The effect of the Ca2+-ATPase of sarcoplasmic reticulum upon activities of Na+, K+, and H3O+ ions.

Activities of single ions in concentrated suspensions of sarcoplasmic reticulum vesicles have been measured using ion-specific electrodes. With activation of the Ca2+-stimulated ATPase by addition of ATP in the presence of equimolar Mg2+ and activating concentrations of Ca2+, activities of Na+ and H3O+ increased to maxima, then declined to reach new steady levels; the activity of K+ decreased to a minimum then rose to a new steady state level. In the absence of Mg2+ and the presence of the ionophore A23187, vesicles failed to accumulate Ca2+ at all; extrema in activities of Na+, K+, and H3O+, however, were still observed, and the final levels of activities of Na+ and K+ were the same as their initial levels. The final pH was always lower than the initial pH because protons were produced by the hydrolysis of ATP. In fresh vesicles which accumulated and retained Ca2+, activities of Na+ and H3O+ were held at higher levels and the activity of K+ at a lower level than those reached in the absence of Ca2+ accumulation. When vesicles were solubilized by addition of Triton X-100, the minimum in pH was still observed. These findings are discussed in terms of a molecular model of active transport in which the driving force for transport is developed by a change in the structure of a small volume of water enclosed within a narrow cleft of the pump unit.