Calcium activates muscle contraction in all muscles investigated (Ebashi and Endo, Prog. Biophys. Molec. Biol. 18: 123-183, 1968). The central questions for each muscle are how myoplasmic calcium concentration is regulated and how elevated intracellular calcium brings about cross-bridge interaction and muscle contraction. In most muscles, intracellular calcium is regulated by its release from and active transport into the sarcoplasmic reticulum (SR), the smooth endoplasmic reticulum of muscle. In addition, cardiac, smooth, and some skeletal muscles have voltage-dependent calcium channels in the cell membraane allowing calcium fluxes from the extracellular fluid during activation, but in most cases it is probably the calcium released from the intracellular stores that brings about contraction (Fabiato and Fabiato, Ann. Rev. Physiol., 41: 473–484, 1979).
KeywordsSarcoplasmic Reticulum Calcium Release Thin Filament Calcium Sensitivity Smooth Endoplasmic Reticulum
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