Summary
Myocardial cells from left ventricles of beating hearts of rats were fixed by immersion in an osmium tetroxide solution containing potassium pyroantimonate to study the electron-microscopic distribution of calcium, the cation being precipitated as an electron-opaque salt (calcium antimonate) by this cytochemical technique. The observed myocytes could be divided into two groups according to their contractile state, evaluated by sarcomere length measurements. In contracted cells (mean sarcomere length 1.43 μm) the intramyoflbrillar precipitate was confined to areas of I-bands bordering the A-bands, the intermyofibrillar space showing scarce content in reaction product. Relaxed cells (mean sarcomere length 1.69 μm) presented a heavy deposition of reaction product over the sarcomeres, the electron-opaque dots being absent on the H and Z bands. The sarcotubular system and mitochondria were also clearly marked by the reaction product. This second pattern of calcium distribution has not been previously described in heart muscle cells and is interpreted as corresponding to the phase of rise of intracellular calcium which is mediated by membrane depolarization. Our results suggest that different bands of heart sarcomeres show different abilities to bind calcium. The I bands retain the cation even in cells under sustained contraction, probably due to their content in calmodulin; Z and M bands are apparently not involved in calcium sequestration, whereas the content in calcium of the A bands seems to be dependent on the contraction-relaxation cycle of heart myocytes.
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Supported by Grant HL 06975 from the National Heart, Lung and Blood Institute and AM 18141 from the National Institute of Arthritis, Metabolism and Digestive Diseases
The authors are grateful to Maria Kapuscinski, Luther B. Joseph, Elisabeth Lawson, Robert Linsmair and Miriam Alojipan for skilled technical assistance
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Aguas, A.P., Nickerson, P.A. Distribution of calcium differs in relaxed and contracted myocardial cells of the rat. Cell Tissue Res. 221, 295–302 (1981). https://doi.org/10.1007/BF00216733
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DOI: https://doi.org/10.1007/BF00216733