Characterization of a (Ca²⁺ +Mg²⁺)-ATPase system in the osteoblast plasma membrane

A high affinity, calmodulin-sensitive (Ca² + Mg²)-ATPase was demonstrated in the plasma membrane preparation of three different osteosarcoma cell lines previously demonstrated to respond to parathyroid hormone with an increase in cytosolic calcium and a decrease in pH. The maximal velocity of the ezyme activity in the membrane preparations ranged from 0.83 to 2.42 nmol Pi released per min per mg protein with half-saturation constants of 26 nM of free Ca. The enzyme activity was not affected by Na⁺, K⁺, ouabain and azide, and exhibited an absolute requirement for Mg²⁺ ions. These results suggest a possible role for a membrane Ca² + Mg²⁺-ATPase in initiating and perpetuating the ionic control of osteoblastic function.