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PubMed1978

Effect of tetramethylammonium ions on conformational changes of DNA in the premelting temperature range.

The reversible conformational change of DNAs and polydeoxyribonucleotides occurring before melting was followed by circular dichroism. deltatheta/deltaT, the rate of change of ellipticity theta with temperature, was used mainly as a measure of this premelting phenomenon. If sodium ions were replaced by tetramethylammonium ions deltatheta/deltaT decreased for poly (dA) poly (dT) and poly (dA.dT) poly (dT.dA), but increased for poly (dG.dC) poly (dC.dG). DNAs of different base composition showed no more premelting (deltatheta/deltaT approximately 0) even at low molarities of TMACl provided the Na/TMA ratio was very small. For all cases studied the theta values at 0 degrees C and at a given ionic strength were smaller in NaCl than in TMACl. When studying the series of ammonium ions from NH4+ to (C2H5)4N+, the deltatheta/deltaT values first decreased, going through zero with TMA+ ions, and then increased again. A tentative and qualitative explanation of our results can be given: (a) Hydration of the polymers increases in presence of TMA ions and their average stability decreases; locally, however, (AT) pairs are preferentially stabilized by TMA ions owing to a specific interaction at the level of O2 of thymine. (b) In order to explain the different behaviour of (AT) polymers and DNA, it is assumed that only the B structure is able to accommodate TMA ions in the small groove of the double stranded helix.

Chemical Phenomena↗
PubMed1990

Cation radius effects on the helix-coil transition of DNA. Cryptates and other large cations.

Most polyelectrolyte theories of the effect of ions on the thermal melting of DNA assume that the predominant influence of the cations comes through their charge. Ion size and structure are treated, for analytic convenience, as negligible variables. We have examined the validity of this assumption by measuring the melting temperature of calf thymus DNA as a function of salt concentration with four univalent cations of different hydrated radii. These are K+ (3.3 A), (n-Pr)4N+ (4.5 A), (EtOH)4N+ (4.5 A), and C222-K+ (5 A). C222-K+ is a complex of cryptand C222 with K+. With K+ as the sole cation, Tm varies linearly with the log of ionic strength over the range 0.001-0.1 M. With all the K+ sequestered by an equimolar amount of C222, Tm is depressed by 10-20 degrees C and the slope of Tm vs. ionic strength is lower. At low ionic strength, an even greater reduction in Tm is achieved with (n-Pr)4N+; but the similar-sized (EtOH)4N+ gives a curve more similar to K+. Theoretical modeling, taking into account cation size through the Poisson-Boltzmann equation for cylindrical polyelectrolytes, predicts that larger cations should be less effective in stabilizing the double helix; but the calculated effect is less than observed experimentally. These results show that valence, cation size, and specific solvation effects are all important in determining the stability of the double-helical form of DNA.

Bridged Bicyclo Compounds↗
PubMed1969

Ammonium ion currents in the squid giant axon.

Voltage-clamp studies on intact and internally perfused squid giant axons demonstrate that ammonium can substitute partially for either sodium or potassium. Ammonium carries the early transient current with 0.3 times the permeability of sodium and it carries the delayed current with 0.3 times the potassium permeability. The conductance changes observed in voltage clamp show approximately the same time course in ammonium solutions as in the normal physiological solutions. These ammonium ion permeabilities account for the known effects of ammonium on nerve excitability. Experiments with the drugs tetrodotoxin (TTX) and tetraethyl ammonium chloride (TEA) demonstrate that these molecules block the early and late components of the current selectively, even when both components are carried by the same ion, ammonium.

Action Potentials↗
PubMed1977

Effects of strychnine on the potassium conductance of the frog node of Ranvier.

The nature of the block of potassium conductance by strychnine in frog node of Ranvier was investigated. The block is voltage-dependent and reaches a steady level with a relaxation time of 1 to several ms. Block is increased by depolarization or a reduction in [K+]O as well as by increasing strychnine concentration. A quaternary derivative of strychnine produces a similar block only when applied intracellularly. In general and in detail, strychnine block resembles that produced by intracellular application of the substituted tetraethylammonium compounds extensively studied by C.M. Armstrong (1969. J. Gen Physiol. 54:553-575. 1971. J. Gen. Physiol. 58:413-437). The kinetics, voltage dependence, and dependence on [K+]O of strychnine block are of the same form. It is concluded that tertiary strychnine must cross the axon membrane and block from the axoplasmic side in the same fashion as these quaternary amines.

