THE EFFECTS OF MG-2+ ON RAT-LIVER MICROSOMAL CA-2+ SEQUESTRATION

The effects of Mg2+ on the hepatic microsomal Ca2+-sequestering system was tested. Ca2+-ATPase activity and Ca2+ uptake were both dependent on the concentration of free Mg2+, reaching maximum levels at 2 mM. The effects of Mg-ATP were also influenced by the concentration of free Mg2+, being maximally effective at a ratio of 1:1. The results suggest that Mg2+ influences Ca2+ sequestration at various steps, namely in addition to forming the substrate of the Ca2+-ATPase reaction, Mg-ATP, Mg2+ stimulates the reaction at an additional step, as indicated by its stimulatory effect on the Ca2+-ATPase reaction and on Ca2+ uptake, even at optimal Mg-ATP levels. The stimulatory effect of Mg2+ was evident at various pH levels tested, and it was nucleotide specific. The stimulatory effect of Mg2+ might be exerted at the dephosphorylation step of the enzymatic reaction or at an other, yet undetined, site. The results demonstrate a plural effect of Mg2+ on the hepatic microsomal sequestration system. This indicates that, depending on its magnitude, changes in Mg2+ distribution might influence cytosolic Ca2+ levels. © 1990.