Ca 2+ Sparks in Embryonic Mouse Skeletal Muscle Selectively Deficient in Dihydropyridine Receptor α1S or β1a Subunits

Abstract

Ca 2+ sparks are miniature Ca 2+ release events from the sarcoplasmic reticulum of muscle cells. We examined the kinetics of Ca 2+ sparks in excitation-contraction uncoupled myotubes from mouse embryos lacking the β 1 subunit and mdg embryos lacking the α 1S subunit of the dihydropyridine receptor. Ca 2+ sparks occurred spontaneously without a preferential location in the myotube. Ca 2+ 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 ± 1.1 μm and the full duration of individual sparks, Δt, was 145 ± 64 ms. In β-null myotubes ( n = 284 sparks), ΔF/Fo = 1.9 ± 0.4, FWHM = 5.1 ± 1.5 μm, and Δ t = 168 ± 43 ms. In mdg myotubes ( n = 426 sparks), ΔF/Fo = 1 ± 0.5, the FWHM = 2.5 ± 1.1 μm, and Δ t = 97 ± 50 ms. Thus, Ca 2+ sparks in mdg myotubes were significantly dimmer, smaller, and briefer than Ca 2+ 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 Ca 2+ sparks of resting embryonic muscle originated from spontaneous openings of ryanodine receptor channels. We conclude that dihydropyridine receptor α 1S and β 1 subunits participate in the control of Ca 2+ sparks in embryonic skeletal muscle. However, excitation-contraction coupling is not essential for Ca 2+ spark formation in these cells.