Developmental changes in rabbit juxtamedullary proximal convoluted tubule water permeability

The mammalian proxi-mal tubule reabsorbs the bulk of the glomerular filtrate in a nearly isosmotic fashion due to the high osmotic water permeability (P_f) of this segment. Although the characteristics of proximal tubule water transport have been studied in the adult proximal tubule, little is known about the neonatal segment. The present study directly measured the P_f and diffusional water permeability (P_Dw) of neonatal (10 ±2 day old) and adult rabbit juxtamedullary proximal convoluted tubules (PCT) using in vitro microperfusion. The P_f of neonatal juxtamedullary PCT was greater than the P_f of adult juxtamedullary PCT. In contrast, the P_Dw was not different between the two groups. The P_f and P_Dw values of both neonatal and adult tubules were inhibited to the same degree by p-chloromercuribenzene sulfonate and had identical activation energies. The transepithelial reflection coefficients of NaCl and NaHCO₃ were also found to be similar in both the neonatal and adult proximal tubules. Thus neonatal and adult juxtamedullary PCT have many characteristics of water transport that are identical; however, neonatal P_f is three to five times that of the adult value. This difference in P_f with identical P_Dw values may give an insight into the transepithelial pathway for water movement in the neonatal tubule.