Ca2+ sparks in embryonic mouse skeletal muscle selectively deficient in dihydropyridine receptor alpha(1S) or beta(1a) subunits

Ca2+ sparks are miniature Ca2+ release events from the sarcoplasmic reticul um of muscle cells. We examined the kinetics of Ca2+ sparks in excitation-c ontraction uncoupled myotubes from mouse embryos lacking the beta(1) subuni t and mdg embryos lacking the alpha(1S) subunit of the dihydropyridine rece ptor. 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 n ormal myotubes (n = 248 sparks), the peak fluorescence ratio, Delta F/Fo, w as 1.6 +/- 0.6 (mean +/- SD), the full spatial width at half-maximal fluore scence (FWHM) was 3.6 +/- 1.1 mu m and the full duration of individual spar ks, 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 md g 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 s ignificantly dimmer, smaller, and briefer than Ca2+ sparks in normal or bet a-deficient myotubes. In all cell types, the frequency of sparks, Delta F/F o, and FWHM were gradually decreased by tetracaine and increased by caffein e. Both results confirmed that Ca2+ sparks of resting embryonic muscle orig inated from spontaneous openings of ryanodine receptor channels. We conclud e that dihydropyridine receptor alpha(1S) and beta(1) subunits participate in the control of Ca2+ sparks in embryonic skeletal muscle. However, excita tion-contraction coupling is not essential for Ca2+ spark formation in thes e cells.