The effects of Mg2+ on rat liver microsomal Ca2+ 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 undefined, 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.
Zhang
G H
GH
Department of Physiology and Cell Biology, University of Texas Medical School, Houston.
Kraus-Friedmann
N
N
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D008274:Magnesium / Q000494:pharmacology*; D008297:Male; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism* / Q000648:ultrastructure; D051381:Rats; D011919:Rats, Inbred Strains
D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium