Neonatal rabbit juxtamedullary proximal convoluted tubule acidification

The present in vitro microperfusion study examined apical membrane Na+/H+ antiporter and basolateral membrane Na(HCO3)3 symporter activity in newborn and adult juxta-medullary proximal convoluted tubules. Proton fluxes were determined from the initial rate of change of intracellular pH after a change in the luminal or bathing solution, buffer capac-ity, and tubular volume of newborn and adult tubules. Intracel-lular pH (pH;) was measured fluorometrically using the pH-sensitive dye (2',7')-bis (carboxyethyl)-(5,6)-carboxyfluores-cein (BCECF). Apical Na+/H+ antiporter proton flux, assayed by the effect of sodium removal (147-- 0 meq/liter) on pH1, was one-third the adult level for the first 2 wk and doubled in the 3rd wk of life. Adult levels were achieved by 6 wk of age. Na+/H+ antiporter activity was not detected on the basolateral membrane of 1-wk-old newborns, indicating that polarity of this transporter was already present. Basolateral membrane Na(HCO3)3 proton flux, assayed by the effect of a bath bicar-bonate change (25-.5 meq/liter) and by a bath sodium change (147 0 meq/liter) on pH,, was 50-60 % of adult values in 1-wk-old newborns. Basolateral membrane Na(HCO3)3 proton flux assayed by a bath bicarbonate change (25-. 5 meq/liter) remained at 50-60 % of adult values for the 1st mo of life and increased to adult levels by 6 wk of age. This transporter not only plays a role in net acidification, but is an important deter-minant of cell pH in newborn juxtamedullary proximal convo-luted tubules. (J. Clin. Invest. 1990.85:499-506.) intracellular pH- Na+/H+ antiporter- Na(HCO3) symporter * proton flu