Effects Of Polyvalent Cations Research Articles

Spontaneous Condensation of RNA into Nanoring and Globular Structures.

The effect of polyvalent cations, like spermine, on the condensation of DNA into very well-defined toroidal shapes has been well studied and understood. A great effort has been made to obtain similar condensed structures from RNA molecules, but so far, it has been elusive. In this work, we show that single-stranded RNA (ssRNA) molecules can easily be condensed into nanoring and globular structures on a mica surface, where each nanoring structure is formed mostly by a single RNA molecule. The condensation occurs in a concentration range of different cations, from monovalent to trivalent, but at a higher concentration, globular structures appear. RNA nanoring structures were observed on mica surfaces by atomic force microscopy (AFM). The samples were observed in tapping mode and were prepared by drop evaporation of a solution of RNA in the presence of one type of the different cations used. As far as we know, this is the first time that nanorings or any other well-defined condensed RNA structures have been reported in the presence of simple salts. The RNA nanoring formation can be understood by an energy competition between the hydrogen bonding forming hairpin stems—weakened by the salts—and the hairpin loops. This result may have an important biological relevance since it has been proposed that RNA is the oldest genome-coding molecule, and the formation of these structures could have given it stability against degradation in primeval times. Even more, the nanoring structures could have the potential to be used as biosensors and functionalized nanodevices.

Rna Compaction in the Presence of Polyvalent Cations

The effects of polyvalent cations on the effective size and charge of double-stranded DNA (dsDNA) have been well studied. In the presence of polyvalent cations, dsDNA in dilute solution undergoes a single-molecule, first-order phase transition, otherwise called condensation: more explicitly, upon onset of 90% neutralization of the phosphate backbone, the DNA undergoes discontinuous compaction into tightly wound toroids. However, the effects of these cations on long single-stranded RNAs (ssRNA) have not been well characterized. In this study we use centrifugation methods to examine the effective size of long ssRNAs in solutions of increasing concentration of the tetravalent cation spermine. In contrast to the case of dsDNA, we find only a continuous decrease in the size of ssRNA upon increase in spermine concentration. However, the decrease is significant enough to suggest that RNA molecules longer than viral genomes can be packaged in vitro into virus-like vectors for gene delivery.

Effects of pH and calcium on soil organic matter dynamics in Alaskan tundra

In Northern Alaska (AK), large variation in biogeochemical cycling exists among landscapes underlain by different aged geologic substrates deposited throughout the Pleistocene. Younger, less weathered landscapes have higher pH (6.5 vs. 4.5), ten-fold higher exchangeable cation concentrations, and slower rates of microbial activity than older, more weathered landscapes. To tease apart the effects of polyvalent cations vs. pH on microbial activity and organic matter solubility and stabilization, we conducted a soil incubation experiment. We collected soils near Toolik Lake, Alaska from replicated sites along a chronosequence of landscape ages ranging from 11,000 to 4.8 million years since glaciation and manipulated soil pH and calcium (Ca, the dominant polyvalent cation across all landscape ages) using a factorial experimental design. As expected, microbial respiration was inhibited by high Ca concentrations at both pH 6.5 and 4.5. In contrast, soils with circumneutral pH (but similar Ca concentrations) exhibited higher rates of microbial respiration than soils with acidic pH, opposite of in situ patterns. Manipulated soils with acidic (4.5) pH (but similar Ca concentrations) exhibited higher cumulative dissolved organic nitrogen (DON) in leachates than soils with circumneutral (6.5) pH, similar to in situ patterns of leaching among landscape ages, but there was no consistent effect of pH on dissolved organic carbon (DOC) in leachates across landscape ages. Increasing Ca concentration inhibited cumulative DOC in leachates at circumneutral pH as expected, but had no effect on DOC or DON in leachates at acidic pH. Our results indicate that both polyvalent cation concentration and pH likely influence microbial activity in tundra soils, suggesting that heterogeneity in geochemical factors associated with landscape age should be considered in models of tundra biogeochemistry.

In Situ Cross-Linking of Polyanionic Polymers to Sustain the Drug Release from Theophylline Tablets

The aim of this study was to develop an extended-release tablet formulation using a new in situ cross-linking method. The effects of polyvalent cations on theophylline release from tablets made with the polyanionic polymers sodium alginate and sodium carboxymethylcellulose, were investigated. Different miliequivalents of the di and tri-valent cation, Ca 2+ and Al 3+ , were added to tablet formulations. The results of the dissolution study showed that incorporation of cations sustained the drug release. This is due to an in situ cross-linking between the polyanionic polymers and the added cation in tablet formulation. The drug release prolongation and the release kinetics were dependent on the nature of the polymers and the cations’ concentrations and valences. The drug release rate decreased by an increase in cation concentration. The combination of the two investigated polymers decreased the drug release rate to a higher extent in comparison with formulations containing each polymer alone. A zero-order drug release kinetic was observed in formulations containing 1:1:1 ratio of drug: Na alginate: NaCMC, and the investigated cations. These results showed that the in situ cross-linking by polyanionic polymers can be used for controlling the drug release rate.

