Membrane crystals of Ca2+‐ATPase in sarcoplasmic reticulum of fast and slow skeletal and cardiac muscles

Crystalline arrays of Ca2+ transport ATPase develop in sarcoplasmic reticulum membranes after treatment with Na3VO4 in a calcium‐free medium [Dux, L. and Martonosi, A. (1983) J. Biol. Chem. 258, 2599–2603). The proportion of vesicles containing Ca2+‐ATPase crystals in microsome preparations isolated from rat muscle of different fiber types (semimembranosus, levator ani, extensor digitorum longus, diaphragm, soleus, and heart) correlates well with the Ca2+‐ATPase content and Ca2+‐modulated ATPase activity. This implies that the concentration of Ca2+‐ATPase in sarcoplasmic reticulum membranes of fast and slow skeletal or cardiac muscles. differs only slightly, and the low Ca2+ transport activity of ‘sarcoplasmic reticulum’ preparations isolated from slow‐twitch skeletal and cardiac muscles is due to the presence of large amount of non‐sarcoplasmic‐reticulum membrane elements. This is in accord with the relatively small differences in the density of 8.5‐nm intramembranous particles seen by freeze‐etch electron microscopy in sarcoplasmic reticulum of red and white muscles. The dimensions of the Ca2+‐ATPase crystal lattice are similar in sarcoplasmic reticulum membranes of different fiber types; therefore if structural differences exist between ‘isoenzymes’ of Ca2+‐ATPase, these are not reflected in the crystal‐lattice.