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In this novel motif, thallium and gold atoms define a complex three-dimensional [AuTl(7)] polyanion in which the barium atoms fill pentagonal cages that are part of highly condensed infinite channels. Relativistic effects appear to be significant in the short Au-Tl and Tl-Tl distances and the bonding by the more electronegative gold.
Both tri- and tetra(ethylene glycol) linked bis-chromium carbene complexes have been synthesized. These carbene complexes were photolyzed with N-protected imidazolines to give protected azapenams. These were transformed into polyether-linked basket dioxocyclams 4a,b and bis-dioxocyclams 5a,b. These compounds have cavities for the complexation of both "hard" and "soft" metal ions. The complexes of Ni, Ba, and Gd were synthesized.
1. The left upper quadrant neurons L2-L6 in the abdominal ganglion of Aplysia californica were voltage clamped in order to examine effects of acetylcholine on voltage-dependent Ca and Ca-dependent K currents. 2. "Puffed" application of 10-100 microM acetylcholine reduced both the early inward and late outward phases of the current elicited by depolarizing voltage steps. An identical effect of the peptide FMRFamide was previously found to result from a suppression of the Ca and Ca-dependent K currents. 3. This effect of acetylcholine was obscured by the simultaneous activation of a previously described K current resembling the "S" current. Extracellular tetraethylammonium (TEA) and 4-aminopyridine could not be used to eliminate this current, because both compounds also appeared to block the acetylcholine receptor mediating the putative suppression of Ca and Ca-dependent K currents. 4. The acetylcholine-induced "S"-like and other K currents could, however, be reduced or eliminated by injection of TEA+ or Cs+ into the cell, replacement of extracellular Ca2+ with Ba2+, and by shifting the K+ equilibrium potential so as to null K currents at the potential used to record Ca current, revealing in each case a partial (10-40%) suppression of the Ca (or Ba) current by acetylcholine. 5. The reduction of the outward phase of depolarization-activated current was confirmed to represent suppression of the Ca-dependent K current by acetylcholine. This effect was indirect, secondary to the suppression of Ca current, since acetylcholine had no effect on Ca-dependent K current elicited by direct injection of Ca2+ into the cell. 6. Activation of the "S"-like K current and suppression of the Ca current by FMRFamide are likely to be important in its proposed role as an agent of presynaptic inhibition in Aplysia. Since acetylcholine has identical effects, it too may have such a function.
The X-ray structure analysis of three compounds of interest as enzyme substrates is reported. They are the hydrated forms of (I) DL-2-amino-4-arsonobutanoic acid [HO-AsO2--CH2-CH2-CH(NH3+)-CO2H], (II) DL-2-amino-4-phosphonobutanoic acid [HO-PO2--CH2-CH2-CH(NH3+)-CO2H] and the hydrated barium salt of (III) D-3-phosphoglycerate [HO-PO2--O-CH2-CH(OH)-CO2-]. The structures were fully refined to R factors of 0.033, 0.053 and 0.046. For the compounds (I) and (II) the charge distribution was directly determined by locating all H atoms. The co-ordination around As and P is approximately tetrahedral, with the valency angle between the two charged O atoms enlarged to 112 degrees in compound (I), 166 degrees in compound (II) and 122 degrees in compound (III). The As-X bond distances are increased relative to P-X to accommodate the increased atomic radius. The analysis establishes that the compounds are structural analogues. Tables of co-ordinates for H atoms, anisotropic thermal parameters, bond lengths and bond angles for the three compounds have been deposited as Supplementary Publication SUP 50122 (5 pages) with the British Library Lending Division, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained directly [see Biochem J. (1983) 209, 5].