Electrostatic Interaction of Internal Mg2+ with Membrane PIP2 Seen with KCNQ K+ Channels

Activity of KCNQ (Kv7) channels requires binding of phosphatidylinositol 4,5-bisphosphate (PIP2) from the plasma membrane. We give evidence that Mg2+ and polyamines weaken the KCNQ channel–phospholipid interaction. Lowering internal Mg2+ augmented inward and outward KCNQ currents symmetrically, and raising Mg2+ reduced currents symmetrically. Polyvalent organic cations added to the pipette solution had similar effects. Their potency sequence followed the number of positive charges: putrescine (+2) < spermidine (+3) < spermine (+4) < neomycin (+6) < polylysine (≫+6). The inhibitory effects of Mg2+ were reversible with sequential whole-cell patching. Internal tetraethylammonium ion (TEA) gave classical voltage-dependent block of the pore with changes of the time course of K+ currents. The effect of polyvalent cations was simpler, symmetric, and without changes of current time course. Overexpression of phosphatidylinositol 4-phosphate 5-kinase Iγ to accelerate synthesis of PIP2 attenuated the sensitivity to polyvalent cations. We suggest that Mg2+ and other polycations reduce the currents by electrostatic binding to the negative charges of PIP2, competitively reducing the amount of free PIP2 available for interaction with channels. The dose–response curves could be modeled by a competition model that reduces the pool of free PIP2. This mechanism is likely to modulate many other PIP2-dependent ion channels and cellular processes.

Effects of polyvalent cations on the folding of an rRNA three-way junction and binding of ribosomal protein S15.

The Bacillus stearothermophilus ribosomal protein S15 binds to a phylogenetically conserved three-way junction formed by the intersection of helices 20, 21, and 22 of eubacterial 16S ribosomal RNA, inducing a large conformational change in the RNA. Like many RNA structures, this three-way junction can also be folded by the addition of polyvalent cations such as magnesium, as demonstrated by comparing the mobilities of the wild-type and mutant junctions in the absence and presence of polyvalent cations in nondenaturing polyacrylamide gels. Using a modification interference assay, critical nucleotides for folding have been identified as the phylogenetically conserved nucleotides in the three-way junction. NMR spectroscopy of the junction reveals that the conformations induced by the addition of magnesium or S15 are extremely similar. Thus, the folding of the junction is determined entirely by RNA elements within the phylogenetically conserved junction core, and the role of Mg2+ and S15 is to stabilize this intrinsically unstable structure. The organization of the junction by Mg2+ significantly enhances the bimolecular association rate (k(on)) of S15 binding, suggesting that S15 binds specifically to the folded form of the three-way junction via a tertiary structure capture mechanism.

Comparison of the effects of Ca2+ and those of Mg2+ and La3+ on endplate currents in mouse interosseus muscles

Acetylcholine-induced currents were measured in dissociated muscle fibres from the mouse interosseus. Fast and slow desensitization could be distinguished, the latter ending in a steady-state current. The time constants of the two phases of current decrease and the steady-state current were decreased when the superfusing solution contained di- or trivalent cations in addition to the 0.2 mM Mg2+ present in the control solution. When Ca2+, Mg2+ or La3+ was added at the time at which the current had settled to a near steady-state, there was a fall of current, the time course of which was much faster than that during desensitization. The amplitude and the time course of current change were nearly the same for Ca2+ and Mg2+, showing that the effect was not specific for any particular cation. No evidence of intracellular action of the cations was found. The [ACh]-current relationship was shifted to a higher [ACh] by raising [Mg2+] or [La3+]. Recovery of the ability to react to ACh was slowed by increasing [La3+], but the initial recovery of fast and slow time constants of desensitization was not much changed. Recovery was impeded by the presence of a low (conditioning) [ACh]; this effect was enhanced by La3+. It was concluded that the effects of polyvalent cations are similar to those of non-specific blockers.

Effects of polyvalent cations and dihydropyridine calcium channel blockers on recovery of CNS white matter from anoxia

The effects of anoxic injury on the functional integrity of mammalian central white matter were studied electrophysiologically using the rat optic nerve model. Previous studies on this model have shown that extracellular Ca2+ is critical to the production of irreversible anoxic injury, and suggest that during anoxia Ca2+ crosses the membrane to enter the intracellular compartment. We attempted to elucidate the mechanism by which this damaging Ca2+ influx occurs. The inorganic Ca2+ channel blockers Mn2+ (1 mM), Co2+ (1 mM) or La3+ (0.1 mM) had no effect on recovery of the area under the compound action potential after a standard 60 min period of anoxia; only Mg2+ (10 mM) significantly improved recovery (54.9 +/- 8.9% vs. 28.7 +/- 10.1%, P less than 0.005). Treatment with organic Ca2+ channel blockers of the dihydropyridine class, nifedipine (1-10 microM) or nimodipine (1-40 microM), also had no effect on recovery from anoxia. We conclude that Ca2+ influx during anoxia does not occur via conventional Ca2+ channels sensitive to polyvalent cations or dihydropyridines.

