Developmental changes in rabbit juxtamedullary proximal convoluted tubule bicarbonate permeability

The bicarbonate transport rate in neonatal rabbit juxtamedullary proximal convoluted tubules (JMPCT) is lower than that in adults. The reduced rate of transport could be due to a decrease in active bicarbonate transport or an increase in the passive permeability of the tubule to bicarbonate. The present in vitro microperfusion study directly measured the bicarbonate permeability of neonatal and adult JMPCT. Bicarbonate permeability was measured at both slow and fast perfusion rates to simulate the neonatal and adult proximal tubule flow rates, respectively. At 38°C in tubules perfused at 3 nL/min, bicarbonate permeability was 0.29 ±0.11 × 10^-5 cm/s in neonates and 1.70 ± 0.49 × 10^-5 cm/s in adult PCT (p < 0.05). At a perfusion rate of 10 nL/min, bicarbonate permeability was 0.11 ± 0.27 × 10^-5 cm/s in neonatal PCT and 2.31 ± 0.15 × 10^-5 cm/s in adult PCT (p < 0.05). These results demonstrate that bicarbonate permeability in neonatal JMPCT is significantly lower than that in adult JMPCT. Thus, the lower rate of bicarbonate transport in neonatal PCT is entirely due to a lower rate of active bicarbonate transport.