Maturation of rat proximal tubule chloride permeability

We have previously shown that neonate rabbit tubules have a lower chloride permeability but comparable mannitol permeability compared with adult proximal tubules. The surprising finding of lower chloride permeability in neonate proximals compared with adults impacts net chloride transport in this segment, which reabsorbs 60% of the filtered chloride in adults. However, this maturational difference in chloride permeability may not be applicable to other species. The present in vitro microperfusion study directly examined the chloride and mannitol permeability using in vitro perfused rat proximal tubules during postnatal maturation. Whereas there was no maturational change in mannitol permeability, chloride permeability was 6.3 ± 1.3 × 10^-5 cm/s in neonate rat proximal convoluted tubule and 16.1 ± 2.3 × 10^-5 cm/s in adult rat proximal convoluted tubule (P < 0.01). There was also a maturational increase in chloride permeability in the rat proximal straight tubule (5.1 ± 0.6 × 10^-5 cm/s vs. 9.3 ± 0.6 × 10^-5 cm/s, P < 0.01). There was no maturational change in bicarbonate-to-chloride permeabilities (P _HCO3/P_Cl) in the rat proximal straight tubules (PST) and proximal convoluted tubules (PCT) or in the sodium-to-chloride permeability (P_Na/P_Cl) in the proximal straight tubule; however, there was a significant maturational decrease in proximal convoluted tubule P _Na/ P_Cl with postnatal development (1.31 ± 0.12 in neonates vs. 0.75 ± 0.06 in adults, P < 0.001). There was no difference in the transepithelial resistance measured by current injection and cable analysis in the PCT, but there was a maturational decrease in the PST (7.2 ± 0.8 vs. 4.6 ± 0.1 Ω · cm², P < 0.05). These studies demonstrate there are maturational changes in the rat paracellular pathway that impact net NaCl transport during development.