title: Chronic regulation of transepithelial Na+ transport by the rate of apical Na+ entry.
abstract: In several settings in vivo, prolonged inhibition of apical Na+ entry reduces and prolonged stimulation of apical entry enhances the ability of renal epithelial cells to reabsorb Na+, an important feature of the load-dependent regulation of renal tubular Na+ transport. To model this load dependency, apical Na+ entry was inhibited or stimulated for 18 h in A6 cells and vectorial transport was measured as short-circuit current (Isc) across monolayers on filter-bottom structures. Basal amiloride-sensitive Isc represents the activity of apical Na+ channels, whereas Isc after permeabilization of the apical membrane to cations with nystatin represents maximal activity of the basolateral Na(+)-K(+)-ATPase. Chronic inhibition of apical Na+ entry by 18-h apical exposure to amiloride or replacement of apical Na+ with tetramethylammonium (TMA+), followed by washing and restoration of normal apical medium, revealed a persistent decrease in Isc that remained despite exposure to nystatin. Both basal and nystatin-stimulated Isc recovered progressively after restoration of normal apical medium. In contrast, chronic stimulation of apical Na+ entry by short circuiting the epithelium increased Isc in the absence and presence of nystatin, indicating upregulation of both apical Na+ channels and basolateral Na(+)-K(+)-ATPase. Basolateral equilibrium [3H]ouabain binding was reduced to 67 +/- 5% in TMA+ vs. control cells, whereas values in 18-h short-circuited cells increased by 42 +/- 19%. The results demonstrate that load dependency of tubular Na+ transport can be modeled in vitro and indicate that the regulation of Na(+)-K(+)-ATPase observed in these studies occurs in part by changes in the density of functional transporter proteins within the basolateral membrane.
authors.affiliation: Laboratory of Epithelial Cell Biology, University of Pittsburgh Medical Center, Pennsylvania, USA.
authors.forename: M D
authors.initials: MD
authors.lastname: Rokaw
authors.forename: E
authors.initials: E
authors.lastname: Sarac
authors.forename: E
authors.initials: E
authors.lastname: Lechman
authors.forename: M
authors.initials: M
authors.lastname: West
authors.forename: J
authors.initials: J
authors.lastname: Angeski
authors.forename: J P
authors.initials: JP
authors.lastname: Johnson
authors.forename: M L
authors.initials: ML
authors.lastname: Zeidel
mesh_terms: D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002461:Cell Line, Transformed; D002462:Cell Membrane / Q000378:metabolism; D004848:Epithelium / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism; D007668:Kidney / Q000166:cytology / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology; D012964:Sodium / Q000037:antagonists & inhibitors / Q000378:metabolism*; D015222:Sodium Channels / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D013997:Time Factors; D014982:Xenopus laevis
chemical_list: D000644:Quaternary Ammonium Compounds; D015222:Sodium Channels; D000584:Amiloride; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; C027917:tetramethylammonium