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Spontaneous calcium release from sarcoplasmic reticulum. Effect of local anesthetics.
Full text linkSpontaneous calcium release from purified light sarcoplasmic reticulum has been previously described (Palade, P., Mitchell, R. D., and Fleischer, S. (1983) J. Biol. Chem. 258, 8098-8107 ) and found to be distinct from several other forms of Ca2+ release. Ca2+ release occurs after a lag period following active Ca2+ preloading and depletion of extravesicular Ca2+. In the present study, we find that local anesthetics inhibit spontaneous Ca2+ release, in a time-dependent manner, varying considerably in the preincubation time required to exert maximal effect. At pH 7.0, hydrophilic and mostly charged local anesthetics, such as procaine, procainamide, and N-(2,6-dimethylphenyl carbamoyl methyl)triethyl ammonium bromide, inhibit Ca2+ release only after long preincubations (hours), whereas more hydrophobic local anesthetics are effective after only a short incubation (minutes) with sarcoplasmic reticulum. The more hydrophobic anesthetics take somewhat longer to reach equilibrium, as studied by inhibition of unidirectional Ca2+ efflux, and there is a direct relationship between hydrophobic partition coefficient and half-time to reach equilibrium. Agents known to inhibit permeability pathways for monovalent cations i.e. K+ channel blockers (decamethonium and n-dodecane-1, 12-N,N,N,N',N',N'-hexamethyl-bis-ammonium) or the anion blocker (4,4'-diisothiocyanostilbene-2,2'-disulfonic acid), do not inhibit spontaneous Ca2+ release. Carbonyl cyanide m-fluorophenylhydrazone, a protonophore, and gramicidin D, a monovalent cation ionophore, have no effect on Ca2+ release whether local anesthetics are present or not, while the Ca2+ ionophore A23187 relieves inhibition of Ca2+ release by local anesthetics. Ruthenium red does not inhibit spontaneous Ca2+ release. These findings suggest that the binding site(s) for local anesthetics is located on the inner face of the sarcoplasmic reticulum membrane and that local anesthetics interact directly with a Ca2+ channel rather than with other permeability pathways which might indirectly influence Ca2+ channel gating
Biochemical characterization, integrity, and sidedness of purified skeletal muscle triads.
Full text linkThe release of Ca2+ from the terminal cisternae of sarcoplasmic reticulum in muscle fiber triggers muscle contraction. The signal for Ca2+ release is mediated via the triad junction, i.e. the junctional association of terminal cisternae and transverse tubule. Recently, highly purified morphologically intact triads were isolated from rabbit skeletal muscle (Mitchell, R. D., Palade, P., and Fleischer, S. (1983) J. Cell Biol. 96, 1008-1016). In this study, biochemical characterization of two variants of purified triad preparations (Pyrophosphate and Standard) is provided. Terminal cisternae of triads sequester Ca2+ at rates comparable to those of purified heavy sarcoplasmic reticulum which is referable to terminal cisternae (Meissner, G. (1975) Biochim. Biophys. Acta 389, 51-68). The permeability for calcium ions, as reflected by a 2-3-fold stimulation of (Ca2+, Mg2+)-ATPase activity in the presence of the Ca2+ ionophore A23187, and by the Ca2+ leak rate, is comparable in triads and heavy sarcoplasmic reticulum. Several transverse tubule characteristics are present in triads. Four of them, i.e. cholesterol content, ouabain binding, dihydroalprenolol binding (beta-adrenergic receptor), and ouabain-sensitive (Na+, K+)-ATPase activity, are comparably enriched in the Pyrophosphate triads and therefore appear to be quantitative indices of the amount of transverse tubule. Adenylate cyclase and basal ATPase are unreliable in this regard. Methodology for analyzing membrane integrity and sidedness was applied (adenylate cyclase activity) and modified (ouabain-sensitive (Na+, K+)-ATPase activity) to characterize the transverse tubule of the triad. In addition, a new method was developed making use of ouabain binding to study sidedness. These studies show that the transverse tubule is largely sealed and inside out in orientation, i.e. with the cytoplasmic face exposed. This report indicates that the t-tubule and sarcoplasmic reticulum components of the triads possess transport capability and retain permeability barriers for ions. Therefore, the isolated triads appear to be suitable for studying the physiological Ca2+ release process in vitro
Pharmacologic differentiation between inositol-1,4,5-trisphosphate-induced Ca2+ release and Ca2+- or caffeine-induced Ca2+ release from intracellular membrane systems.
