ATP-driven active transport of calcium in the membranes of the sarcoplasmic reticulum

During the uptake of calcium by the isolated vesicles of the sarcoplasmic reticulum from oxalate-containing solutions, calcium is accumulated inside the vesicles as calcium oxalate, until the product of the active concentrations of calcium and oxalate inside is about 5000 times higher than outside. The increase of the product of the ions involved is brought about by a calcium pump and not by an oxalate pump. The uptake of calcium is strictly correlated with the calcium-induced splitting of ATP (extra ATPase) as well as with the calcium-induced P-exchange between ATP and ADP. Both reactions depend on the concentration of the free calcium ions outside and cease when the level of the free calcium ions has been reduced to values about 10$^{-8}$ to 10$^{-9}$ M. Vesicles subfractionated take up 2 moles of calcium for every mole of ATP split as long as the concentration of free calcium exceeds 10$^{-7}$ M. The ratio of calcium stored to ATP split declines to a value of 1 when the concentration of free calcium is reduced below 10$^{-7}$ M. 12 $\mu $moles calcium (mg N)$^{-1}$ min$^{-1}$ are accumulated by the most active fraction of the vesicles at 20 degrees C