Na to Cl permeability in newborn rabbit superficial and juxtamedullary proximal convoluted tubules

: It has been previously demonstrated that superficial (SF) versus juxtamedullary (JM) proximal convoluted tubules (PCT) of rabbit have different intrinsic transport characteristics: Na is less permeable than Cl in the latter portions of the SFPCT, while Cl is less permeable than Na throughout the JMPCT. These permeability differences have major influences on the mechanism of salt reabsorption across the proximal tubules. However, both populations of PCT have the same embryological origin. Studies therefore were designed to examine whether two distinct populations of PCT exist at birth or whether a second population of tubules develops with delivery as a result of some unidentified acute change in humoral factor affecting epithelial transport properties. Both morphological and electrophysiological studies were conducted on PCTs from rabbits within 36 h of birth. Both transmission and scanning electron microscopy studies clearly disclosed that SFPCT are less mature than the JMPCT. Also the SFPCT had a lower Na permeability than Cl (0.55 +/- 0.06) while the JMPCT had a higher Na permeability than Cl (1.37 +/- 0.11). Thus these studies demonstrate that intrinsic heterogeneity of PCT is present at birth. Since the SF Na to Cl permeability approximates that of free diffusion these studies suggest that epithelial discrimination in PCT is part of a maturation process.