Action Of Mg2 Research Articles

The blocking effect of magnesium on the secretion of adrenal catecholamines induced by the omission of sodium from the extracellular medium.

The blocking action of Mg++ on catecholamine release induced by the substitution of extracellular Na+ by an osmotic equivalent amount of sucrose was studied in isolated, perfused bovine adrenal glands. Perfusing glands with 10 mM Mg++ produced at 51.1% inhibition on catecholamine release evoked by Na+ omission. Increasing the concentration of Mg++ to 20 mM this inhibitory effect was enhanced to 90.3%. D-600 (0.3 mM) promoted a marked blockade of acetylcholine-induced release of catechol hormones that was partially and significantly reverted increasing the concentration of Ca++ in the perfusion medium. D-600 (0.3 mM) failed to inhibit the catecholamine-releasing effect of Na+ deprivation. In adrenal glands previously perfused with D-600 (0.3 mM) and then exposed to a Locke solution containing D-600 (0.3 mM) + Mg++ (10 or 20 mM) the inhibition of the secretory responses evoked by the omission of Na+ was of the same magnitude as that obtained when the glands were perfused with Mg++ (10 or 20 mM) in the absence of D-600. These results are compatible with the view that the blocking effect of Mg++ may involve an intracellular site of action and that the access of Mg++ into the chromaffin cell may not be mediated through the Ca++ channels.

The interaction of spectrin-actin and synthetic phospholipids. II. The interaction with phosphatidylserine

Sonicated vesicles of phosphatidylserine and phosphatidylserine/phosphatidylcholine mixtures were recombined with spectrin-actin from human erythrocyte ghosts. Morphological properties and physicochemical characteristics of the recombinates were studied with freeze etch electron microscopy, 31P NMR and differential scanning calorimetry. Sonicated dimyristoyl phosphatidylserine vesicles show a decrease in enthalpy change of the lipid phase transition upon addition of spectrin-actin. These vesicles collapse and fuse, into multilamellar structures in the presence of spectrin-actin, as demonstrated by freeze fracturing and NMR. Spectrin-actin cannot prevent the salt formation between phosphatidylserine and Ca2+, all phosphatidylserine is withdrawn from the lipid phase transition. In contrast a protection against the action of Mg2+ could be observed. Mixed bilayers of dimyristoyl phosphatidylserine/dimyristoyl phosphatidylcholine show phase separations at molar ratios above 1/1 (van Dijck, P.W.M., de Kruijff, B., Verkleij, A.J., van Deenen, L.L.M. and de Gier, J. (1978) Biochim. Biophys. Acta 512, 84--96). These phase spearations can be prevented by spectrin-actin. Ca2+-induced lateral phase separations in cocrystallizing phosphatidylserine/phosphatidylcholine mixtures, can be reduced by spectrin-actin. Formation of the Ca2+-phosphatidylserine salt, occurring in addition to lateral phase separation when mixtures contain more than 30 mol % phosphatidylserine, cannot be prevented by spectrin-actin.

Action of Mg 2+ at low concentrations on the response to habituating stimuli and spontaneous activity of frog spinal cord motoneurons

Action of Mg 2+ at low concentrations on the response to habituating stimuli and spontaneous activity of frog spinal cord motoneurons

Magnesium-phenethylbiguanide competition in Mg 2+-depleted hepatic mitochondria

Guinea pig liver mitochondria 40–70 per cent depleted of magnesium are three times more sensitive to phenethylbiguanide inhibition of state 3 respiration than are comparable controls. Similar results are obtained whether the depletion is accomplished by the divalent cation ionophore A23187 or by incubation with inorganic phosphate. In contrast to controls, inhibition in Mg-depleted mitochondria is competitively reversed by Mg 2+. These phenomena are observed with NAD + -linked substrates, stimulated by either ADP or Ca 2+, but not with succinate. The Mg-depleted mitochondria can be restored to the normal degree of phenethylbiguanide sensitivity by incubation with high levels of Mg 2+ followed by washing. As in controls, phenethylbiguanide inhibition in Mg-repleted mitochondria is not reversed by Mg 2+. These results suggest a dual action of Mg 2+: (1) a structural role of bound magnesium which regulates the activity of cation transporters, and (2) competition between Mg 2+ in solution and guanidinium derivatives for a cation carrier.