Action Potentials↗
PubMed1987

Influence of tetraalkyl ammonium ions on the structure of poly (rG-dC).poly (rG-dC): unexpected transitions among the Z, A and B conformations.

The conformation of the double-stranded, mixed ribodeoxyribo polynucleotide, poly (rG-dC).poly (rG-dC), has been examined in the presence of tetraalkyl ammonium ions. Tetramethyl ammonium ion stabilizes the "low salt" Z conformation (1) of the polymer from submillimolar to molar concentrations of the counterion. In the presence of tetraethyl and tetrapropyl ammonium ions the polymer exists in the low salt Z form up to 2 mM concentration of the counterions and then flips to the right hand helical A form. With tetrabutyl ammonium counterions the polymer is in an A conformation at low ion concentrations and converts to a B form at concentrations greater than thirty millimolar. These results are interpreted in terms of electrostatic and solvent interactions of the polynucleotide.

Chemical Phenomena↗
PubMed2000

NEt4OH.5H2O containing hydroxide-water layers

In the title compound, tetraethylammonium hydroxide pentahydrate, C(8)H(20)N(+).OH(-).5H(2)O, layers of approximately hexagonally close-packed NEt(4)(+) cations and anionic layers of hydroxide and water molecules are stacked alternately along the b axis. All hydroxide and water H atoms are in ordered positions, giving rise to a network of hydrogen bonds [O.O 2.633 (1)-2.947 (2) A] with four- and six-membered rings. The hydroxide ion accepts four hydrogen bonds from four water molecules but does not act as a proton donor.

Journal Article↗
PubMed1969

Effects of quaternary ammonium compounds on choline transport in red cells.

1. Neuromuscular blocking agents have been shown to be powerful inhibitors of choline transport in human erythrocytes. Ganglionic blocking agents were weaker inhibitors.2. The affinity of the choline transport site for alkyltrimethylammonium compounds was considerably higher than its affinity for alkyl-bis-(trimethylammonium) compounds of similar chain length. The affinity increases with increasing length of the alkyl chain in both series.3. Tetramethylammonium (TMA), ethyl- and propyltrimethylammonium appear to enter the cells on the choline carrier while the larger monoquaternary compounds, and the bisquaternary compounds, bind to the carrier but are unable to cross the cell membrane.4. Radioactively labelled carbachol, acetylcholine and decamethonium do not enter the cells on the choline carrier.5. Choline transport in erythrocytes from patients with myasthenia gravis is normal, suggesting that this disorder is not associated with a generalized defect of choline transport.

Acetylcholine↗
PubMed1985

Synaptic block of a transmitter-induced potassium conductance in Aplysia neurones.

A voltage-clamp study was made of a slow excitatory post-synaptic potential (slow e.p.s.p.) that can be elicited in the medial cells of the left pleural ganglion of Aplysia californica by the firing of at least three different presynaptic neurones (labelled I, II and III). Each of these three neurones elicits other permeability changes in addition to the slow e.p.s.p., and all elements of these synaptic responses were shown to be mediated monosynaptically. The slow e.p.s.p., associated with an increase in membrane resistance, was shown to be due to a decrease in K permeability. When the slow e.p.s.p. was present spontaneously, it could be blocked by three compounds (tetraethylammonium (TEA), phenyltrimethylammonium (PTMA), or methylxylocholine (beta-TM 10], all previously shown to block the cholinergic receptor that mediates an increase in K conductance in the medial cells (see Kehoe, 1972b). Furthermore, in ganglia in which no slow e.p.s.p. was seen in response to firing of the neurones I, II, and III, such a response became manifest when agonists capable of activating the cholinergic receptor were applied (e.g. acetylcholine (ACh), carbachol, arecoline, or F2268). The slow e.p.s.p. thus appears to result from the reduction, induced by any one of three 'blocking neurones', of a cholinergically controlled K conductance. Finally, when presynaptic neurone I (the only neurone tested) was fired shortly before or during the activation of presynaptic neurone IV, previously shown to be cholinergic (Kehoe, 1972b), the K component of the cholinergic post-synaptic inhibitory potential was markedly reduced. The concentration at which a given agonist caused the manifestation of the synaptic diminution in K conductance (i.e. the slow e.p.s.p.) was found to be the same as that at which it caused a reduction in the synaptically activated, cholinergic, K-dependent conductance elicited by presynaptic neurone IV. Intracellularly injected adenosine 3',5'-cyclic monophosphate (cyclic AMP) imitated the effect of the 'blocking neurones' on the K conductance activated by bath-applied cholinomimetics. This effect was superimposed on a cyclic-AMP-induced, voltage-dependent inward current that disappeared when the cell was bathed in Na-free sea water, or when the extracellular Ca concentration was increased to 60 mM. The effect of cyclic AMP on the cholinergic K conductance remained even after this cyclic-AMP-activated inward current was eliminated.(ABSTRACT TRUNCATED AT 400 WORDS)

1-Methyl-3-isobutylxanthine↗
PubMed1995

Rhythmic electrical and mechanical activity in stomach of toad and frog.

