BASAL MG2-DEPENDENT ATPASE ACTIVITY OF RAT-LIVER MICROSOMES IS NOT INFLUENCED BY AMBIENT FREE CA2+()

The potent inhibitor of Ca2+, Mg2+-ATPase activity, thapsigargin, has been used to investigate the effect of ambient free Ca2+ on basal Mg2-dependent ATPase activity in rat liver microsomes. Thapsigargin non-c ompetitively inhibited both Ca2+-stimulated ATP-dependent Ca2+ uptake and Ca2+-stimulated Mg2+-dependent ATPase activity. At a concentration of 1 muM, thapsigargin completely inhibited the Ca2+-pump activity, w ithout affecting Mg2+-dependent ATPase activity measured in the absenc e of Ca2+. Increasing the ambient free Ca2+ concentration did not alte r the basal Mg2+-dependent ATPase activity. The data presented indicat e that ATPase activity measured in the absence of Ca2+ is a reliable m easure for the basal Mg2+-dependent ATPase activity and that, conseque ntly, the Ca2+-stimulated Mg2+-dependent ATPase activity can indeed be defined as the difference between die ATPase activity measured in the presence and the absence of Ca2+.