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  • 1
    Electronic Resource
    Electronic Resource
    Effects of cyclopiazonic acid on Ca2+ regulation by the sarcoplasmic reticulum in saponin-permeabilized skeletal muscle fibres (1998)
    Springer
    Pflügers Archiv 436 (1998), S. 104-111 
    Details
    ISSN: 1432-2013
    Keywords: Key words Cyclopiazonic acid ; Fura-2 ; Sarcoplasmic reticulum ; Skeletal muscle
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  The effects of the sarcoplasmic reticulum (SR) Ca2+ pump inhibitor cyclopiazonic acid (CPA) were studied in saponin-permeabilized frog skeletal muscle fibres. Release of Ca2+ from the SR was triggered by brief (2 s) applications of 40 mM caffeine at 2-min intervals. Changes in [Ca2+] within the fibre were monitored continuously using Fura-2 fluorescence. At a bathing [Ca2+] of 100 nM, introduction of 20 μM CPA induced a slow release of Ca2+ from the SR. The following one to two caffeine-induced Ca2+ transients were markedly increased in amplitude and duration. Thereafter, the caffeine-induced Ca2+ transients decreased progressively and were barely detectable 6–7 min after introduction of CPA. However, increasing the bathing [Ca2+] or increasing the Ca2+ loading period resulted in a partial recovery of the caffeine-induced Ca2+ transients, suggesting that pump inhibition is incomplete, even in the presence of 100 μM CPA. The slow Ca2+ efflux induced by CPA was insensitive to ryanodine, but absent following abolition of SR Ca2+ pump activity by ATP withdrawal. These results suggest that the caffeine-induced Ca2+ transient reflects a balance between efflux via the SR Ca2+ channel and reuptake by the Ca pump. Ca2+ release upon addition of CPA may result from inhibition of SR Ca2+ uptake, which reveals a tonic Ca2+ efflux that is independent of the Ca2+ release channels.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050610
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  • 2
    Electronic Resource
    Electronic Resource
    Effects of cytosolic Ca2+ on the Ca2+ content of the sarcoplasmic reticulum in saponin-permeabilized rat ventricular trabeculae (1998)
    Springer
    Pflügers Archiv 435 (1998), S. 555-563 
    Details
    ISSN: 1432-2013
    Keywords: Key words Heart muscle ; Extracellular [Ca2+] ; Intracellular [Ca2+] ; Sarcoplasmic reticulum ; Ca-uptake
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  Rat ventricular trabeculae were mounted for isometric tension recording, and then permeabilized with saponin. The Ca2+ concentration ([Ca2+]) within the permeabilized preparation (cytosolic [Ca2+]) was monitored continuously using Indo-1 and the integrals of Ca2+ transients resulting from brief caffeine application used as an index of the sarcoplasmic reticulum (SR) Ca2+ content. The relationship between SR Ca2+ content and cytosolic [Ca2+] was studied within the reported physiological range (i.e. 50–250 nmol · l–1 Ca2+). Increasing cytosolic [Ca2+] from 50 nmol · l–1 to 250 nmol · l–1 increased the steady-state SR Ca2+ content about threefold. However, increasing [Ca2+] above 250 nmol · l–1 typically resulted in spontaneous SR Ca2+ release, with no further increase in SR Ca2+ content. The SR Ca2+ content increased only slowly when cytosolic [Ca2+] was increased; it was unchanged 20 s after a rapid increase in cytosolic [Ca2+], but increased progressively to a new steady-state level during the following 1–2 min. In a parallel series of experiments using intact papillary muscles, increasing extracellular [Ca2+] (from 0.5 to 5 mmol · l–1) significantly increased twitch tension within 20 s of the solution change. These results support previous suggestions that the SR Ca2+ content may increase when diastolic cytosolic [Ca2+] rises during inotropic interventions such as increased stimulus rate or extracellular [Ca2+]. However, the rate at which SR Ca2+ responds to changes in cytoplasmic [Ca2+] within the diastolic range does not appear rapid enough to explain the early potentiation of twitch tension in intact preparations after an increase in extracellular [Ca2+].
