Comparison between Strontium and Calcium Uptake by the Fragmented Sarcoplasmic Reticulum

Abstract

The ATP-supported uptake of Sr by the fragmented [rabbit skeletal muscle] sarcoplasmic reticulum is monophasic and proceeds more rapidly than the fast uptake of Ca. Sr uptake is not activated by Pi. The accumulation of Sr is nearly proportional to the external Sr concentration even in the millimolar range. Internal and external Sr quickly equilibrate. One mole of Sr is stored for every mole of ATP split by the Sr2+-activated ATPase. In the absence of oxalate most of the Sr is taken up with a transport ratio of 1. The transport ratio of Ca 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 Ca resulting in the interruption of the fast uptake. After depletion of ATP most of the stored Sr is released and the remaining fraction appears to be nonexchangeable. Sr activates the slow uptake of Ca, but reduces the amplitude of the fast uptake. The Ca induced release of Sr, and vice versa, is partial and transient. The Sr activated ATPase does not transport Ca at low ionic Ca concentrations.