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PubMed1989

The influence of mono- and divalent cations on the cardiac metabolism of arachidonic acid.

Our previous study indicated that, in the isolated rabbit heart, perfusion with Ca2+ free Krebs Henseleit buffer (KHB) results in increased conversion of exogenous arachidonic acid to PGE2 and 6-keto-PGF1 alpha, probably as the result of increased availability of substrate to cyclooxygenase. Since perfusion with Ca2+ free buffer is known to cause alterations in the cardiac content of various mono- and divalent cations, the present study was performed to determine: a) The relationship between the conversion of exogenous arachidonic acid to prostaglandins and cardiac content of Na+, K+, Ca2+ and Mg2+; and b) Whether enhanced arachidonic acid conversion to prostaglandins during Ca2+ free perfusion is due to reduced incorporation of this fatty acid into tissue lipids. Perfusion of the rabbit heart with Ca2+ free buffer produced a significant reduction in the tissue content of Na+, K+, Ca2+ and Mg2+. However, the production of 6-keto-PGF1 alpha from exogenous arachidonic acid was linearly correlated with tissue Mg2+. These observations, together with our finding that perfusion with Ca2+ free KHB reduced the incorporation of [3H] arachidonic acid into tissue lipids, suggests that Ca2+ free perfusion may, by reducing the activity of arachidonyl CoA synthetase (a Mg2+ dependent enzyme), decrease the acylation of arachidonic acid into lipids, thus increasing the availability of arachidonic acid to cyclooxygenase.

6-Ketoprostaglandin F1 alpha↗

PubMedDate not supplied

Insight into the binding of divalent cations to Sepia eumelanin from IR absorption spectroscopy.

IR absorption spectroscopy is used to examine the binding of the divalent cations Mg(II), Ca(II), Zn(II) and Cu(II) to melanin granules isolated from the ink sacs of Sepia officinalis. The functional groups of the melanin granules interacting with the bound metal ions are deduced by examining the effect of metal concentration on transition frequencies associated with the COOH, NH and OH moieties of the pigment. The coordinating groups vary with metal ion and with concentration. For the experimental conditions used (initial solution pH of 4, ionic strength of 100 mM and a melanin concentration of 1 mg mL(-1)) Mg(II), Ca(II) and Zn(II) bind to carboxylate groups and Cu(II) binds predominantly to phenolic (catechol) groups However, at a concentration of 10 mM Cu(II) also shows evidence of binding to carboxylate and amine groups, reflecting a secondary binding site that becomes populated as the catechol sites are depleted.

Animals↗

PubMed1984

Effects of divalent cations on the inhibitory potentials in the duodenal smooth muscle cells of the guinea-pig.

The effects of divalent cations and some Ca2+ antagonists on the non-adrenergic inhibitory potential (i.p.) in the duodenal smooth muscle cells of the guinea-pig were investigated intracellularly. The membrane potential was a function of the external Ca2+ (0.25-7.5 mM) and Mg2+ (1.2-24 mM) concentrations. The latency and the time to peak of the i.p. were prolonged by low Ca2+ and excess Mg2+. The amplitude and the rate of hyperpolarization of the i.p. were reduced by low Ca2+ and excess Mg2+. The effects of Mn2+ (5-10 mM) on the i.p. were similar to those by excess Mg2+ except for the small depolarization. The i.p. evoked in the Ca2+-free solution was considerably restored by the addition of Ba2+ (1.25 mM) and Sr2+ (2 mM). The actions of Ba2+ and Sr2+ on the i.p. were inhibited by verapamil (10(-4)g/ml). Verapamil (2 X 10(-5)-2 X 10(-4)g/ml) and gentamicin (10(-3)g/ml) reduced the amplitude and the rate of hyperpolarization of the i.p. The latency and the time to peak of the i.p. were prolonged by verapamil but not by gentamicin. In nifedipine (10(-4)g/ml), the two-peaked i.p. was evoked by a single stimulus. This potential was similar to that evoked by a paired pulse in normal solution. The results obtained suggest that the released of the non-adrenergic inhibitory substance requires Ca2+ which moves into the non-adrenergic inhibitory neurons through the Ca2+ conductance pathway.

Animals↗

PubMed1994

Differential effects of divalent cations on specific 3H-GR 65630 binding to 5-HT3 receptors in rat cortical membranes.

