Calcium and Magnesium Movements Through Sarcoplasmic Reticulum, Endoplasmic Reticulum, and Mitochondria

  • A. V. Somlyo
  • M. Bond
  • R. Broderick
  • A. P. Somlyo
Part of the Advances in Experimental Medicine and Biology book series (AEMB, volume 232)


Free cytoplasmic [Ca2+] is normally maintained at or below micromolar levels by actively transporting cell membranes and organelles, and by binding to high affinity, Ca-binding proteins. Until recently, the structural identity of the organelles that normally regulate cytoplasmic Ca2+ has been the subject of considerable debate (A.P. Somlyo, 1984). The primary role, in skeletal muscles, of the well-organized sarcoplasmic reticulum (SR) is generally accepted (Ebashi and Endo, 1968; Martonosi and Beeler, 1983) but, because of the smaller volume of SR in smooth muscle (A.V. Somlyo, 1980; A.P. Somlyo, 1985a) and in cardiac muscle (Sommer and Johnson, 1979), its dominant role in regulating cytoplasmic Ca2+ was not as readily recognized. It had been suggested, erroneously in retrospect, that in these muscles and in nonmuscle cells, mitochondria play a major role in Ca2+ regulation. This conclusion was based largely on studies of isolated organelles and on compartmental analysis of 45Ca fluxes, but the question whether the mitochondria or the endoplasmic reticulum (ER) regulate cytoplasmic Ca2+ could be conclusively resolved only by direct measurements of their calcium content in situ. This became possible with electron probe X-ray microanalysis (EPMA) (reviewed in A.P. Somlyo, 1985b) of rapdily frozen tissues. The results of these studies, that we summarize here, have shown that the ER, not mitochondria, are primarily responsible for physiological Ca2+ regulation and also revealed unexpected, and possibly physiologically significant, changes in mitochondrial magnesium.


Sarcoplasmic Reticulum Rough Endoplasmic Reticulum American Physiological Society Inositol Trisphosphate Electron Probe Analysis 


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Copyright information

© Plenum Press, New York 1988

Authors and Affiliations

  • A. V. Somlyo
    • 1
  • M. Bond
    • 1
  • R. Broderick
    • 1
  • A. P. Somlyo
    • 1
  1. 1.Pennsylvania Muscle InstituteUniversity of PennsylvaniaPhiladelphiaUSA

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