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PubMed1996

Mechanism of oligonucleotide release from cationic liposomes.

We propose a mechanism for oligonucleotide (ODN) release from cationic lipid complexes in cells that accounts for various observations on cationic lipid-nucleic acid-cell interactions. Fluorescent confocal microscopy of cells treated with rhodamine-labeled cationic liposome/ fluorescein-labeled ODN (F-ODN) complexes show the F-ODN separates from the lipid after internalization and enters the nucleus leaving the fluorescent lipid in cytoplasmic structures. ODN displacement from the complex was studied by fluorescent resonance energy transfer. Anionic liposome compositions (e.g., phosphatidylserine) that mimic the cytoplasmic facing monolayer of the cell membrane released ODN from the complex at about a 1:1 (-/+) charge ratio. Release was independent of ionic strength and pH. Physical separation of the F-ODN from monovalent and multivalent cationic lipids was confirmed by gel electrophoresis. Fluid but not solid phase anionic liposomes are required, whereas the physical state of the cationic lipids does not effect the release. Water soluble molecules with a high negative linear charge density, dextran sulfate, or heparin also release ODN. However, ATP, spermidine, spermine, tRNA, DNA, polyglutamic acid, polylysine, bovine serum albumin, or histone did not release ODN, even at 100-fold charge excess (-/+). Based upon these results, we propose that the complex, after internalization by endocytosis, induces flip-flop of anionic lipids from the cytoplasmic facing monolayer. Anionic lipids laterally diffuse into the complex and form a charged neutralized ion-pair with the cationic lipids. This leads to displacement of the ODN from the cationic lipid and its release into the cytoplasm.

Base Sequence↗
PubMed2000

Synthetic receptors as models for alkali metal cation-pi binding sites in proteins.

The alkali metal cations Na(+) and K(+) have several important physiological roles, including modulating enzyme activity. Recent work has suggested that alkali metal cations may be coordinated by pi systems, such as the aromatic amino acid side chains. The ability of K(+) to interact with an aromatic ring has been assessed by preparing a family of synthetic receptors that incorporate the aromatic side chains of phenylalanine, tyrosine, and tryptophan. These receptors are constructed around a diaza-18-crown-6 scaffold, which serves as the primary binding site for an alkali metal cation. The ability of the aromatic rings to coordinate a cation was determined by crystallizing each of the receptors in the presence of K(+) and by solving the solid state structures. In all cases, complexation of K(+) by the pi system was observed. When possible, the structures of the unbound receptors also were determined for comparison. Further proof that the aromatic ring makes an energetically favorable interaction with the cation was obtained by preparing a receptor in which the arene was perfluorinated. Fluorination of the arene reverses the electrostatics, but the aromaticity is maintained. The fluorinated arene rings do not coordinate the cation in the solid state structure of the K(+) complex. Thus, the results of the predicted electrostatic reversal were confirmed. Finally, the biological implications of the alkali metal cation-pi interaction are addressed.

Binding Sites↗
PubMed1996

A guanine nucleotide-independent inwardly rectifying cation permeability is associated with P2Y1 receptor expression in Xenopus oocytes.

