Ca2+ sparks in embryonic mouse skeletal muscle selectively deficient in dihydropyridine receptor α1S or β1a subunits

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 beta(1) subunit and mdg embryos lacking the alpha(1S) 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, Delta F/Fo, was 1.6 +/- 0.6 (mean +/- SD), the full spatial width at half-maximal fluorescence (FWHM) was 3.6 +/- 1.1 mu m and the full duration of individual sparks, at, was 145 +/- 64 ms. In beta-null myotubes (n = 284 sparks), Delta F/Fo = 1 +/- 0.5, FWHM = 5.1 +/- 1.5 mu m, and Delta t = 168 +/- 43 ms. In mdg myotubes (n = 426 sparks), Delta F/Fo = 1 +/- 0.5, the FWHM = 2.5 +/- 1.1 mu m, and Delta t = 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, Delta F/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 alpha(1S) and beta(1) 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.