Ca ” ’ + Mg 2 + ) - ATPase Activity in Plasma Membrane of Circulating Mononuclear Cells

The in vivo effect of vitamin D on (Ca2+ + Mg2+)ATPase activity was examined in a plasma membrane fraction of rat circulating mononuclear cells (MPM). Although there was no significant difference in the ATPase activities in red blood cell ghosts, (Ca”’ + Mg2+)-ATPase activity in MPM was significantly higher (p < 0.05) in long-term vitamin DB-replete rats (100 IU/day for 6 months) than that in vitamin Ddeplete rats (for 6 months). In rats maintained on vitamin D-deficient diets for 5-7 weeks, in vivo administration of either vitamin D3, 2,000 IU orally, 5 days prior to killing or 1,25-dihydroxyvitamin D3, 2.4 nmol, intraperitoneally, 24 h prior to killing failed to show any significant effect on (Caz+ + Mg+)-ATPase activity in MPM. (Ca2+ + M$+)-ATPase activity in MPM from rats maintained on vitamin D-deficient diet with high calcium content (1.8%) was significantly higher (p c 0.05) than that from rats maintained on vitamin D-deficient diet with low calcium content (0.3%). Moreover, in vitro addition of vitamin D3 metabolites did not show any effect on (Ca” + Mg+)ATPase activity in MPM. These data suggest that decreased (Ca?” + M$+)-ATPase activity in MPM from long-term vitamin D-deplete rats resulted from an adaptation to low extracellular calcium rather than vitamin D depletion.