Involvement of multiple intracellular release channels in calcium sparks of skeletal muscle

In many types of muscle, intracellular Ca2+ release for contraction consist s of brief Ca2+ sparks. Whether these result from the opening of one or man y channels in the sarcoplasmic reticulum is not known. Examining massive nu mbers of sparks from frog skeletal muscle and evaluating their Ca2+ release current, we provide evidence that they are generated by multiple channels. A mode is demonstrated in the distribution of spark rise times in the pres ence of the channel activator caffeine. This finding contradicts expectatio ns for single channels evolving reversibly, but not for channels in a group , which collectively could give rise to a stereotyped spark. The release ch annel agonists imperatoxin A, ryanodine, and bastadin 10 elicit fluorescenc e events that start with a spark, then decay to steady levels roughly propo rtional to the unitary conductances of 35%, 50%, and 100% that the agonists , respectively, promote in bilayer experiments. This correspondence indicat es that the steady phase is produced by one open channel. Calculated Ca2+ r elease current decays 10- to 20-fold from spark to steady phase, which requ ires that six or more channels be open during the spark.