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At least 865 records · Page 48Linked to original sources
PubMed1983

Inhibitory effects of sodium and other monovalent cations on purified versus membrane-bound kallikrein.

Glandular kallikrein is a serine proteinase implicated in epithelial ion transport processes. The present studies show that the enzymatic activities (alpha-N-tosyl-L-arginine methyl ester esterase and kininogenase) and the immunoreactivities of rat and human urinary kallikreins are inhibited by monovalent cations. The degree of inhibition, which exceeds 80% with physiological concentrations of cations, is dependent on temperature and preincubation time. The inhibitory effects are reversed when ions are removed via gel filtration. Bovine serum albumin or polyethylene glycol attenuate cationic inhibition, but sucrose or mannitol do not. Detergents, including deoxycholate, 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonate, Triton X-100, and Lubrol PX also attenuate cationic inhibition of enzymatic activity or immunoreactivity. Membrane-bound kallikrein of renal cortical microsomal fractions or detergent-treated purified kallikrein lose less than 30% of their activity, even in the presence of 200 mM cation. These results show that both catalytic activity and immunoreactivity of purified versus membrane-bound kallikreins are affected, but to a different extent, by cations. The data suggest that monovalent cations may play a role in the regulation of soluble versus membrane-bound kallikrein activity.

Animals↗
PubMed1980

Mechanisms of organic cation transport in kidney plasma membrane vesicles: 1. Countertransport studies.

Transport for organic cations has been described in both luminal and antiluminal membranes. The two transport systems difer from one another on the basis of kinetic parameters and countertransport. By using N1-[3H]methylnicotinamide as the indication cation, a series of organic cations (which were known to be secreted) were tested for their capacity to stimulate countertransport. The phenomenon of countertransport was symmetrical in that it was observed for both influx and efflux in both membranes. However, under appropriate conditions, certain organic cations were effective in producing "uphill" transport of N1-[3H]methylnicotinamide in the luminal membrane and the effect was limited to that membrane exclusively. Detailed analysis of countertransport in the luminal membrane showed quantitative differences in the maximal stimulation produced by the various cations: giving rise to the relationship that the concentration of organic cation which gave one-half maximal N1-[3H]methylnicotinamide countertransport approximated its affinity constant. The data were interpreted to suggest that all the members of the organic cation series tested are translocated across the luminal membrane by the same carrier but that they are moved at different rates.

Animals↗
PubMed1981

The role of cations in avian liver phosphoenolpyruvate carboxykinase catalysis. Activation and regulation.

The activation of the avian liver phosphoenolpyruvate carboxykinase catalyzed reversible decarboxylation of oxalacetate by Mn2+ has been studied. The Mn2+ facilitates the interaction of oxalacetate to the enzyme. At saturating ITP and oxalacetate concentrations, Mn2+ has a Km = 2 microM. The cation Mg2+ can substitute for Mn2+ with 50% Vmax in the oxalacetate decarboxylation and 2% Vmax in the oxalacetate formation. The Km for Mg2+ is 3 orders of magnitude greater than the Km of Mn2+, however. Of the other cations tested (Ca2+, Zn2+, Fe2+, and Cd2+), Co2+ was the only other cation found to activate the enzyme in both directions. In the presence of Mg2+, the enzyme is extremely sensitive to trace metal contaminants which can cause activation. At 1 mM Mg2+, 20 microM Mn2+ causes a 15-fold activation of activity. The apparent Km for Mn2+ (2 microM) at high concentrations of Mg2+ is the identical value calculated for free Mn2+. In a mixed metal (Mg2+ and Mn2+) assay, the Km values for phosphoenolpyruvate and for oxalacetate are independent of the concentration of Mg2+ but decrease upon an increase in Mn2+. The kinetic results demonstrate two roles for the divalent cations for activity. The cation forms a metal . nucleotide complex which serves as the substrate. The results indicate that MgITP is a better substrate than MnITP, but MnIDP is a better substrate than MgIDP. The cation also binds to the enzyme to form an enzyme . metal complex which is the active form of the enzyme. This cation functions to aid in the interaction of either oxalacetate or phosphoenolpyruvate to the enzyme. The activation by micromolar amounts of Mn2+ at millimolar concentrations of Mg2+ suggests that this enzyme, and thus the pathway of gluconeogenesis, can be modulated by changes in concentration of Mn2+ within mitochondria.