The functional properties of the G protein-coupled P2Y1 receptor were investigated in Xenopus oocytes. Incubation of oocytes expressing either the human or turkey P2Y1 receptor with adenine nucleotide agonists resulted in an increase in Cl- current and activation of a novel cation current with an inwardly rectifying current-voltage relationship. Activation of either the human P2Y2 (P2U-purinergic) or M1 muscarinic receptor expressed in oocytes resulted in an increase in Cl- current similar to that observed in P2Y1 receptor-expressing oocytes but had no effect on cation current. P2 receptor agonists stimulated both the cation current and Cl- current in P2Y1 receptor-expressing oocytes with EC50 values and an order of potency (2-methylthioadenosine diphosphate > 2-methylthioadenosine triphosphate (2MeSATP) > ATP > UTP) that were similar to those previously observed for activation of phospholipase C in 1321N1 human astrocytoma cells stably expressing the human or turkey P2Y1 receptor. The P2Y receptor antagonists suramin and pyridoxal phosphate 6-azophenyl-2'-4'-disulfonic acid both shifted to the right the concentration-response relationship for 2MeSATP for stimulation of oocyte currents. Although injection of oocytes with either GDPbetaS (guanyl-5'-yl thiophosphate) or GTPgammaS (guanosine 5'-3-O-(thio)triphosphate) resulted in loss of adenine nucleotide-promoted Cl- channel activation, neither guanine nucleotide altered the 2MeSATP-stimulated cation current. These data are consistent with the view that activation of the novel cation current by the P2Y1 receptor does not involve a G protein. Tail current analysis of the novel P2Y1 receptor-associated cation conductance revealed that the open channel current-voltage relationship was outwardly rectifying with a reversal potential of -38 mV for the turkey P2Y1 receptor and -36 mV for the human P2Y1 receptor. Replacement of Na+ with K+ ions in the bathing solution produced a shift in reversal potential to near zero mV, but significant outward rectification remained. The cation current was not permeable to either Ca2+ or Ba2+ and exhibited steady-state inactivation at holding potentials below -60 mV. These results indicate that the P2Y1 receptor exhibits both metabotropic properties and a novel G protein-independent ionotropic response when expressed in Xenopus oocytes.

Animals↗
PubMed2006

Effect of alkali metal cation on the anaerobic hydrolysis and acidogenesis of vegetable waste.

Five batch testing scenarios were designed to evaluate the effects of alkali metal cations on anaerobic hydrolysis and acidogenesis. These scenarios were A (c=0 g l(-1)), B (cNa+=25 g l(-1)), C (cNa+=50 g l(-1)), D (cK+=25 g l(-1)), and E (cK+=50 g l(-1), pH 7.0). A solution pH of 7.0 or above favored protein hydrolysis, higher proteinase activity and higher ammonia production. However, such a pH suppressed carbohydrate hydrolysis, as indicated by low alpha-amylase activity. Cation interference at pH 5.0-6.0 seemed not to affect carbohydrate hydrolysis, as showed by the unimpaired alpha-amylase activity at 50 g l(-1) K+. Acidogenesis was more sensitive to alkali metal cations, so acid production and the drop in pH were lowest in a 25-50 g l(-1) Na+, acidic environment (pH 4.0-6.0). It was insensitive to cations when the pH was maintained at 7.0-8.0. When the pH was uncontrolled and decreased freely to acidic values, 25 g l(-1) of cation inhibited the action of the microbes, which rapidly acclimated, as presented by the slow transformation of soluble polymers to soluble metabolites. However, acidogenetic microbes could not easily recover from inhibition by 50 g l(-1) of cation. When the pH was maintained at over 7.0, the microbes were not inhibited by cation (50 g l(-1)) as indicated by the more active acidogenesis. The metabolic pathways to lactate, acetate and alcohols were not fully coupled.

Bacteria, Anaerobic↗
PubMed1972

Inorganic cations in the cell nucleus. Selective accumulation during meiotic prophase in mouse testis.

Earlier reports indicated the presence of significant amounts of inorganic salts in the nucleus. In the present study the possibility that this might be related to the transcription process was tested on seminiferous epithelium of the adult mouse, using potassium pyroantimonate as a fixative. The results indicated that a correlation exists between the inorganic cations comprising the pyroantimonate-precipitable fraction and the RNA synthetic activity. During meiotic prophase an accumulation of cation-antimonate precipitates occurs dispersed through the middle pachytene nuclei, the stage in which RNA synthesis reaches a maximum. At other stages (zygotene to diplotene), where RNA synthesis falls to a low level, that pattern is not seen; cation-antimonate deposits are restricted to a few masses in areas apparently free of chromatin. The condensed sex chromosomes, the heterochromatin of the "basal knobs," the axial elements, and the synaptonemal complexes are devoid of antimonate deposits during the meiotic prophase. The Sertoli cells, active in RNA synthesis in both nucleoplasm and nucleolus, show cation-antimonate deposits at these sites. In the nucleoplasm some "patches" of precipitates appear coincident with clusters of interchromatin granules; in the nucleolus the inorganic cations are mainly located in the fibrillar and/or amorphous areas, whereas relatively few are shown by the granular component. The condensed chromatin bodies associated with the nucleolus were always free of antimonate precipitates. It is suggested that the observed sites of inorganic cation accumulation within the nucleus may at least partially indicate the presence of RNA polymerases, the activity of which is dependent on divalent cations.

