Voltage‐ and ligand‐gated ryanodine receptors are functionally separated in developing C2C12 mouse myotubes

1 In order to further understand the role of voltage‐ and ligand‐gated ryanodine receptors in the control of intracellular Ca 2+ signalling during myogenesis, changes in cytosolic free calcium concentration ([Ca 2+ ] i ) were investigated by fura‐2 videoimaging in C2C12 mouse myotubes developing in vitro . 2 A synchronous [Ca 2+ ] i increase was observed after depolarisation with high [K + ], while the Ca 2+ response propagated as a wave following caffeine administration. Application of the two stimuli to the same myotube often revealed the existence of cellular zones that were responsive to depolarisation but not to caffeine. 3 Focal application of high [K + ] promoted a [Ca 2+ ] i response detectable only in the cellular areas close to the pipette tip, while focal application of caffeine elicited a [Ca 2+ ] i increase which spread as a Ca 2+ wave. Buffering of [Ca 2+ ] i by BAPTA did not affect the pattern of the depolarisation‐induced [Ca 2+ ] i transient but abolished the Ca 2+ waves elicited by caffeine. 4 When high [K + ] and caffeine were applied in sequence, reciprocal inhibition of the [Ca 2+ ] i responses was observed. 5 Our results suggest that the different spatial patterns of [Ca 2+ ] i responses are due to uneven distribution of voltage‐ and ligand‐gated ryanodine receptors within the myotube. These two types of receptor control two functionally distinct Ca 2+ pools which are part of a common intracellular compartment. Finally, the two differently operated ryanodine receptor channels appear to be independently activated, so that a mechanism of Ca 2+ ‐induced Ca 2+ release is not required to sustain the global response in C2C12 myotubes.