Changes in ryanodine receptor-mediated calcium release during skeletal muscle differentiation

We have observed a disparity between the actions of caffeine and ryanodine, two agents known to affect the same site of intracellular calcium (Ca2+) release in muscle. The site of intracellular Ca2+ release, the ryanodine receptor (RyR), is established as the route of Ca2+ movement from the sarcoplasmic reticulum (SR) to the cytosol during excitation-contraction coupling. We measured Ca2+ release fluorimetrically in both saponin-permeabilized and intact L6 cells, in response to known modulators (i.e., caffeine and ryanodine), during differentiation in vitro. The undifferentiated L6 cells showed little response to caffeine. However, a substantial caffeine-induced calcium release (caffCR) was evident by Day 3 of differentiation, and was nearly maximal by Day 7 of differentiation. By contrast, ryanodine failed to stimulate Ca2+ release until Day 4, lagging behind the caffeine response. Ryanodine-stimulated Ca2+ release was also maximal by Day 7. Higher concentrations of ryanodine, known to inhibit Ca2+ release, only began to affect caffCR at Day 4, indicating that cells were insensitive to both ryanodine stimulation and ryanodine inhibition prior to this time. Most of the results could be obtained both in permeabilized and intact cells. Using intact cells, we measured the time course of K+ -dependent (i.e., depolarization-induced) Ca2+ release. This time course matched caffeine and not ryanodine-induced Ca2+ release suggesting the action of caffeine was not due to Ca2+ release unrelated to excitation-contraction coupling. These findings suggest that ryanodine binding sites on the RyR may not be functional at early stages of muscle development, that ryanodine sensitivity is a poor indicator of Ca2+ flux through the RyR, or that other proteins are involved in Ca2+ release under certain circumstances.