Sodium-dependent recovery of ionised magnesium concentration following magnesium load in rat heart myocytes

Abstract

Our objectives were to investigate regulation of intracellular ionised Mg2+ concentration ([fMg2+]i) in cardiac muscle and cardiac Na+/Mg2+ antiport stoichiometry. [fMg2+]i was measured at 37 degrees C in isolated rat ventricular myocytes with mag-fura-2. Superfusion of myocytes with Na+ and Ca2+ free solutions containing 30 mM Mg2+ for 15 min more than doubled [fMg2+]i from its basal level (0.75 mM). Re-addition of Na+ caused [fMg2+]i to fall exponentially with time to basal level, the rate increasing linearly with [Na+]. Log(recovery rate) increased linearly with log([Na+]), the slope of 1.06 (95% confidence limits, 0.94-1.17) suggesting one Na+ ion is exchanged for each Mg2+. [fMg2+]i recovery was complete even if the membrane potential was depolarised to 0 mV or if superfusate [Mg2+] was increased to 3 mM. Recovery was rapid in normal Tyrode (0.3 min(-1)) with a Q10 of 2.2. It was completely inhibited by 200 microM imipramine but was unaffected by 20 microM KB-R7943 or 1 microM SEA0400, suggesting the Na+ /Ca2+ antiporter is not involved. Membrane depolarisation by increasing superfusate [K+] to 70 mM, or voltage clamp to 0 mV, increased recovery rate in Na+ containing solutions more than threefold. We conclude [fMg2+]i recovery is by Mg2+ efflux on a 1 Na+:1 Mg2+ antiport.

KB-R7943 inhibits Na+-dependent Mg2+ efflux in rat ventricular myocytes

“…The results of this study are apparently in contrast with those reported by Almulla et al [19], who found little effect of 20 lM KB-R7943 on the time constants of the Na ? -dependent fall in [Mg 2? ]…”

“…efflux was significantly accelerated by membrane depolarization from -80 to -0 mV, although the effect was not significant at -40 mV. Almulla et al [19] also found significant facilitation of Mg 2? efflux by depolarization from -80 to 0 mV at 37°C.…”

“…-loaded rat ventricular myocytes at 37°C. It should be noted, however, that Almulla et al [19] loaded the cells with Mg 2? to reach, on average, 4.8 mM [Mg 2? ]…”

“…-dependent Mg 2? efflux [3,19], a part of the inhibitory effect of KB-R7943 described above may be caused by a change in membrane potential, i.e., hyperpolarization, rather than the drug's direct effect on the transport. To examine this possibility, the myocytes were patch-clamped, and the membrane potential was measured under the current-clamp mode (Fig.…”

Human gene SLC41A1 encodes for the Na+/Mg2+ exchanger

“…Particularly, the free intracellular [Mg 2ϩ ] ([Mg 2ϩ ] i ) is a cofactor for enzymes and signal-transduction proteins and regulates bioenergetics, ion transport, growth, and proliferation (18,28,29,49). Thus optimal [Mg 2ϩ ] i values between 0.5 and 1.2 mM (1,6,37,39) have to be maintained under various conditions. It is long thought that a key element in this process is regulated Mg 2ϩ efflux either via an anion-dependent mechanism (36) or a putative Na ϩ /Mg 2ϩ exchanger (28).…”