Localization of calcium‐accumulating structures in the anterior byssal retractor muscle of Mytilus edulis and their role in the regulation of active and catch contractions.

Abstract

The localization of Ca-accumulating structures in the anterior byssal retractor muscle (ABRM) of M. edulis and their role in the contraction-relaxation cycle were studied by fixing the ABRM at rest or during various phases of mechanical activity with a 1% osmium tetroxide solution containing 2% potassium pyroantimonate. In the resting ABRM, electron-opaque pyroantimonate precipitate was observed at the inner surface of the plasma membrane, the vesicles and the mitochondria. Electron probe X-ray microanalysis showed Ca in the precipitate, indicating that the precipitate provides a valid measure for Ca localization. In the ABRM fixed at the peak of mechanical response to Ca-removal or to acetylcholine, the precipitate was diffusely distributed in the myoplasm in the form of a number of particles. At the completion of spontaneous relaxation, the precipitate was seen at the inner surface of the plasma membrane. During the catch state the precipitate was re-accumulated in the peripheral structures with a corresponding decrease of the precipitate in the myoplasm. Ca-accumulating structures are involved in the contraction-relaxation cycle in the ABRM; the transition from active to catch contractions is probably related to a decrease in myoplasmic free Ca2+ concentration.