Calcium uptake in glycerol‐extracted rabbit psoas muscle fibers II. Electron microscopic localization of uptake sites

An effort has been made to ascertain the site of calcium uptake in rabbit psoas muscle by electron microscopy. Following a brief glycerol extraction, small groups of fibers were exposed in a suitable electrolyte environment to calcium and ATP. Conditions were such that calcium was accumulated as an insoluble oxalate salt. Conventional preparatory procedures for electron microscopy were modified so that the calcium oxalate deposits were retained in situ in ultrathin sections. These were found localized in the longitudinal component of the sarcoplasmic reticulum including the terminal dilatations which form the outer elements of the triads. The possible participation of the intermediate element of the triad in the uptake of calcium could not be determined. The results are regarded as direct evidence that the sarcoplasmic reticulum is intimately concerned with the uptake and release of calcium associated with the contraction‐relaxation cycle.