Apical membrane Na⁺/H⁺ exchange in rat medullary thick ascending limb. pH(i)-dependence and inhibition by hyperosmolality

Apical membrane Na⁺/H⁺ exchange mediates virtually all of transepithelial HCO₃/^- absorption in the rat medullary thick ascending limb (MTAL). Regulation of the apical exchanger by intracellular pH (pH(i)) and hyperosmolality was studied in the isolated, perfused MTAL by measurement of pH(i) using the fluorescent probe 2',7'-bis-(carboxyethyl)-5,6- carboxyfluorescein. Under isosmotic conditions (290 mosmol/kg H₂O), the Na⁺/H⁺ exchange rate increased sigmoidally over the pH(i) range 7.8 to 6.5 (Hill coefficient = 2.1), consistent with cooperative activation of the exchanger by internal H⁺. The exchanger had a high apparent affinity for intracellular H⁺ (apparent pK = 7.36), which resulted in the exchanger being maximally active at resting pH(i) and insensitive to changes in pH(i) over the physiologic pH(i) range (6.5-7.2). Hyperosmolality (590 mosmol/kg H₂O) inhibited Na⁺/H⁺ exchange by at least 35% at all pH(i) values studied and induced pH(i) dependence of the exchanger between 6.5 and 7.2. The inhibition by hyperosmolality appeared to be the result of an acid shift of the pH(i) dependence curve of the exchanger. These functional properties of apical membrane Na⁺/H⁺ exchange can account for our previous observations that hyperosmolality inhibited net HCO₃/^- absorption and that the rate of HCO₃/^- absorption did not correlate with pH(i). Apical membrane Na⁺/H⁺ exchange in the MTAL differs functionally from Na⁺/H⁺ exchange in other cell types in which exchanger activity is stimulated rather than inhibited by hyperosmolality.