Maturation of rabbit proximal convoluted tubule chloride permeability

Chloride transport in the rabbit proximal convoluted tubule (PCT) has components of active, transcellular, and passive, paracellular transport. The preferential reabsorption of bicarbonate and organic solutes by the early proximal tubule leaves the luminal fluid with a higher chloride concentration than that in the peritubular capillaries. Previous studies have suggested that solute permeability of the paracellular pathway may be higher in the neonatal PCT and that the neonatal proximal tubule reabsorbs solutes by passive mechanisms to a greater extent than the adult segment. A higher chloride permeability would provide a mechanism for the greater rate of passive NaCl transport by the neonatal proximal tubule. The purpose of the present in vitro microperfusion study was to directly examine the chloride permeability of neonatal and adult PCT. Superficial and juxtamedullary, neonatal and adult PCT were perfused with a high chloride perfusate without organic solutes, simulating late proximal tubular fluid, at 20°C, and bathed in a serum-like albumin solution. Chloride concentrations in the perfusate and the collected fluid were measured by electrometric titration. Neonatal juxtamedullary PCT chloride permeability (P(Cl)) was significantly lower than adult juxtamedullary PCT P(Cl)) (0.15 ± 0.25 x 10^-5 cm/s versus 5.23 ± 0.57 x 10^-5 cm/s, p < 0.001). The P(Cl) of neonatal superficial PCT was not different from that of adult superficial PCT (0.81 ± 0.48 x 10^-5 cm/s versus 0.05 ± 0.62 x 10^-5 cm/s). thus, there is a maturational increase in juxtamedullary PCT P(Cl), whereas superficial PCT P(Cl) remains very low. The passive diffusion of chloride in neonatal PCT is extremely low and is not a mechanism to explain a higher rate of passive NaCl transport in this segment.