Animals↗
PubMed1983

Two Ca2+ functions are demonstrated by the substitution of specific divalent and lanthanide cations for the Ca2+ required by the aggregation factor complex from the marine sponge, Microciona prolifera.

Multivalent cations were tested for their ability to replace the Ca2+ requirements of aggregation factor (AF) complex in activity, stability, and integrity assays. The ability of each cation to replace the Ca2+ required for the cell aggregation-enhancing activity of AF was examined by replacing the usual 10 mM Ca2+ with the test cation at various concentrations in the serial dilution assay of the AF. The other alkaline earth cations, Mg2+, Sr2+, and Ba2+, could not replace Ca2+; two transition elements, Mn2+ and Cd2+, partially replaced calcium. All 15 of the available lanthanides (including La3+ and Y3+) produced normal activity but only at 10-400-fold lower cation concentrations than Ca2+. An AF preparation is stable and remains active for months in 1 mM Ca2+ but decays rapidly when Ca2+ is lowered. Sr2+ and Ba2+ at 20 mM but not at 1 mM could replace 1 mM Ca2+ and give long term stability. AF was not stable in the presence of Mg2+, even at 100 mM. High Mn2+ concentrations did not stabilize AF even though AF was partially active in Mn2+. Cd2+ gave full stability at 75 mM and La3+ at about 0.1 mM. When Ca2+ is chelated, the macromolecular subunits of the AF slowly dissociate. Permeation chromatography and analytical ultracentrifugation showed that the cations that stabilized activity maintained the integrity of AF complex while those that failed to stabilize activity allowed the complex to dissociate into subunits, indicating that these two Ca2+ requirements are related. The cation specificities for activity and for stability-integrity are different indicating that these are separate Ca2+-dependent functions.

Animals↗
PubMed1982

The effect of cations on the activity of human urinary kallikrein.

We studied the effect of ions on the ability of purified human urinary kallikrein to cleave its natural substrate (kininogen) as well as two synthetic substrates, tosylarginine [3H]methyl ester and Pro-Phe-Arg-[3H]benzylamide. The kininogenase activity of kallikrein is markedly dependent upon the concentration of cations in vitro. Kininogenase activity is very low when measured in a low electrolyte buffer. The addition of cations to the reaction mixture increases activity by up to 27-fold. Maximum activity is achieved with 100 mM sodium, 100 mM potassium, or 20 mM magnesium. The activity is stable at higher concentrations of cation. Renal kallikrein is believed to act within distal tubular fluid in vivo. The concentration of cations in this fluid varies widely in response to alterations in salt and water metabolism. Thus, the relationship of kininogenase activity to the concentration of cations demonstrated in vitro may be relevant to the activity of kallikrein at its presumed site of action in the kidney. In separate experiments, we evaluated the effect of ions on the amidase and esterase activities of kallikrein which are the basis of several assays in routine use for physiological studies. In contrast to their stimulatory effect on kininogenase activity, cations inhibit amidase and to a lesser extent esterase activity. Additional studies indicate that urinary cations probably account entirely for the well known ability of normal urine to inhibit the amidase and esterase activities of kallikrein.

Anions↗
PubMed1993

Nonpolar amino acid substitutions of potential cation binding residues glu-955 and glu-956 of the rat alpha 1 isoform of Na+, K(+)-ATPase.

