We have used rat cortical collecting tubules perfused in vitro to study the effects of antidiuretic hormone (ADH) and desoxycorticosterone (DOCA) on the unidirectional fluxes of sodium. We found that in the basal state, lumen-to-bath flux (Jib) and bath-to-lumen flux (JbJ) of 22Na were approximately equal, 39.5±3.9 and 41.8±11.0 pmol min-1 mm-', respectively, resulting in no net flux. Addition of 100 1U/ml ADH to the bath produced a stable increase in Jib to 583±4.7 pmol* min-' * mm-'. Addition of ADH to a tubule from a DOCA-pretreated rat caused an increase in Jib to 144.1±12.0 pmol* min-' mm-'. Neither hormone had an effect on Jbi. Thus ADH produced a greater absolute and fractional increase in Jib when the animal was pretreated with DOCA, and the ADH-induced increase over baseline was greater than the DOCA-induced increase. Both the ADHand DOCA-induced stimulation of Jib were completely abolished by 10-5 M luminal amiloride, suggesting that the route of sodium transport stimulated by both hormones involves apical sodium channels. However, ADH and DOCA have very different time courses of action; ADH acted within minutes, while aldosterone and DOCA are known to require 90-180 min. The facilitating action of ADH on DOCA-induced stimulation of sodium transport may be important for maximal sodium reabsorption and for the ability to achieve a maximally concentrated urine.

This paper describes experiments designed to evaluate Na+ and C1-transport 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 10-4 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 Cl-concentration 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 C1-solutions, V, may become positive when tubular fluid C-concentrations rise during volume reabsorption. The permeability coefficients PN and PcI computed from tracer fluxes were, respectively, 0.23 X 10-4 and 0.73 X 10 -4 cm s -1. 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 present experiments were designed to evaluate the effective thickness of the unstirred layers in series with native and porous (i.e., in the presence of amphotericin B) lipid bilayer membranes and, concomitantly, the respective contributions of membranes and unstirred layers to the observed resistances to the diffusion of water and nonelectrolytes between aqueous phases. The method depended on measuring the tracer permeability coefficients for the diffusion of water and nonelectrolytes (PD,, cm sec -') when the aqueous phase viscosity (7) was increased with solutes having a unity reflection coefficient, such as sucrose or dextran. The effective thickness of the unstirred layers (at, cm) and the true, or membrane, permeability coefficients for diffusion of water and nonelectrolytes (P, , cm sec -1 ) were computed from, respectively, the slope and intercept of the linear regression of 1/PDi on a. In both the native and porous membranes, at was approximately 110 X 10 -4 cm. The ratio of Pt, the osmotic water permeability coefficient (cm sec -') to PmH,, was 1.22 in the native membranes and 3.75 in the porous membranes. For the latter, the effective pore radius, computed from Poiseuille's law, was approximately 5.6 A. A comparison of P,i and PDj indicated that the porous membranes accounted for 16, 25, and 66% of the total resistance to the diffusion of, respectively, H 2 0, urea, and glycerol, while the remainder was referable to the unstirred layers.