Changes in the effects of Mg++-ions on mitochondrial respiration after wounding of potato tubers

Summary Mg ++ ions have two effects on mitochondrial NADH-consuming respiration: the first is a stimulation of state III respiration. This effect decreases after wounding and subsequent incubation of the tissue. The site of action of Mg ++ is at the external NADH dehydrogenase; the decrease in stimulation after wounding is probably not due to an increase in the Mg ++ concentration in the intermembrane space. The second effect of Mg ++ on NADH-consuming respiration is a reduction of the R.C. ratio to about 1, this effect being found only with mitochondria from wounded tissue. The same result is found with respect to succinate-consuming respiration when NAD is added to the reaction mixture. This effect of Mg ++ is not due to an uncoupling of respiration or a stimulation of ATP-ase activity. Following the addition of Mg ++ (and NAD), a net production of ADP and ATP was found to have taken place in state II prior to the addition of external ADP. This is the consequence of the formation of AMP in the preparation, whereby the presence of Mg ++ is necessary for the stimulation of the activity of adenylate kinase, which converts the AMP into ADP. The origin of the formation of AMP is located outside the true mitochondria, as the compound is not found following the purification of the mitochondria on a sucrose gradient. NAD-breakdown cannot account for the AMP formed. It is possible that AMP is produced as a result of the degradation of particulate, (poly)ribosomal RNA present in the preparations after wounding of the tissue.

Na'sites of the ( [formula omitted])-dependent ATPase

The concentration of NaCl for half-maximal activation of the ( Na + + K + )-dependent ATPase (ATP phosphohydrolase, EC 3.6.1.3) the K 0.5 for Na +, decreased as the concentration of KCl was reduced, as if K + competed with Na +, Extrapolation to the absence of KCl gave a K 0.5 for Na + of 1.7 mM and a K i for K + of 5.1 mM. The K 0.5 for Na + also decreased with the Mg · ATP concentration, and extrapolation to the absence of ATP gave a K 0.5 for Na + of 0.26 mM and a K i for K + of 0.85 mM; the calculated K i for Mg-ATP toward Na + and K + was 0.5–0.6 mM, in accord with an action of Mg · ATP through the low-affinity substrate sites of the enzyme. A plot of −log K 0.5 for Na + against pH suggested that a group with a pK a near 8.1 influenced activation by Na +. The K 0.5 for KCl as an activator of the ATPase, through “β-sites” for K +, varied with the NaCl concentration, with an extrapolated K 0.5 for K + of 0.11 mM and a K i for Na + of 14 mM. However, the K 0.5 for KCl as an activator of the related K +-dependent phosphatase reaction of the enzyme, through “α-sites” for K +, was 2.0 mM; the K i for NaCl as a competitor was 5.5 mM. These latter values also decreased with substrate concentration, and extrapolation to the absence of substrate gave a K 0.5 for K + of 1 mM and a K i for Na + of 3.5 mM. The Na + sites and the α-sites for K + could also be distinguished in terms of the changes in their kinetic parameters in the presence of 10% dimethylsulfoxide or 0.1 mM tetraphenyl boron, although lowering the incubation temperature from 37 to 23° C generally reduced all values similarly. These results indicate that the Na + sites are distinguishable from both classes of K + sites; a formulation relating internal Na + sites with external α-sites for K +, in a flip-flop dimeric transport model, is suggested.

