A freeze-fracture study of sarcoplasmic reticulum from fast and slow muscle of the mouse.

Sarcoplasmic reticulum (SR) of fast extensor digitorum longus (EDL) and slow soleus (SOL) muscles of the mouse was examined by freeze-fracture techniques. A distinctive feature of sarcoplasmic reticulum from the EDL is the presence of hillocks on the A-face within the terminal cisterns. These hillocks are usually arranged in a single row which is deployed parallel to the long axis of the adjacent T-tubule. Center-to-center spacing of hillocks within a row is about 70-75 nm. Hillocks are also found scattered within the collar region. The EDL of ten week mice was characterized by sheet-like terminal and intermediate cisterns, the latter being replaced in 37 week animals by thin tubular longitudinal elements of the SR which contain no hillocks or dimples. Hillocks occur only occasionally in SR from 10 or 20 week SOL muscle. In such cases the hillocks occur singly rather than in rows as in the terminal cisterns of EDL. The predominant form of SR in the SOL contains no hillocks. Total particles in the A-face of EDL-SR (2996 particles/mu2; S.D. = +/- 287) slightly exceeded that of SOL-SR (2558 particles/mu2; S.D. = 274 8 NM). Packing density of 8 nm particles was slightly higher for EDL (750/mu2) VS. SOL (700/mu2). The possible significance of these features of SR in fast and slow muscle is discussed.