Functional roles of apical membrane Na₊/H⁺ exchange in rat medullary thick ascending limb

The medullary thick ascending limb (MTAL) of the rat actively absorbs both HCO₃ and ammonium. The roles of apical membrane Na⁺/H⁺ exchange in these processes and in determining steady-state intracellular pH (pH_i) were examined in MTAL perfused in vitro with solutions containing 146 mM Na⁺ and 25 mM HCO₃ (pH 7.4). Addition of 1 mM amiloride or 50 μM ethylisopropylamiloride (EIPA) to the lumen decreased HCO₃ absorption (J_HCO3) from 10.6 ± 0.5 to 2.3 ± 0.3 pmol·min ₁· mm ₁ (P < 6.001) and pH_i from 7.10 ± 0.02 to 6.86 ± 0.03 (P < 0.001). The combination of lumen Na⁺ replacement plus amiloride abolished J_HCO3. Chronic metabolic acidosis (CMA) caused a 32% increase in J_HCO3 that was inhibited by luminal amiloride. Addition of 4 mM NH₄Cl to perfusate and bath markedly decreased pH_i (from 7.10 to 6.70) but did not stimulate luminal H secretion as assessed by HCO₃ absorption. With 4 mM NH₄Cl in perfusate and bath, luminal addition of amiloride decreased pH_i from 6.70 ± 0.06 to 6.50 ± 0.05 (P < 0.005) but had no effect on net ammonium absorption. These results demonstrate that 7) apical membrane Na⁺/H⁺ exchange mediates virtually all of HCO₃⁺ absorption and is an important determinant of steadystate pH; in the MTAL; 2) the adaptive increase in HCO₃ absorption in CMA is mediated by an increase in apical membrane Na⁺/H⁺ exchange; 3) ammonium markedly acidifies the cells but does not stimulate luminal acidification, suggesting that pH_i is not a predominant influence on apical Na₊/H⁺ exchange activity and that H⁺ generated in the cells as the result of transcellular ammonium absorption is extruded across the basolateral membrane; and 4) apical membrane Na⁺/H⁺ exchange is not important for ammonium absorption.