心筋細胞膜のカルシウム結合とATPase活性

Cell membrane was isolated from porcine ventricles and its Ca2+ binding and ATPase activities were investigated. The cell membrane (heart sarcolemma) exhibits Na+-K+activated ATPase and Ca2+ stimulated. Mg-ATPase activities which suggest the presence of “Ca-pump”system.Ca2+ binding to the cell membrane was dependent on Ca2+ concentration and analysis of binding curve revealed that the existence of two kinds of binding sites (high and low affinity sites). Mg2+ increased Ca2+ binding at the high affinity site whereas Na+ decreased it. These actions of Mg2+ and Na+ may be physiologically important for the maintenance and transient change of intracellular Ca2+ concentration at rest and excitation of the membrane

Calcium dependent action potentials in skeletal muscle fibres of a beetle larva, Xylotrupes dichotomus

The ionic requirement for generating action potentials in ventral longitudinal muscle fibers dissected from beetle larvae was examined by conventional electrophysiological techniques. Muscle fibers that generated only graded responses in physiological saline were able to generate an all-or-none action potential when the potassium permeability of the membrane was inhibited by tetraethylammonium + added to the saline. The peak of the action potential thus elicited was intimately related to the external Ca ++ concentration. The action potential was blocked by Co ++ which is known as a competitive inhibitor of Ca-spikes. Neither tetrodotoxin (3 μM) nor a Na-free condition effectively blocked the generation of the action potential. Mg ++ induced a shift in the peak of the action potential; this was, however, due to the stabilizing action of Mg ++ but not due to the penetration of Mg ++ through the muscle membrane. No action potential was elicited in the muscle fiber when immersed in a Ca-free, EGTA saline even when a high concentration of either Mg ++, Na +, or tetraethylammonium + was present. The action potential of the larval muscle fiber was thus concluded to be a Ca-spike, through the channel of which Na + or Mg ++ did not penetrate.

Iodide quenching of tryptophan fluorescence in phosphorylase kinase

1. 1. The quenching of tryptophan fluorescence by iodide in rabbit muscle phosphorylase kinase was investigated. 2. 2. It has been shown that the action of Mg 2+-ATP brought a change in the Stern-Volmer constant. 3. 3. The pH influence of tryptophan fluorescence have been demonstrated.

Heterogeneity in the fluidity of intact erythrocyte membrane and its homogenization upon hemolysis

Intact erythrocytes were spin-labeled with various classes of phospholipid label. The ESR spectrum for phosphatidylcholine spin label was distinctly different from those for phosphatidylserine, phosphatidylethanolamine, phosphatidylglycerol and phosphatidic acid spin labels. The overall splitting for the former (52.5 G) was markedly larger than those for the others (approx. 47 G), suggesting a more rigid phosphatidylcholine bilayer phase and more fluid phosphatidylethanolamine and phosphatidylserine phases in the erythrocyte membrane. Evidence for asymmetric distribution of phospholipids in the membrane was obtained. Spin-labeled phosphatidylcholine incorporated into erythrocytes was reduced immediately by cystein and Fe 3+, while the reduction of spin-labeled phosphatidylserine was very slow. The present results therefore suggest asymmetric fluidity in erythrocyte membrane; a more rigid outer layer and a more fluid inner layer. The heterogeneity in the lipid structure was also manifested in the temperature dependence of the fluidity. The overall splitting for phosphatidylcholine spin label showed two inflection points at 18 and 33 °C, while that for phosphatidylserine spin label had only one transition at 30 °C. When the spin-labeled erythrocytes were hemolyzed, the marked difference in the ESR spectra disappeared, indicating homogenization of the heterogeneous fluidity. Mg 2+ or Mg 2+ + ATP prevented the hemolysis-induced spectral changes. Ca 2+ did not prevent the homogenization and acted antagonistically to Mg 2+. The heterogeneity preservation by Mg 2+ was nullified by trypsin, pronase or N- ethylmaleimide added inside the cell. Some inner proteins may therefore be involved in maintaining the heterogeneous structure. The protecting action of Mg 2+ was dependent on hemolysis temperature, starting to decrease at 18 °C and vanishing at 40 °C. The present study suggests that the heterogeneity in the fluidity of intact erythrocyte membranes arises from interactions between lipids and proteins in the membrane and also from interactions between the membrane constituents and the inner proteins. Concentration of cholesterol in the outer layer may also partly contribute to the heterogeneity.

