Stoichiometry of Na+-HCO-3 cotransport in basolateral membrane vesicles isolated from rabbit renal cortex
Abstract:
The major pathway for HCO3- transport across the basolateral membrane of the proximal tubule cell is electrogenic Na+-HCO3- cotransport. In this study, we have determined the stoichiometry of the Na+-HCO3- cotransport system in basolateral membrane vesicles that were isolated from rabbit renal cortex by Percoll gradient centrifugation. When the membrane potential is approximated by the Nernst potential for K+, as in the presence of the K+ ionophore valinomycin, equilibrium thermodynamics predicts that the Na+-HCO3- cotransport system should come to equilibrium and mediate no net flux when (Na)i/(Na)o = [(HCO3)o/(HCO3)i]n[(K)o/(K)i]n-1, where n is the HCO3-:Na+ stoichiometry. Our experimental approach was to impose transmembrane Na+, HCO3-, and K+ gradients of varying magnitude and direction, and then to measure the net flux of Na+ over the subsequent 3-s period. In this way, we could determine the conditions for equilibrium of the transport system and thereby calculate n. The results of these experiments indicate that the value of n is greater than 2.6 and less than 3.5, consistent with a stoichiometry of 3 HCO3-:1 Na+, or a thermodynamically equivalent process. Based on reported intracellular potentials and ion activities, this value for the stoichiometry indicates that the inside-negative membrane potential is sufficient to drive HCO3- exit against the inward concentration gradients of HCO3- and Na+ that are present across the basolateral membrane of the intact proximal tubule cell under physiologic conditions.
More Related Videos
07:38Functional Characterization of Na+/H+ Exchangers of Intracellular Compartments Using Proton-killing Selection to Express Them at the Plasma Membrane
Published on: March 30, 2015
11:27Methods to Study Epithelial Transport Protein Function and Expression in Native Intestine and Caco-2 Cells Grown in 3D
Published on: March 16, 2017
Related Concept Videos
COP Coated Vesicles
Reabsorption and Secretion in the PCT
Transport mechanisms involving sodium ions (Na+) contribute significantly to solute reabsorption. These mechanisms include symport and antiport processes.
A key example is the...
Reabsorption and Secretion in the Loop of Henle
Reabsorption and Secretion in the DCT and Collecting Duct
The distal part of the DCT, along with the...
Renal Regulation of Acid-Base Balance
In the kidneys, cells within the proximal convoluted tubules (PCT) and the collecting ducts secrete hydrogen ions (H+) into the tubular fluid. Specifically, in the PCT, Na+/H+ antiporters secrete H+ while reabsorbing Na+.
However, the intercalated cells in...
Renal Drug Excretion: Tubular Secretion
