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PubMed1993

Single calcium channels and acetylcholine release at a presynaptic nerve terminal.

The relationship between calcium influx and the gating of transmitter release was examined at the release face of a cholinergic presynaptic nerve terminal using a technique that allows the simultaneous recording of both calcium channels at the single-channel level and quantal acetylcholine secretion. Acetylcholine release occurred during large inward calcium currents through many simultaneously open channels but was also gated by very small calcium transients, admitting less than 200 ions, when only one channel was open at a time. These findings provide functional support for a highly structured model of the transmitter release face in which the synaptic vesicle release mechanism is closely tethered to one or more presynaptic calcium channels and the opening of only one of these may be sufficient to trigger quantal secretion.

Acetylcholine↗
PubMed1992

Effects of methoxamine and barium on 45Ca2+ fluxes in the rat vas deferens.

The aim of this study was to determine whether methoxamine and barium stimulate 45Ca2+ uptake or efflux in the rat vas deferens in a manner that correlates with their contractile activity, and whether 45Ca2+ movements are inhibited by verapamil or nifedipine. Basal La(3+)-resistant (cellular) 45Ca2+ uptake was significantly greater in the epididymal half (791 +/- 27 nmol g-1) than in the prostatic half (654 +/- 14 nmol g-1) of the rat vas deferens and was unaffected by verapamil (61 microM) or nifedipine (14 microM). Methoxamine (8 microM) was without effect on 45Ca2+ uptake in either half but BaCl2 (1 mM) increased 45Ca2+ uptake by 31% in the prostatic half and by 22% in the epididymal half. The barium-induced increases in 45Ca2+ uptake were markedly reduced or abolished by verapamil (2 microM) or nifedipine (0.3 microM), which at these concentrations have no effect on the rhythmic contractions but abolish the initial small phasic contraction induced by barium. The basal rate of 45Ca2+ efflux from the intact vas deferens (into Ca2+ containing Krebs-Henseleit solution or into Ca-free Krebs-Henseleit solution +/- EGTA 0.05 mM) was not affected by verapamil (61 microM) or nifedipine (14 microM). Methoxamine (8 microM) produced a marked, transient and reversible increase in 45Ca2+ efflux into 2.5 mM CaCl2 Krebs-Henseleit in 50% of the intact vasa deferentia examined which was augmented by verapamil (61 microM). BaCl2 (1 mM) produced a small increase in 45Ca2+ efflux into Ca(2+)-containing and Ca(2+)-free Krebs-Henseleit solutions from some intact vasa deferentia and this was not inhibited by nifedipine (14 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1996

Electric quadrupole interaction of 137Ba in barium fluorohalides.

The X-ray storage phosphor materials BaFC1 and BaFBr differ greatly in their 137Ba nuclear electric quadrupole interaction, despite a common crystal structure. The quadrupole coupling constant in BaFCl is relatively small and increases rapidly with temperature, in an unusual manner: [CQ[ = 1.7 MHz at T = 296 K, CQ-1(delta CQ/delta T)p = 1.7 x 10(-3) K-1. The coupling BaFBr is [CQ[ = 17.0 MHz at 296 K. The difference in quadrupole coupling at room temperature is explained using an electrostatic point-charge model of the crystals.

Barium↗
PubMed2004

The development of a continuous isothermal titration calorimetric method for equilibrium studies.

A continuous isothermal titration calorimetry (cITC) method for microcalorimeters has been developed. The method is based on continuous slow injection of a titrant into the calorimetric vessel. The experimental time for a cITC binding experiment is 12-20 min and the number of data points obtained is on the order of 1000. This gives an advantage over classical isothermal titration calorimetry (ITC) binding experiments that need 60-180 min to generate 20-30 data points. The method was validated using two types of calorimeters, which differ in calorimetric principle, geometry, stirring, and way of delivering the titrant into the calorimetric vessel. Two different experimental systems were used to validate the method: the binding of Ba(2+) to 18-crown-6 and the binding of cytidine 2'-monophosphate to RNAse A. Both systems are used as standard test systems for titration calorimetry. Computer simulations show that the dynamic range for determination of equilibrium constants can be increased by three orders of magnitude compared to that of classical ITC, making it possible to determine high affinities. Simulations also show an improved possibility to elucidate the actual binding model from cITC data. The simulated data demonstrate that cITC makes it easier to discriminate between different thermodynamic binding models due to the higher density of data points obtained from one experiment.

