Neonatal rabbit proximal tubule basolateral membrane Na⁺/H⁺ antiporter and Cl^-/base exchange

The present in vitro microperfusion study examined the maturation of Na⁺/H⁺ antiporter and Cl^-/base exchanger on the basolateral membrane of rabbit superficial proximal straight tubules (PST). Intracellular pH (pH(i)) was measured with the pH-sensitive fluorescent dye 2',7'-bis(2-carboxyethyl)- 5(6)-carboxyfluorescein in neonatal and adult superficial PST. Na⁺/H⁺ antiporter activity was examined after basolateral Na⁺ addition in tubules initially perfused and bathed without Na⁺. Neonatal Na⁺/H⁺ antiporter activity was ~40% that of adult segment (9.7 ± 1.5 vs. 23.7 ± 3.2 pmol · mm^-1 · min^-1; p < 0.001). The effect of bath Cl^- removal on pH(i) was used to assess the rates of basolateral Cl^-/base exchange. In both neonatal and adult PST, the Cl^-/base exchange activity was significantly higher in the presence of 25 mM HCO₃ than in the absence of HCO₃ and was inhibited by cyanide and acetazolamide, consistent with Cl^-/HCO₃ exchange. The proton flux rates in the presence of bicarbonate in neonatal and adult tubules were 14.1 ± 3.6 and 19.5 ± 3.5 pmol. mm^-1 min^-1, respectively (P = NS), consistent with a mature rate of Cl^-/HCO₃ exchanger activity in neonatal tubules. Basolateral Cl^-/base exchange activity in the absence of CO₂ and HCO₃, with luminal and bath cyanide and acetazolamide, was greater in adult than in neonatal PST and inhibited by bath DIDS consistent with a maturational increase in Cl^-/OH^- exchange. We have previously shown that the rates of the apical membrane Na⁺/H⁺ antiporter and Cl^-/base exchanger were approximately fivefold lower in neonatal compared with adult rabbit superficial PST. These data demonstrate that neonatal PST basolateral membrane Na⁺/H⁺ antiporter and Cl^-/base exchanger activities are relatively more mature than the Na⁺/H⁺ antiporter and Cl^-/base exchangers on the apical membrane.