Animals↗
PubMed1961

Antibacterial effects associated with changes in bacterial cytology produced by cationic polypeptides.

The antibacterial action of cationic polypeptides has been found to be correlated with changes which they produce in the cytochemical reactions of the bacterial cells. Bacterial cells ordinarily stain with cationic dyes at pH values above 7. Treatment with cationic polypeptides rendered them stainable with the anionic dye fast green at pH 8.1. This change occurred parallel to loss of bacterial viability. An anionic, biological substance, mucin, was found to inhibit the usual conversion of bacterial cells from strong basophilia to strong acidophilia in the presence of cationic polypeptide. This inhibitory action was paralleled by inhibition of the toxicity of the cationic polypeptides for the bacterial cells. It was found that the basic protein, globin, which has a lesser pK than total calf thymus histone, was less toxic for E. coli and less effective in causing the bacterial cells to become stainable with the anionic dye at high pH. Staining with fast green at pH 8.1 appeared to provide presumptive evidence of lethal interaction between certain bacterial cells and cationic polypeptides such as total histones. It is suggested that the method may prove to be useful in the detection of lethal interaction between cationic polypeptides of the host and bacteria residing in host tissues.

Anti-Bacterial Agents↗
PubMed1993

Ca(2+)-blockable, poorly selective cation channels in the apical membrane of amphibian epithelia. UO2(2+) reveals two channel types.

This study deals with the effect of mucosal UO2(2+) on the Ca(2+)-blockable, poorly selective cation channels in the apical membrane of frog skin and toad urinary bladder. Our data show that UO2(2+) inhibits the Na+ currents through the amiloride-insensitive cation pathway and confirm a previously described stimulatory effect on the amiloride-blockade Na+ transport. Noise analysis of the Ca(2+)-blockable current demonstrates that the divalent also depresses the low-frequency Lorentzian (fc = 11.7 Hz) in the power density spectrum (PDS) and reveals the presence of high-frequency relaxation noise (fc = 58.5 Hz). The action of UO2(2+) is not reversed upon washout and is not accompanied by noise, typically induced by reversible blockers. The divalent merely depresses the plateau of the low-frequency Lorentzian, demonstrating a decrease in the number of conductive cation channels. Similarly, with mucosal K+ and Rb+, UO2(2+) also unmasks the high-frequency Lorentzian by depressing the noise from the slowly fluctuating cation channels (type S). In all experiments with mucosal Cs+, the PDS contains high-frequency relaxation noise (fc = 75.1 Hz in Rana temporaria, and 65.4 Hz in Rana ridibunda). An effect of UO2(2+) on the Cs+ currents and Lorentzian plateaus could not be demonstrated, suggesting that this monovalent cation does not pass through type S channels. Experiments with the urinary bladder revealed only a UO2(2+)-insensitive pathway permeable for Na+, K+, Rb+, and Cs+. We submit that in frog skin two cation-selective channels occur, distinguished by their spontaneous gating kinetics, their sensitivity to UO2(2+), and their permeability for Cs+. In toad urinary bladder, only one kind of cation-selective channel is observed, which resembles the UO2(2+)-insensitive channel in frog skin, with fast open-closed kinetics (type F).

Amiloride↗
PubMed2004

Polyvalent cations constitute the voltage gating particle in human connexin37 hemichannels.