Barium Compounds↗
PubMed2006

Synthesis of 4-aryl-3,4-dihydropyrimidin-2(1H)-thione derivatives as potential calcium channel blockers.

Biginelli reaction which involves condensation of methyl 3-oxopentanoate, aromatic aldehydes and thiourea with a catalytic amount of HCl at reflux temperature has been used for the synthesis of 4-aryl-6-ethyl-5-(methoxycarbonyl)-3,4-dihydropyrimidin-2(1H)-thiones (1-16). In addition, Lewis acids such as FeCl(3).6H(2)O and/or H(3)BO(3) were also used as catalysts for one-pot synthesis of the products. Compared to the classical Biginelli reaction conditions, the usage of Lewis acids has the advantage of good yields and short reaction times. The calcium channel blocker activities of 1-16 were screened by the tests performed on isolated rat ileum and thoracic aorta. Although product 11, 2-nitrophenyl-derivative, has potent antispasmodic activity on BaCl(2)-stimulated rat ileum, it does not have vasodilator activity on KCl-stimulated rat thoracic aorta. Product 15, 2-ethoxyphenyl-derivative, exhibited significant antispasmodic and vasodilator activities in both screening paradigms.

Animals↗
PubMed2004

Calorimetric vs. van't Hoff binding enthalpies from isothermal titration calorimetry: Ba2+-crown ether complexation.

The 1:1 complexation reaction between Ba(2+) and 18-crown-6 ether is re-examined using isothermal titration calorimetry (ITC), with the goal of clarifying previously reported discrepancies between reaction enthalpies estimated directly (calorimetric) and indirectly, from the temperature dependence of the reaction equilibrium constant K (van't Hoff). The ITC thermograms are analyzed using three different non-linear fit models based on different assumptions about the data error: constant, proportional to the heat and proportional but correlated. The statistics of the fitting indicate a preference for the proportional error model, in agreement with expectations for the conditions of the experiment, where uncertainties in the delivered titrant volume should dominate. With attention to proper procedures for propagating statistical error in the van't Hoff analysis, the differences between Delta H(cal) and Delta H(vH) are deemed statistically significant. In addition, statistically significant differences are observed for the Delta H(cal) estimates obtained for two different sources of Ba(2+), BaCl(2) and Ba(NO(3))(2). The effects are tentatively attributed to deficiencies in the standard procedure in ITC of subtracting a blank obtained for pure titrant from the thermogram obtained for the sample.

Barium Compounds↗
PubMed2005

Evidence that inward rectifier K+ channels mediate relaxation by the PGI2 receptor agonist cicaprost via a cyclic AMP-independent mechanism.

OBJECTIVE: We investigated the role of the inward rectifier potassium (KIR) channel and the cyclic AMP-dependent pathway in mediating vasorelaxation induced by the prostacyclin analogue cicaprost. METHODS: Small vessel myography was used to assess responses to cicaprost in segments of rat tail artery contracted with phenylephrine. Microelectrode recordings were made from helical strips to assess effects on membrane potential. RESULTS: Cicaprost caused relaxation and hyperpolarisation that were significantly inhibited by Ba2+ (30-100 microM), a known blocker of KIR channels. Raising extracellular K+ from 5 to 15 mM elicited membrane hyperpolarisation and an endothelium-independent relaxation that was blocked by Ba2+ (30-100 microM), suggesting the existence of functional KIR channels on the smooth muscle. In contrast, neither glibenclamide (10 microM), a blocker of ATP-sensitive K+ channels, nor fluoxetine hydrochloride (100 microM), a blocker of G-protein-gated inward rectifier K+ channels, nor pertussis toxin (PTX; 1 microg/ml), which irreversibly inhibits Gi/Go, reduced relaxation to cicaprost. Indeed, PTX significantly potentiated responses. Relaxation to cicaprost was not mediated by NO but was partially endothelium-dependent, consistent with a similar inhibition by a combination of charybdotoxin (0.1 microM) and apamin (0.5 microM), blockers of endothelium-derived hyperpolarising factor (EDHF). However, relaxation was unaffected by adenylyl cyclase (SQ22536, dideoxyadenosine) or protein kinase A (Rp-2-O-monobutyryl-cAMP) inhibitors, consistent also with Ba2+ only weakly inhibiting relaxation to the adenylyl cyclase activator forskolin. CONCLUSION: We conclude that cicaprost relaxes rat tail artery by activating KIR channels with some involvement from EDHF. The mechanism appears to be largely independent of cyclic AMP and Gi/Go, although the latter appears to counteract relaxation through an unknown pathway and/or receptor.