Effect of inorganic salt additives on the viscosity of cellulose acetate solutions

A study of the effect of Li, Na, Mg, Ca, Sr, Ba, La perchlorates and Li chloride and nitrate on the viscosity of 22% secondary cellulose acetate (CA) solutions in acetone, aqueous acetone mixtures and DMF shows that on increasing the amount of salt dissolved, the higher the valency of the cation, the greater the extent to which solution viscosity increases. This is the result of the formation of donor-acceptor bonds between cations of salts and donor solvents and between cations and donor groups of the CA chain; as a consequence solution cross linking increases. The anions are unable to provide solvated complexes with donor solvents (DMF and acetone) and therefore have no effect in practice on the viscosity of CA solutions in these solvents. In the presence in the solution of fairly large amounts of water the dependence of solution viscosity on the amount of salt added is expressed by a curve with a maximum, which is due to the competing ordering action of Mg 2+ cations and the disorganizing effect of perchlorate of the anion.

The comparative effects of (Ca2+) and (Mg2+) on on tension generation in the fibers of skinned frog skeletal muscle and mechanically disrupted rat ventricular cardiac muscle.

The effects of intracellular Mg2+ on Ca2+-activated isometric tension generation in rat cardiac muscle fibers and frog skeletal muscle were compared. The membranous sarcolemmal barrier was removed from rat cardiac muscle fibers by mechanical disruption and from frog skeletal muscle by skinning. Tension was recorded in the fibers in bathing solutions of different Ca2+ concentrations and either 5 X 10(-5) M or 1 X 10(-3) M Mg2+ concentration (the same concentrations used in a previous study on single skinned frog skeletal muscle fibers [3]). The amount of Ca2+ required to activate the muscle increased with Mg2+ concentration for both rat ventricular muscle and frog skeletal muscle. These data indicate that intracellular Mg2+ concentration could strongly modulate Ca2+-activated tension in cardiac muscle and that very similar molecular mechanisms are responsible for Ca2+-activated tension in rat ventricular muscle and frog skeletal muscle. The possible sites of action of Mg2+ on Ca2+-activated tension are discussed.

The effect of magnesium at motoneurons of the isolated spinal cord of the frog

The action of Mg on membrane excitability and on synaptic transmission at motoneurons of the isolated spinal cord of the frog has been studied. Impulse propagation along presynaptic fibers and antidromically elicited spikes at motoneurons were unaffected by [Mg] in the concentration range of 0–11 mM. This range of [Mg] also had no effect on motoneuron input impedance; however, intracellular current intensities required for spike generation were increased with increasing [Mg]. Iontophoretic application of Mg near intracellular recording sites reversibly depressed elicited PSPs. With 5–11 m M [Mg] and 1 m M [Ca], synaptic transmission was attenuated, but was blocked if the [Ca] was lowered to 0.1 m M [Ca]. In 4 cells, iontophoretic application of Mg caused a reversible increase in PSP amplitudes which was associated with membrane hyperpolarization. We interpret this effect to be due to suppression of tonic excitatory synaptic input. High concentrations of Mg had no effect on the frequency of spontaneously occurring synaptic potentials recorded in the presence of tetrodotoxin.

Effet de l'ion Mg 2+ sur la croissance des têtards de Discoglossus pictus (OTTH) et sur la capacite de leurs microsomes a synthetiser les proteines in vitro

1. 1. Inhibition of growth of tadpoles was induced by an excess or lack of Mg 2+ ions in the breeding medium (mineral solutions). It was correlated with a decrease in the ability of the isolated microsomes to synthesize proteins. 2. 2. The action of Mg 2+ ions added in vitro to the incubation system suggested that the inhibition of protein synthesis was due to an excess or a lack of microsomal Mg 2+ content. 3. 3. The comparative action of Mg 2+ and Mn 2+ ions has been studied. Mn 2+ ions were comparable to Mg 2+ ions but at concentrations ten- or hundredfold less. 4. 4. Preliminary experiments dealing with the action of Mg 2+ ions on the formation of leucyl-tRNA were made.

