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

 To investigate the Mg2+ regulation in neuropile glial (NG) cells and pressure (P) neurones of the leech Hirudo medicinalis the intracellular free Mg2+ ([Mg2+]i) and Na+ ([Na+]i) concentrations, as well as the membrane potential (Em), were measured using Mg2+- and Na+-selective microelectrodes. The mean steady-state values of [Mg2+]i were found to be 0.91 mM (mean Em=–63.6 mV) in NG cells and 0.20 mM (mean Em=–40.6 mV) in P neurones with a [Na+]i of 6.92 mM (mean Em=–61.6 mV) and 7.76 mM (mean Em=–38.5 mV), respectively. When the extracellular Mg2+ concentration ([Mg2+]o) was elevated, [Mg2+]i in P neurones increased within 5–20 min whereas in 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 extracellular Na+ concentration ([Na+]o) decreased the inwardly directed Na+ gradient 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 solutions 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 that the major [Mg2+]i-regulating mechanism in both cell types is Na+/ Mg2+ antiport. In P neurones a second, Na+-independent Mg2+ extrusion system may exist.