Mg-Malate Co-transport, a Mechanism for Na-Independent Mg Transport in Neurons of the Leech Hirudo medicinalis

Mg-extrusion from Mg-loaded neurons of the leech, Hirudo medicinalis, is mediated mainly by Na/Mg antiport. However, in a number of leech neurons Mg is extruded in the nominal absence of extracellular Na, indicating the existence of an additional, Na-independent Mg transport mechanism. This mechanism was investigated using electrophysiological and microfluorimetrical techniques The rate of Na-independent Mg extrusion from Mg-loaded leech neurons was found to be independent of extracellular Ca, K, NO3, HCO3, SO4, HPO4 and of the intraand extracellular pH. Na-independent Mg extrusion was not inhibited by DIDS, furosemide, ouabain, vanadate, iodoacetate, 4-amino-hippurate or α-cyano-4-hydroxycinnamate and was not influenced by changes in the membrane potential in voltage-clamp experiments. Na-independent Mg extrusion was, however, inhibited by the application of 2 mM probenecid, a blocker of organic anion transporters, suggesting that Mg might be co-transported with organic anions. Extracellularly, of all organic anions tested (malate, citrate, lactate, αketoglutarate, and 4-amino-hippurate) only high, but physiological, concentrations of malate (30 mM) had a significant inhibitory effect on Na-independent Mg extrusion. Intracellularly, iontophoretically injected malate, citrate or fura-2 but not Cl, α-ketoglutarate, glutamate, succinate or urate were stimulating Na-independent Mg extrusion from those neurons that initially did not extrude Mg in Na-free solutions. Our data indicate that Mg is co-transported with organic anions, preferably with malate, the predominant extracellular anion in the leech. The proposed model implies that under experimental condition malate drives Mg extrusion, while under physiological conditions malate is actively taken up, driven by Mg, so that malate can be metabolized.