A high-affinity plasma membrane Ca2+-ATPase in Dictyostelium discoideum: its relation to cAMP-induced Ca2+ fluxes.

Abstract Chemotactic stimulation of Dictyostelium discoideum induces an uptake of Ca 2+ by the cells followed by a release of Ca 2+ . In this study we investigated the mechanism of Ca 2+ release and found that it was inhibited by La 3+ , Cd 2+ and azide. Ca 2+ release occurred in the absence of external Na + , indicating that an Na + Ca 2+ exchange was not involved. Plasma membranes contained high- and low-affinity ATPase activities. Apparent K 0.5 values were 8 μM for the major Mg 2+ -ATPase and 1.1 μM for the high-affinity Ca 2+ -ATPase, respectively. The Mg 2+ -ATPase activity was inhibited by elevated concentrations of Ca 2+ , whereas both Ca 2+ -ATPases were active in the absence of added Mg 2+ . The activities of the Ca 2+ -ATPases were not modified by calmodulin. The high-affinity Ca 2+ -ATPase was competitively inhibited by La 3+ and Cd 2+ ; we suggest that this high-affinity enzyme mediates the release of Ca 2+ from D. discoideum cells.