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  • 1
    Electronic Resource
    Electronic Resource
    Re-evaluation of the P/Q Ca2+ channel components of Ba2+ currents in bovine chromaffin cells superfused with solutions containing low and high Ba2+ concentrations (1996)
    Springer
    Pflügers Archiv 432 (1996), S. 1030-1038 
    Details
    ISSN: 1432-2013
    Keywords: Key words Calcium channels ; Chromaffin cells ; ω-Agatoxin IVA ; ω-Conotoxin ; GVIA ; ω-Conotoxin MVIIC ; Furnidipine
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  This study was undertaken to reassess the set of voltage-dependent Ca2+ channel subtypes expressed by bovine adrenal chromaffin cells maintained in primary cultures. Previous views on the pharmacology of such channels had to be revised in the light of the novel data which arose from the use in this study of low and high micromolar concentrations of ω-agatoxin IVA, and low (2 mM) and high (10 mM) concentrations of the charge carrier Ba2+. Whole-cell Ba2+ currents (IBa) through Ca2+ channels were elicited in voltage-clamped chromaffin cells, with a holding potential of –80 mV and depolarising pulses to 0 mV. Mean peak I Ba was 425 pA in 2 mM Ba2+ (59 cells) and 787 pA in 10 mM Ba2+ (42 cells). In 2 mM Ba2+, ω-conotoxin MVIIC (3 μM) inhibited I Ba by 79%; in 10 mM Ba2+, the blockade developed much more slowly and reached only 44%. A low concentration of ω-agatoxin IVA (20 nM) inhibited I Ba by 9%; 2 μM inhibited I Ba by 60%. This blockade was similar in low and high Ba2+ concentrations. After giving furnidipine (3 μM) and ω-conotoxin GVIA (1 μM), 2 μM ω-agatoxin IVA inhibited the remaining current (about 40–45%); this blockade was independent of the Ba2+ concentration. The current could be fully blocked by the cocktail furnidipine/ω-conotoxin GVIA/high ω-agatoxin IVA, both in low and high Ba2+ concentrations. The large Q-type channel component of I Ba is blocked by micromolar concentrations of ω-agatoxin IVA and ω-conotoxin MVIIC. While solutions with a high Ba2+ concentration strongly delayed the development of blockade by ω-conotoxin MVIIC, the blockade by high concentrations of ω-agatoxin IVA was equally effective in solutions with a low or a high Ba2+ concentration. Hence, the use of appropriate Ba2+ and toxin concentrations in this study reveals that P-type Ca2+ channels are poorly expressed in bovine chromaffin cells; in contrast, a robust component of the current depends on Q-type Ca2+ channels. An R-type residual current is not present in these cells.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050231
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  • 2
    Electronic Resource
    Electronic Resource
    A1PO4-Al2O3 catalysts with low alumina content. III. Surface basicity of catalysts obtained in aqueous ammonia (1993)
    Springer
    Catalysis letters 19 (1993), S. 137-142 
    Details
    ISSN: 1572-879X
    Keywords: AlPO4 ; AlPO4-Al2O3 ; surface basicity ; Langmuir adsorption ; Knoevenagel condensation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The amount of basic sites of A1PO4-Al2O3 (APA1-A, 5–15 wt% Al2O3) catalysts at two basic strengths was measured by studying the liquid-phase adsorption of two acidic molecules (benzoic acid (BA, pK = 4.2) and phenol (PH, pKa = 9.9) from cyclohexane solutions, through the application of a spectrophotometric method. The data obtained follow the Langmuir adsorption isotherm and the monolayer coverage at equilibrium (at 298 K),X m, is assumed as the amount of basic sites corresponding to the specific pK of the acid used as titrant. The amount of basic sites of any AlPO4-Al2O3 catalyst is higher than that of AlPO4, but lower than that of Al2O3. Besides, an increase in the Al2O3 content from 10 wt% gradually increases the basicity of the APA1-A catalyst. Moreover, calcination at increasing temperatures does not practically affect the surface basicity of APAl-A-5 and APAl-A-10 catalysts. However, for AlPO4 content higher than 10 wt% we observe a decrease in surface basicity, this decrease depends on alumina content, i.e. it is higher as the amount of alumina increases. The basic sites of APAl-A systems catalyze the Knoevenagel condensation ofp-methoxybenzaldehyde and malononitrile at room temperature and in the absence of solvent.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00771748
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  • 3
    Electronic Resource
    Electronic Resource
    Analogies and differences between ω-conotoxins MVIIC and MVIID: binding sites and functions in bovine chromaffin cells (1997)
    Springer
    Pflügers Archiv 435 (1997), S. 55-64 
    Details
    ISSN: 1432-2013
