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 fast uptake shows a maximum amplitude which remains unchanged in the presence of phosphate. The slow uptake leads to a nearly complete depletion of the external calcium, and its rate is proportional to the phosphate concentration, even at physiological range. The sarcoplasmic ATPase liberates inorganic phosphate and the slow uptake is an autocatalytic process.