Differential effects of superoxide on luminal and basolateral Na+/H+ exchange in the thick ascending limb.

Superoxide (O2-) increases Na+ reabsorption in the thick ascending limb (THAL) by enhancing Na/K/2Cl cotransport. However, the effects of O2- on other THAL transporters, such as Na(+)/H+ exchangers, are unknown. We hypothesized that O2- stimulates Na(+)/H+ exchange in the THAL. We assessed total Na(+)/H+ exchange activity by measuring recovery of intracellular pH (pH(i)) after acid loading in isolated perfused THALs before and after adding xanthine oxidase (XO) and hypoxanthine (HX). We found that XO and HX decreased total pH(i) recovery rate from 0.26 +/- 0.05 to 0.21 +/- 0.04 pH units/min (P < 0.05), and this net inhibition decreased steady-state pH(i) from 7.52 to 7.37. Because THALs have different Na(+)/H+ exchanger isoforms on the luminal and basolateral membrane, we tested the effects of xanthine oxidase and hypoxanthine on luminal and basolateral Na(+)/H+ exchange by adding dimethylamiloride to either the bath or lumen. Xanthine oxidase and hypoxanthine increased luminal Na(+)/H+ exchange from 3.5 +/- 0.8 to 6.7 +/- 1.4 pmol.min(-1).mm(-1) (P < 0.01) but decreased basolateral Na(+)/H+ exchange from 10.8 +/- 1.8 to 6.8 +/- 1.1 pmol.min(-1).mm(-1) (P < 0.007). To ascertain whether these effects were caused by O2- or H2O2, we examined the ability of tempol, a superoxide dismutase mimetic, to block these effects. In the presence of tempol, xanthine oxidase and hypoxanthine had no effect on luminal or basolateral Na(+)/H+ exchange. We conclude that O2- inhibits basolateral and stimulates luminal Na(+)/H+ exchangers, perhaps because different isoforms are expressed on each membrane. Inhibition of basolateral Na(+)/H+ exchange may enhance stimulation of luminal Na(+)/H+ exchange by providing additional protons to be extruded across the luminal membrane. Together, the effects of O2- on Na(+)/H+ exchange may increase net HCO3- reabsorption by the THAL.