Side chain oxygens are critical in the binding of cations by several macromolecules, and chemical labeling studies suggest that the carboxyl oxygens of E953 of the pig kidney Na, K-ATPase are essential for Na+ and K+ ligation. An adjacent residue, E954, is highly conserved and may also be important in cation binding. Replacement of the corresponding glutamates in the rat alpha 1 isoform (E955 and E956) with glutamines or aspartic acids has only a modest effect on enzyme activity, but both substituted amino acids contain at least one side chain oxygen, and may fulfill the role of the wild-type glutamates in binding cations. Since substitutions of amino acids lacking an oxygenated side chain have not been made at positions 955 and 956, nonpolar amino acid replacements (E955A, E956A, E955A-E956A, and E955L-E956L) were made, and the effects of the substitutions on the cation dependence properties of the mutant enzymes were examined. The substitutions were made using site-directed mutagenesis of the rat alpha 1 cDNA (which encodes a ouabain-resistant alpha subunit), followed by transfection of wild-type and mutant cDNAs into ouabain-sensitive HeLa cells, with subsequent expression of the altered proteins. Transfection with all cDNA constructs resulted in the ouabain resistance of transfected HeLa cells, demonstrating that the modified Na, K-ATPase in each case was functional. Ouabain resistance of Na, K-ATPase activity was increased 1,000-fold in microsomal membranes isolated from cells transfected with wild-type rat alpha 1 (WT) and all mutant cDNA constructs, compared to activity in membranes prepared from untransfected HeLa cells. This confirmed the expression of a ouabain-resistant enzyme in all transfected cell lines, and the enhanced ouabain resistance permitted the study of the exogenous, expressed Na, K-ATPase in the presence of 5.0 microM ouabain. Cation stimulation of exogenous Na, K-ATPase activity was not affected by the E955A substitution and only slightly by the E956A replacement (K0.5(Na+) of 3.40 +/- 0.21, 3.30 +/- 0.22, and 6.60 +/- 0.55 mM NaCl for WT, E955A and E956A, respectively; K0.5(K+) of 0.78 +/- 0.01, 0.74 +/- 0.10, and 0.56 +/- 0.11 mM KCl). Even doubly substituted enzymes had only mild alterations in cation dependence properties (K0.5(Na+) of 7.54 +/- 0.23, 7.14 +/- 0.10 mM NaCl for E955A-E956A, E955L-E956L, respectively; K0.5(K+) of 0.43 +/- 0.13, 1.83 +/- 0.13 mM KCl). The results demonstrate that there are no requirements for an oxygenated side chain at positions 955 and 956 for normal or nearly normal cation dependence.

Animals↗
PubMed1996

Stereospecificity in contraluminal and luminal transporters of organic cations in the rat renal proximal tubule.

The effect of chirality on the interaction of substrates with the organic cation transporters in the proximal tubule of rat kidney was investigated. The apparent Ki values of the enantiomers/diastereomers of ephedrine and norephedrine and the stereoisomers of deprenyl, tranylcypromine, disopyramide, verapamil, pindolol and quinine/quinidine were determined against the contraluminal organic cation transporter, the luminal H+/organic cation exchanger and the luminal choline+ transporter, using the stop-flow luminal or contraluminal capillary microperfusion method. The ephedrine/norephedrine enantiomers/diastereomers had apparent Ki values against the contraluminal organic cation transporter in the range of 0.8 to 2.4 mM, and only norpseudoephedrine showed significant enantioselectivity. The same substrates had apparent Ki values against the luminal H+/organic cation exchanger between 3.0 and 15.0 mM, and ephedrine, norephedrine and norpseudoephedrine showed enantioselectivity. The Ki values against the luminal choline+ transporter were even higher (7.2-19.1 mM) and demonstrated no enantioselectivity. The verapamil and deprenyl enantiomers showed selectivity against the luminal choline+ transporter, as did quinine and quinidine against the contraluminal organic cation transporter. In all other instances enantioselectivity was not observed. In no case was the difference in the Ki values of the enantiomers/isomers greater than a factor of 3. The data confirm the high degree of nonspecificity of the renal organic cation transporters. Evaluation of three-dimensional molecular models of the ephedrine enantiomers/diastereomers suggests that the spatial orientations of the amino group and, to a lesser extent, the OH group and possibly the terminal CH3 group are of importance for different interactions with the transporters.

