Spatiotemporal characterization of short versus long duration calcium transients in embryonic muscle and their role in myofibrillogenesis

Intracellular calcium (Ca2+) signals are essential for several aspects of muscle development, including myofibrillogenesis—the terminal differentiation of the sarcomeric lattice. Ryanodine receptor (RyR) Ca2+ stores must be operative during this period and contribute to the production of spontaneous global Ca2+ transients of long duration (LDTs; mean duration ∼80 s). In this study, high-speed confocal imaging of intracellular Ca2+ in embryonic myocytes reveals a novel class of spontaneous Ca2+ transient. These short duration transients (SDTs; mean duration ∼2 s) are blocked by ryanodine, independent of extracellular Ca2+, insensitive to changes in membrane potential, and propagate in the subsarcolemmal space. SDTs arise from RyR stores localized to the subsarcolemmal space during myofibrillogenesis. While both LDTs and SDTs occur prior to myofibrillogenesis, LDT production ceases and only SDTs persist during a period of rapid sarcomere assembly. However, eliminating SDTs during this period results in only minor myofibril disruption. On the other hand, artificial extension of LDT production completely inhibits sarcomere assembly. In conjunction with earlier work, these results suggest that LDTs have at least two roles during myofibrillogenesis—activation of sarcoplasmic regulatory cascades and regulation of gene expression. The distinct spatiotemporal patterns of LDTs versus SDTs may be utilized for differential regulation of cytosolic cascades, control of nuclear gene expression, and localized activation of assembly events at the sarcolemma.