refold urea denatured proteins if it is six times less than that of urea, and (iii) taurine regulates perfect counteraction in a protein specific manner; 1.5∶2.0, 1.2∶2.0 and 1.0∶2.0 ([taurine]:[urea]) ratios for RNase-A, lysozyme and α-lactalbumin, respectively.

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.

Osmotic regulation of the betaine metabolism in immortalized renal cells

Betaine plays an important role in the osmoregulation of various renal cells. In the kidney betaine synthesis seems to be highest in the cortex, whereas osmotically regulated accumulation seems to play a crucial role in the inner medulla. Therefore, the influence of betaine synthesis on the long-term osmotic regulation of betaine content was investigated in epithelial SV40 transfected cell culture, derived from the outer medullary thick ascending limb of the loop of Henle (TALH) of rabbit kidney. Under hyperosmotic conditions the betaine content of TALH was significantly increased from 218 +/- 35 mumol/g protein (300 mOsm/liter; control) to 334 +/- 27 mumol/g (600 mOsm/liter; P < 0.0005). In addition the intracellular accumulation of 14C-betaine from 14C-choline was significantly elevated from 4.3 +/- 1.0 mumol/g protein x hr) to 8.2 +/- 1.0 mumol/g protein x hr; P < 0.001) under hyperosmotic conditions. Synthesis of betaine was also influenced by the extracellular betaine content. In a betaine free medium the synthesis of betaine was increased by 7% (300 mOsm/liter; NS) or 40% (600 mOsm/liter; P < 0.0001) when compared to betaine containing medium. The alteration of betaine synthesis is presumably caused by osmotic regulation of the betaine aldehyde dehydrogenase. Activity of this enzyme was significantly higher under hyperosmotic conditions compared to isoosmotic control conditions (Vmax 4.1 +/- 0.8 U/g protein; 600 mOsm/liter) versus 1.4 +/- 0.1 U/g (300 mOsm/liter; P < 0.0001), while the affinity to betaine aldehyde remained unaltered. These results demonstrate that during long-term adaptation, betaine synthesis in TALH cells of the outer medulla of rabbit kidney can be regulated by extracellular osmolarity.

Ammonium chloride-induced acidification in renal TALH SVE.1 cells monitored by 31P-NMR

The aim of this study was to investigate the effect of NH4+ on the intracellular pH in TALH SVE.1 cells derived from the medullary thick ascending limb of Henle's loop (TALH) of rabbit kidney. These cells are specialized to perform NH4+ transport in vivo. Intracellular pH was monitored by 31P-NMR. The steady state intracellular pH (pHi) under standard conditions was 7.24 +/- 0.04 (n = 46). Exposure to NH4Cl resulted in an initial intracellular acidification of the TALH SVE.1 cells, followed by a recovery to the initial steady-state pHi value. The NH4(+)-induced acidification followed saturation kinetics up to 20 mM NH4Cl (delta pHmax = 0.2 pHunits). Half-maximal acidification was observed at 0.6 mmol/l. The intracellular acidification due to NH4Cl exposure was completely inhibited by 0.1 mM of the diuretic bumetanide, an inhibitor of the Na+/K+/2Cl- cotransporter. The effect of bumetanide was dose-dependent and a Ki value of 8.10(-7) M was calculated. NH4+ influx via K+ channels or the (Na+ + K+)ATPase could not be detected. pHi recovery to the initial value was caused mainly by amiloride-sensitive Na+/H+ exchange and to a lesser extent by an amiloride-insensitive system, which was not studied in detail. In the presence of bumetanide, pulses of high concentrations of NH4Cl induced small intracellular alkalinizations. From these experiments, an intrinsic buffer capacity (beta i) in TALH SVE.1 cells of 26 +/- 3 mM x pH-1 (pHi = 7.65) was determined. It could also be shown that the TALH SVE.1 cells exhibit maximal 'functional buffer capability' between pHout 6.9 and 7.3. Within these limits the cells can maintain their intracellular pH at a constant level, even though the extracellular pH changes. These data strongly suggest that the Na+/K+/2Cl- cotransporter is the main site of NH4+ entry into rabbit thick ascending limb cells in culture. A high intracellular buffer capacity and potent acid extrusion mechanism cooperate in counteracting the intracellular acidification caused by NH4+ influx into the cell.