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The ascidian myocardium: Sarcoplasmic reticulum and excitation-contraction coupling

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Summary

The ascidian myocardium is composed of small striated myoepithelial cells. The sarcoplasmic reticulum (SR) of these cells was reconstructed from serial sections. T-tubules are absent, but subsarcolemmal cisternae of the SR, that may be the counterpart of terminal cisternae, form couplings with the sarcolemma. Longitudinal SR tubules, parallel to the myofilaments, are interconnected near the middle of the A-band and form a transverse collar.

Cinematographic photography of spontaneous contractions in fresh myocardial preparations produced records that could be analyzed frame by frame. Contractions are typically limited to parts of the myofilament field of single cells. They are locally symmetrical with respect to Z-bands; either both A-bands on each side of a Z-band converge on it (contractions), or neither A-band moves with respect to the Z-band.

It is suggested that the spontaneous contractions are the result of local Ca++ release from randomly distributed subsarcolemmal cisternae. It is proposed that the symmetry of contraction is due to a rapid sequestering of Ca++ by the SR collars at the middle of the A-bands and a possible diffusion barrier at that level.

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Authors and Affiliations

  1. Department of Zoology, University of Washington, Seattle, Washington, USA

    Lynn W. Oliphant & Richard A. Cloney

  2. Department of Veterinary Anatomy, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

    Lynn W. Oliphant

Authors
  1. Lynn W. Oliphant
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  2. Richard A. Cloney
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Additional information

This investigation was supported in part by an NIH Fellowship (1-F1-GM-40, 488-01) from the National Institute of General Medical Sciences and National Science Foundation Research Grant GB 5394. The authors wish to thank Dr. Albert M. Gordon for his suggestions and stimulating discussion, and for critically reading the manuscript.

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Oliphant, L.W., Cloney, R.A. The ascidian myocardium: Sarcoplasmic reticulum and excitation-contraction coupling. Z. Zellforsch 129, 395–412 (1972). https://doi.org/10.1007/BF00307296

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  • DOI: https://doi.org/10.1007/BF00307296

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