title: Modulation of Na+/Mg²+ exchanger stoichiometry ratio by Cl⁻ ions in basolateral rat liver plasma membrane vesicles.
abstract: The Na+/Mg²+ exchanger represents the main Mg²+ extrusion mechanism operating in mammalian cells including hepatocytes. We have previously reported that this exchanger, located in the basolateral domain of the hepatocyte, promotes the extrusion of intravesicular trapped Mg²+ for extravesicular Na+ with ratio 1. This electrogenic exchange is supported by the accumulation of tetraphenyl-phosphonium within the vesicles at the time when Mg²+ efflux occurs. In this present study, the role of extra- and intra-vesicular Cl⁻ on the Na+/Mg²+ exchange ratio was investigated. The results reported here suggest that Cl⁻ ions are not required for the Na+ to Mg²+ exchange to occur, but the stoichiometry ratio of the exchanger switches from electrogenic (1Na(in)+ :1Mg(out)²+) in the presence of intravesicular Cl⁻ to electroneutral (2Na(in)+ :1 Mg(out)²+) in their absence. In basolateral liver plasma membrane vesicles loaded with MgCl₂ labeled with ³⁶Cl⁻, a small but significant Cl⁻ efflux (~30 nmol Cl⁻/mg protein/1 min) is observed following addition of NaCl or Na-isethionate to the extravesicular medium. Both Cl⁻ and Mg²+ effluxes are inhibited by imipramine but not by amiloride, DIDS, niflumic acid, bumetanide, or furosemide. In vesicles loaded with Mg-gluconate and stimulated by Na-isethionate, an electroneutral Mg²+ extrusion is observed. Taken together, these results suggest that the Na+/Mg²+ exchanger can operate irrespective of the absence or the presence of Cl⁻ in the extracellular or intracellular environment. Changes in trans-cellular Cl⁻ content, however, can affect the modus operandi of the Na+/Mg²+ exchanger, and consequently impact "cellular" Na+ and Mg²+ homeostasis as well as the hepatocyte membrane potential.
authors.affiliation: Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, 10900 Euclid Ave, Cleveland, OH 44106-4970, USA.
authors.forename: C
authors.initials: C
authors.lastname: Cefaratti
authors.forename: A
authors.initials: A
authors.lastname: Romani
mesh_terms: D000818:Animals; D017920:Antiporters / Q000378:metabolism*; D001485:Basement Membrane / Q000378:metabolism*; D002712:Chlorides / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D017920:Antiporters; D002712:Chlorides; C046250:sodium-magnesium antiporter; D008274:Magnesium