Connexins oligomerize to form intercellular channels that gate in response to voltage and chemical agents such as divalent cations. Historically, these are believed to be two independent processes. Here, data for human connexin37 (hCx37) hemichannels indicate that voltage gating can be explained as block/unblock without the necessity for an independent voltage gate. hCx37 hemichannels closed at negative potentials and opened in a time-dependent fashion at positive potentials. In the absence of polyvalent cations, however, the channels were open at relatively negative potentials, passing current linearly with respect to voltage. Current at negative potentials could be inhibited in a concentration-dependent manner by the addition of polyvalent cations to the bathing solution. Inhibition could be explained as voltage-dependent block of hCx37, with the field acting directly on polyvalent cations, driving them through the pore to an intracellular site. At positive potentials, in the presence of polyvalent cations, the field favored polyvalent efflux from the intracellular blocking site, allowing current flow. The rate of appearance of current depended on the species and valence of the polyvalent cation in the bathing solution. The rate of current decay upon repolarization depended on the concentration of polyvalent cations in the bathing solution, consistent with deactivation by polyvalent block, and was rapid (time constants of tens of milliseconds), implying a high local concentration of polyvalents in or near the channel pore. Sustained depolarization slowed deactivation in a flux-dependent, voltage- and time-independent fashion. The model for hCx37 voltage gating as polyvalent block/unblock can be expanded to account for observations in the literature regarding hCx37 gap junction channel behavior.

Animals↗
PubMed2004

Thermodynamic analyses of the specific interaction between T:T mismatch base pair and mercury (II) cation: toward the efficient detection of single nucleotide polymorphism (1).

We examined the effect of mercury (II) cation on the thermal stability of heteroduplex and homoduplex. Addition of mercury (II) cation increased the melting temperature of heteroduplex containing T:T mismatch base pair by about 4 degrees C. The thermal stability of homoduplex and heteroduplexes containing other kinds of mismatch base pairs was not significantly changed by the addition of mercury (II) cation. Isothermal titration calorimetric study demonstrated that mercury (II) cation directly bound to T:T mismatch base pair in hcteroduplex at a molar ratio of 1:1. The binding constant and the enthalpy change for the binding of mercury (II) cation to T:T mismatch base pair was approximately 10(6) M(-1) and -6 kcal mol(-1), respectively. We conclude that mercury (II) cation directly binds to T:T mismatch base pair in heteroduplex with high affinity and specificity. Our results certainly support the idea that the addition of mercury (II) cation to T:T mismatch base pair in heteroduplex could be a convenient strategy for heteroduplex analysis and may eventually lead to progress in single nucleotide polymorphism genotyping.

Base Pair Mismatch↗
PubMed1982

Characterization of the Ca2+-gated cation channel in sarcoplasmic reticulum vesicles.

Two types of sarcoplasmic reticulum (SR) vesicles, i.e., normal (NSR) and heavy (HSR), were prepared by using a centrifugation technique, and further characterization of the cation transport in the two types of SR vesicles was performed by measuring the choline influx. The choline influx in SR vesicles was measured by following the change in light scattering intensity using a stopped flow apparatus. Analysis of the choline influx gave the following results. (1) There exists a Ca2+-gated cation channel in SR vesicles other than the K+-selective, voltage-dependent cation channel. The Ca2+-gated cation channel was activated by both extravesicular Ca2+ and caffeine, and inhibited by procaine and N,N'-dicyclohexylcarbodiimide. (2) About 70% of the HSR vesicles have Ca2+-gated cation channels, whereas only 20% of NSR vesicles have these channels. On the other hand, 60% of HSR vesicles also have the voltage-gated cation channels and 80% of NSR vesicles have these channels. On the basis of various properties of the Ca2+-gated cation channel, it is suggested that this channel must be the one which mediates the Ca2+-induced Ca2+ release.

Animals↗
PubMed1999

Increased concentrations of eosinophilic cationic protein in whole-gut lavage fluid from children with inflammatory bowel disease.

