INHIBITION OF RAT GLOMERULAR MESANGIAL CELL SODIUM/HYDROGEN EXCHANGE BY HYDROGEN PEROXIDE

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Abstract
1. pHi regulation in glomerular mesangial cells (GMC) includes both Na+/H+ and Cl-/HCO3-exchange. As a fall in pHi may protect against H2O2-mediated GMC damage during ischaemia-reperfusion, the involvement of these mechanisms in the GMC pH1 response to H2O2 was assessed using confluent GMC grown in RPMI medium with 20% fetal calf serum (10-15 passages). 2. Cells were loaded with BCECF-AM and pH1 evaluated using standard fluorometric-ratio techniques. In HEPES buffer, GMC exposure to H2O2 dose-dependently (25 mumol/L-1 mmol/L) decreased pHi over 10 min from 7.3 +/- 0.1 to 6.7 +/- 0.1 (at 100 mumol/L) partly due to rapid non-competitive inhibition of amiloride-sensitive Na+/H+ exchange. 3. BCECF fluorescence in free solution was unchanged by H2O2 and averaged 100 +/- 9 nmol/2.6 x 10(6) cells/pH unit. Similarly, zero-Na+/high-K+ buffer, used to minimize passive H+ entry, did not prevent the fall in pHi while GMC H+-formation/extrusion, assessed by the rate of extracellular acidification in low-capacity buffer (0.05 mmol/L), was rapidly inhibited. 4. In contrast, following only a brief 3 min exposure to 1 mmol/L H2O2, HCO3-/CO2 buffer potentiated the inhibition of Na+/H+ exchange from 50 to 80% of control and reduced the acidification from pHi 6.6 +/- 0.1 to 7.15 +/- 0.05. This effect was reversed (to pHi 6.8 +/- 0.07) by pretreatment with 200 mumol/L DIDS, an inhibitor of Cl-/HCO3- exchange. 5. Thus, the decrease in GMC pHi in response to H2O2 in HEPES, partly mediated by inhibition of Na+/H+ exchange and a possible redistribution of intracellular H+, is antagonized in HCO3-/CO2 through a DIDS-sensitive Cl-/HCO3- exchange mechanism. This may act to negate potentially protective effects of low pHi and potentiate oxidative damage to membrane lipids, enzymes and intracellular organelles on reperfusion.