Limits...
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+.

Show MeSH

Related in: MedlinePlus

Amplitudes and rise times of simulated sparks. Dots plot detected amplitude, as vs. rise time in the line scan, T, for sparks generated at random locations in the simulation volume, using a current of 30 pA and release durations Ξ between 0.5 and 35 ms. The sparks represented had as > 0.3. Green circles, average values (±SEM) in bins of T containing 300 sparks each. Line, single exponential fit to bin averages (Eq. 7, with b = 0.9409 and k = 0.7337 ms−1). Note that for sparks of constant release flux amplitude increases with T in a saturating manner. Pink symbols, bin averages of as for a set of 8,000 sparks simulated with current of 30 pA and Ξ = 5 ms. Note that while Ξ is constant, T varies in a narrow range, being in most cases greater than Ξ. Orange circles, bin averages of as of a set of 4,667 sparks simulated assuming the inverse relationship between Ξ and m3. (Ξ was exponentially distributed, with a minimum of 0.5 ms. m3 was calculated from Ξ by an approximate solution of Eq. 6 and Ξ ∼ T). Note that the dependence between averaged amplitude and T reflects well the inverse relationship between flux and release time assumed in the simulation.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2279174&req=5

fig5: Amplitudes and rise times of simulated sparks. Dots plot detected amplitude, as vs. rise time in the line scan, T, for sparks generated at random locations in the simulation volume, using a current of 30 pA and release durations Ξ between 0.5 and 35 ms. The sparks represented had as > 0.3. Green circles, average values (±SEM) in bins of T containing 300 sparks each. Line, single exponential fit to bin averages (Eq. 7, with b = 0.9409 and k = 0.7337 ms−1). Note that for sparks of constant release flux amplitude increases with T in a saturating manner. Pink symbols, bin averages of as for a set of 8,000 sparks simulated with current of 30 pA and Ξ = 5 ms. Note that while Ξ is constant, T varies in a narrow range, being in most cases greater than Ξ. Orange circles, bin averages of as of a set of 4,667 sparks simulated assuming the inverse relationship between Ξ and m3. (Ξ was exponentially distributed, with a minimum of 0.5 ms. m3 was calculated from Ξ by an approximate solution of Eq. 6 and Ξ ∼ T). Note that the dependence between averaged amplitude and T reflects well the inverse relationship between flux and release time assumed in the simulation.

Mentions: The simulations aimed on one hand to test whether and how different models of spark release current and/or duration affected the distribution of amplitudes and rise times determined in line scans, thus testing whether experimental distributions could be used to evaluate such models. A second purpose of the simulations was to test predictions (about these amplitude distributions) derived from a theory of confocal scanning (Ríos et al., 2001). For the first goal, two distributions of parameters were used, with results represented in Fig. 5. To represent sparks satisfying the calcium inactivation model of Scheme 1, release durations were randomly assigned according to an exponential distribution with a mean of 5 ms. Release current m3 was then calculated for each spark from its release duration T by approximate numerical inversion of Eq. 6 subject to an upper bound (m3 = 40 pA*(2.163 ms/T − 0.061); m3 ≤ 37 pA). Then, to generate a comparable representation of a “ model,” another set of simulated sparks was generated with a distribution of release current duration that was the same as in the calcium inactivation model, but with constant release current.


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)

Amplitudes and rise times of simulated sparks. Dots plot detected amplitude, as vs. rise time in the line scan, T, for sparks generated at random locations in the simulation volume, using a current of 30 pA and release durations Ξ between 0.5 and 35 ms. The sparks represented had as > 0.3. Green circles, average values (±SEM) in bins of T containing 300 sparks each. Line, single exponential fit to bin averages (Eq. 7, with b = 0.9409 and k = 0.7337 ms−1). Note that for sparks of constant release flux amplitude increases with T in a saturating manner. Pink symbols, bin averages of as for a set of 8,000 sparks simulated with current of 30 pA and Ξ = 5 ms. Note that while Ξ is constant, T varies in a narrow range, being in most cases greater than Ξ. Orange circles, bin averages of as of a set of 4,667 sparks simulated assuming the inverse relationship between Ξ and m3. (Ξ was exponentially distributed, with a minimum of 0.5 ms. m3 was calculated from Ξ by an approximate solution of Eq. 6 and Ξ ∼ T). Note that the dependence between averaged amplitude and T reflects well the inverse relationship between flux and release time assumed in the simulation.
© Copyright Policy
Related In: Results  -  Collection

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

fig5: Amplitudes and rise times of simulated sparks. Dots plot detected amplitude, as vs. rise time in the line scan, T, for sparks generated at random locations in the simulation volume, using a current of 30 pA and release durations Ξ between 0.5 and 35 ms. The sparks represented had as > 0.3. Green circles, average values (±SEM) in bins of T containing 300 sparks each. Line, single exponential fit to bin averages (Eq. 7, with b = 0.9409 and k = 0.7337 ms−1). Note that for sparks of constant release flux amplitude increases with T in a saturating manner. Pink symbols, bin averages of as for a set of 8,000 sparks simulated with current of 30 pA and Ξ = 5 ms. Note that while Ξ is constant, T varies in a narrow range, being in most cases greater than Ξ. Orange circles, bin averages of as of a set of 4,667 sparks simulated assuming the inverse relationship between Ξ and m3. (Ξ was exponentially distributed, with a minimum of 0.5 ms. m3 was calculated from Ξ by an approximate solution of Eq. 6 and Ξ ∼ T). Note that the dependence between averaged amplitude and T reflects well the inverse relationship between flux and release time assumed in the simulation.
Mentions: The simulations aimed on one hand to test whether and how different models of spark release current and/or duration affected the distribution of amplitudes and rise times determined in line scans, thus testing whether experimental distributions could be used to evaluate such models. A second purpose of the simulations was to test predictions (about these amplitude distributions) derived from a theory of confocal scanning (Ríos et al., 2001). For the first goal, two distributions of parameters were used, with results represented in Fig. 5. To represent sparks satisfying the calcium inactivation model of Scheme 1, release durations were randomly assigned according to an exponential distribution with a mean of 5 ms. Release current m3 was then calculated for each spark from its release duration T by approximate numerical inversion of Eq. 6 subject to an upper bound (m3 = 40 pA*(2.163 ms/T − 0.061); m3 ≤ 37 pA). Then, to generate a comparable representation of a “ model,” another set of simulated sparks was generated with a distribution of release current duration that was the same as in the calcium inactivation model, but with constant release current.

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