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Calcium-dependent inactivation terminates calcium release in skeletal muscle of amphibians.

Ríos E, Zhou J, Brum G, Launikonis BS, Stern MD - J. Gen. Physiol. (2008)

Bottom Line: In groups of thousands of sparks occurring spontaneously in membrane-permeabilized frog muscle cells a complex relationship was found between amplitude a and rise time T, which in sparks corresponds to the active time of the underlying Ca2+ release.Using every method, it was found that T and flux were inversely correlated, roughly inversely proportional.Considering these results and other available evidence it is concluded that Ca2+-dependent inactivation, or CDI, provides the crucial mechanism for termination of sparks and cell-wide Ca2+ release in amphibians.

View Article: PubMed Central - PubMed

Affiliation: Section of Cellular Signaling, Department of Molecular Biophysics and Physiology, Rush University, Chicago, IL 60612, USA.

ABSTRACT
In skeletal muscle of amphibians, the cell-wide cytosolic release of calcium that enables contraction in response to an action potential appears to be built of Ca2+ sparks. The mechanism that rapidly terminates this release was investigated by studying the termination of Ca2+ release underlying sparks. In groups of thousands of sparks occurring spontaneously in membrane-permeabilized frog muscle cells a complex relationship was found between amplitude a and rise time T, which in sparks corresponds to the active time of the underlying Ca2+ release. This relationship included a range of T where a paradoxically decreased with increasing T. Three different methods were used to estimate Ca2+ release flux in groups of sparks of different T. Using every method, it was found that T and flux were inversely correlated, roughly inversely proportional. A simple model in which release sources were inactivated by cytosolic Ca2+ was able to explain the relationship. The predictive value of the model, evaluated by analyzing the variance of spark amplitude, was found to be high when allowance was made for the out-of-focus error contribution to the total variance. This contribution was estimated using a theory of confocal scanning (Ríos, E., N. Shirokova, W.G. Kirsch, G. Pizarro, M.D. Stern, H. Cheng, and A. González. Biophys. J. 2001. 80:169-183), which was confirmed in the present work by simulated line scanning of simulated sparks. Considering these results and other available evidence it is concluded that Ca2+-dependent inactivation, or CDI, provides the crucial mechanism for termination of sparks and cell-wide Ca2+ release in amphibians. Given the similarities in kinetics of release termination observed in cell-averaged records of amphibian and mammalian muscle, and in spite of differences in activation mechanisms, CDI is likely to play a central role in mammals as well. Trivially, an inverse proportionality between release flux and duration, in sparks or in global release of skeletal muscle, maintains constancy of the amount of released Ca2+.

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Rise time vs. different estimators of release flux. Black symbols, T vs. m1, calculated from binned averages in Fig. 1 B by Eq. 1 (same values as in graph in Fig. 2 B). Green, T vs. m2, calculated according to Eq. 2 for the same spark averages. Red, T vs. m3, release current calculated by volume integration of flux density derived for the same spark averages by the backward method. Continuous curve, best fit to T vs. m3 by Eq. 6. Best fit parameters: kiβ, 2.3 mM−1; kr, 0.061 ms−1; A* = 0.115.
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fig4: Rise time vs. different estimators of release flux. Black symbols, T vs. m1, calculated from binned averages in Fig. 1 B by Eq. 1 (same values as in graph in Fig. 2 B). Green, T vs. m2, calculated according to Eq. 2 for the same spark averages. Red, T vs. m3, release current calculated by volume integration of flux density derived for the same spark averages by the backward method. Continuous curve, best fit to T vs. m3 by Eq. 6. Best fit parameters: kiβ, 2.3 mM−1; kr, 0.061 ms−1; A* = 0.115.

Mentions: To overcome these weaknesses, two additional estimations of release flux were used, none of which involved explicit division by T. One approximated release flux as the rate of change of signal mass, or spark mass, M. Following Chandler et al. (2003), M was calculated assuming that sparks are spherically symmetric, as(2)\documentclass[10pt]{article}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{pmc}\usepackage[Euler]{upgreek}\pagestyle{empty}\oddsidemargin -1.0in\begin{document}\begin{equation*}M(t){\sim}1.206\hspace{.167em}\frac{{\mathrm{{\Delta}}}F}{F_{0}}(t,\hspace{.167em}x=0)\hspace{.167em}FWHM(t)^{3}.\end{equation*}\end{document}In this equation fluorescence F and its initial value F0 are evaluated on the spark averages at each bin of rise time at the location x of the spark peak; FWHM (t) represents the full width at half maximum of , calculated from the Gaussian standard deviation σ of the spatial profile at time t as 2(2ln2)0.5σ. Peak rate of change, calculated on M (t), was close in value to its average over the duration of the rising phase of M(t). This average was calculated for all bins of T in the set of sparks represented in Fig. 1 (A and B). Denoted as m2, it is plotted against T by green symbols in Fig. 4. In the same figure, m1, calculated by Eq. 1, is represented in black.


