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 (pHi) 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 pHi 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[Formula: see text] than in the absence of[Formula: see text] and was inhibited by cyanide and acetazolamide, consistent with Cl-/[Formula: see text]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-1min-1, respectively ( P = NS), consistent with a mature rate of Cl-/[Formula: see text]exchanger activity in neonatal tubules. Basolateral Cl-/base exchange activity in the absence of CO2 and[Formula: see text], 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.