1-Methyl-4-phenylpyridinium↗
PubMed1998

Oligodeoxyribonucleotide uptake in primary human hematopoietic cells is enhanced by cationic lipids and depends on the hematopoietic cell subset.

The use of antisense oligodeoxyribonucleotides (ODN) is a potential method to switch off gene expression. The poor cellular uptake of ODN in primary cells still is a limiting factor that may contribute to the lack of functional efficacy. Various forms of cationic lipids have been developed for efficient delivery of nucleic acids into different cell types. We examined the two cationic lipids DOTAP and DOSPER to improve uptake of ODN into primary human hematopoietic cells. Using a radiolabeled 23-mer, ODN uptake into blood-derived mononuclear cells could be increased 42- to 93-fold by DOTAP and 440- to 1,025-fold by DOSPER compared with application of ODN alone. DOTAP was also effective for delivery of ODN into leukocytes within whole blood, which may resemble more closely the in vivo conditions. As assessed by fluorescein isothiocyanate-conjugated ODN both cationic lipids enhanced cytoplasmic accumulation of ODN in endosome/lysosome-like structures with a partial shift of fluorescence to the whole cytoplasm and the nucleus following an incubation of 24 hours. ODN uptake by cationic lipids into different hematopoietic cell subsets was examined by dual-color immunofluorescence analysis with subset-specific monoclonal antibodies. We found a cell type-dependent delivery of ODN with greatest uptake in monocytes and smallest uptake in T cells. CD34+ cells, B cells, and granulocytes took up ODN at an intermediate level. Uptake of ODN into isolated CD34+ cells could be increased 100- to 240-fold using cationic lipids compared with application of ODN alone. Stimulation of CD34+ cells by interleukin-3 (IL-3), IL-6, and stem cell factor did not significantly improve cationic lipid-mediated ODN delivery. Sequence-specific antisense effects in clonogenic assays could be shown by transfection of bcr-abl oncogene-directed antisense ODN into primary cells of patients with chronic myelogenous leukemia using this established protocol. In conclusion, cationic lipids may be useful tools for delivery of antisense ODN into primary hematopoietic cells. These studies provide a basis for clinical protocols in the treatment of hematopoietic cells in patients with hematologic malignancies and viral diseases by antisense ODN.

Antibodies, Monoclonal↗
PubMed1998

Functional characterization of an organic cation transporter (hOCT1) in a transiently transfected human cell line (HeLa).

Recently, a polyspecific organic cation transporter, hOCT1, was cloned from human liver. To date, limited studies examining the functional characteristics of the transporter have been performed. The purpose of the present study was to develop a mammalian expression system for hOCT1 and to characterize the interactions of various compounds with the cloned transporter. Lipofection was used to transiently transfect the hOCT1 plasmid DNA in a human cell line, HeLa. We tested the interaction of an array of organic cations and other compounds with hOCT1 by determining Ki values in inhibiting 14C-tetraethylammonium (TEA) transport in the transfected cells. Transient expression of hOCT1 activity was observed between 24 and 72 hr post-transfection, with maximal expression at approximately 40 hr. TEA transport was temperature dependent and saturable with Vmax and K(m) values of 2.89 +/- 0.448 nmol/mg protein/30 min and 229 +/- 78.4 microM, respectively. 14C-TEA uptake in hOCT1 plasmid DNA-transfected HeLa cells was trans-stimulated by unlabeled TEA and 1-methyl-4-phenyl-pyridinium. Organic cations, including clonidine, quinine, quinidine and verapamil (0.1 mM), significantly inhibited 14C-TEA uptake, whereas the organic anion, p-aminohippuric acid (5 mM), did not. The neutral compounds, corticosterone (Ki, 7.0 microM) and midazolam (Ki, 3.7 microM) potently inhibited 14C-TEA uptake. The Ki values of several compounds in interacting with hOCT1 differed substantially from the corresponding values for the rat organic cation transporter, rOCT1, and the human kidney-specific organic cation transporter, hOCT2, determined in previous studies. Transiently transfected HeLa cells represent a useful tool in studying the interactions and kinetics of organic cations and other xenobiotics with hOCT1 and in understanding the molecular events involved in organic cation transport.