We have examined the ability of divalent cations to inhibit 3H-GR 65630 binding to 5-HT3 receptor recognition sites in rat brain cortical membranes. Among the divalent metal cations tested, Cd2+, Zn2+ and Cu2+ inhibited the specific 3H-GR 65630 binding activity to 5-HT3 receptors at a concentration of 0.1-1 mM. The other divalent metal cations tested (i.e. Co2+, Ni2+, Ba2+, Ca2+, Mg2+ and Mn2+) produced no inhibition of the specific 3H-GR 65630 binding. Cd2+, Zn2+ and Cu2+ did not change the Bmax value of the binding activity, but significantly increased the Kd value. It was suggested that these cations inhibited the binding activity by reducing affinity of the 5-HT3 receptor for the antagonist, resulting in apparent inhibition of the binding activity. As to the binding association rate, Cd2+, Cu2+ and Zn2+ were found to have an inhibitory effect. The binding dissociation rate, however, was shown to be decreased by Cu2+ but not by Cd2+ and Zn2+. Furthermore, the Zn(2+)-induced inhibition of 3H-GR 65630 binding was suggested to be antagonized by both concanavalin A and wheatgerm agglutinin. The Cu(2+)-induced inhibition, however, was not influenced by these lectins, indicating that Cu2+ has a different lectin sensitivity for its inhibitory effect. The different mechanism of action between Cu2+ and Zn2+ was suggested in their inhibitory effect on the specific 3H-GR 65630 binding activity.

Animals↗

PubMed1997

Regulation of the mitochondrial Ca2+ uniporter by external adenine nucleotides: the uniporter behaves like a gated channel which is regulated by nucleotides and divalent cations.

We have previously used measurements of uncoupler-enforced reverse activity to demonstrate that the mitochondrial Ca2+ uniporter is strongly inhibited by external EGTA plus free Mg2+, following a brief period of rapid activity. Using the same approach, we now show that in addition to divalent cations, the uniporter is regulated by external adenine nucleotides and by other components of the cytosol. Inhibition produced by EGTA plus free Mg2+ is reversed by spermine (EC0.5 approximately 40 microM) and reduced when mitochondria are purified by an isoosmotic density-gradient method. Under either condition, inhibition is restored by external adenine nucleotides in a concentration-dependent manner. The order of effectiveness is ATP > ADP > AMP, with the nucleoside adenosine being ineffective. Among nucleotide triphosphates, the order is ATP > CTP approximately UTP > GTP. The effectiveness of ATP (EC50 approximately 0.6 mM) is the same in mitochondria and mitoplasts, the same as that of AMPPNP, and is not altered by the presence of oligomycin, carboxyatractyloside, or AP5A, used alone or in combinations. These findings indicate that ATP acts at a site located on the outer surface of the inner membrane through a mechanism which does not require its hydrolysis. Phosphate also inhibits reverse uniport under some conditions (EC50 approximately 20 microM). The sites at which free ATP and free Mg2+ inhibit the uniporter can be distinguished by chymotrypsin treatment of mitoplasts, which eliminates the action of Mg2+ but does not affect the action of ATP. Data are interpreted within the context of a model in which the uniporter is considered to be a gated channel that is controlled, in part, by specific external effector sites that accept divalent cations or nucleotides. The possible consequences of the model for cell Ca2+ regulation by mitochondria and regulation of TCA cycle activity by the matrix free Ca2+ concentration are considered.

Adenine Nucleotides↗

PubMed1982

Chemiluminescence of human neutrophils induced by soluble stimuli: effect of divalent cations.

The effect of three soluble stimuli, phorbol myristate acetate, concanavalin A, and the calcium ionophore A23187, on the luminol-enhanced chemiluminescence of human neutrophils was investigated. All three stimuli elicited a strong burst of chemiluminescence which was dose dependent. The effect of phorbol myristate acetate was independent of the presence of divalent cations in the medium and, in fact, was greater in the presence of ethylene glycol-bis(beta-aminoethyl ether)-N,N-tetraacetic acid. The effect of concanavalin A was greatly stimulated by the presence of calcium in the medium, although some reaction was observed in the absence of this cation. In contrast, chemiluminescence induced by A23187 was absolutely dependent upon the presence of extracellular calcium. These results suggest that the mobilization of calcium into the cell is a sufficient, but not necessary, stimulus for initiation of the respiratory burst. Compounds such as phorbol myristate acetate, which act independently of extracellular divalent cations, may function by altering the intracellular ratio of bound/free calcium.

