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 transpo rt. The preferential reabsorption of bicarbonate and organic solutes b y the early proximal tubule leaves the luminal fluid with a higher chl oride concentration than that in the peritubular capillaries. Previous studies have suggested that solute permeability of the paracellular p athway may be higher in the neonatal PCT and that the neonatal proxima l tubule reabsorbs solutes by passive mechanisms to a greater extent t han the adult segment. A higher chloride permeability would provide a mechanism for the greater rate of passive NaCl transport by the neonat al proximal tubule. The purpose of the present in vitro microperfusion study was to directly examine the chloride permeability of neonatal a nd adult PCT. Superficial and juxtamedullary, neonatal and adult PCT w ere perfused with a high chloride perfusate without organic solutes, s imulating late proximal tubular fluid, at 20 degrees C, and bathed in a serum-like albumin solution. Chloride concentrations in the perfusat e and the collected fluid were measured by electrometric titration. Ne onatal juxtamedullary PCT chloride permeability (P-Cl) was significant ly 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 neonata l 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). Thu s, there is a maturational increase in juxtamedullary PCT P-Cl, wherea s superficial PCT P-Cl remains very low. The passive diffusion of chlo ride in neonatal PCT is extremely low and is not a mechanism to explai n a higher rate of passive NaCl transport in this segment.