Carrier Proteins↗
PubMed1998

Structural changes associated with the coupling of ATP hydrolysis and cation transport by the Na pump.

Most of the residues associated with cation coordination seem to reside within transmembrane segments of the alpha-subunit of the Na,K-ATPase, whereas amino acids which appear to be involved in the coordination of ATP are found in the major cytoplasmic loop between transmembrane segments M4 and M5 (see Lingrel & Kuntzweiler, 1994; Lutsenko & Kaplan, 1995). The coupling of the two functions of cation transport and ATP hydrolysis involved in the active transport of Na and K ions must involve interactions between these two structural units. This paper summarizes recent experimental results and conclusions of studies on the renal Na,K-ATPase which have employed controlled proteolysis in the presence of physiological ligands, chemical modification with a range of reagents and a variety of functional assays. The data provide evidence for movements between specific transmembrane segments associated with cation-binding conformations and coupled changes which take place in the ATP binding domain. The binding of different cations in the cation-binding domain is sensed in the ATP binding domain and manifested as a change in reactivity. This occurs at amino acid residues which are widely spaced in primary structure. It is apparent that structural changes are transmitted through much of the ATP-binding domain as a consequence of the occupancy of the cation-binding domain. We also provide evidence that both the number and identity of cations bound are also sensed in the ATP-binding domain.

Adenosine Triphosphate↗
PubMed1986

Changes in conformation, stability and condensation of DNA by univalent and divalent cations in methanol-water mixtures.

Circular dichroism spectroscopy, absorption spectroscopy, measurements of Tm values, sedimentation analysis and electron microscopy were used to study properties of calf thymus DNA in methanol-water mixtures as a function of monovalent cation (Na+ or Cs+) concentration and also in the presence of divalent cations Ca2+, Mg2+, and Mn2+. In the absence of divalent cations only slight conformational changes occurred and no condensation and/or aggregation could be detected. The Tm values depend on the amount of methanol and on the nature and concentration of cations. In methanol-water mixtures higher thermal stability was observed in solutions containing Cs+ ions. Up to 40% (v/v) methanol the addition of divalent ions leads to DNA stabilization. At methanol concentration higher than 50% the presence of divalent cations causes DNA condensation and denaturation even at room temperature. The denaturation is reversible with respect to EDTA addition indicating that no separation of complementary strands occurred and the resulting form of DNA is probably similar to the P form. DNA destacking appears to be a direct consequence of stronger cation binding by the condensed DNA in methanol-water mixtures.

Animals↗
PubMedDate not supplied

Enhanced firefly bioluminescent assay of adenosine 5'-triphosphate using liposomes containing cationic cholesterols.

Cationic liposomes composed of phosphatidylcholine and cationic cholesterols were prepared by extrusion technique. Dimethylaminoethyl-carbamoyl cholesterol (DMAE-chol) and diethylaminoethyl-carbamoyl cholesterol (DEAE-chol) were synthesized as a cationic cholesterol. Cationic liposomes containing DMAE-chol and DEAE-chol enhanced the intensity of maximum light emission from the firefly bioluminescent (BL) reaction. The sensitivity for ATP in the presence of cationic liposomes containing DMAE-chol and DEAE-chol was improved by a factor of 10 times compared to that in water alone. The detection limit for ATP upon using cationic liposomes was 1.0 pmol/L in an aqueous standard solution. The BL enhancement in the presence of cationic liposomes could be explained in terms of BL emitters and electrostatic interaction between the liposomes surface and BL reactants.

Adenosine Triphosphate↗
PubMed2004

Neovascular targeting chemotherapy: encapsulation of paclitaxel in cationic liposomes impairs functional tumor microvasculature.

