MATURATION OF PROXIMAL TUBULAR ACIDIFICATION

Neonatal juxtamedullary proximal convoluted tubules (PCTs) transport b icarbonate at one-third the rate of adult rabbit PCTs. The lower rate of bicarbonate transport could be due to a greater permeability of the neonatal PCT to bicarbonate or to a lower rate of active bicarbonate transport. This review discusses potential factors which could result in a lower rate of bicarbonate transport by the neonatal PCT. In isola ted perfused PCT, bicarbonate permeability is lower in neonatal than a dult PCT, and thus it does not account for the lower rate of bicarbona te transport in neonatal PCT. In the adult PCT, apical proton secretio n occurs via the Na+/H+ antiporter and H+-ATPase; basolateral bicarbon ate exit occurs via the Na(HCO3)(3) symporter. The activity of transpo rters can be ascertained by measuring intracellular pH with the fluore scent dye BCECF. Apical Na+/H+ antiporter, apical H+-ATPase and basola teral Na(HCO3)(3) symporter activity are all significantly lower in ne onatal PCT. The factors which stimulate PCT maturation are unknown, ho wever glucocorticoids have been postulated to play an important role i n this process. Administration of dexamethasone to pregnant does resul ts in higher rates of PCT volume absorption, bicarbonate transport, Na +/H+ antiporter and Na(HCO3)(3) symporter activities than in PCT from vehicle-treated controls. Thus, the lower rate of neonatal PCT bicarbo nate transport is due to lower activities of the apical Na+/H+ antipor ter, apical H+-ATPase and basolateral Na(HCO3)(3) symporter. Glucocort icoids may be an important factor in the maturation of PCT acidificati on.