ADP binding to (Na + + K +)-activated ATPase

[ 14C]ADP binding to EDTA-washed ox brain cell membranes was increased by Na +, but decreased by K +, Mg 2+ and Ca 2+. Na + abolished the effect of K + on ADP binding by a competitive mechanism, but could not reverse the inhibitory action of Mg 2+ and Ca 2+. It is concluded that the cation-induced changes in ADP binding reflect properties of (Na + + K +)-activated ATPase.

Effect of divalent cations on the interaction of adenosine 5'-triphosphate (ATP) with histamine release induced by compound 48-80.

AbstractThe inhibition exerted by ATP on histamine release caused by compound 48/80 from isolated rat mast cells was counteracted by Ca++, Mg++, Ba++, and Sr++, but not by Be++. In addition, after the inhibitory effect had been obtained by pretreatment of mast cells with ATP, addition of Mg++ restored the sensitivity of the cells towards 48/80. The spontaneous restoration of the sensitivity of the mast cells towards 48/80 observed after prolonged pretreatment of the cells with ATP (< 10 μM.) was enhanced by the presence of glucose. Glucose did not effect the inhibition per se. EDTA blocked 1) the protecting effect of divalent cations on the action of ATP on mast cells in regard to their sensitivity towards 48/80, 2) the action of Mg++ to restore the sensitivity of mast cells towards 48/80 after rendered insensitive by treatment with ATP, 3) the spontaneous restoration of the sensitivity of ATP‐treated mast cells towards 48/80 observed after prolonged incubation alone, as well as the enhanced restoration observed in the presence of glucose. The results suggest an interaction between divalent cations and ATP on the plasma membrane.

Effects of magnesium on calcification of young bone in tissue culture.

1. Incubation of calvaria from new-born mice for 73 hours with low-Mg medium (0.08 mM Mg) resulted in a progressive reduction in the medium concentration of Ca, as compared with controls (0.90 mM Mg), when the initial medium concentration of Ca was 1.60 mM. The low-Mg medium also increased the uptake of45Ca into bones. At 0.16 mM Ca a net release of Ca from the bones took place, and the release of Ca was reduced by the low-Mg medium. These changes were observed with both living bones and bones killed by heat-inactivation. 2. Short-time45Ca-efflux experiments carried out with a superfusion system revealed an immediate inhibitory effect of low-Mg medium on the release of45Ca incorporated into bones of new-born mice 2–7 days earlier. This was not seen in old animals 1–2 hours after labelling. Conversely, 5 mM Mg increased the efflux of45Ca in the latter, but not in the former situation. 3. It is concluded that the inhibitory action of Mg in physiological concentrations on the calcification of calvaria from young mice is basically of a physico-chemical nature and that it bears some relevance to living conditions. Apparantly Ca in bone exists in at least two phases which are different with respect to their reactivity to Mg. The results suggest that Mg in the bones of young animals inhibits the transformation of amorphous calcium phosphate to the relatively Ca-rich hydroxyapatite.

Oxytocin-sensitive Adenylate Cyclase in Frog Bladder Epithelial Cells: ROLE OF CALCIUM, NUCLEOTIDES, AND OTHER FACTORS IN HORMONAL STIMULATION

