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modulates
shrinkage-induced activation of Na+/H+
exchanger in rat mesangial cells
Department of Nephrology, Jichi Medical School, Minamikawachi, Kawachi, Tochigi 329-0498, Japan
To examine the
effect of hyperosmolality on Na+/H+ exchanger
(NHE) activity in mesangial cells (MCs), we used a
pH-sensitive dye,
2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein-AM, to measure intracellular pH (pHi) in a single MC from rat
glomeruli. All the experiments were performed in
CO2/HCO
3-free HEPES
solutions. Exposure of MCs to hyperosmotic HEPES solutions (500 mosmol/kgH2O) treated with mannitol caused cell
alkalinization. The hyperosmolality-induced cell alkalinization was
inhibited by 100 µM ethylisopropylamiloride, a specific NHE
inhibitor, and was dependent on extracellular Na+. The
hyperosmolality shifted the Na+-dependent acid extrusion
rate vs. pHi by 0.15-0.3 pH units in the
alkaline direction. Removal of extracellular Cl by
replacement with gluconate completely abolished the rate of cell
alkalinization induced by hyperosmolality and inhibited the Na+-dependent acid extrusion rate, whereas, under isosmotic
conditions, it caused no effect on Na+-dependent
pHi recovery rate or Na+-dependent acid
extrusion rate. The Cl-dependent cell alkalinization
rate under hyperosmotic conditions was partially inhibited by
pretreatment with 5-nitro-2-(3-phenylpropylamino)benzoic acid, DIDS,
and colchicine. We conclude: 1) in MCs, hyperosmolality activates NHE to cause cell alkalinization, 2) the acid
extrusion rate via NHE is greater under hyperosmotic conditions than
under isosmotic conditions at a wide range of pHi,
3) the NHE activation under hyperosmotic conditions, but not
under isosmotic conditions, requires extracellular
Cl, and 4) the
Cl-dependent NHE activation under hyperosmotic
conditions partly occurs via Cl channel and
microtubule-dependent processes.
intracellular pH; chloride ion; chloride channel; microtubule
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