Adenine↗
PubMed2005

Potassium transmembrane fluxes in anoxic hepatocytes from goldfish (Carassius auratus L.).

Despite the fact that anoxic goldfish hepatocytes can maintain the transmembrane gradients of Na(+), H(+) and Ca(2+), cyanide (CN) intoxication leads to a rapid breakdown of K(+) homeostasis. In this study, [(86)Rb(+)] K(+) fluxes across the plasma membrane of goldfish hepatocytes were studied in order to identify the possible causes of this imbalance. Four minutes of cyanide incubation induced an acute and stable 61% decrease of K(+) influx (mostly driven by Na,K-ATPase activity), whereas K(+) efflux increased by 24.3%, this imbalance yielding a net K(+) efflux of 0.279+/-0.024 nmol 10(-6) cells(-1) min(-1). This uncoupling was not observed when glycolytic ATP production was inhibited with iodoacetic acid. Although the CN-induced decrease of K(+) influx was fully reversible upon washout of the inhibitor, it could not be prevented by any of the following treatments: (1) addition of 2% bovine serum albumin, which binds extracellular fatty acids known to activate specific K(+) channels; (2) addition of ascorbate, which acts as a radical scavenger; (3) inclusion of 5 mM glucose as an extracellular carbon source; and (4) removal of medium oxygen (obtained by nitrogen bubbling). Regarding the elevation of K(+) efflux in the presence of CN, neither ATP-dependent K(+) channels nor the KCl cotransporter appeared to be activated, whereas BaCl(2), an inhibitor of voltage-gated K(+) channels, decreased K(+) efflux of CN-intoxicated cells to control levels. In summary, these results indicate that, in goldfish hepatocytes, the CN-induced K(+) imbalance results from acute Na,K-ATPase inhibition together with the activation of voltage-dependent K(+) channels, the latter probably resulting from transient membrane depolarization.

Animals↗
PubMed2004

Effects of flowable resin on bond strength and gap formation in Class I restorations.

OBJECTIVES: This study measured resin/dentin bond strengths and evaluated gap formation at the pulpal floor of occlusal composite restorations using a flowable resin as a liner with either bulk or incremental placement of the restorative composite. METHODS: Class I occlusal preparations were made in 16 extracted molars, and were randomly divided into two groups for restoration using either One-Step adhesive (OS, BISCO) and Renew composite (RW, BISCO) or OptiBond SOLO adhesive (SOLO, Kerr) and Herculite XRV composite (XRV, Kerr). A flowable resin [AEliteFlo (AE, BISCO) or Revolution (RV, Kerr)] was applied as a liner in half of the specimens of each group. The lined and unlined groups were restored with composite cured in bulk, or cured in 1.5 mm increments. After 24 h in water, the teeth were sectioned into 0.7-mm thick slabs and were examined with an optical microscope (400 x) to determine the presence of internal resin-dentin gaps. The gap-free specimens were trimmed and were subjected to microtensile bond strength testing. Specimens that contained gaps had Knoop hardness of the restorative material measured. RESULTS: All incrementally filled restorations were well adapted to the pulpal floor but nearly all bulk-cured specimens had gaps between the flowable and the hybrid composites. The use of a flowable resin increased the bond strength of OS, but not SOLO. SIGNIFICANCE: The incremental technique prevented gap formation regardless of the use of a flowable resin. In contrast, the use of a flowable composite did not guarantee gap-free restorations or improved bond strength of resin to dentin in bulk-filled restorations.

Barium Compounds↗
PubMed2005

How filler properties, filler fraction, sample thickness and light source affect light attenuation in particulate filled resin composites.