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050552
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  • 3
    Electronic Resource
    Electronic Resource
    Measurement of SR Ca2+ content in the presence of caffeine in permeabilised rat cardiac trabeculae (1998)
    Springer
    Pflügers Archiv 437 (1998), S. 139-148 
    Details
    ISSN: 1432-2013
    Keywords: Key words Ca2+ ; Ca2+ -ATPase ; Caffeine ; Cardiac ; Heart ; Ryanodine ; Sarcoplasmic reticulum
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  This study was designed to measure the Ca2+ content of rat cardiac sarcoplasmic reticulum (SR) after equilibration with normal diastolic levels of Ca2+ (100 nM), in the absence and presence of caffeine. Measurements of [Ca2+] based on Fura-2 fluorescence were made from a limited bath volume (230 nl) containing individual saponin-permeabilised rat cardiac trabeculae. Injection of caffeine (5–40 mM) into this volume caused an initial release of Ca2+ from the SR, but within 30 s the SR was able to re-accumulate a significant proportion of the Ca2+. Ca2+ re-accumulation into the SR could be prevented by removal of ATP to inhibit the SR Ca2+ pump. Incubation of the preparation in an ATP-containing solution containing caffeine (5–40 mM) and 100 nM Ca2+ indicated that the SR’s ability to retain Ca2+ depends inversely on the dose of caffeine. The relative Ca2+ content of the SR after preincubation with caffeine was 86.7±3.5% at a caffeine concentration of 5 mM, 62.5±5.1% at 10 mM caffeine, 37.8±8.1% at 20 mM caffeine and 7.1±1.9% at 40 mM caffeine. Measurement of the SR Ca2+ release in the presence of different BAPTA concentrations was used to calculate (1) the Ca2+-binding capacity of the preparation (equivalent to 245±10 µM BAPTA) and (2) the Ca2+ content of the SR accessed by caffeine after equilibration with 100 nM Ca2+ (186±11 µmol/l cell volume or 5.6 mmol/l SR volume).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050758
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  • 4
    Electronic Resource
    Electronic Resource
    The effects of caffeine and Ca2+ on rigor tension in triton-treated rat ventricular trabeculae (1992)
    Springer
    Pflügers Archiv 421 (1992), S. 343-349 
    Details
    ISSN: 1432-2013
    Keywords: Cardiac muscle ; Rigor tension ; Ca2+ ; Caffeine
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Ventricular trabeculae from rat heart were chemically skinned with Triton-X100, which disrupts all cellular membranes including the sarcoplasmic reticulum. Trabeculae developed a maintained rigor contracture when adenosine triphosphate was withdrawn from the bathing medium. In all preparations, the final level of rigor force developed in the presence of caffeine (10–40 mM) was greater than under control conditions. However, caffeine failed to increase rigor tension when applied after contracture had fully developed. The effect of caffeine on rigor was maximal at about 15 mM; concentrations greater or less than 15 mM were less effective. On average, caffeine decreased the time required to develop half-maximum rigor force. The caffeine-induced potentiation of rigor force occurred in the effective absence of Ca2+ (10−9 M), in solutions strongly Ca2+-buffered with [ethylenebis(oxonitrilo)]tetraaceticacid (10–50 mM). In all preparations, rigor force was found to be independent of [Ca2+] over the range 10−10 M to about 10−7 M. These results suggest that caffeine affects rigor force by a direct effect on the myofilaments via a mechanism that is independent of Ca2+.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00374222
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  • 5
    Electronic Resource
    Electronic Resource
    Effects of cAMP and forskolin on caffeine-induced contractures and myofilament Ca-sensitivity in saponin-treated rat ventricular trabeculae (1992)
    Springer
    Journal of muscle research and cell motility 13 (1992), S. 146-152 
    Details
    ISSN: 1573-2657
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary Trabeculae from the right ventricle of rat were chemically ‘skinned’ with saponin (50 μg ml−1 for 30 min). Caffeine (10mm) induced a transient contracture as a consequence of Ca2+-release from the sarcoplasmic reticulum and subsequent activation of the myofilaments. The amplitude of the caffeine contracture was used as an index of the Ca2+-content of the sarcoplasmic reticulum. cAMP (0.1–10 μm) markedly potentiated the caffeine-induced response under standardized conditions. This was interpreted as a cAMP-induced increase in Ca2+ accumulation by the sarcoplasmic reticulum during the Ca2+ loading period before the application of caffeine. After removal of cAMP, the amplitude of the regularly evoked contractures slowly declined towards standard levels. The characteristic effects of cAMP on the caffeine contracture were mimicked by addition of forskolin (10−6 m), a substance known to stimulate cAMP production by adenylate cyclase. The results suggest that a significant quantity of functional adenylate cyclase persists after saponin treatment. In these preparations cAMP had no effect on the apparent Ca2+-sensitivity of the myofilaments. If the preparations were exposed briefly to saponin, cAMP caused a decrease in the apparent Ca2+ sensitivity of the contractile proteins. However, further exposure to saponin, or to Triton X-100, caused a marked increase in maximum Ca2+-activated force (Cmax). It was concluded that ‘briefly’ saponin-treated preparations, exhibiting a reduction in Ca2+ sensitivity in response to cAMP, are not uniformly permeable to the bathing solution.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01874151
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