Maturation of rabbit proximal straight tubule chloride/base exchange

The present in vitro microperfusion study compared the mechanism and rates of NaCl transport in neonatal and adult rabbit proximal straight tubules. In proximal straight tubules perfused with a late proximal tubular fluid and bathed in a serumlike albumin solution, the rate of volume absorption (J(v)) was 0.54 ± 0.10 and 0.12 ± 0.05 nl·mm^-1·min^-1 in adults and neonates, respectively (P < 0.05). With the addition of 10^-5 M bath ouabain, J(v) decreased to 0.27 ± 0.07 and -0.03 ± 0.04 nl·mm^{- 1}·min^-1 in adult and neonatal tubules, respectively (P < 0.05), consistent with lower rates of active and passive NaCl transport in the neonatal proximal straight tubule. The effect of luminal sodium and chloride removal on intracellular pH was used to assess the relative rates of Na⁺/H⁺ and Cl^-/base exchange. The rates of Na⁺/H⁺ and Cl^-/base exchange were approximately fivefold less in neonatal proximal straight tubules than adult tubules. In both neonatal and adult proximal straight tubules, the rate of Cl^-/base exchange was not affected by formate, bicarbonate, or cyanide and acetazolamide, consistent with Cl^-/OH^- exchange. These data demonstrate an increase in proximal straight tubule NaCl transport during postnatal renal development.