    Keywords: Key words Calcium channels ; Q channels ; Chromaffin cells ; ω-Conotoxin MVIIC ; ω-Conotoxin MVIID ; 45Ca2+ uptake ; Catecholamine release
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  The characteristics of the binding sites for the Conus magus toxins ω-conotoxin MVIIC and ω-conotoxin MVIID, as well as their effects on K+-evoked 45Ca2+ entry and whole-cell Ba2+ currents (I Ba), and K+-evoked catecholamine secretion have been studied in bovine adrenal chromaffin cells. Binding of [125I] ω-conotoxin GVIA to bovine adrenal medullary membranes was displaced by ω-conotoxins GVIA, MVIIC and MVIID with IC50 values of around 0.1, 4 and 100 nM, respectively. The reverse was true for the binding of [125I] ω-conotoxin MVIIC, which was displaced by ω-conotoxins MVIIC, MVIID and GVIA with IC50 values of around 30, 80 and 1.200 nM, respectively. The sites recognized by ω-conotoxins MVIIC and MVIID in bovine brain exhibited higher affinities (IC50 values of around 1 nM). Both ω-conotoxin MVIIC and MVIID blocked I Ba by 70–80%; the higher the [Ba2+]o of the extracellular solution the lower the blockade induced by ω-conotoxin MVIIC. This was not the case for ω-conotoxin MVIID; high Ba2+ (10 mM) slowed down the development of blockade but the maximum blockade achieved was similar to that obtained in 2 mM Ba2+. A further difference between the two toxins concerns their reversibility; washout of ω-conotoxin MVIIC did not reverse the blockade of I Ba while in the case of ω-conotoxin MVIID a partial, quick recovery of current was produced. This component was irreversibly blocked by ω-conotoxin GVIA, suggesting that it is associated with N-type Ca2+ channels. Blockade of K+-evoked 45Ca2+ entry produced results which paralleled those obtained by measuring I Ba. Thus, 1 μM of each of ω-conotoxin GVIA and MVIIA inhibited Ca2+ uptake by 25%, while 1 μM of each of ω-conotoxin MVIIC and MVIID caused a 70% blockade. K+-evoked catecholamine secretory responses were not reduced by ω-conotoxin GVIA (1 μM). In contrast, at 1 μM both ω-conotoxin MVIIC and MVIID reduced the exocytotic response by 70%. These data strengthen the previously established conclusion that Q-type Ca2+ channels that contribute to the regulation of secretion and are sensitive to ω-conotoxins MVIIC and MVIID are present in bovine chromaffin cells. These channels, however, seem to possess binding sites for ω-conotoxins MVIIC and MVIID whose characteristics differ considerably from those described to occur in the brain; they might represent a subset of Q-type Ca2+ channels or an entirely new subtype of voltage-dependent high-threshold Ca2+ channel.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050483
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  • 4
    Electronic Resource
    Electronic Resource
    Q-type Ca2+ channels are located closer to secretory sites than L-type channels: functional evidence in chromaffin cells (1998)
    Springer
    Pflügers Archiv 435 (1998), S. 472-478 
    Details
    ISSN: 1432-2013
    Keywords: Key words Ca2+ channels ; Exocytosis ; Chromaffin cells ; Catecholamine release ; ω-toxins ; Furnidipine ; Lubeluzole
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  This study uses a new strategy to investigate the hypothesis that, of the various Ca2+ channels expressed by a neurosecretory cell, a given channel subtype is coupled more tightly to the exocytotic apparatus than others. The approach is based on the prediction that the degree of inhibition of the secretory response by various Ca2+ channel blockers will differ at low (0.5 mM) and high (5 mM) extracellular Ca2+ concentrations ([Ca2+]o). So, at low [Ca2+]o the K+-evoked catecholamine release from superfused bovine chromaffin cells was depressed 60–70% by 2 μM ω-agatoxin IVA (P/Q-type Ca2+ channel blockade), by 3 μM ω-conotoxin MVIIC (N/P/Q-type Ca2+ channel blockade), or by 3 μM lubeluzole (N/P/Q-type Ca2+ channel blockade); in high [Ca2+]o these blockers inhibited the responses by only 20–35%. At 1–3 μM ω-conotoxin GVIA (N-type Ca2+ channel blockade) or 3 μM furnidipine (L-type Ca2+ channel blockade), secretion was inhibited by 30 and 50%, respectively; such inhibitory effects were similar in low or high [Ca2+]o. Combined furnidipine plus ω-conotoxin MVIIC, ω-agatoxin IVA or ω-conotoxin GVIA exhibited additive blocking effects at both Ca2+ concentrations. The results suggest that Q-type Ca2+ channels are coupled more tightly to exocytotic active sites, as compared to L-type channels. This hypothesis if founded in the fact that external Ca2+ that enters the cell through a Ca2+ channel located near to chromaffin vesicles will saturate the K+ secretory response at both [Ca2+]o, i.e. 0.5 mM and 5 mM. In contrast, Ca2+ ions entering through more distant channels will be sequestered by intracellular buffers and, thus, will not saturate the secretory machinery at lower [Ca2+]o.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050541
