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.

The biphasic active transport of calcium by the fragmented sarcoplasmic reticulum as revealed by the flow dialysis method.

The calcium accumulated by the fast uptake has an apparent association constant of 0.8 X 10(6) 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 calcium concentration when the latter ranges from 5 to 50 muM.

The effect of calcium and phosphate on the biphasic calcium uptake by the sarcoplasmic reticulum.

The amplitude of the fast uptake and the initial rate of the slow uptake increase with increasing free calcium concentrations, up to 30 muM. In that range, both processes are correlated to each other. At higher concentrations, the slow uptake is more inhibited than the fast uptake.

The biphasic Ca2+-uptake by the fragmented sarcoplasmic reticulum.

The non-equilibrium dialysis has been used for kinetic studies of ATP dependent calcium uptake by the sarcoplasmic reticulum. The uptake displays two phases, which are defined as fast and slow uptake. The former is an exponential function of time, with a half-life time of approximately 15--20 sec, the latter presents the characteristics of an autocatalytic reaction.