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PubMed1995

Effects of divalent cations on muscarinic receptor cationic current in smooth muscle from guinea-pig small intestine.

1. Effects of Mg2+ and Ca2+ on muscarinic receptor cationic current (Icat) in guinea-pig ileal smooth muscle cells have been studied using patch-clamp techniques (whole-cell recording). Icat was activated either by externally applied carbachol or, to bypass receptors, by intracellular GTP-gamma-S. 2. Independently of the main permeant cation the current-voltage (I-V) relation for Icat was U-shaped between the reversal potential (usually 0 mV) and very negative potentials such as -120 mV where current could be virtually lost. Adding Ca2+ to Ca(2+)- and Mg(2+)-free external solution reduced inward current and made it less U-shaped whereas adding Mg2+ reduced inward current and shifted more positively the potential at which maximum inward current occurred. 3. Activation of the conductance underlying Icat could be described by the Boltzmann relation which was shifted positively by adding Ca2+ or Mg2+. Extracellular Ca2+ also distorted the relation by increasing the slope factor; maximal conductance was reduced in all cases. Icat relaxation at negative potentials was accelerated by increasing Mg2+ and slowed down by Ca2+. 4. These data suggest the presence of fixed negative surface charges on or near the muscarinic receptor cationic channel, which allow its modulation through alteration of surface potential. Additional more direct ion binding to and blocking of the channel cannot be ruled out. Some additional effects of Ca2+ (if compared with Mg2+) could be explained on the assumption that the Ca(2+)-binding activation site known to be present on the internal side of the channel can be accessible to Ca2+ entering through the open channel during muscarinic receptor stimulation, as Ca2+ ions contribute to a limited extent to Icat. 5. We conclude that voltage-dependent gating of muscarinic receptor cationic channels is an intrinsic channel property and that Ca2+ and Mg2+ have strong modulatory effects.

Animals↗
PubMed1979

Effect of divalent cation concentrations on the antibiotic susceptibilities of nonfermenters other than Pseudomonas aeruginosa.

The effects of supplementing Mueller-Hinton broth with calcium and magnesium on the minimal inhibitory concentrations (MICs) of eight aminoglycosides, colistin, tetracycline, and carbenicillin for 11 nonfermenters other than Pseudomonas aeruginosa were studied and compared with the effects for Escherichia coli and P. aeruginosa. MICs were simultaneously performed in unsupplemented Mueller-Hinton broth and Mueller-Hinton broth supplemented to contain 5 mg of calcium and 2.5 mg of magnesium per dl. Changes in MICs were expressed as the increases in the number of log(2) concentrations caused by supplementation. The usual increases in MICs of aminoglycosides caused by supplementation were: zero concentrations for E. coli, one to six concentrations for P. aeruginosa, and one to two concentrations for most other nonfermenters. The largest increases (five to six concentrations) were observed with gentamicin and P. aeruginosa. The usual increases in MICs of colistin were: zero concentrations for E. coli, two concentrations for P. aeruginosa, and one to two concentrations for other nonfermenters. Increases in MICs of tetracycline were: one to five concentrations for all organisms tested. The usual increases in MICs of carbenicillin were: zero concentrations for E. coli and P. aeruginosa and zero to two concentrations for other nonfermenters. These observations indicated that supplementation of Mueller-Hinton broth to contain recommended concentrations of calcium and magnesium had little effect on MICs of aminoglycosides and colistin for E. coli but increased MICs for most nonfermenters, increased MICs of tetracycline for E. coli and all nonfermenters, and had little effect on MICs of carbenicillin for E. coli and P. aeruginosa but increased the MICs for several nonfermenters other than P. aeruginosa.

Anti-Bacterial Agents↗
PubMed1985

Inactivation of glycerol dehydrogenase of Klebsiella pneumoniae and the role of divalent cations.

Anaerobically induced NAD-linked glycerol dehydrogenase of Klebsiella pneumoniae for fermentative glycerol utilization was reported previously to be inactivated in the cell during oxidative metabolism. In vitro inactivation was observed in this study by incubating the purified enzyme in the presence of O2, Fe2+, and ascorbate or dihydroxyfumarate. It appears that O2 and the reducing agent formed H2O2 and that H2O2 reacted with Fe2+ to generate an activated species of oxygen which attacked the enzyme. The in vitro-oxidized enzyme, like the in vivo-inactivated enzyme, showed an increased Km for NAD (but not glycerol) and could no longer be activated by Mn2+ which increased the Vmax of the native enzyme but decreased its apparent affinity for NAD. Ethanol dehydrogenase and 1,3-propanediol oxidoreductase, two enzymes with anaerobic function, also lost activity when the cells were incubated aerobically with glucose. However, glucose 6-phosphate dehydrogenase (NADP-linked), isocitrate dehydrogenase, and malate dehydrogenase, expected to function both aerobically and anaerobically, were not inactivated. Thus, oxidative modification of proteins in vivo might provide a mechanism for regulating the activities of some anaerobic enzymes.

