The ultrastructural localization of the Ca2+ + Mg2+-dependent ATPase of sarcoplasmic reticulum in rat gracilis muscle was determined by indirect immunoferritin labeling of ultrathin frozen sections. Simultaneous visualization of ferritin particles and of adsorption-stained cellular membranes showed that the Ca2+ + Mg2+-ATPase was concentrated in the longitudinal sarcoplasmic reticulum and in the nonjunctional regions of the terminal cisternae membrane but was virtually absent from mitochondria, plasma membranes, transverse tubules, and junctional sarcoplasmic reticulum. Ferritin particles were found preponderantly on the cytoplasmic surface of the membrane, in agreement with published data showing an asymmetry of the Ca2+ + Mg2+-ATPase within the sarcoplasmic reticulum membrane. Comparison of the density of ferritin particles in fast and slow myofibers suggested that the density of the Ca2+ + Mg2+-ATPase in the sarcoplasmic reticulum membrane in a fast myofiber is approximately two times higher than in a slow myofiber.

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