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J Neurophysiol ; 94(1): 441-53, 2005 Jul.
Article in English | MEDLINE | ID: mdl-15788520

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.


Subject(s)
Biological Transport/physiology , Hirudo medicinalis/physiology , Magnesium/metabolism , Malates/metabolism , Neurons/metabolism , Sodium/metabolism , Analysis of Variance , Animals , Antiporters/pharmacology , Chlorpromazine/pharmacology , Disaccharides/pharmacology , Dose-Response Relationship, Drug , Extracellular Space/metabolism , Fura-2/pharmacokinetics , Hydrogen-Ion Concentration , In Vitro Techniques , Iontophoresis/methods , Magnesium/pharmacology , Membrane Potentials/drug effects , Membrane Potentials/physiology , Microelectrodes , Models, Biological , Neurons/classification , Neurons/drug effects , Organ Preservation Solutions , Patch-Clamp Techniques , Sodium/pharmacology , Sucrose/pharmacology , Temperature
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