Cytochemical studies on sarcoplasmic reticulum of heart and skeletal muscle

Ultrastructural aspects and cytochemical localization of Ca uptake and of -SH groups involved in the calcium transport of vesicular fragments of sarcoplasmic reticulum (SR) from rabbit skeletal muscle and from dog myocardium were studied by positive and various negatively-staining techniques. Size, shape, and membrane structure as well as proportion of calcium-containing vesicles vary a great deal depending on the method applied. On negatively stained material sarcoplasmic reticulum derivatives are identifiable in both skeletal muscle as well as in heart preparations by the presence on the surface of the vesicular membrane of 40-A particles and the capability of accumulating calcium. The amount of these vesicles is lower in cardiac SR than in the skeletal SR; however, no qualitative differences can be observed. Vesicles of cardiac SR labeled with the electron dense -SH group reagent, Hg-phenyl azoferritin, also bind asymmetrically the marker only at the outer surface of the membrane-like microsomes of skeletal muscle.