In this paper, we report a structural study both in the solid state and in solution of barium complexes with the diamine N,N'-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L(2)), that allows us to rationalize the template effect of the metal ion in the synthesis of Schiff-base lateral macrobicycles resulting from the condensation of L(2) with different dicarbonyl compounds. The X-ray crystal structures of [Ba(L(2))(ClO(4))](ClO(4)) (3) [triclinic space group P1 with Z = 2, a = 10.467(2) A, b = 10.4755(2) A, c = 16.9911(3) A, alpha = 85.075(1) degrees, beta = 80.907(1) degrees, and gamma = 61.627(4) degrees ] and [Ba(L(2))(NCS)(H(2)O)](SCN) (4) [monoclinic space group P2(1)/n with Z = 4, a = 9.954(5) A, b = 29.193(5) A, c = 11.313(5) A, and beta = 91.371(5) degrees ] demonstrate that in the solid state the barium(II) ion induces an anti conformation of the receptor in the complexes. Variable temperature (1)H and (13)C NMR data point out that in solution compounds 3 and 4 exist as a mixture of syn and anti isomers. The presence of the syn isomer in solution, independent of the counterion employed (perchlorate or thiocyanate), accounts for the effectiveness of the barium(II) ion as a template agent in the synthesis of the lateral macrobicycles resulting from the condensation of L(2) with different dicarbonyl compounds. Density functional theory calculations (at the B3LYP/LanL2DZ level) for [Ba(L(2))](2+) predict the syn conformation to be more stable both in vacuo and in solution (PCM model). In order to asses which of the two isomers predominates in acetonitrile solution, the (13)C NMR shielding tensors of the two isomers of [Ba(L(2))](2+) were calculated for the in vacuo optimized structures by using the GIAO method, and the results were compared with the experimental ones. According to these analyses, a syn stereochemistry is assigned to the major species in solution.
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PURPOSE: The authors previously reported that rat retinal pigment epithelial (RPE) cells exhibit an ionic current through voltage-operated calcium channels that is dihydropyridine sensitive. They attempted to record the same current from freshly isolated fetal or adult primate RPE cells, as well as from cultured cells. METHODS: The whole-cell version of the patch-clamp technique was applied to RPE cells freshly isolated by enzymatic dissociation from fetal human and adult human and monkey eyes, as well as cultured fetal and adult human RPE cells. The cells were loaded with cesium to minimize potassium-related current and were bathed in an extracellular solution containing 40 mM barium to intensify the calcium currents. RESULTS: Freshly isolated cells, both fetal and adult, showed sustained and inward-going barium current through voltage-operated calcium channels with membrane depolarizations from a high holding voltage of -60 mV. This current was affected by dihydropyridine compounds. Cultured human RPE cells showed no sign of a calcium current of this type. CONCLUSIONS: Freshly isolated fetal and adult human RPE cells, as well as adult monkey, exhibit calcium current through voltage-operated, dihydropyridine-sensitive channels, similar to the neuronal L-type, just as in rat RPE cells.
A new inorganic ion exchanger, zirconium vanadate, has been synthesized and characterized. Elemental analysis suggests that the probable formula of the compound is ZrO2, V2O5, 2H2O. The exchanger is highly stable in thermal, radiation and chemical environments. Radiochemical separation schemes for the 134Cs and 133Ba pair and also for separating the short-lived daughter 137Ba from its parent 137Cs using this newly synthesized ion exchanger have been developed.
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Mechanical changes in the superior cervical ganglion (SCG) of the guinea pig, evoked by stimulating the pre- or postganglionic nerve, were determined by using a mechano-electric transducer. The mechanical change evoked by either one of these nerves was monophasic, an initial phase of rapid swelling (about 1 dyn/cm2) being followed by a slow relaxation. The mechanism of these mechanical changes was examined by treating the preparations with agents which are known to suppress or facilitate synaptic transmission in the SCG. Mechanical changes elicited by preganglionic nerve stimulation were suppressed by agents that reversibly block ganglionic transmission (Ca2(+)-free high Mg2+, d-tubocurarine, hexamethonium, TEA, TTX). On the other hand, agents which are known to facilitate synaptic transmission (high Ca2+) increased both the amplitude and the duration of swelling. In a Ba2(+)-containing medium, a large, long-lasting shrinkage of the ganglion was observed. It is suggested that the mechanical changes recorded in this study are associated with postsynaptic electro-genesis.