BACKGROUND: Eosinophils may be involved in the pathogenesis of inflammation in inflammatory bowel disease. The purpose of this study was to verify whether concentrations of eosinophilic cationic protein in gut lavage fluid from children with inflammatory bowel disease correlate with clinical and laboratory indexes of disease activity. METHODS: Twenty-three children with Crohn's disease, 14 with ulcerative colitis, and 22 age-matched control subjects entered the study. Radioimmunoassay and sandwich enzyme-linked immunosorbent assay techniques were used to measure eosinophilic cationic protein, total immunoglobulin G and interleukin-1beta, respectively. RESULTS: Gut lavage eosinophilic cationic protein levels were significantly (p < 0.005) higher in patients with Crohn's disease and ulcerative colitis than in control subjects. Intestinal eosinophilic cationic protein levels decreased in three of four children with Crohn's disease who were fed an elemental diet. There was a significant (p < 0.001) correlation between eosinophilic cationic protein concentrations and immunoglobulin G and interleukin-1beta levels in gut lavage fluid. CONCLUSIONS: Elevated intestinal eosinophilic cationic protein levels in inflammatory bowel disease suggest that eosinophils are involved in the gastrointestinal inflammation in this disease. Intestinal eosinophilic cationic protein concentration is another marker with which to discriminate between active and inactive inflammatory bowel disease.

Adolescent↗
PubMed1999

Cation distribution and crystal chemistry of Y3Al5-xGaxO12 (0 </= x </= 5) garnet solid solutions.

Five single crystals of Y(3)Al(5-x)Ga(x)O(12) (0 </= x </= 5) garnet solid solutions with the compositions x = 0.0, 1.0, 2.0, 3.0 and 4.0 were synthesized using a flux method. The compositional dependence of the lattice constants of the garnet solid solutions shows a large deviation from Vegard's law. Investigation of the cation distributions of these garnets using single-crystal X-ray diffraction shows that Ga(3+), which is larger than Al(3+), preferentially occupies the tetrahedral (four-coordinate) site rather than the octahedral (six-coordinate) site. On the basis of the results obtained from structure refinements, geometric analyses of the polyhedral distortions were carried out. The results imply that the cation-cation repulsive force across the polyhedral shared edges decreases with increasing substitution of Ga(3+). Moreover, the proportion of covalent bonding in the cation-oxygen bonds was estimated from the bond strength; the results indicate that the covalency of the Ga-O bond is greater than that of the Al-O bond. The peculiar cation distributions observed in the Y(3)Al(5-x)Ga(x)O(12) garnet solid solutions are most probably caused by the strong covalency of the Ga-O bond and also simultaneously induced by the need to decrease the cation-cation repulsive force. Crystal data: cubic, Ia3;d, Z = 8, Mo Kalpha, lambda = 0.71069 Å; at x = 0.0 (triyttrium pentaaluminium dodecaoxide): a(0) = 12.0062 (5) Å, V = 1730.7 (2) Å(3), D(x) = 4.56 Mg m(-3), M(r) = 593.613, µ = 21.21 mm(-1), F(000) = 2224, R = 0.029 for 294 reflections; at x = 1.0 (triyttrium tetraaluminium gallium dodecaoxide): a(0) = 12.0432 (7) Å, V = 1746.7 (3) Å(3), D(x) = 4.84 Mg m(-3), M(r) = 636.351, µ = 24.09 mm(-1), F(000) = 2368, R = 0.022 for 124 reflections; at x = 2.0 (triyttrium trialuminium digallium dodecaoxide): a(0) = 12.0926 (9) Å, V = 1768.3 (4) Å(3), D(x) = 5.10 Mg m(-3), M(r) = 679.089, µ = 26.85 mm(-1), F(000) = 2512, R = 0.018 for 144 reflections; at x = 3.0 (triyttrium dialuminium trigallium dodecaoxide): a(0) = 12.1552 (6) Å, V = 1795.9 (3) Å(3), D(x) = 5.34 Mg m(-3), M(r) = 721.827, µ = 29.43 mm(-1), F(000) = 2656, R = 0.018 for 184 reflections; at x = 4.0 (triyttrium aluminium tetragallium dodecaoxide): a(0) = 12.2123 (8) Å, V = 1821.3 (4) Å(3), D(x) = 5.58 Mg m(-3), M(r) = 764.565, µ = 31.97 mm(-1), F(000) = 2800, R = 0.014 for 159 reflections.

