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  • 1
    Electronic Resource
    Electronic Resource
    The contribution of the sarcoplasmic reticulum Ca2+-transport ATPase to caffeine-induced Ca2+ transients of murine skinned skeletal muscle fibres (1996)
    Springer
    Pflügers Archiv 432 (1996), S. 717-726 
    Details
    ISSN: 1432-2013
    Keywords: Key words Sarcoplasmic reticulum ; SR Ca2+-transport ATPase ; Cyclopiazonic acid ; Mouse muscle ; Skinned fibres ; Caffeine
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The present study was carried out to investigate the contribution of the Ca2+-transport ATPase of the sarcoplasmic reticulum (SR) to caffeine-induced Ca2+ release in skinned skeletal muscle fibres. Chemically skinned fibres of balb-C-mouse EDL (extensor digitorum longus) were exposed for 1 min to a free Ca2+ concentration of 0.36 μM to load the SR with Ca2+. Release of Ca2+ from the SR was induced by 30 mM caffeine and recorded as an isometric force transient. For every preparation a pCa/force relationship was constructed, where pCa = −log10 [Ca2+]. In a new experimental approach, we used the pCa/force relationship to transform each force transient directly into a Ca2+ transient. The calculated Ca2+ transients were fitted by a double exponential function: Y 0 + A 1⋅exp (−t/t 1) + A 2⋅exp(t/t 2), with A 1 〈 0 〈 A 2, t 1 〈 t 2 and Y 0, A 1, A 2 in micromolar. Ca2+ transients in the presence of the SR Ca2+-ATPase inhibitor cyclopiazonic acid (CPA) were compared to those obtained in the absence of the drug. We found that inhibition of the SR Ca2+-ATPase during caffeine-induced Ca2+ release causes an increase in the peak Ca2+ concentration in comparison to the control transients. Increasing CPA concentrations prolonged the time-to-peak in a dose-dependent manner, following a Hill curve with a half-maximal value of 6.5 ± 3 μM CPA and a Hill slope of 1.1 ± 0.2, saturating at 100 μM. The effects of CPA could be simulated by an extended three-compartment model representing the SR, the myofilament space and the external bathing solution. In terms of this model, the SR Ca2+-ATPase influences the Ca2+ gradient across the SR membrane in particular during the early stages of the Ca2+ transient, whereas the subsequent relaxation is governed by diffusional loss of Ca2+ into the bathing solution.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050190
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  • 2
    Electronic Resource
    Electronic Resource
    Ca2+-movements in muscle modulated by the state of K+-channels in the sarcoplasmic reticulum membranes (1987)
    Springer
    Pflügers Archiv 409 (1987), S. 374-380 
    Details
    ISSN: 1432-2013
    Keywords: Skeletal muscle ; Sarcoplasmic reticulum ; K+ conductance ; Ca2+ loading ; Skinned muscle fibres ; K+ channel blockers
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract 1. A procedure has been developed to load Ca2+ reversibly by the sarcoplasmic reticulum (SR) of mechanically skinned muscle fibres from the toadBufo marinus under controlled conditions and was employed to investigate the effects of conditions known to reduce the K+ conductance located in the SR-membrane during Ca2+-loading on the amount of Ca2+ releasable by caffeine. 2. The amount of releasable Ca2+ was markedly increased compared to controls when 4-aminopyridine (4AP) (6 μM to 2 mM), tetraethylammonium (TEA), decamethonium (0.5 mM) or procaine (1 mM) were present in the Ca2+-loading solutions. All these substances are known to act as SR-K+-channel blockers. 3. The increased amount of releasable Ca2+ in the presence of the K+-channel blocker 4AP was observed both at low (0.3 mM) and at higher (1 mM) Mg2+ concentrations and was not affected by verapamil (20 μM), a known Ca2+-channel blocker of the sarcolemma nor by the Na+−K+ pump inhibitor, ouabain (1 mM). 4. In the presence of 0.1–5 μM ruthenium red, a known inhibitor of Ca2+ induced Ca2+ release from the SR, the amount of releasable Ca2+ was greatly increased by up to 300%. Addition of between 50 μM and 1.6 mM 4AP to ruthenium red Ca2+ loading solution modified differently the amount of releasable Ca2+, suggesting that the mechanism of action of 4AP is different from that of ruthenium red. 5. When all K+ ions in the loading solution were replaced by the less permeant Na+ ions the amount of releasable Ca2+ ions was also increased. 6. These results indicate that the amount of releasable Ca2+ from the SR is consistently modified under conditions aimed at interfering with the state of SR-K+-channels, suggesting that SR-K+-channels may play an important physiological role in the modulation of excitation-contraction coupling. One possible mechanism involving SR-K+-channels which could explain our results is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00583791
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  • 3
    Electronic Resource
    Electronic Resource
    The influence of ionic strength upon relaxation from rigor induced by flash photolysis of caged-ATP in skinned murine skeletal muscle fibres (1995)
    Springer
    Pflügers Archiv 430 (1995), S. 994-1003 
    Details
    ISSN: 1432-2013