Complete and homogeneous distribution of gastrointestinal (GI) contrast media are important factors for their effective use in computed tomography as well as in magnetic resonance (MR) imaging. A radiographic method (using fluoroscopy or spot films) could be effective for monitoring intestinal filling with GI contrast agents for MR imaging (GICMR), but it would require the addition of a radiopaque agent to most GICMR. This study was conducted to determine the minimum amount of barium additive necessary to be radiographically visible and to evaluate whether this additive influences the signal characteristics of the GICMR. A variety of barium sulfate preparations (3-12% wt/vol) were tested in dogs to determine the minimum quantity needed to make the administered agent visible during fluoroscopy and on abdominal radiographs. Solutions of 10 different potential GI contrast agents (Gd-DTPA, ferric ammonium citrate, Mn-DPDP, chromium-EDTA, gadolinium-oxalate, ferrite particles, water, mineral oil, lipid emulsion, and methylcellulose) were prepared without ("nondoped") and with ("doped") the barium sulfate additive. MR images of the solutions in tubes were obtained at 0.38 T using 10 different spin-echo pulse sequences. Region of interest (ROI) measurements of contrast agent signal intensity (SI) were made. In addition, for the paramagnetic contrast media, the longitudinal and transverse relaxivity (R1 and R2) were measured. A 6% wt/vol suspension of barium was the smallest concentration yielding adequate radiopacity in the GI tract. Except for gadolinium-oxalate, there was no statistically significant difference in SI for doped and non-doped solutions with most pulse sequences used. In addition, the doped and nondoped solutions yielded R1 and R2 values which were comparable. We conclude that barium sulfate 6% wt/vol added to MR contrast agents produces a suspension with sufficient radiodensity to be viewed radiographically, and it does not cause significant alteration in the MR signal appearance of most GICMR. These formulations can be useful for achieving optimal filling of the gastrointestinal tract prior to MRI.
A novel potassium-selective channel which is active at membrane potentials between -100 mV and +40 mV has been identified in peripheral myelinated axons of Xenopus laevis using the patch-clamp technique. At negative potentials with 105 mM-K on both sides of the membrane, the channel at 1 kHz resolution showed a series of brief openings and closings interrupted by longer closings, resulting in a flickery bursting activity. Measurements with resolution up to 10 kHz revealed a single-channel conductance of 49 pS with 105 mM-K and 17 pS with 2.5 mM-K on the outer side of the membrane. The channel was selective for K ions over Na ions (PNa/PK = 0.033). The probability of being within a burst in outside-out patches varied from patch to patch (> 0.2, but often > 0.9), and was independent of membrane potential. Open-time histograms were satisfactorily described with a single exponential (tau o = 0.09 msec), closed times with the sum of three exponentials (tau c = 0.13, 5.9, and 36.6 msec). Sensitivity to external tetraethylammonium was comparatively low (IC50 = 19.0 mM). External Cs ions reduced the apparent unitary conductance for inward currents at Em = -90 mV (IC50 = 1.1 mM). Ba and, more potently, Zn ions lowered not only the apparent single-channel conductance but also open probability. The local anesthetic bupivacaine with high potency reduced probability of being within a burst (IC50 = 165 nM). The flickering K channel is clearly different from the other five types of K channels identified so far in the same preparation. We suggest that this channel may form the molecular basis of the resting potential in vertebrate myelinated axons.
The effect on the afferent synaptic transmission of Ba2+, Sr2+, tetraethylammonium (TEA) and 4-aminopyridine (4-AP) has been investigated in the isolated frog labyrinth by intracellularly recording the posterior canal resting and evoked receptor discharge. BaCl2 (0.3 mM) or SrCl2 (1.8 mM) substitution for normal external CaCl2 restored the afferent activity without affecting the membrane potential of the sensory fibres. On further increasing Ba2+ concentration (0.5-5 mM) a dose-dependent increase in the EPSP and spike discharges was observed in all the units examined. Ba2+ (1.8-4 mM) removed the depression of the sensory activity operated by CoCl2 (3 mM), while its facilitatory effect was completely antagonized by raising Ca2+ concentration (up to 10 mM). TEA (20 mM) elicited a clear-cut increase in the EPSP and spike discharges which, however, was less consistent than that produced by Ba2+ (1 mM). The increment in spike frequency produced by TEA and Ba2+ proved to be inversely related to the initial resting firing level of the different units. The 4-AP (4-20 mM) effect resulted in a decrease of the sensory activity, which was fully restored by TEA or Ba2+. In normal saline a linear relationship was found between the mean unit resting discharge and the respective excitatory peak response during sinusoidal rotation (0.1-0.3 Hz). This result suggest that the mechanical response is mainly determined by the unit resting level. Consistent evoked responses were obtained under TEA and Ba2+ treatment which proved to depend linearly on the new mean resting discharge of the different units. Conversely, a reduced evoked response was invariably observed in all the fibres tested in the presence of 4-AP. The present results suggest that Ba2+ and Sr2+ may substitute for Ca2+ in the transmitter release process at the cyto-neural junction, the ability of Ba2+ being even larger than that of Sr2+ and Ca2+ itself. The effects of TEA and 4-AP are discussed in the light of their possible interaction with the presynaptic K+-currents recently described in hair cells.
