(Ca²⁺ + Mg²⁺)-ATPase activity in plasma membrane of circulating mononuclear cells: Lack of a direct effect of vitamin D

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, (Ca²⁺ + Mg²⁺)-ATPase activity in MPM was significantly higher (p < 0.05) in long-term vitamin D₃-replete rats (100 IU/day for 6 months) than that in vitamin D-deplete 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 (Ca²⁺ + Mg²⁺)-ATPase activity in MPM. (Ca²⁺ + Mg²⁺)-ATPase activity in MPM from rats maintained on vitamin D-deficient diet with high calcium content (1.8%) was significantly higher (p < 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 (Ca²⁺ + Mg²⁺)-ATPase activity in MPM. These data suggest that decreased (Ca²⁺ + Mg²⁺)-ATPase activity in MPM from long-term vitamin D-deplete rats resulted from an adaptation to low extracellular calcium rather than vitamin D depletion.