Calcium-dependent inactivation terminates calcium release in skeletal muscle of amphibians.

Ríos E, Zhou J, Brum G, Launikonis BS, Stern MD - J. Gen. Physiol. (2008)

Rise time vs. different estimators of release flux. Black symbols, T vs. m1, calculated from binned averages in Fig. 1 B by Eq. 1 (same values as in graph in Fig. 2 B). Green, T vs. m2, calculated according to Eq. 2 for the same spark averages. Red, T vs. m3, release current calculated by volume integration of flux density derived for the same spark averages by the backward method. Continuous curve, best fit to T vs. m3 by Eq. 6. Best fit parameters: kiβ, 2.3 mM−1; kr, 0.061 ms−1; A* = 0.115.
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fig4: Rise time vs. different estimators of release flux. Black symbols, T vs. m1, calculated from binned averages in Fig. 1 B by Eq. 1 (same values as in graph in Fig. 2 B). Green, T vs. m2, calculated according to Eq. 2 for the same spark averages. Red, T vs. m3, release current calculated by volume integration of flux density derived for the same spark averages by the backward method. Continuous curve, best fit to T vs. m3 by Eq. 6. Best fit parameters: kiβ, 2.3 mM−1; kr, 0.061 ms−1; A* = 0.115.
Mentions: To overcome these weaknesses, two additional estimations of release flux were used, none of which involved explicit division by T. One approximated release flux as the rate of change of signal mass, or spark mass, M. Following Chandler et al. (2003), M was calculated assuming that sparks are spherically symmetric, as(2)\documentclass[10pt]{article}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{pmc}\usepackage[Euler]{upgreek}\pagestyle{empty}\oddsidemargin -1.0in\begin{document}\begin{equation*}M(t){\sim}1.206\hspace{.167em}\frac{{\mathrm{{\Delta}}}F}{F_{0}}(t,\hspace{.167em}x=0)\hspace{.167em}FWHM(t)^{3}.\end{equation*}\end{document}In this equation fluorescence F and its initial value F0 are evaluated on the spark averages at each bin of rise time at the location x of the spark peak; FWHM (t) represents the full width at half maximum of , calculated from the Gaussian standard deviation σ of the spatial profile at time t as 2(2ln2)0.5σ. Peak rate of change, calculated on M (t), was close in value to its average over the duration of the rising phase of M(t). This average was calculated for all bins of T in the set of sparks represented in Fig. 1 (A and B). Denoted as m2, it is plotted against T by green symbols in Fig. 4. In the same figure, m1, calculated by Eq. 1, is represented in black.

Bottom Line: In groups of thousands of sparks occurring spontaneously in membrane-permeabilized frog muscle cells a complex relationship was found between amplitude a and rise time T, which in sparks corresponds to the active time of the underlying Ca2+ release.Using every method, it was found that T and flux were inversely correlated, roughly inversely proportional.Considering these results and other available evidence it is concluded that Ca2+-dependent inactivation, or CDI, provides the crucial mechanism for termination of sparks and cell-wide Ca2+ release in amphibians.

View Article: PubMed Central - PubMed

Affiliation: Section of Cellular Signaling, Department of Molecular Biophysics and Physiology, Rush University, Chicago, IL 60612, USA.

ABSTRACT
In skeletal muscle of amphibians, the cell-wide cytosolic release of calcium that enables contraction in response to an action potential appears to be built of Ca2+ sparks. The mechanism that rapidly terminates this release was investigated by studying the termination of Ca2+ release underlying sparks. In groups of thousands of sparks occurring spontaneously in membrane-permeabilized frog muscle cells a complex relationship was found between amplitude a and rise time T, which in sparks corresponds to the active time of the underlying Ca2+ release. This relationship included a range of T where a paradoxically decreased with increasing T. Three different methods were used to estimate Ca2+ release flux in groups of sparks of different T. Using every method, it was found that T and flux were inversely correlated, roughly inversely proportional. A simple model in which release sources were inactivated by cytosolic Ca2+ was able to explain the relationship. The predictive value of the model, evaluated by analyzing the variance of spark amplitude, was found to be high when allowance was made for the out-of-focus error contribution to the total variance. This contribution was estimated using a theory of confocal scanning (Ríos, E., N. Shirokova, W.G. Kirsch, G. Pizarro, M.D. Stern, H. Cheng, and A. González. Biophys. J. 2001. 80:169-183), which was confirmed in the present work by simulated line scanning of simulated sparks. Considering these results and other available evidence it is concluded that Ca2+-dependent inactivation, or CDI, provides the crucial mechanism for termination of sparks and cell-wide Ca2+ release in amphibians. Given the similarities in kinetics of release termination observed in cell-averaged records of amphibian and mammalian muscle, and in spite of differences in activation mechanisms, CDI is likely to play a central role in mammals as well. Trivially, an inverse proportionality between release flux and duration, in sparks or in global release of skeletal muscle, maintains constancy of the amount of released Ca2+.

Show MeSH
Related in: MedlinePlus