Initiation and termination of calcium sparks in skeletal muscle

Three main paths to derive the Ca2+ release flux underlying Ca2+ sparks are reviewed here: Some properties of release flux can be inferred from an examination of spark morphology. Others from model simulations, which generate sparks assuming an ion source within a cytoplasm-like medium. Finally, the release flux can be derived from the fluorescence transient by generalizing an algorithm developed earlier for global or whole cell signals. The transient and spatially limited nature of sparks adds many uncertainties to the process. These methods yield estimates between 1.4 and 30 pA, not clearly greater in skeletal than in cardiac muscle. At their low end, the estimates are consistent with generation of sparks by one or two ryanodine receptor channels, but the results are easier to explain if several channels, from as little as four to as many as 60, cooperate in their generation. How release flux determines spark shape and time course has been understood largely through simulations. The rise time of sparks corresponds to active release time. Both release flux and release time may vary among individual sparks, leading to their varied size and shape. Release flux turns off abruptly, therefore the decay of sparks is determined by Ca2+ removal and diffusion. Spatial width increases with release time (rise time). That its experimentally determined value is too large compared with simulations, remains the single most important question in the interpretation of shape. Sparks are not the sole form of local fluorescence transients. When channel opening drugs are present, or sometimes spontaneously, sparks may be prolonged by embers. If the release flux calculated during an ember corresponds to a single open channel, then the release underlying a spark must require many open channels. The continued examination of Ca2+ release flux appears to be an essential requisite for the interpretation of sparks and their place in calcium signaling.