Journal Article↗
PubMed1969

The effect of chronic osmotic disturbance on the concentrations of cations in cerebrospinal fluid.

1. Adult cats were rendered hypo- and hypernatraemic by peritoneal dialysis. These states were maintained for periods of 2-5 days.2. The concentrations in cerebrospinal fluid (c.s.f.) of the cations, potassium, calcium and magnesium all decreased in the hyponatraemic animals and increased in the hypernatraemic animals. These shifts in c.s.f. cation concentrations did not relate to plasma changes in the same cations, which were often in the opposite direction.3. The relations of the cation concentrations to c.s.f. sodium were not linear and, in the cases of calcium and magnesium, the relevant cation concentration related better to the square rather than the first power of the c.s.f. sodium concentration.4. Brain water changed much less in the hypo- and hypernatraemic animals than might be anticipated from the shifts in blood osmolarity, plasma sodium concentration and muscle water.5. Isotonicity of the fluids in brain with blood plasma and c.s.f. appeared to be largely maintained by loss or gain of sodium and chloride ions by this tissue.6. The c.s.f. results may be partly due to a constant influx of the cation in question being diluted with more formed c.s.f. in hyponatraemia and less c.s.f. in hypernatraemia, but the deviations from linearity in the plots of c.s.f. cation against c.s.f. sodium suggest the influence of other factors.

Animals↗
PubMed1991

Monovalent cation conductance in the ryanodine receptor-channel of sheep cardiac muscle sarcoplasmic reticulum.

1. The ryanodine receptor protein of sheep cardiac muscle sarcoplasmic reticulum membranes functions as a ligand-regulated ion channel following solubilization with the zwitterionic detergent CHAPS (3-[(3-cholamidopropyl)-dimethylammonio]-1- propane sulphonate); purification by density gradient centrifugation, reconstitution into proteo-liposomes and incorporation into planar phospholipid bilayers. 2. In the absence of divalent cations, measurable conductance is observed with the group 1a cations and with some larger organic cations. In symmetric 210 mM solutions the following conductance sequence was determined: K+ greater than Rb+ = NH4+ greater than Na+ = Cs+ greater than Li+ much greater than Tris+. 3. Other organic cations, e.g. TEA+, do not produce measurable current under these conditions. 4. Single-channel conductance saturates with increasing ionic activities of K+, Na+ and Li+. Saturation curves are described by Michaelis-Menten kinetic schemes with the following values of maximal conductance and apparent dissociation constant: K+ 900 pS, 19.9 mM; Na+ 516 pS, 17.8 mM; Li+ 248 pS, 9.1 mM. 5. The channel displays only minor differences in permeability amongst the group 1a cations. Relative permeability, monitored under bi-ionic conditions, yields the following sequence: Na+, 1.15 greater than K+, 1.00 = Li+, 0.99 greater than Rb+, 0.87 greater than Cs+, 0.61. Under similar conditions the permeability ratio of NH4+ to K+ was found to be 1.32 and that for Tris+ to K+ was 0.22. 6. The K+ conductance is reduced by low concentrations of the impermeant cation TEA+. Block appears as a smooth reduction in single-channel current amplitude and the degree of block is dependent upon applied voltage. These observations are consistent with a single-site blocking scheme in which TEA+ has access to a site within the voltage drop of the channel from only the cytosolic face of the channel protein and interacts with a site located approximately 90% of the electrical distance across the channel. The zero-voltage dissociation constant for TEA+ block is 50 mM. 7. Single-channel conductance measurements in mixtures of K(+)-Na+ and K(+)-Li+ reveal no anomalous behaviour as the mole fraction of the ions is varied. 8. With monovalent cations as permeant species, the sheep cardiac sarcoplasmic reticulum ryanodine receptor protein functions as a poorly selective, ligand-regulated channel. Under the conditions described here the channel functions as a single-ion pore. It is proposed that discrimination is largely dependent upon the strength of interaction of the permeant ion with a binding site in the conduction pathway.

