title: Change of apparent stoichiometry of proximal-tubule Na(+)-HCO3- cotransport upon experimental reversal of its orientation.
abstract: Electrogenic cotransport of Na+ with HCO3- has been reported in numerous tissues. It has always been shown with a net transfer of negative charge, but in some situations achieves net outward transport of both species with a stoichiometry of at least three HCO3- ions per Na+ ion (3:1), and in other situations achieves net inward transport of both species and has a stoichiometry of at most two HCO3- ions per Na+ ion (2:1). This suggests either that there may be more than one protein responsible for Na(+)-HCO3- cotransport in different tissues or that if there is a single protein, its stoichiometry may differ depending on the orientation of net transport. The present study, using conventional or double-barreled ion-selective microelectrodes to follow basolateral membrane potential and intracellular pH or Na+ activity in Necturus proximal convoluted tubule in vivo, shows that the orientation of the basolateral Na(+)-HCO3- cotransporter can be reversed upon switching from a perfusate simulating normal acid-base conditions to one that imposes peritubular isohydric hypercapnia. Moreover, accompanying the reversal of orientation is a change of apparent stoichiometry from 3:1 to 2:1. Given that the observed change of orientation and accompanying change of apparent stoichiometry occur within seconds and in the same preparation, these results suggest that a single transport protein is responsible for both types of behavior.
authors.affiliation: Institut National de la Santé et de la Recherche Médicale Unité 323, Faculté de Médecine, Paris, France.
authors.forename: G
authors.initials: G
authors.lastname: Planelles
authors.forename: S R
authors.initials: SR
authors.lastname: Thomas
authors.forename: T
authors.initials: T
authors.lastname: Anagnostopoulos
mesh_terms: D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid / Q000494:pharmacology; D000818:Animals; D001464:Barium / Q000494:pharmacology; D001639:Bicarbonates / Q000378:metabolism*; D001692:Biological Transport / Q000187:drug effects; D006863:Hydrogen-Ion Concentration; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D009337:Necturus; D012964:Sodium / Q000378:metabolism*; D013816:Thermodynamics
chemical_list: D001639:Bicarbonates; D001464:Barium; D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid; D012964:Sodium