The interaction of intracellular Mg2+ and pH on Cl- fluxes associated with intracellular pH regulation in barnacle muscle fibers

Modulation of Connexin-36 Gap Junction Channels by Intracellular pH and Magnesium Ions

Connexin-36 (Cx36) protein forms gap junction (GJ) channels in pancreatic beta cells and is also the main Cx isoform forming electrical synapses in the adult mammalian brain. Cx36 GJs can be regulated by intracellular pH (pHi) and cytosolic magnesium ion concentration ([Mg2+]i), which can vary significantly under various physiological and pathological conditions. However, the combined effect and relationship of these two factors over Cx36-dependent coupling have not been previously studied in detail. Our experimental results in HeLa cells expressing Cx36 show that changes in both pHi and [Mg2+]i affect junctional conductance (gj) in an interdependent manner; in other words, intracellular acidification cause increase or decay in gj depending on whether [Mg2+]i is high or low, respectively, and intracellular alkalization cause reduction in gj independently of [Mg2+]i. Our experimental and modelling data support the hypothesis that Cx36 GJ channels contain two separate gating mechanisms, and both are differentially sensitive to changes in pHi and [Mg2+]i. Using recombinant Cx36 we found that two glutamate residues in the N-terminus could be partly responsible for the observed interrelated effect of pHi and [Mg2+]i. Mutation of glutamate at position 8 attenuated the stimulatory effect of intracellular acidification at high [Mg2+]i, while mutation at position 12 and double mutation at both positions reversed stimulatory effect to inhibition. Moreover, Cx36*E8Q lost the initial increase of gj at low [Mg2+]i and double mutation lost the sensitivity to high [Mg2+]i. These results suggest that E8 and E12 are involved in regulation of Cx36 GJ channels by Mg2+ and H+ ions.

Intracellular Cl- dependence of Na-H exchange in barnacle muscle fibers under normotonic and hypertonic conditions

We previously showed that shrinking a barnacle muscle fiber (BMF) in a hypertonic solution (1,600 mosM/kg) stimulates an amiloride-sensitive Na-H exchanger. This activation is mediated by a G protein and requires intracellular Cl-. The purpose of the present study was to determine (a) whether Cl- plays a role in the activation of Na-H exchange under normotonic conditions (975 mosM/kg), (b) the dose dependence of [Cl-]i for activation of the exchanger under both normo- and hypertonic conditions, and (c) the relative order of the Cl-- and G-protein-dependent steps. We acid loaded BMFs by internally dialyzing them with a pH-6.5 dialysis fluid containing no Na+ and 0-194 mM Cl-. The artificial seawater bathing the BMF initially contained no Na+. After dialysis was halted, adding 50 mM Na+ to the artificial seawater caused an amiloride-sensitive pHi increase under both normo- and hypertonic conditions. The computed Na-H exchange flux (JNa-H) increased with increasing [Cl-]i under both normo- and hypertonic conditions, with similar apparent Km values ( approximately 120 mM). However, the maximal JNa-H increased by nearly 90% under hypertonic conditions. Thus, activation of Na-H exchange at low pHi requires Cl- under both normo- and hypertonic conditions, but at any given [Cl-]i, JNa-H is greater under hyper- than normotonic conditions. We conclude that an increase in [Cl-]i is not the primary shrinkage signal, but may act as an auxiliary shrinkage signal. To determine whether the Cl--dependent step is after the G-protein-dependent step, we predialyzed BMFs to a Cl--free state, and then attempted to stimulate Na-H exchange by activating a G protein. We found that, even in the absence of Cl-, dialyzing with GTPgammaS or AlF3, or injecting cholera toxin, stimulates Na-H exchange. Because Na-H exchange activity was absent in control Cl--depleted fibers, the Cl--dependent step is at or before the G protein in the shrinkage signal-transduction pathway. The stimulation by AlF3 indicates that the G protein is a heterotrimeric G protein.

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.