Rhythmic slow spikes and contractions at 2-5/min occur in frog/toad stomach when longitudinal muscle is present, not in the circular layer after removal of the longitudinal layer. Interstitial cells of Cajal occur in frog stomach in the longitudinal layer and in toad stomach mainly in the longitudinal layer, with some between layers. They were identified by staining with methylene blue, the fluorescent dye rhodamine 123, toluidine blue, and by electron microscopy. Interstitial cells resemble in ultrastructure those observed in mammalian intestine by small cell diameter, long processes forming a network, large nuclei, mitochondria, caveolae, endoplasmic reticulum, and absence of fibrillae. Rhythmic activity in stomachs stained with methylene blue is abolished by bright illumination. Rhythmic slow spikes and contractions are reduced and frequency is lowered when Ca2+ concentration in the medium is significantly decreased; in high Ca2+ these functions are increased. Threshold concentration of Ca2+ for maintaining slow spikes was between 10(-7) and 10(-6) M Ca2+. Ba2+ and 2.5 mM increases slow spike frequency and amplitude; Ba2+ increases contractions proportionately more than spikes. Slow spikes are reduced or blocked by verapamil, nifedipine, and Cd2+ and are enhanced by BAY K 8644. Quaternary ammonium compounds tetraethylammonium and tetrapentyl-ammonium bromide prolong slow spikes and enhance contractions. Replacement of NaCl by LiCl, methyl glutamine, or treatment with amiloride reduces slow spike amplitude and to a lesser extent frequency. This indicates efflux of Ca2+ by Na+/Ca2+ exchange. Contractions, but not spikes, are reduced by the calmodulin blocker trifluoroperazine. Contractions are enhanced, but slow spikes are not altered by ouabain. This suggests retention of Ca2+ when a tonic Na+/K+ pump is blocked. A model for rhythmicity includes influx of Ca2+ via L-channels, repolarization by IKCa2+, efflux of Ca2+ by Na+/Ca2+ exchange, and efflux of Na+ by a Na+/K+ pump. Frequency is determined by rate constants for both influx and efflux of Ca2+.

Animals↗
PubMedDate not supplied

Determination of ten N-nitrosoamino acids in cured meat products.

A rapid, sensitive, and accurate solid-phase extraction method was developed for the measurement of 10 N-nitrosoamino acids (NAAs) in cured meat products. In the procedure, the comminuted meat was mixed with sulfamic acid and Celite, and then added to a glass column containing anhydrous sodium sulfate. The column was washed with pentane, and the NAAs were eluted with ethyl acetate. The eluate was concentrated, then derivatized with diazomethane followed by acetic anhydride-pyridine reagent. The NAA methyl esters and their acylated hydroxy derivatives were separated by gas chromatography on a DB-5 fused silica capillary column and quantitated with a thermal energy analyzer, a chemiluminescence detector specific for nitric oxide derived from the thermal denitrosation of nitrosamines. Recovery of 10 of the NAAs exceeded 75% at the 10 ppb level. The method is applicable to a wide range of cured meat products.

Amino Acids↗
PubMed1989

[Ionic mechanisms evoked by acetylcholine-, nicotine-, and muscarine-induced depolarization of Helix lucorum RPa4 neurons].

Three cholinomimetics: acetylcholine, nicotine and muscarine were applied locally on soma of Helix lucorum RPa4 neuron to study ionic nature of its depolarization. Substances that inhibit Na-, K-, Ca- and Cl-conductances were used. Acetylcholine increased Na-, Ca- and Cl-conductances. As the chlorine response part was small due to the proximity of the resting membrane potential and chlorine equilibrium potential, it may be concluded that the dominating response to acetylcholine is connected with the chemocontrolled input of Na+ and Ca2+ ions into a cell. Nicotine induced mainly Na-conductance and muscarine induced Ca-conductance.

Acetylcholine↗