Anti-Bacterial Agents↗

PubMed2002

Divalent cations reduce the electrogenic transport of monovalent cations across rumen epithelium.

The rumen epithelium of sheep and goats showed an increase in short circuit current ( Isc) and transepithelial conductance (gt) upon mucosal removal of divalent cations. A divalent-sensitive Isc and gt were present in Na+, K+ or Rb+ buffer, but nearly abolished in mucosal NMDG+ (N-methyl-D-glucamine) buffer. High K buffer, addition of BaCl2 or of ouabain on the serosal side also reduced or abolished the divalent-sensitive Isc. Mucosal Ca2+ was more potent in blocking Isc, but had the same potency as Mg2+ in blocking gt. A prolonged mucosal deprivation of Mg2+ ions increased gt, potential difference and basal as well as the Ca2+-sensitive Isc. Mucosal addition of Mg2+ had a smaller effect on gt after serosal preincubation with Ba. The data suggest that rumen epithelial cells exhibit an apical non-selective cation conductance, which permits the passage of monovalents in the mucosal absence of divalents. The development of a divalent-sensitive Isc in Na buffer requires Na+/K+ pumps and K+ recycling through Ba2+-sensitive K+ conductances on the basolateral side. This Isc is blocked by extracellular Ca2+ and both extracellular and intracellular Mg2+ ions. A prolonged deprivation of mucosal Mg2+ alone seems to affect intracellular Mg2+ in this Mg2+-absorbing tissue.

Animals↗

PubMed1994

Fura-2 signals evoked by kainate in leech glial cells in the presence of different divalent cations.

The glutamate-agonist kainate evokes Ca2+ transients in both neurones and glial cells. Owing to the membrane depolarization elicited by kainate, a Ca2+ influx could occur through voltage-gated Ca2+ channels or through the kainate-gated cation channels directly. We have measured ratio signals of the calcium indicator dye fura-2, injected into giant glial cells of the leech Hirudo medicinalis, as response to kainate (5-20 microM) in the presence of different divalent cations. The responses to kainate increased during the first 2-4 kainate applications, both in unclamped and in voltage-clamped cells. The fura-2 fluorescence ratio (F350/F380) still increased when Ca2+ was replaced by Ba2+ but was suppressed in Ca(2+)-free saline and in the presence of Ni2+ (2 mM). Co2+ and Mn2+ (2 mM) also reduced the kainate-induced fura-2 fluorescence signals, due to entry of these divalent cations into the cells and subsequent quenching the fluorescence of the intracellular dye. It is concluded that Ni2+ blocks the kainate-induced membrane depolarization and Ca2+ transient but apparently does not enter the cells, while Ba2+, Co2+, and Mn2+ appear to permeate the membrane, presumably through the kainate-gated channels.

Animals↗

PubMed1980

The limitations of chlorotetracycline as a fluorescent probe of divalent cations associated with membranes.

The interaction of chlorotetracycline with magnesium chloride, detergents and ATP and ADP has been investigated. At various pH the fluorescence of chlorotetracycline, or the chlorotetracycline complex, is enhanced in the presence of the negatively charged detergent sodium dodecyl sulphate, the uncharged detergent Triton X-100, or the positively charged detergent cetyltrimethylammonium bromide. Also ternary fluorescent complex is formed between chlorotetracycline, magnesium and ATP or ADP. In a biological membrane, it would not seem possible to distinguish between the fluorescence signal arising from chlorotetracycline acting as a probe of divalent cations bound to specific sites on the membrane and that arising from the complex of chlorotetracycline with divalent cations acting as a probe of neutral or negative regions of the membrane.

Adenosine Diphosphate↗

PubMed1978

In vitro studies of the interaction of calcium ions and other divalent cations with the Lp(a) lipoprotein and other isolated serum lipoproteins.

