Basal Mg(2+)-dependent ATPase activity of rat liver microsomes is not influenced by ambient free Ca2+

The potent inhibitor of Ca2+, Mg(2+)-ATPase activity, thapsigargin, has been used to investigate the effect of ambient free Ca2+ on basal Mg(2+)-dependent ATPase activity in rat liver microsomes. Thapsigargin non-competitively inhibited both Ca(2+)-stimulated ATP-dependent Ca2+ uptake and Ca(2+)-stimulated Mg(2+)-dependent ATPase activity. At a concentration of 1 microM, thapsigargin completely inhibited the Ca(2+)-pump activity, without affecting Mg(2+)-dependent ATPase activity measured in the absence of Ca2+. Increasing the ambient free Ca2+ concentration did not alter the basal Mg(2+)-dependent ATPase activity. The data presented indicate that ATPase activity measured in the absence of Ca2+ is a reliable measure for the basal Mg(2+)-dependent ATPase activity and that, consequently, the Ca(2+)-stimulated Mg(2+)-dependent ATPase activity can indeed be defined as the difference between the ATPase activity measured in the presence and the absence of Ca2+.