Multi-barrelled ion-sensitive microelectrodes and their application in micro-droplets and biological systems

Quantification of Mg2+ extrusion and cytosolic Mg2+-buffering in Xenopus oocytes

Intracellular Mg(2+) buffering and Mg(2+) extrusion were investigated in Xenopus laevis oocytes. Mg(2+) or EDTA were pressure injected and the resulting changes in the intracellular Mg(2+) concentration were measured simultaneously with Mg(2+)-selective microelectrodes. In the presence of extracellular Na(+), injected Mg(2+) was extruded from the oocytes with an estimated v(max) and K(M) of 74 pmol cm(-2)s(-1) and 1.28 mM, respectively. To investigate genuine cytosolic Mg(2+) buffering, measurements were carried out in the nominal absence of extracellular Na(+) to block Mg(2+) extrusion, and during the application of CCCP (inhibiting mitochondrial uptake). Under these conditions, Mg(2+) buffering calculated after both MgCl(2) and EDTA injections could be described by a buffer equivalent with a concentration of 9.8mM and an apparent dissociation constant, K(d-app), of 0.6mM together with an [ATP](i) of 0.9 mM with a K(d-app) 0.12 mM. Xenopus oocytes thus possess highly efficient mechanisms to maintain their intracellular Mg(2+) concentration.

Mg2+-Malate Co-Transport, a Mechanism for Na+-Independent Mg2+Transport in Neurons of the LeechHirudo medicinalis

Mg2+-extrusion from Mg2+-loaded neurons of the leech, Hirudo medicinalis, is mediated mainly by Na+/Mg2+antiport. However, in a number of leech neurons, Mg2+is extruded in the nominal absence of extracellular Na+, indicating the existence of an additional, Na+-independent Mg2+transport mechanism. This mechanism was investigated using electrophysiological and microfluorimetrical techniques. The rate of Na+-independent Mg2+extrusion from Mg2+-loaded leech neurons was found to be independent of extracellular Ca2+, K+, NO3−, HCO3−, SO42−, HPO42−, and of intra- and extracellular pH. Na+-independent Mg2+extrusion was not inhibited by 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS), furosemide, ouabain, vanadate, iodoacetate, 4-amino-hippurate, or α-cyano-4-hydroxycinnamate and was not influenced by changes in the membrane potential in voltage-clamp experiments. Na+-independent Mg2+extrusion was, however, inhibited by the application of 2 mM probenecid, a blocker of organic anion transporters, suggesting that Mg2+might be co-transported with organic anions. Extracellularly, of all organic anions tested (malate, citrate, lactate, α-ketoglutarate, and 4-amino-hippurate) only high, but physiological, concentrations of malate (30 mM) had a significant inhibitory effect on Na+-independent Mg2+extrusion. Intracellularly, iontophoretically injected malate, citrate, or fura-2, but not Cl−, α-ketoglutarate, glutamate, succinate, or urate, were stimulating Na+-independent Mg2+extrusion from those neurons that initially did not extrude Mg2+in Na+-free solutions. Our data indicate that Mg2+is co-transported with organic anions, preferably with malate, the predominant extracellular anion in the leech. The proposed model implies that, under experimental conditions, malate drives Mg2+extrusion, whereas under physiological conditions, malate is actively taken up, driven by Mg2+, so that malate can be metabolized.

Activation of AMPA/kainate receptors but not acetylcholine receptors causes Mg2+ influx into Retzius neurones of the leech Hirudo medicinalis

In Retzius neurones of the medicinal leech, Hirudo medicinalis, kainate activates ionotropic glutamate receptors classified as AMPA/kainate receptors. Activation of the AMPA/kainate receptor-coupled cation channels evokes a marked depolarization, intracellular acidification, and increases in the intracellular concentrations of Na+ ([Na+]i) and Ca2+. Qualitatively similar changes are observed upon the application of carbachol, an activator of acetylcholine receptor-coupled cation channels. Using multibarrelled ion-selective microelectrodes it was demonstrated that kainate, but not carbachol, caused additional increases in the intracellular free Mg2+ concentration ([Mg2+]i). Experiments were designed to investigate whether this kainate-induced [Mg2+]i increase was due to a direct Mg2+ influx through the AMPA/kainate receptor-coupled cation channels or a secondary effect due to the depolarization or the ionic changes. It was found that: (a) Similar [Mg2+]i increases were evoked by the application of glutamate or aspartate. (b) All kainate-induced effects were inhibited by the glutamatergic antagonist DNQX. (c) The magnitude of the [Mg2+]i increases depended on the extracellular Mg2+ concentration. (d) A reduction of the extracellular Ca2+ concentration increased kainate-induced [Mg2+]i increases, excluding possible Ca2+ interference at the Mg2+-selective microelectrode or at intracellular buffer sites. (e) Neither depolarizations evoked by the application of 30 mM K+, nor [Na+]i increases induced by the inhibition of the Na+/K+ ATPase caused comparable [Mg2+]i increases. (f) Inhibitors of voltage-dependent Ca2+ channels did not affect the kainate-induced [Mg2+]i increases. Moreover, previous experiments had already shown that intracellular acidification evoked by the application of 20 mM propionate did not cause changes in [Mg2+]i. The results indicate that kainate-induced [Mg2+]i increases in leech Retzius neurones are due to an influx of extracellular Mg2+ through the AMPA/kainate receptor-coupled cation channel. Mg2+ may thus act as an intracellular signal to distinguish between glutamatergic and cholinergic activation of leech Retzius neurones.

Apparent intracellular Mg2+ buffering in neurons of the leech Hirudo medicinalis

The apparent intracellular Mg2+ buffering, or muffling (sum of processes that damp changes in the free intracellular Mg2+ concentration, [Mg2+](i), e.g., buffering, extrusion, and sequestration), was investigated in Retzius neurons of the leech Hirudo medicinalis by iontophoretic injection of H+, OH-, or Mg2+. Simultaneously, changes in intracellular pH and the intracellular Mg2+, Na+, or K+ concentration were recorded with triple-barreled ion-selective microelectrodes. Cell volume changes were monitored measuring the tetramethylammonium (TMA) concentration in TMA-loaded neurons. Control measurements were carried out in electrolyte droplets (diameter 100-200 microm) placed on a silver wire under paraffin oil. Droplets with or without ATP, the presumed major intracellular Mg2+ buffer, were used to quantify the pH dependence of Mg2+ buffering and to determine the transport index of Mg2+ during iontophoretic injection. The observed pH dependence of [Mg2+](i) corresponded to what would be expected from Mg2+ buffering through ATP. The quantity of Mg2+ muffling, however, was considerably larger than what would be expected if ATP were the sole Mg2+ buffer. From the decrease in Mg2+ muffling in the nominal absence of extracellular Na+ it was estimated that almost 50% of the ATP-independent muffling is due to the action of Na+/Mg2+ antiport.