title: Mg2+-malate co-transport, a mechanism for Na+-independent Mg2+ transport in neurons of the leech Hirudo medicinalis.
abstract: Mg2+-extrusion from Mg2+-loaded neurons of the leech, Hirudo medicinalis, is mediated mainly by Na+/Mg2+ antiport. However, in a number of leech neurons, Mg2+ is extruded in the nominal absence of extracellular Na+, indicating the existence of an additional, Na+-independent Mg2+ transport mechanism. This mechanism was investigated using electrophysiological and microfluorimetrical techniques. The rate of Na+-independent Mg2+ extrusion from Mg2+-loaded leech neurons was found to be independent of extracellular Ca2+, K+, NO3-, HCO3-, SO4(2-), HPO4(2-), and of intra- and extracellular pH. Na+-independent Mg2+ extrusion was not inhibited by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), furosemide, ouabain, vanadate, iodoacetate, 4-amino-hippurate, or alpha-cyano-4-hydroxycinnamate and was not influenced by changes in the membrane potential in voltage-clamp experiments. Na+-independent Mg2+ extrusion was, however, inhibited by the application of 2 mM probenecid, a blocker of organic anion transporters, suggesting that Mg2+ might be co-transported with organic anions. Extracellularly, of all organic anions tested (malate, citrate, lactate, alpha-ketoglutarate, and 4-amino-hippurate) only high, but physiological, concentrations of malate (30 mM) had a significant inhibitory effect on Na+-independent Mg2+ extrusion. Intracellularly, iontophoretically injected malate, citrate, or fura-2, but not Cl-, alpha-ketoglutarate, glutamate, succinate, or urate, were stimulating Na+-independent Mg2+ extrusion from those neurons that initially did not extrude Mg2+ in Na+-free solutions. Our data indicate that Mg2+ is co-transported with organic anions, preferably with malate, the predominant extracellular anion in the leech. The proposed model implies that, under experimental conditions, malate drives Mg2+ extrusion, whereas under physiological conditions, malate is actively taken up, driven by Mg2+, so that malate can be metabolized.
authors.affiliation: Institut für Klinische Physiologie, Heinrich-Heine-Universität Düsseldorf, Germany. dorothee.guenzel@charite.de
authors.forename: Dorothee
authors.initials: D
authors.lastname: Günzel
authors.forename: Karin
authors.initials: K
authors.lastname: Hintz
authors.forename: Simone
authors.initials: S
authors.lastname: Durry
authors.forename: Wolf-Rüdiger
authors.initials: WR
authors.lastname: Schlue
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D017920:Antiporters / Q000494:pharmacology; D001692:Biological Transport / Q000502:physiology*; D002746:Chlorpromazine / Q000494:pharmacology; D004187:Disaccharides / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005110:Extracellular Space / Q000378:metabolism; D016257:Fura-2 / Q000493:pharmacokinetics; D047709:Hirudo medicinalis / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007478:Iontophoresis / Q000379:methods; D008274:Magnesium / Q000378:metabolism* / Q000494:pharmacology; D008293:Malates / Q000378:metabolism*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008839:Microelectrodes; D008954:Models, Biological; D009474:Neurons / Q000145:classification / Q000187:drug effects / Q000378:metabolism*; D019188:Organ Preservation Solutions; D018408:Patch-Clamp Techniques; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D013395:Sucrose / Q000494:pharmacology / Q000031:analogs & derivatives; D013696:Temperature
chemical_list: D017920:Antiporters; D004187:Disaccharides; D008293:Malates; D019188:Organ Preservation Solutions; C067074:sodium lactobionate sucrose solution; D013395:Sucrose; C030298:malic acid; D012964:Sodium; D008274:Magnesium; D016257:Fura-2; D002746:Chlorpromazine