Cationic liposomes have been shown to be internalized selectively by angiogenic tumor endothelial cells after intravenous injection. Therefore, encapsulation of cytotoxic substances in cationic liposomes is a new approach to target tumor vasculature. It was the aim of our study to quantify the effects of paclitaxel encapsulated in cationic liposomes (MBT-0206) on tumor microvasculature and growth in vivo. Experiments were performed in the dorsal skinfold chamber preparation of Syrian Golden hamsters bearing syngeneic A-Mel-3 melanomas. Tumors were treated with intravenous infusion of MBT-0206 (20 mM) resulting in an effective paclitaxel dose of 5 mg/kg body weight (b.w.). Control animals received conventional paclitaxel in Cremophor EL (Taxol(R); 5 mg/kg b.w.), unloaded cationic liposomes (20 mM) or the solvent 5% glucose, respectively. Using intravital microscopy, tumor growth and effects on intratumoral microvasculature were analyzed. Tumor growth was significantly retarded after treatment with MBT-0206 compared to the treatment with paclitaxel. Analysis of intratumoral microcirculation revealed a reduced functional vessel density in tumors after application of liposomal paclitaxel. At the end of the observation time, vessel diameters were significantly smaller in animals treated with paclitaxel encapsulated in cationic liposomes while red blood cell velocity was less affected. This resulted in a significantly reduced blood flow in vessel segments and a reduced microcirculatory perfusion index in these animals. Histochemical TUNEL stain was vessel-associated after treatment with liposomal paclitaxel in contrast to few apoptotic tumor cells in the control groups. Our data demonstrate that encapsulation of paclitaxel in cationic liposomes significantly increased the antitumoral efficacy of the drug. Remarkable microcirculatory changes indicate that encapsulation of paclitaxel in cationic liposomes resulted in a mechanistic switch from tumor cell toxicity to an antivascular therapy.

Animals↗
PubMed2007

Double cation adduction in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of electron deficient anthraquinone derivatives.

Six anthraquinone derivatives were analyzed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS). Clear (pseudo) molecular ions were observed for all the compounds. Interestingly, for some derivatives, strong ions with double cation adduction were also recorded in the positive mode. It is remarkable that all these ions are singly charged. In this work, possible mechanisms for the double cation adduction were investigated and discussed. It appears that the double cation adduction was due to the electron deficient nature of the derivatives, and formed by taking up two singly charged cations and one electron. Substituents on the anthraquinone ring were found to have a significant effect on the double cation adduction. In contrast, no considerable influence of the acidity of MALDI matrix/solution was observed, even on the double proton adduction. Furthermore, it was demonstrated that double cation adduction might occur in the MALDI gas-phase plume. In addition to the anthraquinones, three more electron deficient compounds of different types, i.e. a perylene bisimide derivative (PB), 3,7-decanoylamino-4,8-dihydrobenzo[1,2-b:4,5-b']dithiophene-4,8-dione (TQ) and 6,6-phenyl C61-butyric acid methyl ester (PCBM), were also analyzed with MALDI TOF MS. The results indicate that the 'abnormal' double cation adduction might be a 'normal' phenomenon in the MALDI TOF MS analysis of many electron deficient compounds.

Journal Article↗
PubMed1977

Mechanism of histamine binding II: effect of alkali metal and alkaline earth cations on histamine binding to peptide H.

The association constants of histamine with Mg2+, Ca2+, Sr2+, and Ba2+ were determined in buffer solutions at constant pH using an ion selective electrode. These cations enabled histamine to bind to peptide H. A minimum cation binding concentration was required for histamine binding. A linear relationship existed between the minimum cation binding concentration and the log of the equilibrium constant for the histamine-cation complexes, indicating that the specificity of the alkaline earth cations in promoting histamine binding was due to the difference in their ability to complex with histamine. The monovalent cations, Nat, Kt, and Cst inhibited histamine binding to peptide H, with the extent of inhibition dependent on cation concentration. An ion-exchange mechanism or a conformation change in the peptide may account for the inhibition.

Barium↗
PubMed1994

Poorly selective cation channels in the apical membrane of A6 cells.