Extracellular Mg(2+)-dependent Na+, K+, and Cl- efflux in squid giant axons

An extracellular Na+ (Nao)-dependent Mg2+ efflux process that requires intracellular ATP has been proposed as the sole mechanism responsible for Mg2+ extrusion in internally dialyzed squid axons (12). We have shown that this exchanger can also "reverse" and mediate an extracellular Mg2+ (Mgo)-dependent Na+ efflux (16). We have extended these studies and found that, in the presence of ouabain, bumetanide, tetrodotoxin, and K+ channel blockers and in the absence of extracellular Na+, K+, and bicarbonate, intracellular K+ and Cl- are also involved in the Mgo-dependent Na+ efflux process. Two main observations support this view: 1) operation of the Mgo-dependent Na+ efflux requires the presence of intracellular K+ and Cl-, and 2) Mgo removal produces a reversible and nearly identical reduction in the magnitude of the simultaneous efflux of the ionic pairs K(+)-Na+ and Cl(-)-Na+. These results suggest that the putative bumetanide-insensitive Na-Mg exchanger also transports K+ and Cl-.

Extracellular magnesium-dependent sodium efflux in squid giant axons

Experiments were designed to determine whether the putative Na(+)-Mg2+ exchanger previously demonstrated to mediate Mg2+ efflux (R. DiPolo and L. Beagué. Biochim. Biophys. Acta 946: 424-428, 1988) could also mediate the efflux of Na+ (presumably a Na+ efflux-Mg2+ influx exchange) in squid giant axons. The effects of external Mg2+ (Mg(o)) on 22Na efflux were measured in internally dialyzed, ATP-fueled axons in which the contribution to Na+ efflux by other pathways was inhibited. To facilitate measurement of Mg(o)-dependent Na+ efflux, the intracellular concentration of Na+ was increased. To prevent Na(+)-Na+ exchange, external Na+ was replaced by tris(hydroxymethyl)aminomethane. To assess the effect of Mg(o) on Na+ efflux without altering the total divalent cation concentrations, Mg(o) was replaced mole-for-mole by external Ba2+ (Ba(o)). This manipulation produced reversible reductions in Na+ efflux. These reductions were neither due to membrane hyperpolarization nor to a direct effect of Bao but were due instead to the reduction in Mg(o). The Mg(o)-dependent Na+ efflux was inhibited by external amiloride but was spared by bumetanide. In the absence of external Na+, the Mgo-dependent Na+ efflux increased as a function of external Mg2+ with Michaelis-Menten kinetics. These results indicate that the Na(+)-Mg2+ exchange can mediate the efflux of Na+ (operate in Na+ efflux-Mg2+ influx mode of exchange).

A study of the ouabain-insensitive sodium efflux in barnacle muscle fibres using phorbol dibutyrate as a probe

1. The resting ouabain-insensitive Na+ efflux in muscle fibres isolated from the barnacle, Balanus nubilus, is stimulated by external or internal application of phorbol 12,13-dibutyrate (PD). The response occurs fairly promptly and may not decay at all, or more commonly, decay rather slowly. The magnitude of the response to external or internal application of PD is dose-dependent, the minimum effective concentration being about 10(-8) M. 2. The response to PD fails to occur in the nominal absence of external Ca2+. Sudden removal of external Ca subsequent to peak stimulation by PD leads to almost complete reversal of the response. The response to PD of fibres suspended in Li(+)-ASW (artificial sea water) is similar in magnitude to that of fibres suspended in Na(+)-ASW. However, it differs in that it is of a sustained nature. 3. Calcium channel blockers, e.g. verapamil, completely prevent the response to PD from occurring. Both Cd2+ and Co2+ are less effective than verapamil. 4. Pre- but not post-injection of EGTA reduces the response to PD. Pre- or post-injection of Mg2+ reduces the response considerably. 5. Fibres pre-injected with GTP show a reduced response to PD. Fibres pre-injected with PD show a reduced response to GTP. Pre-injection of protein kinase inhibitor is without effect on the response to PD. 6. Furosemide, piretanide and bumetanide are without effect on the response to PD. 7. DIDS (4,4'-diisothiocyanostilbene-2,2-disulphonic acid) is a potent inhibitor of the response to PD but not amiloride. Pyridoxal 5-phosphate and benzolamide are also powerful inhibitors. Pyridoxal 5-phosphate in combination with benzolamide fails to completely abolish or reverse the response to PD. 8. Luminescence from aequorin is promptly increased by PD in a dose-dependent manner, the minimal effective concentration being in the nanomolar range. The signal is monophasic or multiphasic in shape, and is often less than 5 min in duration. Not infrequently, however, the aequorin response fails to completely decay and the new level of resting glow remains above the original baseline level. 9. Collectively, these observations accord with a tentative general hypothesis stating that the stimulatory response of the ouabain-insensitive Na+ efflux to PD is triggered by two mechanisms. One involves a rise in myoplasmic free [Ca2+] resulting from the entry of external Ca2+ via opened Ca2+ channels which is followed by the operation of the Na(+)-Ca2+ exchanger in the reverse mode.(ABSTRACT TRUNCATED AT 400 WORDS)