No full textVarious known Ca2+ channel blockers and intracellular Ca2+ antagonists have been tested for effects of inositol-1,4,5-trisphosphate (IP3)-induced Ca2+ release from isolated canine brain microsomes. In agreement with previous reports, heparin, p-chloromercuribenzoic acid, W-7, cinnarizine, flunarizine, certain local anesthetics, La3+, and Ca2+ inhibit the release of Ca2+ induced by addition of IP3. In addition, we report here pronounced inhibition of IP3-induced Ca2+ release by low levels of Cd2+, by relatively high concentrations of TMB-8, and by phytic acid. In contrast, a number of blockers of other Ca2+ channels (nifedipine, verapamil, dantrolene, dithiothreitol, and ruthenium red) have relatively little or no effect on IP3-induced Ca2+ release from brain microsomes. The relative ineffectiveness of substances that inhibit Ca2+- or caffeine-induced Ca2+ release from skeletal muscle sarcoplasmic reticulum suggests that release of Ca2+ from caffeine- and IP3-sensitive neuronal Ca2+ stores is likely to be mediated by different channels. Further evidence that different channels are involved is presented by way of demonstration of the lack of Ca2+-induced Ca2+ release from these brain microsomes and the lack of effect on sarcoplasmic reticulum caffeine-induced Ca2+ release of certain inhibitors of IP3-induced Ca2+ release used here. Among IP3-induced Ca2+ release blockers, La3+ appeared to be exceptional in its ability to stimulate microsomal Ca2+ uptake sufficiently to attenuate release of Ca2+ induced by IP3. Most blockers of IP3-induced Ca2+ release appear not to function by way of inhibiting K+ counter-ion movements (valinomycin does not reverse the inhibition) but rather by way of direct interaction with the IP3 receptor or the Ca2+ channel that mediates the IP3-induced Ca2+ release. Inhibition of [3H]IP3 binding to the microsomes by phytic acid, heparin, pyrophosphate, p-chloromercuribenzoic acid, and Ca2+ could be demonstrated but not by the other substances tested
Polymer-derived microcellular SiOC foams with magnetic functionality
No full textSiOC microcellular ceramic foams possessing
soft-ferromagnetic properties were produced from a preceramic
polymer, poly-methyl-methacrylate microbeads
(PMMA) (used as sacrificial pore formers) and iron silicide
micro-powders (as functional filler). The interactions
between the matrix and the filler were studied as a function
of the amount of powders introduced and the pyrolysis
temperature. Magnetic and mechanical properties were also
investigated
Direct inhibition of inositol-1,4,5-trisphosphate-induced Ca2+ release from brain microsomes by K+ channel blockers.
No full textDihydropyridine receptor and ryanodine receptor gene expression in long-term denervated rat muscles
No full textFollowing disruption of the nerve supply, extensor digitorum longus (EDL) and soleus (SOL) muscles in rats are known to exhibit alterations in excitation-contraction coupling. After total RNA isolation from the denervated and the contralateral control muscles performed at 25 and 50 days following denervation, RNase protection assays were carried out with four cDNA probes specific for the skeletal and cardiac isoforms of both the DHPR alpha 1-subunit and the RyR. Longterm denervation increased the expression of the mRNA for skeletal DHPR and skeletal RyR in SOL muscle, but it also significantly increased the expression of the mRNA for the cardiac isoform of the DHPR alpha 1 subunit in EDL muscle
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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