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Ca2+ sparks in embryonic mouse skeletal muscle selectively deficient in dihydropyridine receptor alpha1S or beta1a
M W Conklin1, P Powers, R G Gregg
1Department of Physiology, University of Wisconsin Medical School, University of Wisconsin, Madison, Wisconsin 53706, USA.
Abstract:
Ca2+ sparks are miniature Ca2+ release events from the sarcoplasmic reticulum of muscle cells. We examined the kinetics of Ca2+ sparks in excitation-contraction uncoupled myotubes from mouse embryos lacking the beta1 subunit and mdg embryos lacking the alpha1S subunit of the dihydropyridine receptor. Ca2+ sparks occurred spontaneously without a preferential location in the myotube. Ca2+ sparks had a broad distribution of spatial and temporal dimensions with means much larger than those reported in adult muscle. In normal myotubes (n = 248 sparks), the peak fluorescence ratio, DeltaF/Fo, was 1.6 +/- 0.6 (mean +/- SD), the full spatial width at half-maximal fluorescence (FWHM) was 3.6 +/- 1.1 micrometer and the full duration of individual sparks, Deltat, was 145 +/- 64 ms. In beta-null myotubes (n = 284 sparks), DeltaF/Fo = 1.9 +/- 0.4, FWHM = 5.1 +/- 1.5 micrometer, and Deltat = 168 +/- 43 ms. In mdg myotubes (n = 426 sparks), DeltaF/Fo = 1 +/- 0.5, the FWHM = 2.5 +/- 1.1 micrometer, and Deltat = 97 +/- 50 ms. Thus, Ca2+ sparks in mdg myotubes were significantly dimmer, smaller, and briefer than Ca2+ sparks in normal or beta-deficient myotubes. In all cell types, the frequency of sparks, DeltaF/Fo, and FWHM were gradually decreased by tetracaine and increased by caffeine. Both results confirmed that Ca2+ sparks of resting embryonic muscle originated from spontaneous openings of ryanodine receptor channels. We conclude that dihydropyridine receptor alpha1S and beta1 subunits participate in the control of Ca2+ sparks in embryonic skeletal muscle. However, excitation-contraction coupling is not essential for Ca2+ spark formation in these cells.
Insights
Calcium (Ca2+) sparks in embryonic muscle are controlled by dihydropyridine receptor subunits, even without excitation-contraction coupling. These sparks are crucial for muscle cell function.
Area of Science:
- Muscle physiology
- Cellular calcium signaling
Background:
- Calcium (Ca2+) sparks are fundamental to muscle cell function, originating from the sarcoplasmic reticulum.
- Dihydropyridine receptors (DHPRs) are critical for excitation-contraction coupling in muscle.
Purpose of the Study:
- To investigate the role of DHPR alpha1S and beta1 subunits in controlling Ca2+ sparks in embryonic skeletal muscle.
- To determine if excitation-contraction coupling is necessary for Ca2+ spark formation.
Main Methods:
- Examined Ca2+ spark kinetics in myotubes from mice lacking DHPR beta1 or alpha1S subunits.
- Analyzed spark dimensions (peak fluorescence, spatial width, duration) and frequency.
- Utilized tetracaine and caffeine to probe spark origins.
Main Results:
- Ca2+ sparks in myotubes lacking alpha1S (mdg) were significantly dimmer, smaller, and briefer than in normal or beta-null myotubes.
- Sparks occurred spontaneously and were not dependent on excitation-contraction coupling.
- Tetracaine and caffeine modulated spark properties, confirming their origin from ryanodine receptor channels.
Conclusions:
- DHPR alpha1S and beta1 subunits play a role in regulating Ca2+ sparks in embryonic skeletal muscle.
- Excitation-contraction coupling is not essential for Ca2+ spark generation in these cells.