Apical and basolateral Na+/H+ exchange in the rabbit outer medullary thin descending limb of Henle: role in intracellular pH regulation

Abstract

The present study was designed to investigate the apical and basolateral transport processes responsible for intracellular pH regulation in the thin descending limb of Henle. Rabbit thin descending limbs of long-loop nephrons were perfused in vitro and intracellular pH (pHi) was measured using BCECF. Steady-state pHi in HEPES buffered solutions (pH 7.4) was 7.18 +/- 0.03. Following the removal of luminal Na+, pHi decreased at a rate of 1.96 +/- 0.37 pH/min. In the presence of luminal amiloride (1 mM), the rate of decrease of pHi was significantly less, 0.73 +/- 0.18 pH/min. Steady-state pHi decreased 0.18 pH units following the addition of amiloride (1 mM) to the lumen (Na+ 140 mM lumen and bath). When Na+ was removed from the basolateral side of the tubule, pHi decreased at a rate of 0.49 +/- 0.05 pH/min. The rate of decrease of pHi was significantly less in the presence of 1 mM basolateral amiloride, 0.29 +/- 0.04 pH/min. Addition of 1 mM amiloride to the basolateral side (Na+ 140 mM lumen and bath) caused steady-state pHi to decrease significantly by 0.06 pH units. When pHi was acutely decreased to 5.87 +/- 0.02 following NH4Cl removal (lumen, bath), pHi failed to recover in the absence of Na+ (lumen, bath). Addition of 140 mM Na+ to the lumen caused pHi to recover at a rate of 2.17 +/- 0.59 pH/min. The rate of pHi recovery was inhibited 93% by 1 mM luminal amiloride. When 140 mM Na+ was added to the basolateral side, pHi recovered only partially at 0.38 +/- 0.07 pH/min. Addition of 1 mM basolateral amiloride inhibited the recovery of pHi by 97%. The results demonstrate that the rabbit thin descending limb of long-loop nephrons possesses apical and basolateral Na+/N+ antiporters. In the steady state, the rate of Na+-dependent H+ flux across the apical antiporter exceeds the rate of Na+-dependent H+ flux via the basolateral antiporter. Recovery of pHi following acute intracellular acidification is Na+ dependent and mediated primarily by the luminal antiporter.