Cardiac Sarcoplasmic Reticulum
The sarcoplasmic reticulum is a membranous subcellular system in muscle cells. Its principal physiological function is to pump cytoplasmic calcium ions into the SR lumen and, as a consequence of decreased Ca2+ concentration, cause muscle relaxation (Ebashi et al., 1969; MacLennan and Holland, 1975; Tada et al., 1978; Hasselbach, 1979; DeMeis and Inesi, 1982; Ikemoto, 1982). The SR, or part of it, as a major Ca2+ pool inside the muscle cells, may also release Ca2+ to the cytoplasm and consequently bring about muscle contraction (Endo, 1977; Winegrad, 1982; Fabiato, 1983). Ca2+ release from SR has not been studied as thoroughly as its function as a “relaxing factor.” To date, our knowledge of the SR and its physiological function is largely based on the results obtained from studies of skeletal muscle. The SR in cardiac muscle cells possess similar properties, and recent studies of this particular “suborganelle” in cardiac muscles are beginning to reveal some properties of its own. Among these, the most important is the finding that the cardiac SR Ca2+-ATPase protein may be regulated by phospholamban, a 22,000-dalton protein present in SR (Adams and Schwartz, 1980; Tada et al., 1982). Phosphorylation of phospholamban by cAMP-dependent and/or Ca2+- and calmodulin-dependent protein kinase enhances the Ca2+-ATPase and Ca2+-transport activities of the cardiac SR.
KeywordsSarcoplasmic Reticulum Tris Maleate Cardiac Sarcoplasmic Reticulum ATPase Protein Tris Maleate Buffer
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