The interaction of isolated Lp(a) lipoprotein with different divalent cations was studied and compared to that of other isolated lipoprotein classes. Purified Lp(a) lipoprotein was found to be most sensitive to the metal ions tested, and the Lp(a) lipoprotein was the only lipoprotein which was precipitated by calcium ions alone. The precipitation apparently depends on the ionic radii of the cations used as well as on the lipoprotein class tested. The precipitation reaction between calcium ions and the Lp(a) lipoprotein, and the interaction between calcium ions and LDL (without precipitation) seem to follow the known rules for small ion-macromolecule interaction reasonably well. The calcium ion - Lp(a) lipoprotein interaction results in a small aggregate. The binding is of ionic type and the precipitation reaction is initially reversible. It was estimated that LDL particles have a mean of 290 equivalent and non-interacting binding sites for calcium ions. The above observations concerning the Lp(a) lipoprotein may be of interest in view of the significantly higher frequency of early coronary heart disease in Lp(a+) than in Lp(a-) individuals, and in view of the previously reported biochemical differences between individuals of different Lp phenotype.

Calcium↗

PubMed1997

Effects of divalent cations, protons and calmidazolium at the rat P2X7 receptor.

The P2X7 receptor is a uniquely bifunctional molecule through which ATP can open a small cationic channel typical of ionotropic receptors and also induce a large pore permeable to high molecular weight molecules (> 600 Da). Activation of this large pore can lead to cell lysis within 1-2 min. We asked whether pharmacological differences existed between the cationic channel and the cell permeabilizing pore by measuring whole-cell currents and uptake of a propidium dye (YO-PRO; Mw 629) in HEK293 cells stably expressing the rat P2X7 receptor, and comparing the actions of divalent cations and protons in these two assays. Currents in response to 2'-3'-(O)-(4-benzoyl benzoyl) ATP (BzATP, 30 microM) were inhibited by extracellular calcium, magnesium, zinc, copper and protons with half-maximal inhibitory concentrations (IC50) of 2.9 mM, 0.5 mM, 11 microM, 0.5 microM and 0.4 microM, respectively. The inhibition was voltage independent in each case. YO-PRO uptake induced by BzATP was also inhibited with similar IC50 values. The rank order of potency of a range of divalents was Cu2+ > Cd2+ = Zn2+ > Ni2+ >> Mg2+ = Co2+ > Mn2+ > Ca2+ = Ba2+ >> Sr2+. These results suggest that these divalent cations and protons all act primarily as allosteric modulators to alter the affinity of ATP binding to the P2X7 receptor. In contrast, extracellular (but not intracellular) calmidazolium inhibited the BzATP-evoked current by up to 90% (IC50 = 15 nM) but had no effect on YO-PRO uptake. Thus, calmidazolium can block activation of the ionic channel but this does not prevent the formation of the large permeabilizing pore.

Adenosine Triphosphate↗

PubMed1987

Tension activation and relaxation in frog atrial fibres. Evidence for direct effects of divalent cations (Ca2+, Sr2+, Ba2+) on contractile proteins and Na-Ca exchange.

The effect of alkali-earth cations (Ca2+, Sr2+, Ba2+) on the excitation-contraction coupling events of the frog atrial fibres were studied using a double mannitol gap voltage clamp technique coupled with a mechano-electric transducer. Photoremoval of the suppressive effect of nifedipine on the calcium channels allowed to obtain rapid transient Ca2+, Sr2+ or Ba2+ ions current jumps. The effect on the amplitude of the associated contraction was proportional to the current jumps. These results together with the correlation established between the estimated increase in the internal concentration of divalent cations and the amplitude of the phasic tension suggest that the essential source of divalent cations for activation of contraction is the extracellular space. Also Ba2+ ions reduced the tonic tension and strongly slowed the relaxation of the phasic component whereas Sr2+ exhibited smaller effects. Sr2+ ions could be more efficient than Ba2+ ions in substituting for Ca2+ ions in the Na+-Ca2+ exchange mechanism known to regulate these two mechanical events. The conclusions are that the order of effectiveness of these ions (Ca2+ greater than Sr2+ greater than Ba2+) is the same with regard to transarcolemmal exchange for Na+ ions, presumed uptake by a "second relaxing system", activation of contraction, and inactivation of the slow inward current.

Action Potentials↗

PubMed1990

Inhibition of Na(+) -K+ pump activity by divalent cations in intact peritoneal mast cells of the rat.

