SODIUM-MAGNESIUM ANTIPORT IN RETZIUS NEURONS OF THE LEECH HIRUDO-MEDICINALIS

1. Intracellular free magnesium ([Mg2+](i)) and sodium ([Na+](i)) conc entrations were measured in Retzius neurones of the leech Hirudo medic inalis using ion-sensitive microelectrodes. 2. The mean steady-state v alues for [Mg2+](i) and [Na+](i) were 0.46 mM (pMg, 3.34 +/- 0.23; ran ge, 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 (E(m)) of - 35.6 +/- 6.1 mV (n = 32). Thus, [Mg2+](i) is far below the value calcu lated for a passive distribution (16.9 mM) but close to the equilibriu m value calculated for a hypothetical 1 Na+-1 Mg2+ antiport (0.41 mM). 3. Simultaneous measurements of [Mg2+](i), [Na+](i) and E(m) in Retzi us neurones showed that an increase in the extracellular Mg2+ concentr ation ([Mg2+](o)) resulted in an increase in [Mg2+](i), a parallel dec rease in [Na+](i) and a membrane depolarization, while a decrease in [ Mg2+](o) had opposite effects. These results are compatible with calcu lations 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 applicatio n of ouabain or in K+-free solutions. This efflux was blocked by amilo ride. 5. In the absence of extracellular Na+ ([Na+](o)), no Mg2+ influ x occurred. Mg2+ influx at high [Mg2+](o) was even lower than in the p resence 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), eve n 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 (pH(i)) c ould be detected when [Mg2+](o) was varied between 1 and 30 mM. 9. Cha nging pH(i) 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 inhibi ted by low extracellular and/or intracellular Na+ and by amiloride.