The effects of polyamines and aminoglycosides on phosphatidylinositol-specific phospholipase C from human amnion

The effects of polyvalent cations (polyamines and aminoglycoside antibiotics) on Ca 2+-dependent phosphatidylinositol-specific phospholipase C activity of human amnion tissue were examined. In the presence of 1 mM Ca 2+, the hydrolysis of phosphatidylinositol (2 mM) by phospholipase C was increased greatly (240–560% of control) by spermine (0.4 mM), spermidine (1 mM), neomycin (0.1 mM), gentamicin (0.2 mM), kanamycin (0.4 mM) and streptomycin (0.8 mM). Putrescine and cadaverine (0.1–2.0 mM), however, stimulated phospholipase C activity only slightly. The effects of spermidine, spermine and gentamicin on phospholipase C activity were characterized and found to be dependent upon the concentrations of phosphatidylinositol, Ca 2+ and the particular polyvalent cation. At low concentrations of phosphatidylinositol and Ca 2+ the predominant effect of polyamines and aminoglycosides was to inhibit phospholipase C activity. When the concentrations of phosphatidylinositol and Ca 2+ were increased, spermidine, spermine and gentamicin stimulated phospholipase C activity. In the presence of 16 mM Ca 2+, however, phospholipase C activity was maximal and was unaffected by either polyamines or aminoglycosides. At all concentrations of Ca 2+ examined, the maximal stimulation of phospholipase C activity by a given polyvalent cation occurred at a fixed molar ratio of the particular polyvalent cation to phosphatidylinositol. Polyamines and aminoglycosides appeared to modulate the Ca 2+ requirement for phospholipase C activity, but could not substitute completely for Ca 2+. The activities of phospholipase A 2, diacylglycerol lipase, monoacylglycerol lipase and diacylglycerol kinase in amnion tissue were unaffected by any of the polyvalent cations examined. It is proposed that any in vivo influences (stimulatory or inhibitory) of polyamines and aminoglycosides on amnion phospholipase C activity would depend upon the effective concentrations of Ca 2+ and phosphatidylinositol

Effects of polyvalent cations on electrolyte transport and inulin space of rat kidney cortical slices.

Effects of polyvalent cations on electrolyte transport and inulin space of rat kidney cortical slices.

The effect of metal ions and antidiuretic hormone on oxygen consumption in toad bladder.

1. The sodium-dependent oxygen consumption of pieces of toad bladder (Bufo marinus) has been investigated using an oxygen electrode.2. The effect of polyvalent cations (Ca(2+), Sr(2+), Mg(2+), Eu(3+), La(3+) and Mn(2+)) on sodium-dependent oxygen consumption has been measured. All cations inhibited oxygen consumption, the order of effectiveness being Ca(2+) > Sr(2+) > Mg(2+) > Mn(2+) > Eu(3+) > La(3+).3. Treatment of bladder pieces with antidiuretic hormone (50 m-u./ml.) decreased the effectiveness of Ca(2+) and Sr(2+) as inhibitors of sodium-dependent oxygen consumption. Mn(2+), Eu(2+) and La(2+) were more effective after hormonal treatment, while the effectiveness of Mg(2+) was unaltered.4. The results have been interpreted in terms of a model in which sodium entry to the transporting mechanisms of the epithelium is controlled by Ca(2+), and in which antidiuretic hormone alters Ca(2+) binding and so affects sodium transport.

A Dipole Model for Negative Steady-State Resistance in Excitable Membranes

A dipole model is presented for ion flow in excitable membranes. This model considers the membrane to be composed of two distinct regions: a polar region and a nonpolar region. Further, the construction of an electrodiffusive formalism which takes explicit account of the energy of partition required by an ion for passage from external fluid to nonpolar region is presented. In the polar region a cooperative effect is considered which produces a configurational transition of the polar group dependent only on membrane voltage. A resulting change in voltage drop across the polar group is brought about by this configurational transition. This gives rise to a negative steady-state resistance for the equimolar case, in reasonable agreement with observation. The theory, in addition, is in reasonable accord with nonequimolar ion flow, and provides an explanation for such effects as the following: the intercept of the voltage-current characteristic, the ion membrane concentrations inferred from electrodiffusion theories, and the effects of polyvalent cations

Specific Effects in Acid Catalysis by Ion Exchange Resins. III. Some Observations on the Effect of Polyvalent Cations1

Specific Effects in Acid Catalysis by Ion Exchange Resins. III. Some Observations on the Effect of Polyvalent Cations¹