1. The inhibition by the divalent cations magnesium, barium and strontium and the trivalent ion lanthanum of the Na(+) -K+ pump in the plasma membrane of rat peritoneal mast cells was studied in pure mast cell populations by measurement of the ouabain-sensitive uptake of the radioactive potassium analogue, 86rubidium (86Rb+). 2. Exposure of the cells to magnesium induced a time- and concentration-dependent decrease in the ouabain-sensitive K+(86Rb+)-uptake of the cells without influencing the ouabain-resistant uptake. The time-dependent decrease was apparent after incubation of the cells for 10 min or more, but no decrease was observed after 2 min incubation when the cells are supposed to be loaded with sodium due to the cell isolation procedure. 3. Barium and strontium caused concentration-dependent decreases in the ouabain-sensitive K(+) -(86Rb+) -uptake of the cells but the ouabain-resistant uptake was not changed. Half maximum decrease in the ouabain-sensitive K+(86Rb+)-uptake was observed with 1.8 mM magnesium, 1.2mM barium and 0.7 mM strontium. 4. The trivalent ion lanthanum blocked almost completely the ouabain-sensitive K+(86Rb+)-uptake at a concentration of 1 microM as does 1 mM calcium. Combining either of these ions with magnesium had no further inhibitory effect on the ouabain-sensitive uptake. 5. In conclusion, in addition to the previously suggested modulation by calcium of the activity of the Na+ (-)K+ pump, evidence is provided in this investigation that the modulation may be a more general effect of divalent and polyvalent cations present in the extracellular space through their influence on the sodium permeability of the plasma membrane.

Animals↗

PubMed1995

Effects of divalent cations and nucleotides on the 14CO2-oxaloacetate exchange catalyzed by the phosphoenol pyruvate carboxykinase from the moderate halophile, Vibrio costicola.

The phosphoenolpyruvate carboxykinase (PEPCK) from Vibrio costicola catalyzed a 14CO2-oxaloacetate exchange reaction with an unusual nucleotide specificity. ATP gave the higher apparent catalytic efficiency (Vmax/Km, 6.78), followed by GTP (1.30), CTP (0.87) and ITP (0.66). Maximal activity required a divalent cation; CdCl2 and MgCl2 synergistically activated the enzyme, when added in the presence of MnCl2. The sigmoidal saturation curve for MnCl2 (apparent n 2.11) was converted into a hyperbola by 0.01 mM CdCl2 (apparent n 1). The results suggest a double role of the divalent cation in the reaction mechanism, namely as part of the MeATP2- substrate and as free Me2+. Mn2+ would be the best for the first, and Cd2+ for the second role. Preincubation with 0.01 mM CdCl2 increased the activity of the enzyme assayed with MgATP2- through an increase in Vmax; addition of CdCl2 to the reaction mixture elicited further activation, through a 17-fold decrease in the apparent Km for MgATP2-. These results, together with the biphasic curve of activation by CdCl2 when used alone, suggest the existence of two different sites for free Cd2+ on the enzyme.

Adenosine Triphosphate↗

PubMed1975

Magnetic resonance studies on the mitochondrial divalent cation carrier.

