Comparison between strontium and calcium uptake by the fragmented sarcoplasmic reticulum.

The ATP-supported uptake of strontium by the fragmented sarcoplasmic reticulum is monophasic and proceeds more rapidly than the fast uptake of calcium. Strontium uptake is not activated by Pi. The accumulation of strontium is nearly proportional to the external strontium concentration even in the millimolar range. Internal and external strontium quickly equilibrate. One mole of strontium is stored for every mole of ATP split by the Sr2+-activated ATPase. In the absence of oxalate most of the strontium is taken up with a transport ratio of one. On the opposite, the transport ratio of calcium decreases immediately, especially when ADP is not instantaneously phosphorylated to ATP. In this case, energy conversion is uncoupled more effectively by the simultaneous action of ADP and free internal calcium, resulting in the interruption of the fast uptake. After depletion of ATP most of the stored strontium is released and the remaining fraction appears to be not exchangeable. Strontium activates the slow uptake of calcium, but reduces the amplitude of the fast uptake. The calcium induced release of strontium, and vice versa, is partial and transient. The strontium activated ATPase does not transport calcium at low ionic calcium concentrations.