Abstract The effects of Ca2+, Mg2+, pH, temperature, and nucleotides (ATP or GTP) on the oxytocin-sensitive adenylate cyclase of frog bladder epithelial cells were studied. The site of the action of these parameters on the binding step, receptor-cyclase coupling, and the adenylate cyclase system as a whole was deduced from a comparison of their effects on basal, and oxytocin- and fluoride-sensitive activities and on the apparent Km of the system for oxytocin. Minute amounts of Ca2+ were found necessary for the stimulation of adenylate cyclase by oxytocin. Elimination of Ca2+ by treatment with ethylene glycol bis(β-aminoethyl ether)-N,N'-tetraacetic acid led to inhibition of the response, which was completely restored by the addition of 10-6 m free Ca2+. Calcium concentrations higher than 5 x 10-5 m were inhibitory, with no change in the apparent Km for oxytocin. Basal activity remained unchanged throughout the range of Ca2+ concentrations tested. The measurement of oxytocin-sensitive adenylate cyclase activity as a function of the ATP concentration in the medium showed that the hormone increases both the maximum velocity of the adenylate cyclase and its affinity for ATP. When low ATP concentrations were used (between 10-6 and 2.5 x 10-4 m), the activation ratio (stimulated to basal activities) showed an optimum for 10-5 m ATP. When low GTP concentrations (10-8 to 10-6 m) were used, the magnitude of this optimum increased and moved toward the lowest concentrations. These results were taken as evidence for the existence of regulatory sites binding either GTP or ATP; the occupancy of these sites enhances the velocity of the adenylate cyclase reaction to a greater extent in the presence of oxytocin than under basal conditions. Oxytocin increases the affinity of the regulatory site for ATP or GTP. Lowering the pH of the incubation medium from 8.0 to 7.0 resulted in a loss of affinity for oxytocin, with no change either in maximum stimulation or fluoride-sensitive activity, suggesting an effect of pH primarily located at the hormone-binding step. Thermal denaturation at 43° for ½ to 30 min did not make it possible to dissociate the hormonal and fluoride stimulation processes. Magnesium experiments confirmed the existence of two actions of Mg2+ ions acting as cosubstrate with ATP and as a regulatory agent for the cyclase system. Effects of varying Mg2+ concentration on fluoride-sensitive activity must be interpreted with caution because of possible formation of poorly dissociated complexes between (F-, Mg2+, and HPO42-).

Kinetic study of the activation process of -galactosidase from Escherichia coli by Mg 2+ .

The action of Mg2+ ions on β‐galactosidase activity has been studied under strictly controlled conditions with different substrates. The activation effect does not depend on the substrate.In the absence of Mg2+ ions, a residual activity has been found and the catalytic properties of Mg2+‐free enzyme have been determined.The activation process induced by Mg2+ is a slow process, the rate of which depends on the Mg2+ concentration. The deactivation process obtained by adding EDTA is also a slow process. Activation by Mg2+ is not a cooperative process. The apparent dissociation constant of the Mg2+ enzyme complexe is rather small, 0.65 ± 0.05 μM.The kinetics of activation and deactivation have been studied with phenyl galactoside as substrate.The results indicate that activation by Mg2+ operates through a binding with free enzyme. This binding involves at least two steps: the first and more rapid one is a single metal‐protein binding, the following slow step probably involves a conformational change of the enzyme induced by the metal binding.

Biological roles for 3′,5′-cyclic nucleotides: I. Lipolytic agents in isolated rat epididymal adipose cells and substrates for adipose tissue phosphodiesterase

All 3′,5′-cyclic nucleotides (3′,5′-CNS) tested, with the exception of thymine derivatives, were effective lipolytic agents in “permeable” rat epididymal adipose cells (adipocytes), but cells were most sensitive to cyclic AMP and its butyryl derivatives. With the exception of cyclic CMP and the butyryl derivatives of cyclic AMP, all 3′,5′-CNS were substrates for a partially purified adipose tissue phosphodiesterase (PDE); cyclic GMP and IMP were even more effective than cyclic AMP. Potency among 3′,5′-CNS as lipolytic agents was not inversely related to susceptibility to attack by PDE. Kinetic analyses of rates of lipolysis provided no evidence for interconversions among 3′,5′-CNS as a prerequisite for activation of lipolysis. Experiments with combinations of 3′,5 -CNS suggested that all activated the same lipolytic mechanism; only the thymine compounds inhibited basal and cyclic nucleotide-stimulated lipolysis, and these inhibitions were noncompetitive. 2′,3′-CNS did not stimulate lipolysis, but did inhibit the effects of cyclic AMP. Mg 2+, Ca 2+, and K +, in concentrations found in Krebs-Ringer buffers, drastically inhibited cyclic nucleotide-stimulated lipolysis in permeable adipocytes; the action of Mg 2+ appeared to be on inhibition of permeability, rather than on activation of PDE. Theophylline was as effective a lipolytic agent in permeable cells as it was in cells used with standard Krebs-Ringer buffers. Other 3′,5′-CNS did not protect cyclic AMP from attack by adipose tissue and liver PDE's.