Cations, Divalent↗
PubMed1992

Dependence of oxygen-tolerant nitrogenase activity on divalent cations in Azotobacter vinelandii.

Nitrogenase activity of washed Azotobacter vinelandii cells was enhanced by the addition of Ca2+ and Mg2+, and the enhancement increased with the O2 concentration. In assays provided with a level of O2 that was initially supraoptimal and inhibitory to nitrogenase activity, the addition of Ca2+ or Mg2+ affected both the maximum respiration rate (Vmax) of the cells and the apparent affinity [KS(O2)] of cell respiration for O2. Changes in these parameters correlated with changes in nitrogenase activity. Aeration-dependent increases in Vmax and KS(O2) were inhibited by rifampin and chloramphenicol and were also observed in ammonium-grown cultures.

Acetylene↗
PubMed1987

Calmodulin and troponin C: affinity chromatographic study of divalent cation requirements for troponin I inhibitory peptide (residues 104-115), mastoparan and fluphenazine binding.

The different conformations induced by the binding of Mg2+ or Ca2+ to troponin C (TnC) and calmodulin (CaM) results in the exposure of various interfaces with potential to bind target compounds. The interaction of TnC or CaM with three affinity columns with ligands of either the synthetic peptide of troponin I (TnI) inhibitory region (residues 104-115), mastoparan (a wasp venom peptide), or fluphenazine (a phenothiazine drug) were investigated in the presence of Mg2+ or Ca2+. TnC and CaM in the presence of either Ca2+ or Mg2+ bound to the TnI peptide 104-115. The cation specificity for this interaction firmly establishes that the TnI inhibitory region binds to the high affinity sites of TnC (most likely the N-terminal helix of site III) and presumably the homologous region of CaM. Mastoparan interacted strongly with both proteins in the presence of Ca2+ but, in the presence of Mg2+, did not bind to TnC and only bound weakly to CaM. Fluphenazine bound to TnC and CaM only in the presence of Ca2+. When the ligands interacted with either proteins there was an increase in cation affinity, such that TnC and CaM were eluted from the TnI peptide or mastoparan affinity column with 0.1 M EDTA compared with the 0.01 M EDTA required to elute the proteins from the fluphenazine column. The interaction of these ligands with their receptor sites on TnC and CaM require a specific and spatially correct alignment of hydrophobic and negatively charged residues on these proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Bee Venoms↗
PubMed1978

Motoneuronal after-potentials and extracellular divalent cations.

In cats under Dial anesthesia, microiontophoretic injections of Co2+ or Mn2+ outside lumbosacral motoneurones regularly diminish the postspike hyperpolarizing after-potential (AHP) without any clear reduction of the deplorarizing after-potential (ADP). Comparable injections of Sr2+ have the opposite effect of augmenting the AHP. These results indicate that the AHP is indeed initiated by an influx of calcium linked to the action potential, but the ADP is unlikely to be mainly a manifestation of a calcium inward current.

Action Potentials↗
PubMed1979

Effects of some divalent cations on motoneurones in cats.

In cats under Dial, Co, Mn, La, and Sr were injected extracellularly near lumbosacral motoneurones. All tended to improve intracellular recording, but when the membrane potential was initially stable, Mn, and especially Co, had a moderate and reproducible depolarizing action. Both Mn and Co depressed excitatory postsynaptic potentials evoked by dorsal root stimulation. The prominent after-hyperpolarization (a.h.p.), which normally follows the motoneuronal action potential, was consistently and reversibly depressed by Mn and Co (as well as La), the underlying conductance increase being also diminished, but there was no significant reduction in the after-depolarization. By contrast, Sr tended to potentiate the a.h.p., especially when this was depressed by a previous injection of Co or Mn. Unlike the other cations, Co had a marked depressant effect on the action potential, particularly its rate of rise. Since the action potential could be immediately restored by hyperpolarization or by an injection of Sr (in the absence of depolarization), Co may enhance Na inactivation.

Action Potentials↗