Animals↗
PubMed1975

Antibacterial activity of cationic proteins from human granulocytes.

Human granulocytes contain several cationic proteins with a molecular weight of approximately 25,000, almost identical amino acid composition, and complete immunologic identity. These proteins possess a chymotrypsin-like protease activity at a neutral pH. The antibacterial activity of the cationic proteins has been studied. Bactericidal activities are found against both Gram-positive (Streptococcus faecalis and Staphylococcus aureus) and Gram-negative (Escherichia coli and Pseudomonas aeruginosa) organisms. Gram-positive bacteria are, however, the most sensitive. The pH-optimum is near neutrality, and the microbicidal activity shows an inverse relationship to the ionic strength, indicating an ionic interaction between the cationic proteins and the bacterial surface. The microbicidal effect of the cationic proteins is generally independent of the chymotrypsin-like activity of the same proteins since the activity against several bacterial species is heat stable while the chymotrypsin-like activity is heat labile. The surface properties of S. aureus that are determined by protein A do not seem to influence the susceptibility to cationic proteins. The properties of the Gram-negative envelope of E. coli that determine the susceptibility to the lytic action of serum do not influence the sensitivity to the action of cationic proteins. The present study shows that cationic proteins of human granulocytes represent one potential microbicidal mechanism that is independent of hydrogen peroxide and myeloperoxidase.

Blood Bactericidal Activity↗
PubMed1981

Differing sensitivities of Purkinje fibers and myocardium to inhibition of monovalent cation transport by digitalis.

The extent of inhibition of monovalent cation active transport in Purkinje fibers and myocardium in response to toxic and inotropic doses of digitalis were studied in the dog to elucidate the factors underlying the different relative sensitivities of these tissues to the toxic arrhythmogenic effects of digitalis. Monovalent cation transport inhibition was assessed by measuring uptake of the K(+) analog Rb(+) in samples of myocardium and Purkinje fibers after in vitro ouabain exposure and after acute or chronic administration of digoxin in vivo. The active uptake of Rb+ was determined as the difference between total uptake and uptake in the presence of 1.0 mM ouabain. Mean active uptake of Rb(+) by Purkinje fibers from control hearts was 1.62+/-0.11 (SEM) nmol/mg wet wt per 15 min, significantly greater than the value of 0.49+/-0.05 for myocardium (P < 0.001). Concentration-effect curves for inhibition of monovalent cation active transport by in vitro exposure to graded concentrations of ouabain showed that the concentration for half-maximal inhibition of monovalent cation transport, IC(50), for Purkinje fiber transport was 0.4 muM, significantly less than the value of 1.4 muM for myocardial samples. Dogs were given toxic doses of digoxin (0.3 mg/kg i.v.). At onset of sustained ventricular tachycardia, they were killed and monovalent cation transport measured in myocardial and Purkinje fiber samples. Active Rb(+) uptake was inhibited by 44% in myocardial samples, whereas a significantly greater inhibition of 76% was noted in Purkinje fibers (P < 0.01). Similar data were obtained for both transmural myocardial biopsy samples and subendocardial trabecular samples obtained from regions adjacent to Purkinje fibers. Another group of dogs received a nontoxic dose of 0.02 mg/kg i.v. daily for 6 d. Myocardium showed a 17% reduction in Rb(+) active uptake compared to control animals receiving no drug, whereas Purkinje fiber transport was reduced in these dogs to a significantly greater extent averaging 44% (P < 0.001). Thus, both toxic and inotropic (non-toxic) doses of digoxin inhibited monovalent cation transport in Purkinje fibers to a greater extent than in myocardium. This difference in apparent sensitivity of monovalent cation transport to digoxin may contribute to the previously reported tendency of digitalis toxic arrhythmias to arise in Purkinje fibers.

