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The fine structure of the sarcoplasmic reticulum (SR) in the smooth muscle of Mytilus and the localization of intracellular calcium at rest and during acetylcholine-induced contraction were studied using the pyroantimonate-osmium (PAO) technique. 1. The SR showed vesicular and tubular elements which were alternately interconnected along the axis of the fiber. Transverse components were sometimes observed. The vesicular elements of the SR formed specialized junctions (couplings) with the plasma membrane. 2. The electron opaque precipitate which formed in the SR, mitochondria and on the inner surface of the plasma membrane after treatment with PAO was proven to be mainly Ca-pyroantimonate by both energy-dispersive and wavelength-dispersive X-ray microanalysis. 3. In muscle fibers treated with PAO shortly after the peak of contraction, the SR was the last site of the three Ca-accumulating sites in which the precipitate was relocalized. This finding suggests that the SR plays a principal role in the regulation of the contraction and relaxation cycle. 4. Fused paramyosin filaments were not observed during the catch state.

Sarcoplasmic Reticulum and Intracellular Calcium Localization at Rest and during Contraction in <i>Mytilus</i> Smooth Muscle