Identification of Oxalate Precipitates in Striated Muscle Fibers

The location of calcium in muscle fibers is of interest, since the contraction-relaxation cycle is controlled by intracellular movements of this ion (Ebashi and Lipmann, 1962 ; Hasselbach and Makinose, 1963 ; Weber et al., 1963; Podolsky and Costantin, 1964). Treatment of the cell with oxalate produces electron-opaque deposits localized in the terminal sacs of the sarcoplasmic reticulum (SR) that have been assumed to be calcium oxalate (Costantin et al ., 1965) . Although this interpretation is consistent with the observation that the deposits are more abundant in preparations that have been loaded with calcium ions before treatment with oxalate, direct evidence that the precipitate contains calcium is still lacking. In this connection it is worth pointing out that muscle fibers contain, in addition to calcium, appreciable amounts of other metals that form oxalate precipitates in vitro and could possibly give rise to oxalate deposits in the cell, particularly if the ion were concentrated in the SR (Table I) . It seemed worthwhile, therefore, to determine the calcium content of the precipitates resulting from oxalate treatment of the normal, unloaded cell. This was done in the present study by spectroscopic analysis of the X-rays generated by bombardment of the deposits with an electron microprobe (Anderson, 1967 ; Hall, 1968; Hall and Hohling, 1969) .