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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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The relationship between spark rise time and amplitude. (A) Rise time, T, vs. amplitude, a, of 1,887 sparks collected in 700 images of seven frog fibers with permeabilized membrane, placed in standard glutamate solution. Fibers were imaged at 0.135 μm per pixel and 2 ms per line. Sparks were detected and parameters were measured automatically on an interpolated average of three central pixels. Correlation coefficient r2 = 0.015, regression coefficient b = −0.017 ms−1. Inset: top, schematic of the profile F/F0 vs. t at the spatial center of the spark, indicating a and T; bottom, Ca2+ flux during spark is thought to be roughly constant and last for the rise time. (B) Average a vs. average T in “bins” of increasing T for the group of sparks in A. Bars represent ± SEM of a and T. (C) Average a vs. average T, determined as described for B, for a set of 881 sparks detected in 500 images of two frog fibers imaged at 0.14 μm per pixel and 0.5 ms per line. (D) Average a vs. T, for a set of 6,300 sparks in 1,000 images of four permeabilized fibers placed in sulfate-based solution and imaged at 0.23 μm per pixel and 1.875 ms per line.
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fig1: The relationship between spark rise time and amplitude. (A) Rise time, T, vs. amplitude, a, of 1,887 sparks collected in 700 images of seven frog fibers with permeabilized membrane, placed in standard glutamate solution. Fibers were imaged at 0.135 μm per pixel and 2 ms per line. Sparks were detected and parameters were measured automatically on an interpolated average of three central pixels. Correlation coefficient r2 = 0.015, regression coefficient b = −0.017 ms−1. Inset: top, schematic of the profile F/F0 vs. t at the spatial center of the spark, indicating a and T; bottom, Ca2+ flux during spark is thought to be roughly constant and last for the rise time. (B) Average a vs. average T in “bins” of increasing T for the group of sparks in A. Bars represent ± SEM of a and T. (C) Average a vs. average T, determined as described for B, for a set of 881 sparks detected in 500 images of two frog fibers imaged at 0.14 μm per pixel and 0.5 ms per line. (D) Average a vs. T, for a set of 6,300 sparks in 1,000 images of four permeabilized fibers placed in sulfate-based solution and imaged at 0.23 μm per pixel and 1.875 ms per line.

Mentions: The inset of Fig. 1 A represents diagrammatically the fluorescence profile (top trace) and the Ca2+ current (or release flux, bottom) believed to underlie a Ca2+ spark (evidences for this view are summarized by Baylor, 2005, and Klein and Schneider, 2006). Ca2+ current starts abruptly and remains approximately constant, as far as the temporal resolution of various methods can tell, until it terminates, abruptly as well. The time of active release has been shown with simulations to match fairly well the spark rise time T (elapsed from start to peak fluorescence), even for off-focus sparks (e.g., Pratusevich and Balke, 1996; Jiang et al., 1999).


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)

The relationship between spark rise time and amplitude. (A) Rise time, T, vs. amplitude, a, of 1,887 sparks collected in 700 images of seven frog fibers with permeabilized membrane, placed in standard glutamate solution. Fibers were imaged at 0.135 μm per pixel and 2 ms per line. Sparks were detected and parameters were measured automatically on an interpolated average of three central pixels. Correlation coefficient r2 = 0.015, regression coefficient b = −0.017 ms−1. Inset: top, schematic of the profile F/F0 vs. t at the spatial center of the spark, indicating a and T; bottom, Ca2+ flux during spark is thought to be roughly constant and last for the rise time. (B) Average a vs. average T in “bins” of increasing T for the group of sparks in A. Bars represent ± SEM of a and T. (C) Average a vs. average T, determined as described for B, for a set of 881 sparks detected in 500 images of two frog fibers imaged at 0.14 μm per pixel and 0.5 ms per line. (D) Average a vs. T, for a set of 6,300 sparks in 1,000 images of four permeabilized fibers placed in sulfate-based solution and imaged at 0.23 μm per pixel and 1.875 ms per line.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2279174&req=5

fig1: The relationship between spark rise time and amplitude. (A) Rise time, T, vs. amplitude, a, of 1,887 sparks collected in 700 images of seven frog fibers with permeabilized membrane, placed in standard glutamate solution. Fibers were imaged at 0.135 μm per pixel and 2 ms per line. Sparks were detected and parameters were measured automatically on an interpolated average of three central pixels. Correlation coefficient r2 = 0.015, regression coefficient b = −0.017 ms−1. Inset: top, schematic of the profile F/F0 vs. t at the spatial center of the spark, indicating a and T; bottom, Ca2+ flux during spark is thought to be roughly constant and last for the rise time. (B) Average a vs. average T in “bins” of increasing T for the group of sparks in A. Bars represent ± SEM of a and T. (C) Average a vs. average T, determined as described for B, for a set of 881 sparks detected in 500 images of two frog fibers imaged at 0.14 μm per pixel and 0.5 ms per line. (D) Average a vs. T, for a set of 6,300 sparks in 1,000 images of four permeabilized fibers placed in sulfate-based solution and imaged at 0.23 μm per pixel and 1.875 ms per line.
Mentions: The inset of Fig. 1 A represents diagrammatically the fluorescence profile (top trace) and the Ca2+ current (or release flux, bottom) believed to underlie a Ca2+ spark (evidences for this view are summarized by Baylor, 2005, and Klein and Schneider, 2006). Ca2+ current starts abruptly and remains approximately constant, as far as the temporal resolution of various methods can tell, until it terminates, abruptly as well. The time of active release has been shown with simulations to match fairly well the spark rise time T (elapsed from start to peak fluorescence), even for off-focus sparks (e.g., Pratusevich and Balke, 1996; Jiang et al., 1999).

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