Animals↗
PubMed1979

Divalent cation-induced aggregation of chromaffin granule membranes.

Divalent cations induce the aggregation of chromaffin granule ghosts (CG membranes) at millimolar concentrations. Monovalent cations produce the same effect at 100-fold higher concentrations. The kinetics of the dimerization phase were followed by light-scattering changes observed in stopped-flow rapid mixing experiments. The rate constant for Ca2+-induced dimerization (kapp) is 0.86-1.0 x 10(9) M-1sec-1, based on the "molar" vesicle concentration. This value is close to the values predicted by theory for the case of diffusion-controlled reaction (7.02 x 10(9) M-1sec-1), indicating that there is no energy barrier to dimerization. Arrhenius plots between 10 degrees and 42 degrees C support this; the activation energy observed, +4.4 Kcal, is close to the value (4.6-4.8 Kcal) predicted for diffusion control according to theory. Artificial vesicles prepared from CG lipids were also found to have cation-induced aggregation, but the rates (values of kapp) were less than 1/100 as large as those with native CG membranes. Also, significant differences were found with respect to cation specificity. It is concluded that the slow rates are due to the low probability that the segments of membrane which approach will be matched in polar head group composition and disposition. Thus large numbers of approaches are necessary before matched segments come into aposition. The salient features of the chromaffin granule membrane aggregation mechanism are as follows: (a) In the absence of cations capable of shielding and binding, the membranes are held apart by electrostatic repulsion of their negatively charged surfaces. (b) The divalent and monovalent cation effects on aggregation are due to their ability to shield these charges, allowing a closer approach of the membrane surfaces. (c) The major determinants of the aggregation rates of CG membranes are proteins which protrude from the (phospholipid) surface of the membrane and serve as points of primary contact. Transmembrane contact between these proteins does not require full neutralization of the surface charge and surface potential arising from the negatively charged phospholipids. (d) After contact between proteins is established, the interaction between membranes can be strengthened through transmembrane hydrogen bonding of phosphatidyl ethanolamine polar head groups, divalent cation-mediated salt bridging, and segregation of phosphatidylcholine out of the region of contact.

Adrenal Medulla↗
PubMed1991

Effects of various dietary cation-anion balances on response to experimentally induced hypocalcemia in sheep.

We examined the effect of dietary cation-anion balance on bone Ca mobilization, measured by challenging wethers with a 5.6% EDTA infusion at a rate of 1.4 ml kg of BW-1. Dietary cation-anion balance was calculated as milliequivalents [(Na + K) - (Cl + S)]. Six crossbred wethers (average BW, 67.8 kg) were fed rations with high Ca (.74% dry basis) or normal Ca (.45%) and three different dietary cation-anion treatments, which differed in their mineral supplementation. Dietary cation-anion balances were 354, 125, and 32 meq kg of DM-1 for control and two treatments. A 6 x 4 incomplete Latin square design (six treatments, four periods) was used with five 15-d periods of 14 d of adaptation followed by 1 d of EDTA infusion. Concentrations of total and EDTA titratable plasma Ca were not affected by the level of dietary Ca or treatments. The decrease in plasma EDTA titratable Ca at conclusion of EDTA infusion (120 min) was least in wethers fed the lowest cation-anion balance within high Ca, and recovery of plasma EDTA titratable Ca during the 240-min postinfusion period was faster for sheep fed this diet than for the control. The amount of Ca mobilized and its rate of mobilization during the EDTA infusion period tended to be higher for diets with reduced cation-anion balances than for the control. Thus, reducing cation-anion balance increased the sheep's ability to mobilize Ca during hypocalcemia.

Acute Disease↗