Simultaneous bidirectional magnesium ion flux measurements in single barnacle muscle cells by mass spectrometry

Stable isotopes of Mg were used to measure bidirectional magnesium ion fluxes in single barnacle giant muscle fibers immersed in Ca- and Na-free, isosmotic media. Measurements were made using a mass spectrometric technique, thermal ionization mass spectrometry (TIMS), in conjunction with atomic absorption spectroscopy. Kinetic relations based on a first-order model were developed that permit the determination of unidirectional rate coefficients for Mg influx, ki, and efflux, ke, in the same experiment from knowledge of initial conditions and the initial and final ratios of 26Mg/24Mg and 25Mg/24Mg in ambient solutions (i.e., by isotope dilution). Such determinations were made for three values of the external Mg ion concentration: 5, 25, and 60 mM. At the concentration [Mg+2]o = 5 mM, ki and ke were about equal at a value of 0.01 min-1. At the higher values of [Mg+2]o, the values of ke increased along a curve suggesting saturation, whereas the values of ki remained essentially constant. As could be expected on the basis of a constant ki, the initial influx rate varied in direct linear proportion to [Mg+2]o, and was 11.8 pmol/cm2s when [Mg+2]o was 5 mM. However, the initial efflux rate appeared to increase nonlinearly with [Mg+2]o, varying from 13.4 pmol/cm2s ([ Mg+2]o = 5 mM) to approximately 80 pmol/cm2s ([ Mg+2]o = 60 mM). The results are consistent with a model that assumes Mg influx to be mainly an electrodiffusive inward leak with PMg = 0.07 cm/s and Mg efflux to be almost entirely by active transport processes. Where comparisons can be made, the rate coefficients determined from stable isotope measurements agree with those previously obtained using radioactive Mg. The rate coefficients can be used to correctly predict time-dependent changes in total fiber Mg content. The results support the conclusion that nonradioactive tracers can be used to measure ion fluxes and ion flux ratios in excitable cells; it is expected that this method will greatly assist in the study of Mg regulation in general.

Cytosolic free magnesium concentration in cultured chick heart cells

Cytosolic free magnesium (Mgi) was measured in embryonic chick heart cells loaded with one of two newly developed 19F nuclear magnetic reasonance (NMR)-sensitive magnesium chelators, 4-methyl,5-fluoro-2-aminophenol-N,N,O-triacetate (MF-APTRA) and 5-fluoro-2-aminophenol-N,N,O-triacetate (5F-APTRA). The cells, embedded in strands of collagen, were superfused at a rate that allowed for solution changes in 2 min. In this preparation 19F- and 31P-NMR spectra were stable for at least 3.5 h. Because Na-coupled Mg countertransport may be a possible mechanism of Mg transport, in some experiments extracellular Na was reduced to 1 mM (choline substituted). This manipulation caused a 2.5-fold increase in Mgi from the basal level of 0.56 mM. A significant proportion of this increase in Mgi could be secondary to an increase in Cai that occurs with low extracellular Na (Nao) perfusion (Nai-Cao exchange). Perfusing cells with nominally Ca-free, 1 mM Na salt solution substantially attenuated the increase in Mgi that occurred with Ca present (1.25 mM) in the low Na (1 mM) solution. Furthermore, perfusion with 1 mM Na, Mg-free salt solution caused a 1.5-fold increase in Mgi, which cannot be attributable to Nai-Mgo exchange. Therefore attempts to describe the regulation of Mgi in heart cells must differentiate between the effects of Nai-Mgo exchange and competition for binding sites that are secondary to stimulation of ion gradient-coupled mechanisms.