Na+-dependent regulation of the free Mg2+ concentration in neuropile glialcells and P neurones of the leech Hirudo medicinalis

To investigate the Mg2+ regulation in neuropile glial (NG) cells and pressu re (P) neurones of the leech Hirudo medicinalis the intracellular free Mg2 ([Mg2+](i)) and Na+ ([Na+](i)) concentrations, as well as the membrane pot ential (E-m), were measured using Mg2+- and Na+-selective microelectrodes. The mean steady-state values of [Mg2+](i) were found to be 0.91 mM (mean E- m = -63.6 mV) in NG cells and 0.20 mM (mean E-m = -40.6 mV) in P neurones w ith a [Na+](i) of 6.92 mM (mean E-m = - 61.6 mV) and 7.76 mM (mean E-m = -3 8.5 mV), respectively. When the extracellular Mg2+ concentration ([Mg2+](o) ) was elevated, [Mg2+](i) in P neurones increased within 5-20 min whereas i n NG cells a [Mg2+](i) increase occurred only after long-term exposure (6 h ). After [Mg2+](o) was reduced back to 1 mM, a reduction of the extracellul ar Na+ concentration ([Na+](o)) decreased the inwardly directed Na+ gradien t and reduced the rate of Mg2+ extrusion considerably in both NG cells and P neurones. In P neurones Mg2+ extrusion was reduced to 15.4% in Na+-free s olutions and to 6.0% in the presence of 2 mM amiloride. Mg2+ extrusion from NG cells was reduced to 6.2% in Na+-free solutions. The results suggest th at the major [Mg2+](i)-regulating mechanism in both cell types is Na+/Mg2antiport. In P neurones a second, Na+-independent Mg2+ extrusion system may exist.