Frog α- and β-Ryanodine Receptors Provide Distinct Intracellular Ca(2+) Signals in a Myogenic Cell Line

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.