The effect of Mg 2+ on hepatic microsomal Ca 2+ and Sr 2+ transport

The ATP‐dependent uptake of Ca 2+ by rat liver microsomal fraction is dependent upon Mg 2+ . Studies of the Mg 2+ requirement of the underlying microsomal Ca 2+ ‐ATPase have been hampered by the presence of a large basal Mg 2+ ‐ATPase activity. We have examined the effect of various Mg 2+ concentrations on Mg 2+ ‐ATPase activity, Ca 2+ uptake, Ca 2+ ‐ATPase activity and microsomal phosphoprotein formation. Both Mg 2+ ‐ATPase activity and Ca 2+ uptake were markedly stimulated by increasing Mg 2+ concentration. However, the Ca 2+ ‐ATPase activity, measured concomitantly with Ca 2+ uptake, was apparently unaffected by changes in the Mg 2+ concentration. In order to examine the apparent paradox of Mg 2+ stimulation of Ca 2+ uptake but not of Ca 2+ ‐ATPase activity, we examined the formation of the Ca 2+ ‐ATPase phosphoenzyme intermediate and formation of a Mg 2+ ‐dependent phosphoprotein, which we have proposed to be an attribute of the Mg 2+ ‐ATPase activity. We found that Ca 2+ apparently inhibited formation of the Mg 2+ ‐dependent phosphoprotein both in the absence and presence of exogenous Mg 2+ . This suggests that Ca 2+ may inhibit (at least partially) the Mg 2+ ‐ATPase activity. However, inclusion of the Ca 2+ inhibition of Mg 2+ ‐ATPase activity in the calculation of Ca 2+ ‐ATPase activity reveals that this effect is insufficient to totally account for the stimulation of Ca 2+ uptake by Mg 2+ . This suggests that Mg 2+ , in addition to stimulation of Ca 2+ ‐ATPase activity, may have a direct stimulatory effect on Ca 2+ uptake in an as yet undefined fashion. In an effort to further examine the effect of Mg 2+ on the microsomal Ca 2+ transport system of rat liver, the interaction of this system with Sr 2+ was examined. Sr 2+ was sequestered into an A23187‐releasable space in an ATP‐dependent manner by rat liver microsomal fraction. The uptake of Sr 2+ was similar to that of Ca 2+ in terms of both rate and extent. A Sr 2+ ‐dependent ATPase activity was associated with the Sr 2+ uptake. Sr 2+ promoted formation of a phosphoprotein which was hydroxylamine‐labile and base‐labile. This phosphoprotein was in‐distinguishable from the Ca 2+ ‐dependent ATPase phosphoenzyme intermediate. Sr 2+ uptake was markedly stimulated by exogenous Mg 2+ , but the Sr 2+ ‐dependent ATPase activity was unaffected by increasing Mg 2+ concentrations. Sr 2+ uptake and Sr 2+ ‐dependent ATPase activity were concomitantly inhibited by sodium vanadate. In contrast to Ca 2+ , Sr 2+ had no effect on Mg 2+ ‐dependent phosphoprotein formation. Taken together, these data indicate that Mg 2+ stimulated Ca 2+ and Sr 2+ transport by increasing the Ca 2+ (Sr 2+ )/ATP ratio. However, in the case of Ca 2+ at least, these data must be interpreted with caution since Ca 2+ may inhibit the Mg 2+ ‐ATPase activity.