Apparent intracellular Mg2+ buffering in neurons of the leech Hirudo medicinalis

The apparent intracellular Mg2+ buffering, or muffling (sum of processes th at damp changes in the free intracellular Mg2+ concentration, [Mg2+](i), e. g., buffering, extrusion, and sequestration), was investigated in Retzius n eurons of the leech Hirudo medicinalis by iontophoretic injection of H+, OH -, or Mg2+. Simultaneously, changes in intracellular pH and the: intracellu lar Mg2+, Na+, or K+ concentration were recorded with triple-barreled ion-s elective microelectrodes. Cell volume changes were monitored measuring the tetramethylammonium (TMA) concentration in TMA-loaded neurons. Control meas urements were carried out in electrolyte droplets (diameter 100-200 mum) pl aced on a silver wire under paraffin oil. Droplets:with or without ATP, the presumed major intracellular Mg2+ buffer, were used to quantify the pH dep endence of Mg2+ buffering and to determine the transport index of Mg2+ duri ng iontophoretic injecton. The observed pH dependence of [Mg2+](i) correspo nded to what would be expected from Mg2+ buffering through ATP. The quantit y of Mg2+ muffling, however, Was considerably larger than what would be exp ected if ATP were the sole Mg2+ buffer. From the decrease in Mg2+ muffling in the nominal absence of extracellular Nat it was estimated that almost 50 % of the ATP-independent muffling is due to the-action of Na+/MS2+ antiport .