OBJECTIVE: It was hypothesized that by standardizing variables such as light sources, filler types and filler surface treatment, it should be possible to use Beer-Lambert's law to predict light absorption in visible light-cured dental composites. METHODS: Mixture of 50 wt% bisGMA and 50 wt% TEGDMA to which a photo-initiator (0.35 wt% champhorquinone) and a co-initiator (0.7 wt% of dimethylaminoethylmethacrylate) was prepared. Three different filler types (HBB, SBB and KU) were added to that mixture in eight different volume percentage. Filler particles were either silane surface treated or not. Specimens were made with thicknesses of 1-5 mm. Total number of 1200 specimens were made for this study. Light transmission was obtained for halogen source and laser lights, which made the number of observations 2400. The absorbance values of the different materials were analyzed in Matlab with respect to the differences in filler fraction and sample thickness. RESULTS: The obtained results revealed that of the two light sources, more light was absorbed by the composite when the laser light was used. Among different filler types, the HBB filler absorbed most light and the KU filler the least. There were significant differences (p < 0.05) in light absorption between all three filler types. SIGNIFICANCE: By comparing the modeled surfaces generated by Matlab for different materials it was possible to determine how different variables such as filler type, filler surface treatment and light source affect light attenuation. The characteristic of incident light affected the light absorbance, meaning that not only the composite's composition needs to be considered in light absorption studies of dental composites.

Absorption↗
PubMed2005

Evaluation of two Bis-GMA analogues as potential monomer diluents to improve the mechanical properties of light-cured composite resins.

OBJECTIVES: The aim of this study was to investigate the influence of new diluent agents, diluent ratio and filler content, on relevant mechanical properties of several novel composite resins containing Bis-GMA as resin matrices, and to compare these with the properties of composites based on TEGDMA, a conventionally used diluent. METHODS: Two Bis-GMA analogues were synthesized and 20 experimental composite resins were prepared combining three monomer mixtures (Bis-GMA/TEGDMA, Bis-GMA/CH3 Bis-GMA and Bis-GMA/CF3 Bis-GMA), at three dilution rates (85/15, 10/90, 0/100) and three levels of hybrid filler content (barium aluminosilicate glass): 0, 10 and 35%. Flexural strength (FS), modulus of elasticity (ME) and microhardness (VHN) of the composites were evaluated. Five specimens of each material were prepared for each mechanical test, light-cured over 120 s and stored in water at 37 degrees C for 1 week. Three-point bending test was used for FS measurement and VHN was quantified by using a Vickers microindentor. Data were analyzed by ANOVA and Student-Newman-Keuls tests (P<0.05). RESULTS: Materials with CH3 Bis-GMA showed an enhanced VHN. Mean FS was higher for matrices containing TEGDMA. Overall, dilution favored FS and VHN but not ME. Filler loading specially improved ME and VHN. SIGNIFICANCE: Results correlate with an increase in the extent of polymerization due to the higher flexibility of the less viscous comonomer starting system and the hydrophobic character of the Bis-GMA analogues.

Aluminum Silicates↗
PubMed2005

Polymerization shrinkage and contraction stress of dental resin composites.

OBJECTIVE: The aim of this study was to evaluate the shrinkage, contraction stress, tensile modulus, and the flow factor of 17 commercially available dental resin composites. METHOD: The volumetric shrinkage measurements were performed by mercury dilatometry, and the contraction stress and tensile modulus were determined by means of stress-strain analysis. The statistical analysis was conducted by ANOVA and Tukey's post hoc test, and linear regression. RESULTS: Strong linear correlation for most resin composites were found for (i) contraction stress and shrinkage (ii) contraction stress and tensile modulus, and (iii) shrinkage and tensile modules. For most of the materials the unpolymerized resin content determines the amount of shrinkage, contraction stress and tensile modules. The pre-polymerized clusters in Heliomolar results in improved shrinkage/contraction stress properties. The shrinkage/contraction stress for Filtek Z100, Aelite Flo, and Flow-it was too high for the amount of resin in the resin composite. This was rationalized by high polymerization rates, a flow factor, and the nature of the resin. SIGNIFICANCE: High shrinkage and/or high contraction stress may lead to failure of the bond between the resin composites and the tooth structure. This study shows that the unpolymerized resin content determines the amount of shrinkage, contraction stress and tensile modules. Therefore, using pre-polymerized clusters will improve shrinkage/contraction stress properties, as was shown in Heliomolar, while high polymerization rates, and low flow factors have a deteriorative effect on the shrinkage/contraction stress properties.

