The Biphasic Active Transport of Calcium by the Fragmented Sarcoplasmic Reticulum as Revealed by the Flow Dialysis Method

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Abstract
The Ca accumulated by the fast uptake in rabbit skeletal muscle sarcoplasmic reticulum has an apparent association constant of 0.8 .times. 106 M-1 and a maximum of 80 nmol/mg protein. The fast uptake and the initial rate of the slow uptake show a similar dependence on the Ca concentration when the latter ranges from 5-50 .mu.M. The fast uptake is a linear and the slow uptake rate an exponential function of the reticulum concentration. Both uptakes of Ca display a fast and nearly total isotopic equilibration between intra- and extravesicular Ca. After depletion of ATP the Ca accumulated during the slow uptake is released, while that sequestered during the fast uptake is retained by the vesicles, though it remains rapidly exchangeable. After depletion of ITP or acetylphosphate, or addition of ethyleneglycol bis(2-aminoethylether) N-tetraacetic acid, the release is more substantial, but is almost complete only after the addition of ionophore X537A (lasalocid) or deoxycholate. The presence of oxalate strongly reduces the rates of these releases. In the steady state observed after the depletion of ATP, a Ca2+ gradient exists through the sarcoplasmic membrane, and the sarcoplasmic pump works at equilibrium. The fast uptake is an active transport and not an active binding. The slow uptake represents an extension of the Ca capacity of the vesicles due to the phosphate liberated by the sarcoplasmic ATPase.