Characterization of Calcium Oxalate and Calcium Phosphate Deposits in Sarcoplasmic Reticulum Vesicles

Sarcoplasmic reticulum vesicles (SRV)' isolated from skeletal muscle retain a highly efficient ATP-dependent Ca t+ transport system (10, 12) . This system plays a key role in the process of excitation-contraction coupling in muscle cells (18) . Oxalate ions strongly enhance active Ca 2+ transport by the formation of calcium oxalate precipitates within the vesicles (10, 12) . In negatively stained preparations, the calcium oxalate deposits formed inside the vesicles are currently identified as discrete areas of low density not penetrated by the staining (1-4, 8-11, 13, 14, 16, 17) . However, these deposits are highly sensitive to the illuminating current and decompose after a short exposure to the electron beam (2, 3, 16) . The decomposition residues left inside the SRV are difficult to identify . The lability of the calcium oxalate deposits is a serious drawback for the characterization of calcium accumulating vesicles in a given SRV preparation . Orthophosphate also activates the Ca" transport of SRV (5, 8, 10, 15, 19) . As far as we know, the identification of calcium phosphate deposits inside the SRV has not been attempted yet . In this paper it is shown that calcium phosphate deposits can also be identified within isolated SRV and that, in contrast to the calcium oxalate deposits, they are quite resistant to the electron bombardment .