Na AND Cl PERMEABILITY IN NEWBORN RABBIT PROXIMAL CONVOLUTED TUBULES (PCT)

Abstract

Ionic permeabilities differ in adult superficial (SF) and juxtamedullary (JM) PCT with Na>Cl=HCO₃ in JM and Cl>Na=HCO₃ in SF. These permeability differences are known to affect passive reabsorption in these nephron populations. To address the question of when these differences develop, PCT from the corticomedullary junction (JM) and cortical surface (SF) of newborn were perfused in vitro with serum ultrafiltrate and bathed in rabbit serum. The immediate subcapsular PCT were too immature to perfuse (no lumens) so that the immediate subadjacent PCT were studied and designated SF. Potential difference (PD) was recorded before and after introduction of a 50mm outward gradient for NaCl (A-50) and the Na/Cl permeability (PNa/PCl) calculated. Five kidneys were also fixed for ultrastructural studies.

JMPCT had PNa>PCl while SF had PCl>PNa. Also morphologic differences were observed between SF and JM. We conclude: 1) Intrinsic heterogeneity of PCT is present at birth. 2) PNa/PCI ≅approximates free solution mobilities for these ions (0.66) in SF while JM have developed Na permselectivity, 3) Since JM PCT develop earlier than SFPCT these findings suggest that epithelial discrimination between ions is part of the maturation process.

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