Acrylic Resins↗
PubMed2005

Rheological properties of resin composites according to variations in monomer and filler composition.

OBJECTIVES: The aim of this study was to investigate the effects of monomer and filler composition on the rheological properties related to the handling characteristics of composites. METHODS: A resin matrix in which Bis-GMA was blended with TEGDMA at a ratio of 6:4, was mixed with silane-treated Barium glass (0.7 and 1.0 microm), 0.04 microm fumed silica or 0.5 microm round silica. The type and content of incorporated fillers were varied to achieve different viscosity levels of the experimental composites. Steady shear and dynamic oscillatory shear tests were performed with a rheometer. The viscosity (eta) of the resin matrix, the storage shear modulus (G'), loss modulus (G''), complex modulus (G*), loss tangent (tandelta), phase angle delta and complex viscosity (eta*) of the composites were evaluated as a function of frequency omega=0.1-100 rad/s. To investigate the effect of temperature on viscosity, a temperature sweep test was also performed. RESULTS: Resin matrices were Newtonian fluids and all experimental composites exhibited pseudoplasticity. The viscosity exponentially increased as the percentage of filler volume was increased, but decreased with increasing temperature. For identical filler volumes, as the filler size decreased, viscosity increased. The effect of filler size on the viscosity was intensified with increasing filler content, and the increased filler content reduced the tandelta. SIGNIFICANCE: The rheological properties of composites related to handling characteristics were greatly influenced by the formulation of the monomer and filler, shear rate and temperature. The locus of frequency domain phasor plots, G(*)(omega)e(idelta)=G(*)(omega) angledelta, in a complex plane was a valuable method for representing the viscoelastic properties of the composites.

Barium Compounds↗
PubMed2006

Bond strength performance of different resin composites used as core materials around fiber posts.

OBJECTIVES: To evaluate the microtensile bond strengths of different resin composites used as core materials around fiber posts. METHODS: Forty DT Light-Posts (RTD) were randomly divided into eight groups, according to the resin composite used. They included two core materials specifically developed for core build-up--Group 1: Core-Flo (Bisco Inc.) and Group 2: UniFil Core (GC Corp.); three hybrid composites--Group 3: Tetric Ceram (Ivoclar-Vivadent), Group 4: Gradia Direct (GC Corp.), Group 5: Bisfil 2B (Bisco, Inc.); and three flowable composites--Group 6: AEliteflo (Bisco, Inc.), Group 7: Filtek Flow (3M ESPE) and Group 8: UniFil Flow (GC Corp). A cylindrical plastic matrix was placed around the silanized post and filled with the respective resin composite. Each bonded post provided five to eight sticks for microtensile testing. Each stick was loaded to failure under tension at a cross-head speed of 0.5mm/min. One-way ANOVA and Tukey's test were used for statistical analysis. Scanning electron microscopy (SEM) was used to evaluate the interface of the fractured sticks. RESULTS: Resin composites exhibited a significant influence on microtensile bond strength (p<0.05). Core-Flo showed the highest bond strength (11.00+/-0.69 MPa) although it was not statistically significantly different from all groups, except from the flowable composites. Under SEM, all the composites adapted well to the fiber post, with a variable extent of voids observed along the fractured composite interfaces. SIGNIFICANCE: Although good adaptation to the post surface was achieved, bond strength to fiber post remains relatively weak. Core build-up and hybrid composites are better alternatives to flowable composites as core build-up materials.

Barium Compounds↗
PubMed2005

K(+)-induced vasodilation in the rat kidney is dependent on the endothelium and activation of K+ channels.

