Cerebrospinal fluid calcium homeostasis: evidence for a plasma membrane Ca²⁺-pump in mammalian choroid plexus

A major unanswered question in central nervous system physiology concerns the mechanism by which cerebrospinal fluid (CSF) Ca²⁺ homeostasis is maintained in the face of hypo- or hypercalcemia. To address this question, we sought and found a protein of M_r ∼ 140,000 in choroid plexus plasma membranes that forms a phosphorylated intermediate with characteristics of a plasma membrane Ca²⁺-pump. A choroid plexus plasma membrane protein of this molecular weight also bound to a monoclonal antibody prepared against the human erythrocyte plasma membrane Ca²⁺-Mg²⁺ ATPase Ca²⁺-pump. When this monoclonal antibody was used for immunohistochemical localization, the plasma membrane Ca²⁺-pump was found primarily in the CSF-facing membranes of choroid plexus cells from rats, cats, and man. The localization of a plasma membrane Ca²⁺-pump in the CSF-facing membranes of the choroid plexus suggests that the choroid plexus, by mechanisms including this pump, may regulate CSF Ca²⁺ concentrations.