Volume Reabsorption, Transepithelial Potential Differences, and Ionic Permeability Properties in Mammalian Superficial Proximal Straight Tubules

Abstract

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.

CO2-stimulated NaCl absorption in the mouse renal cortical thick ascending limb of Henle. Evidence for synchronous Na +/H+ and Cl-/HCO3- exchange in apical plasma membranes.

“…Furthermore, it can be shown (Schafer et al ., 1974;Hebert et al, 1981a), using L'Hospital's rule, that for Ve in the range of 2.5-10 mV, i.e., the VQ values observed in our experiments, the calculated values of Pi computed from Eq. 5 vary by <10% from those computed using Fick's first law for an uncharged species.…”

“…The net total C02 flux UC0'2, pmol The term total C02 refers to all C02 species present, at equilibrium, in bicarbonatebuffered solutions : dissolved C02, HCO3 , H2CO3, and CO3' . Evidently, at an ambient pH of 7 .4, 95% of the total C02 will be in the form of HCO3--The rate of net volume absorption (,J, nl min -1 mm-1) was measured using exhaustively dialyzed [3H-methoxy]-inulin (New England Nuclear, Boston, MA), as described previously (Schafer et al ., 1974) . ,J was computed from the difference between perfusion and collection rates and normalized for tubule length .…”

“…The ionic permeability coefficients (Pi, cm/s) were computed as described previously (Schafer et al ., 1974;Hebert et al, 1981a) from the expression However, at perfusion rates in excess of 10 nl/min in tubule segments < 1 mm in length, we found that the composition of tubular fluid differed only slightly from that of the perfusate . Thus, since the rates of axial volume delivery exceeded the rates of radial solute transport events, it is reasonable to argue that the spontaneous transepithelial voltage was more or less uniform with respect to tubule length, at least for the tracer ion flux experiments .…”

“…The spontaneous transepithelial voltage (V,, mV, lumen with respect to bath) and zero-current voltages (V) were determined as described previously (Schafer et al, 1974) . During perfusion with symmetric external solutions, the perfusate and bath had nearly identical ionic compositions and the protein concentration of the bathing solution, 0.4 g/dl, was negligible.…”

ATP as a mediator of macula densa cell signalling

“…Once the afferent arteriole enters Bowman's capsule it branches to form glomerular capillaries before coalescing and emerging from the glomerulus as the efferent arteriole. Filtrate processing occurs through distinct sequential tubular segments with differing cellular morphology and functions [2]. The initial tubular segment is the proximal tubule, the "work horse" of the nephron.…”