Calcium release from the sarcoplasmic reticulum.

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Abstract
Ca2+ release from the SR [sarcoplasmic reticulum] is seemingly initiated by 2 different processes in mammalian cardiac muscle and predominantly fast skeletal muscle. In mammalian cardiac muscle, the Ca2+ that crosses the surface membrane during the action potential could initate Ca2+ release from the SR. In contrast, Ca2+ release from the SR of fast skeletal muscles could be initiated by the depolarization of the transverse tubules through a process about which the only well established finding is that it does require Ca2+. Possibly other types of muscles, such as slow skeletal muscles or some smooth muscles, use a combination of both processes in variable proportions. Such a combination is not yet excluded for any type of muscle. The possibility that there is a difference between cardiac and fast skeletal muscles in the process of Ca2+ release from the SR concerns only the initiation of this release.