This paper describes a Ca(2+)-blockable, poorly selective cation pathway in the apical membrane of A6 epithelia. This pathway has properties that resemble the cation-selective channels in the toad urinary bladder and frog skin. Transepithelial short circuit currents (Isc) and power density spectra (PDS) of the fluctuations in current were recorded. The basolateral surface of the tissues was exposed to Cl- or SO4(2-) solutions with Na+ as the major cation. Ca(2+)-blockable inward oriented currents and Lorentzian noise were recorded with isotonic (215 mosmol/kg) mucosal Cl- and hypotonic (144 mos-mol/kg serosal SO4(2-) solution with Na+, K+, Rb+ or Cs+ as the major mucosal cation. Experiments with mucosal K+ demonstrated that the cation-selective channel was markedly activated by serosal hypotonicity. Effects of an increased electrical driving force were excluded on the basis of the results obtained with microelectrode experiments and transepithelial voltage clamping. Cell volume expansion induced by isotonic replacements of serosal sucrose by glycerol or urea also activated the cation-selective pathway. Furthermore, the presence of Cl- in the mucosal solution was a prerequisite for a sustained response to hypotonicity or replacements of the organic compounds. Moreover, we found that the cation-selective channels are mainly expressed in the cells during the early period of epithelial growth.

Animals↗
PubMed1984

Divalent cation effects on acetylcholine-activated channels at the frog neuromuscular junction.

The effects of the divalent cations Ca and Mg on the properties of ACh-activated channels at the frog neuromuscular junction were studied using a two-microelectrode voltage clamp. The divalent cation concentration was varied from 2 to 40 mM in solutions containing 50% normal Na. The reversal potential was determined by interpolation of the acetylcholine (ACh)-induced current versus voltage relationship. The single-channel conductance and the mean channel lifetime were calculated from fluctuation analysis of the ACh-induced end-plate current. Extracellular Na and/or divalent cations affected the reversal potential of endplate channels in a way that cannot be described by the Goldman-Hodgkin-Katz equation or by a simple two-barrier, one-binding site model of the channel if the assumption was made that permeability ratios were constant and not a function of ion concentrations. Increasing the divalent cation concentration decreased the single-channel conductance to approximately 10 pS in solutions with 50% Na and 40 mM divalent cation concentrations. The effect of the divalent cations Ca and Mg on the mean channel lifetime was complex and dependent on whether the divalent cation was Ca or Mg. The mean channel lifetime was not significantly changed in most solutions with increased Ca concentration, while it was slightly prolonged by increased Mg concentration.

Acetylcholine↗
PubMed1983

(Na+,K+)-ATPase of mammalian brain: effects of temperatures on cation and ATP interactions regulating phosphatase activity.

The effects of temperature on interactions between univalent cations or ATP and the p-nitrophenylphosphatase activity associated with brain (Na+,K+)-ATPase were examined. The apparent affinity for K+ activation under conditions favoring the moderate affinity site was temperature dependent, increasing with decreasing temperature. A comparison of univalent cations showed that the negative apparent delta H and delta S for cation binding increased with increasing apparent cation affinity. In contrast to the case with the moderate affinity sites, apparent affinity for the high affinity K+ site was independent of temperature. As temperature decreased, properties of moderate affinity site binding approached those of the high affinity site. The temperature dependence of ATP inhibition was opposite to that for K+ activation, with positive apparent delta H and delta S. The apparent delta H and delta S for cation binding approached those for the overall conformational change to K+-sensitive enzyme as cation affinity increased. These data suggest that E2, the K+-sensitive form of (Na+,K+)-ATPase, is stabilized by forces that require a decrease in entropy, explaining the predominant existence of E1 at physiologic temperatures. A conformational change leading to stabilization of E2 at higher temperatures can be produced by binding of univalent cations to a moderate affinity, presumably intracellular, site. This effect is counteracted by ATP. ATP also appears to alter the selectivity of this site to favor Na+ over K+ binding.

4-Nitrophenylphosphatase↗