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  • 5
    Electronic Resource
    Electronic Resource
    Fluoride treatment of AlPO4-Al2O3 catalysts. II. Poisoning experiments by bases for cyclohexene conversion and cumene cracking (1994)
    Springer
    Catalysis letters 24 (1994), S. 293-301 
    Details
    ISSN: 1572-879X
    Keywords: AlPO4-Al2O3 ; fluoride ion loading ; surface acidity ; cyclohexene conversion ; cumene cracking ; poisoning by bases ; pyridine ; 2,6-dimethylpyridine ; hexamethyldisilazane
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Brønsted acid sites on fluoride-modified AlPO4-Al2O3 (2.5 wt% F; APAl-P-2.5F) catalyst are poisoned by the presence of 2,6-dimethylpyridine (DMPY) and hexamethyldisilazane (HMDS), thus decreasing the catalytic activity for cyclohexene and cumene reaction processes, while the effect of pyridine (PY) was scarce. Besides, the drop in activity for cyclohexene conversion was accompanied by a change in reaction selectivity so that hydrogen transfer sites are much more sensitive to base poisoning (getting greater as the poisoning effect increased) than isomerization sites. Moreover, surface trimethylsilyl (TMS) complexes (formed by covalent reaction of HMDS with surface hydroxyls) decomposed and thus, the activity progressively increased at increasing time intervals, thus reaching greater values (at ca. 4 h) than the unpoisoned APAl-P-2.5F catalyst. So, DMPY was more suitable than PY and HMDS for the poisoning of Brønsted acid sites on APAl-P-F catalyst.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00811802
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  • 6
    Electronic Resource
    Electronic Resource
    Continuous flow toluene methylation over AlPO4 and AlPO4-Al2O3 catalysts (1994)
    Springer
    Catalysis letters 26 (1994), S. 159-167 
    Details
    ISSN: 1572-879X
    Keywords: aluminum orthophosphate (AlPO4) ; AlPO4-Al2O3 ; toluene methylation ; methanol ; activity ; selectivity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Toluene methylation with methanol over AlPO4 (AP) and AlPO4-Al2O3 (APAl) catalysts, obtained through different methods, was carried out in a continuous down-flow fixed bed reactor. The main products were xylenes (XYL), although trimethylbenzenes (TMB) were also found over APAl catalysts. The benzene and ethylbenzene selectivities increased slightly with time on stream at the expense of XYL and TMB selectivities. Isomer distribution was approximately 50, 24 and 26 mol% foro-, m- andp-XYL, and 72, 27 and 0 mol% for 1,2,3-, 1,2,4- and 1,3,5-TMB. The initial reaction rate constants were higher on APA1 catalysts and, furthermore, APAl catalysts exhibited similar catalytic activities, although those obtained in ethylene or propylene oxide are the most active ones. The same occurs on AP catalysts. Moreover, the changes in catalytic activity are similar to the changes in the acidic characteristics measured, in gas phase, versus pyridine. Furthermore, the activity decreased with time on stream due to coke deposition according to the expressionk = k0 exp(-βt). The rate of deactivation, evaluated from the deactivation coefficients (β), was greater for APAl than for AP catalysts.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00824041
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  • 7
    Electronic Resource
    Electronic Resource
    Alkylation of phenol with dimethyl carbonate over AlPO4, Al2O3 and AlPO4-Al2O3 catalysts (1998)
    Springer
    Reaction kinetics and catalysis letters 63 (1998), S. 261-269 
    Details
    ISSN: 1588-2837
    Keywords: Aluminium orthophosphate (AlPO4) ; AlPO4-Al2O3 ; phenol/dimethyl carbonate alkylation ; activity ; anisole-cresol formation ; selectivity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The vapor-phase catalytic alkylation of phenol with dimethyl carbonate over different AlPO4 (Al/P=1), Al2O3 and AlPO4-Al2O3 (5–25 wt.% Al2O3) catalysts produces anisole (O-alkylation) as the major reaction product althougho-cresol (C-alkylation) and methylanisoles were also found. The reaction is first order in phenol while O-and C-alkylation follow parallel processes. As compared with methanol, DMC is far more effective as a methylating agent, and the methylation proceeds at a lower temperature and with higher O-alkylation selectivity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02475397
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