Ca2+ Sparks in Embryonic Mouse Skeletal Muscle Selectively Deficient in Dihydropyridine Receptor α1S or β1a Subunits

AbstractCa2+ 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 β1 subunit and mdg embryos lacking the α1S 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, ΔF/Fo, was 1.6±0.6 (mean±SD), the full spatial width at half-maximal fluorescence (FWHM) was 3.6±1.1μm and the full duration of individual sparks, Δt, was 145±64ms. In β-null myotubes (n=284 sparks), ΔF/Fo=1.9±0.4, FWHM=5.1±1.5μm, and Δt=168±43ms. In mdg myotubes (n=426 sparks), ΔF/Fo=1±0.5, the FWHM=2.5±1.1μm, and Δt=97±50ms. Thus, Ca2+ sparks in mdg myotubes were significantly dimmer, smaller, and briefer than Ca2+ sparks in normal or β-deficient myotubes. In all cell types, the frequency of sparks, ΔF/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 α1S and β1 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