    Keywords: Ionic strength ; Caged-ATP ; Rigor ; Skeletal muscle
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The influence of ionic strength upon relaxation kinetics from rigor in skinned murine extensor digitorum longus (EDL) skeletal muscle fibres was examined using photolysis of caged-ATP at low Ca2+. The ionic strength was adjusted with either KMeSO3 or ethylene glycol bis-(β-aminoethyl ether)N,N,N′,N′-tetraacetic acid, dipotassium salt (K2EGTA) in the range of Г/2=65–215 mM, or I.E. 49–194 mM, where I.E. denotes ionic equivalent. Following rigor development at aГ/2 of 165–215 mM (I.E. 144–194 mM), the liberation of approximately 0.5 mM ATP resulted in an initial 6-to 10-ms detachment phase with a decline in force of approximately 10–20% followed by a 10-to 30-ms reattachment with up to a 60% increase compared to the corresponding rigor level and a final detachment leading to complete relaxation. Interestingly, when similar ATP concentrations were liberated at lower ionic strengths between a Г/2 of 65 mM and 110 mM (I.E. 60–100 mM), the initial detachment phase was shortened and force decreased by only approximately 5–10%, while the following reattachment phase was lengthened and led to an increased steady-state force of approximately 20–80% without final relaxation. ATP-induced detachment and subsequent reattachment were mainly determined by the currently present ionic strength and were relatively independent of the preceding rigor state which had been developed at higher or lower ionic strengths. The effects of phosphate and apyrase on the force transient suggest that reattachment of ADP- binding crossbridges may contribute to the increase in tension at high and even more at low ionic strengths. The study shows that the kinetics of initial fast relaxation and subsequent redevelopment of force following flash photolysis of similar ATP concentrations are markedly modified by the ionic strength in the narrow range of between 65 mM and 215 mM.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01837414
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  • 4
    Electronic Resource
    Electronic Resource
    The effect of ionic strength on the kinetics of rigor development in skinned fast-twitch skeletal muscle fibres (1998)
    Springer
    Pflügers Archiv 435 (1998), S. 753-761 
    Details
    ISSN: 1432-2013
    Keywords: Key words Skeletal fast-twitch muscle ; Rigor ; Diffusion ; In-phase stiffness
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  Recent atomic 3-D reconstructions of the acto-myosin interface suggest that electrostatic interactions are important in the initial phase of cross-bridge formation. Earlier biochemical studies had also given strong evidence for the ionic strength dependence of this step in the cross-bridge cycle. We have probed these interactions by altering the ionic strength (Γ/2) of the medium mainly with K+, imidazole+ and EGTA2– to vary charge shielding. We examined the effect of ionic strength on the kinetics of rigor development at low Ca2+ (experimental temperature 18–22°C) in chemically skinned single fast-twitch fibres of mouse extensor digitorum longus (EDL) muscle. On average the delay before rigor onset was 10 times longer, the maximum rate of rigor tension development was 10 times slower, the steady-state rigor tension was 3 times lower and the in-phase stiffness was 2 times lower at high (230 mM) compared to low (60 mM) ionic strength. These results were modelled by calculating ATP depletion in the fibre due to diffusional loss of ATP and acto-myosin Mg.ATPase activity. The difference in delay before rigor onset at low and high ionic strength could be explained in our model by assuming a 15 times higher Mg.ATPase activity and a threefold increase in K m in relaxing conditions at low ionic strength. Activation by Ca2+ induced at different time points before and during onset of rigor confirmed the calculated time course of ATP depletion. We have also investigated ionic strength effects on rigor development with the activated troponin/tropomyosin complex. ATP withdrawl at maximum activation by Ca2+ induced force transients which led into a ”high rigor” state. The peak forces of these force transients were very similar at low and high ionic strength. The subsequent decrease in tension was only 10% slower and steady-state ”high rigor” tension was reduced by only 27% at high compared to low ionic strength. Addition of 10 mM phosphate to lower cross-bridge attachment strongly suppressed the transient increases in force at high ionic strength and reduced the steady-state rigor tension by 17%. A qualitatively similar but smaller effect of phosphate was observed at low ionic strength where steady-state rigor force was reduced by 10%. The data presented in this study show a very strong effect of ionic strength on rigor development in relaxed fibres whereas the ionic strength dependence of rigor development after thin filament activation was much less. The data confirm the importance of electrostatic interactions in cross-bridge attachment and cross-bridge-attachment-induced activation of thin filaments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050580
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