INTRACELLULAR MG2+ IS REGULATED BY 1 NA+ 1 MG2+ ANTIPORT IN NEURONS OF AN INVERTEBRATE CENTRAL-NERVOUS-SYSTEM/

Triple-barrelled, ion-sensitive microelectrodes were used to investiga te the regulation of intracellular Mg2+ in Retzius neurones of the med icinal leech (Hirudo medicinalis). There is strong evidence that Mg2extrusion from Mg2+-loaded Retzius neurones is mediated by an amilorid e-sensitive Na+/Mg2+-antiport, possibly working at a stoichiometry of 1:1. In contrast, Mg2+ influx pathways are more complex. Mg2+ influx c ould be partially inhibited by the Ca2+ channel blockers Ni2+, Co2+, L a3+, and Gd3+. However, inhibition was especially prominent under cond itions where the voltage-sensitive Ca2+ channels of Retzius neurones a re closed, so that these results indicate the existence of specific Mg 2+ influx pathways. For thermodynamic reasons a 1 Na+/1 Mg2+ antiport should reverse its mode of action in solutions with high Mg2+ content and contribute to Mg2+ influx, while a 2 or 3 Na+/1 Mg2+ antiport woul d still be able to extrude Mg2+ from the cell. As Mg2+ influx into Ret zius neurones was Na+-independent and hardly affected by amiloride, a contribution of Na+/Mg2+-antiport, if present at all, is very small. H owever, an inhibitory effect of amiloride was detectable when, in addi tion, Mg2+ influx was reduced by the application of Ni2+ or Co2+. This reduction of Mg2+ influx by amiloride indicates a 1:1 stoichiometry o f the Na+/Mg2+ antiport. (C) 1997 Elsevier Science Ltd.