Sodium‐magnesium antiport in Retzius neurones of the leech Hirudo medicinalis.

1. Intracellular free magnesium ([Mg2+]i) and sodium ([Na+]i) concentrations were measured in Retzius neurones of the leech Hirudo medicinalis using ion‐sensitive microelectrodes. 2. The mean steady‐state values for [Mg2+]i and [Na+]i were 0.46 mM (pMg, 3.34 +/‐ 0.23; range, 0.1‐1.2 mM; n = 32) and 8.95 mM (pNa, 2.05 +/‐ 0.15; range, 5.1‐15.5 mM, n = 21), respectively, at a mean membrane potential (Em) of ‐35.6 +/‐ 6.1 mV (n = 32). Thus, [Mg2+]i is far below the value calculated for a passive distribution (16.9 mM) but close to the equilibrium value calculated for a hypothetical 1 Na(+)‐1 Mg2+ antiport (0.41 mM). 3. Simultaneous measurements of [Mg2+]i, [Na+]i and Em in Retzius neurones showed that an increase in the extracellular Mg2+ concentration ([Mg2+]o) resulted in an increase in [Mg2+]i, a parallel decrease in [Na+]i and a membrane depolarization, while a decrease in [Mg2+]o had opposite effects. These results are compatible with calculations based on a 1 Na(+)‐1 Mg2+ antiport. 4. Na+ efflux at high [Mg2+]o still occurred when the Na(+)‐K+ pump was inhibited by the application of ouabain or in K(+)‐free solutions. This efflux was blocked by amiloride. 5. In the absence of extracellular Na+ ([Na+]o), no Mg2+ influx occurred. Mg2+ influx at high [Mg2+]o was even lower than in the presence of [Na+]o. Mg2+ efflux was blocked in the absence of [Na+]o. 6. The rate of Mg2+ extrusion was reduced by lowering [Na+]o, even if the Na+ gradient across the membrane remained almost unchanged. 7. Mg2+ efflux was blocked by amiloride (half‐maximal effect at 0.25 mM amiloride; Hill coefficient, 1.3) but not by 5‐(N‐ethyl‐N‐isopropyl)‐amiloride (EIPA). 8. No changes in intracellular Ca2+ and pH (pHi) could be detected when [Mg2+]o was varied between 1 and 30 mM. 9. Changing pHi by up to 0.4 pH units had no effect on [Mg2+]i. 10. The results suggest the presence of an electrogenic 1 Na(+)‐1 Mg2+ antiport in leech Retzius neurones. This antiport can be reversed and is inhibited by low extracellular and/or intracellular Na+ and by amiloride.