The ability of acute application of the neurotoxicant methylmercury (MeHg) to disrupt the function of presynaptic Ca2+ and Na+ channels at intact neuromuscular junctions was examined using mouse triangularis sterni motor nerves. In Ba(2+)-containing solutions, potential changes arising from Na+ and Ca2+ channel function could be recorded from the perineurial sheath surrounding motor neurons when K+ channels were blocked by tetraethylammonium chloride and 3,4-diaminopyridine. MeHg (100 microM) reduced both Na(+)- and Ba(2+)-dependent components to block within 3-5 min at apparently equivalent rates. Time to block was approximately 7 min after exposure to 50 microM MeHg. In 2 of 5 preparations exposed to 50 microM MeHg, the Ca2+ channel-mediated component was blocked prior to the Na+ channel-mediated component. In the remaining three preparations, Na(+)- and Ba(2+)-dependent potentials were blocked at similar times. Following block by MeHg, neither perfusing the preparation in MeHg-free solutions nor increasing the intensity and/or duration of stimulus to the intercostal nerves resulted in recovery of Na+ or Ca2+ potentials. In the presence of K+ channel blockers, repetitive firing of nerves in response to a single stimulus was observed in 20-30% of the triangularis preparations; in the two preparations treated with MeHg in which repetitive firing was observed, it decreased prior to block of the stimulus-induced Na+/Ba2+ potentials. These results corroborate the results obtained in isolated synaptosomes and pheochromocytoma cells, and suggest that MeHg decreases motor nerve excitability by disrupting Na+ channel function and may block neurotransmitter release by disrupting Na+ and Ca2+ channel function.
The contractile effects of barium, potassium, acetylcholine and acetyl-beta-methylcholine were studied in jejunum of 4, 12 and 20 month-old rats. The effects of age on the sensitivity and responsiveness of the jejunum to the agonists were examined using cumulative dose administration and measuring the pD2. The maximum contraction produced by the agonists and the sensitivity to barium and potassium were not modified by aging. In the middle-aged animals, the jejunum sensitivity to acetylcholine decreased while the sensitivity to acetyl-beta-methylcholine was not statistically different from that of 4 month-old rats. Since at this age (12 months) the acetylcholinesterase activity increased, the reduction in the sensitivity to acetylcholine could have been due to the increase in the activity of the metabolizing enzyme. Otherwise, in the jejunum from the older animals the sensitivity for both cholinoceptor agonists, acetylcholine and acetyl-beta-methylcholine, increased 5-fold when compared with the middle-aged animals. However, compared with the jejunum of 4 month-old rats the sensitivity to acetylcholine, to 20 month-old animals, was not altered while the sensitivity to acetyl-beta-methylcholine was increased. At this age (20 months), the jejunum acetylcholinesterase activity was increased when compared with that of 4 month-old rats and was not modified when compared with that of 12 month-old rats. The change in supersensitivity to cholinoceptor agonists could not have been due to an alteration in muscarinic receptor affinity, since the pA2 for atropine was not modified with increasing age. These results suggest that after an increase in acetylcholinesterase activity around month 12, the jejunum develops an adaptative supersensitivity to cholinoceptor agonists.
Effects of hexamethonium (C6) on the excitability of sympathetic ganglion cells were examined by means of intracellular recording. When DC currents were injected, high concentrations of C6 significantly augmented the repetitive firing of the cells without any change in threshold voltage for initiation of the spike. The Ca2+-sensitive component of the after-hyperpolarization following a spike was reduced by C6 in a dose-dependent fashion (0.3 to 10 mM). C6 slightly affected parameters for the spike but neither the resting membrane potential nor the input membrane resistance. The amplitude of the Ca2+ spike (in the presence of 1 microM tetrodotoxin and 10 mM tetraethylammonium) was not affected even by 30 mM C6. Bee venom (0.3 micrograms/ml) which contains apamin showed similar effects. These results suggest that C6 blocks the Ca2+-activated K+ channels, resulting in an increase in excitability of the cells.
A pilot study is described in which the exposed pulps of molar teeth of normal laboratory rats were capped with various metallic compounds in general medical usage. There appeared to be no relationship between the response obtained and the pH of the compounds studied. Aluminum powder, aluminum hydroxide, Aludrox, aluminum oxide, light magnesium oxide, magnesium hydroxide, and titanium dioxide gave poor results. Barium sulfate, calcium carbonate, cupric oxide, ferric oxide, heavy magnesium oxide, and stannic oxide merited more detailed study.