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 similar to 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.