Increased extracellular K+ is reported to cause endothelium-independent vasodilation and K+ has been proposed as an endothelium-derived hyperpolarizing factor. However, the endothelium is endowed with K+ channels that may also be responsive to increased K+. We examined the vasodilator effect of bolus administration of 20, 40 and 60 micromol KCl in the rat isolated kidney in which perfusion pressure was elevated with phenylephrine. KCl produced dose-dependent vasodilator responses that were virtually abolished by removal of the endothelium which also abolished the vasodilator effect of bradykinin without affecting that to nitroprusside. The vasodilator effect of KCl was unaffected by inhibition of cyclooxygenase, nitric oxide synthase or cytochrome P450 but reduced by inhibition of K+ channels with tetraethylammonium (TEA). Barium chloride reduced the vasodilator effects of KCl but charybdotoxin/apamin was without effect. These results indicate that KCl results in endothelium-dependent vasodilation that is independent of nitric oxide (NO), prostaglandins and cytochrome P450 but dependent on activation of endothelial K+ channels.

Animals↗
PubMed2006

Cellular basis for electrocardiographic and arrhythmic manifestations of Andersen-Tawil syndrome (LQT7).

BACKGROUND: Andersen-Tawil syndrome, a skeletal muscle syndrome associated with periodic paralysis and long QT intervals on the ECG, has been linked to defects in KCNJ2, the gene encoding for the inward rectifier potassium channel (I(K1).) OBJECTIVES: The purpose of this study was to examine the cellular mechanisms underlying the ECG and arrhythmic manifestations of Andersen-Tawil syndrome. METHODS: To investigate the effects of KCNJ2 loss-of-function mutations responsible for Andersen-Tawil syndrome, we used barium chloride (BaCl(2)) to inhibit I(K1) in arterially perfused wedge preparation. Transmembrane action potentials (APs) were simultaneously recorded from endocardial, midmyocardial, and epicardial cells, together with a transmural ECG. RESULTS: BaCl(2) (1 to 30 microM) produced a concentration-dependent prolongation of the QT interval, secondary to a homogeneous prolongation of AP duration of the three cell types. QT interval was prolonged without an increase in transmural dispersion of repolarization (TDR). Low extracellular potassium (2.0 mM), isoproterenol (20-50 nM), and an abrupt increase in temperature (36 degrees C-39 degrees C) in the presence of 10 microM BaCl(2) did not significantly increase TDR but increased ectopic extrasystolic activity. Early afterdepolarizations were not observed under any condition. Spontaneous torsades de pointes arrhythmias were never observed, nor could they be induced with programmed electrical stimulation under any of the conditions studied. CONCLUSION: Our results provide an understanding of why QT prolongation associated with Andersen-Tawil syndrome is relatively benign in the clinic and provide further support for the hypothesis that the increase in TDR, rather than QT interval, is responsible for development of torsades de pointes.

Action Potentials↗
PubMed2006

Active ammonia absorption in the midgut of the Tobacco hornworm Manduca sexta L.: transport studies and mRNA expression analysis of a Rhesus-like ammonia transporter.

In this study the mid- and hindgut of Manduca sexta larvae were tested for their ammonia transport properties using a custom-made Ussing chamber. In the presence of 0.1 mmoll(-1) ammonia on both sides of the isolated epithelium, active transepithelial ammonia absorption (aTEPA) was observed in all midgut sections, with greatest transport rates (ca. 140 nmol cm(-2)h(-1)) detected in the median midgut. The hindgut showed no aTEPA. In the median midgut inhibition of energy metabolism by azide blocked aTEPA completely, whereas inhibition of vacuolar H(+)-ATPase by bafilomycin A(1) reduced the active transport by 50%. The imposition of a luminal-directed NH(3)-gradient (pH 6.5 apical, pH 8.5 basal) lowered the aTEPA by approximately 50% but did not reverse its direction. Apical addition of amiloride reduced aTEPA by 90%, suggesting a role of carrier-mediated ammonia transport across the apical membrane via a member of the NHE family. Inhibition of the microtubule network by colchicine reduced aTEPA by ca. 50%. In contrast, blocking basal K(+) channels by Ba(2+) had no effect on aTEPA. Using molecular methods, evidence for intestinal expression of a Rhesus-like ammonia transporter (RhMS) was found with low mRNA expression in midgut tissues, but high expression levels in the hindgut, Malpighian tublules and ganglia.

Amiloride↗