title: Two structurally distinct calcium storage sites in rat cardiac sarcoplasmic reticulum: an electron microprobe analysis study.
abstract: The elemental composition of subcellular organelles in resting rat papillary muscle was measured by electron probe x-ray microanalysis of cryosections of flash-frozen tissue. Nonmitochondrial electron-dense structures (50-100 nm in diameter) with a phosphorous concentration larger than 375 mmol/kg dry wt were identified in the interfibrillar spaces of the I band region. They were not visible in the proximity of transverse tubules. The sodium, magnesium, phosphorus, sulfur, chlorine, and potassium content of the electron dense structures showed a normal distribution, consistent with a uniform composition of a specific subcellular organelle. However, the distribution of the calcium concentrations in these electron-dense structures was bimodal, suggesting that they are composed of at least two subpopulations. One subpopulation had relatively high calcium (up to 53 mmol/kg dry wt) content with a mean value of 12.5 +/- 1.1 mmol/kg dry wt, while the other one had a relatively low calcium content with a mean value of 2.8 +/- 0.3 mmol/kg dry wt. The mean calcium concentration in the junctional sarcoplasmic reticulum (j-SR) in rat papillary muscle with calcium concentrations larger than 6 mmol/kg dry wt was 14.6 +/- 2.0 mmol/kg dry wt. We propose that the electron-dense structures described above correspond to nonjunctional sarcoplasmic reticulum and that the population containing relatively high calcium concentrations is calsequestrin-containing corbular sarcoplasmic reticulum (c-SR) confined to the I band region, while the population containing relatively low calcium concentrations corresponds to anastomosing regions of the network sarcoplasmic reticulum that lack calsequestrin.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Anatomy, Faculty of Medicine, University of Toronto, Canada.
authors.forename: A O
authors.initials: AO
authors.lastname: Jorgensen
authors.forename: R
authors.initials: R
authors.lastname: Broderick
authors.forename: A P
authors.initials: AP
authors.lastname: Somlyo
authors.forename: A V
authors.initials: AV
authors.lastname: Somlyo
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002155:Calsequestrin / Q000378:metabolism; D004577:Electron Probe Microanalysis; D007158:Immunologic Techniques; D008297:Male; D008854:Microscopy, Electron; D009206:Myocardium / Q000378:metabolism* / Q000648:ultrastructure; D010210:Papillary Muscles / Q000378:metabolism / Q000648:ultrastructure; D051381:Rats; D011919:Rats, Inbred Strains; D012518:Sarcomeres / Q000378:metabolism / Q000648:ultrastructure; D012519:Sarcoplasmic Reticulum / Q000378:metabolism* / Q000648:ultrastructure
chemical_list: D002155:Calsequestrin; D002118:Calcium