Measurements of water proton spin relaxation enhancements (epsilon) can be used to discriminate high-affinity binding of Mn-2+ or Gd-3+ to biological membranes, from low-affinity binding. In rat liver mitochondria, epsilon b values of approx. 11 are observed upon binding of Mn-2+ to the inner membrane, while internal or low-affinity binding remains invisible to this technique. Energy-driven Mn-2+ uptake by liver mitochondria results in the subsequent decay of epsilon. Comparison of epsilon with the initial velocity of Mn-2+ uptake in rat liver mitochondria reveals a linear correlation, which holds at all temperatures between 0 degrees C and 40 degrees C, regardless of the mitochondrial protein concentration. Consequently, enhancement appears to reflect the binding of Mn-2+ to the divalent cation pump. Binding of Mn-2+ to blowfly flight muscle also results in substantial epsilon, which is associated with the glycerol-1-phosphate dehydrogenase instead of divalent cation transport. Consequently, no decay in epsilon due to uptake occurs after Mn-2+ is bound. Lanthanide ions are also bound and transported by mitochondria. Addition of Gd-3+ to pigeon heart or rat liver mitochondria results in epsilon b approximately equal to 5-6, which decays with similar kinetics in both systems. The uptake velocity of Gd-3+ in rat liver mitochondria is about 1/6 the rate with which Mn-2+ is transported. Lanthanides also diminish epsilon due to the addition of Mn-2+, and greatly retard the Mn-2+ uptake kinetics. The presence of carbonylcyanide-p-trifluoromethoxyphenylhydrazone depresses epsilon upon addition of Mn-2+ or Gd-3+ and also uncouples energy-driven uptake. On the other hand, prolonged anaerobic incubation in the presence of antimycin and rotenone exhausts the mitochondria of their energy stores, blocks the uptake of Mn-2+, but does not affect epsilon significantly. Evidently, the uncoupler-induced disappearance of divalent cation binding sites is not the result of "de-energization". Measurements of epsilon at several NMR frequencies indicate a correlation time (tau b) for carrier-bound Mn-2+ in rat liver mitochondria between 20 ns and 4 ns as one varies the temperature between 10 degrees C and 30 degrees C. The 13 Kcal/mole activation energy for tau b suggests that the 11 ns time constant at room temperature represents the movement of the Mn-11-carrier comples. On the other hand, tau b is probably approx. 100 times too short to represent the rotational motion of a carrier protein. Apparently, Mn-2+ binds to a small arm of the carrier which moves independent

Animals↗

PubMed1973

Vitamin D and adenosine triphosphatase dependent on divalent cations in rat intestinal mucosa.

Intestinal brush borders prepared from vitamin D-deficient rats demonstrate increased susceptibility in vitro to fragmentation by shear forces or to loss of microvillus enzymes on treatment with EDTA. These effects are relatively nonspecific and are also observed in normal rats starved for 48 h. They may underlie prior observations that purport to demonstrate a vitamin D-dependent increase in brush border Ca-dependent ATPase. In addition, however, vitamin D increases ATPase activity dependent on certain divalent cations, including Ca and Zn, in whole-particulate suspensions pelleted by high-speed centrifugation of mucosal homogenates. This action is independent of changes in other microvillus enzymes, i.e. disaccharidases, and tissue distribution and cation specificity studies support the hypothesis that the mucosal whole-particulate ATPase is related to transport of Ca, Zn, and possibly other divalent cations.

Adenosine Triphosphatases↗

PubMed1985

Light-evoked depolarizations in the retina of Strombus: role of calcium and other divalent cations.

Previous studies indicate that overlapping inward sodium and outward potassium currents play a role in generating the waveform of light-evoked depolarizations (LEDs) in one type of retinal neuron in Strombus luhuanus, a marine gastropod [Chinn, K. S., and Gillary, H. L. (1985). Comp. Biochem. Physiol. 80A:233-245]. This paper concerns the effects of divalent cations on the LED. The LED can exhibit a distinct early phase of depolarization (DE). Increasing the [Ca2+] in the artificial seawater (ASW) bathing medium reduced the amplitude of the entire LED, and omitting Ca2+ increased it. Adding 10 mM Sr2+ or 10 mM Mn2+ to either normal ASW or 0-Ca2+ ASW decreased the LED amplitude. Adding 10 mM Ba2+ to 0-Ca2+ ASW also decreased the LED amplitude, but adding Ba2+ to normal ASW selectively increased DE. Cd2+ (100 microM) selectively reduced DE when added to normal ASW but not when added to 0-Ca2+ ASW. The results show that a variety of divalent cations can alter the currents that underlie the LED. They also suggest that an inward Ca2+ current occurs during DE.

Animals↗

PubMed1999

AMPA/kainate receptors permeable to divalent cations in amphibian central nervous system.

Glutamate receptors have been studied extensively in mammals but less explored in lower vertebrates. These receptors are present in amphibians. Using a recent method based upon agonist-induced cobalt uptake, we were able to detect the presence of functional alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors permeable to divalent cations in tadpoles and in adults. The uptake specificity was checked by co-application of an antagonist. We studied the distribution of receptor-bearing cells in the principal brain regions. The distribution was similar in the two species studied: Rana esculenta (green frog) and Bufo bufo (common toad). The high number of cobalt-positive cells suggests that the AMPA/kainate receptors permeable to divalent cations play an important role in the anuran nervous system.

Animals↗