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Background  In frog skeletal muscle, two ryanodine receptor (RyR) isoforms, α-RyR and β-RyR, are expressed in nearly equal amounts. However, the roles and significance of the two isoforms in excitation-contraction (E-C) coupling remains to be elucidated.    Methodology/Principal Findings  In this study, we expressed either or both α-RyR and β-RyR in 1B5 RyR-deficient myotubes using the herpes simplex virus 1 helper-free amplicon system. Immunological characterizations revealed that α-RyR and β-RyR are appropriately expressed and targeted at the junctions in 1B5 myotubes. In Ca 2+  imaging studies, each isoform exhibited caffeine-induced Ca 2+  transients, an indicative of Ca 2+ -induced Ca 2+  release (CICR). However, the fashion of Ca 2+  release events was fundamentally different: α-RyR mediated graded and sustained Ca 2+  release observed uniformly throughout the cytoplasm, whereas β-RyR supported all-or-none type regenerative Ca 2+  oscillations and waves. α-RyR but not β-RyR exhibited Ca 2+  transients triggered by membrane depolarization with high [K + ] o  that were nifedipine-sensitive, indicating that only α-RyR mediates depolarization-induced Ca 2+  release. Myotubes co-expressing α-RyR and β-RyR demonstrated high [K + ] o -induced Ca 2+  transients which were indistinguishable from those with myotubes expressing α-RyR alone. Furthermore, procaine did not affect the peak height of high [K + ] o -induced Ca 2+  transients, suggesting minor amplification of Ca 2+  release by β-RyR via CICR in 1B5 myotubes.    Conclusions/Significance  These findings suggest that α-RyR and β-RyR provide distinct intracellular Ca 2+  signals in a myogenic cell line. These distinct properties may also occur in frog skeletal muscle and will be important for E-C coupling.

Frog α- and β-Ryanodine Receptors Provide Distinct Intracellular Ca2+ Signals in a Myogenic Cell Line