Simultaneous internalization and binding of calcium during the initial phase of calcium uptake by the sarcoplasmic reticulum calcium pump

The kinetics of Ca2+ transport mediated by the sarcoplasmic reticulum (SR) Ca-ATPase were investigated by rapid kinetic techniques that either measure the disappearance of Ca2+ from the medium [stopped-flow photometry of Ca2+ indicators or rapid filtration (method 1)] or directly detect the changes in the accessibility of Ca2+ to the exterior of the membrane, i.e., occlusion of Ca2+ within the Ca pump and Ca2+ transport into the lumen of SR vesicles [EGTA quench (method 2)]. SR vesicles were preincubated in micromolar Ca2+ to form the E.2Cacyt intermediate of the Ca-ATPase, and then Ca2+ transport was initiated by addition of ATP. It was found that Ca2+ uptake measured by method 1 began with no lag phase, in spite of the prediction of kinetic models of the Ca-ATPase. Instead, the time course of Ca2+ uptake was found to have two components: a fast and a slow phase, similar to that obtained using method 2, although the rate constant of the fast phase determined by method 1 was considerably lower than that measured by method 2. The fast phase of Ca2+ uptake measured by method 1 was not influenced by either Ca2+ ionophore or detergent treatment, whereas the slow phase was diminished.(ABSTRACT TRUNCATED AT 250 WORDS)