Superficial Proximal Straight Tubules

This paper describes experiments designed to evaluate Na+ and C1transport in isolated proximal straight tubules from rabbit kidneys. When the perfusing solution was Krebs-Ringer buffer with 25 mM HCO8 (KRB) and the bath contained KRB plus 6 % albumin, net volume reabsorption (J,, nl min-' mm-') was -0.46 4 0.03 (SEM); V., the spontaneous transepithelial potential difference, was -1.13 + 0.05 mV, lumen negative. Both J, and V. were reduced to zero at 21°C or with 104 M ouabain, but J, was not HCO dependent. Net Na+ reabsorption, measured as the difference between 22Na+ fluxes, lumen to bath and bath to lumen, accounted quantitatively for volume reabsorption, assuming the latter to be an isotonic process, and was in agreement with the difference between lumen to bath 22Na+ fluxes during volume reabsorption and at zero volume flow. The observed flux ratio for Na+ was 1.46, and that predicted for a passive process was 0.99; thus, Na+ reabsorption was rationalized in terms of an active transport process. The Clconcentration of tubular fluid rose from 113.6 to 132.3 mM during volume reabsorption. Since V, rose to +0.82 mV when tubules were perfused with 138.6 mM C1solutions, V, may become positive when tubular fluid Cconcentrations rise during volume reabsorption. The permeability coefficients PN and PcI computed from tracer fluxes were, respectively, 0.23 X 104 and 0.73 X 10 4 cm s1. A PNa/Pcl ratio of 0.3 described NaCl dilution potentials at zero volume flow. The magnitudes of the potentials were the same for a given NaCl gradient in either direction and PNa/PcO was constant in the range 32-139 m'M NaCl. We infer that the route of passive ion permeation was through symmetrical extracellular interfaces, presumably tight junctions, characterized by neutral polar sites in which electroneutrality is maintained by mobile counterions. THE JOURNAL OF GENERAL PHYSIOLOGY VOLUME 64, I974 pages 582-607 582 on Jne 2, 2017 D ow nladed fom Published November 1, 1974