Endocytosis precedes dissolution of basic calcium phosphate crystals by murine macrophages

Murine peritoneal macrophages were incubated with 45Ca-labeled basic calcium phosphate (BCP) crystals in the presence or absence of cytochalasin B. Untreated macrophages solubilized 30-50% of 45Ca-BCP in 24 hours. Dissolution began within 3 hours and was linear thereafter. Twenty-three percent of BCP was cell-associated by 3 hours. Endocytosis of crystal occurred continuously throughout the incubation. Endocytosis of crystal did not affect the migration of macrophages through Percoll density gradients. Addition of cytochalasin B did not prevent cell-association of BCP, but inhibited solubilization in a dose-dependent manner. Virtually all cell-associated BCP was removed when cytochalasin B-treated cells were washed with EDTA, suggesting that the crystals were only bound to the surface. In contrast, cell-associated BCP in untreated cells was only partially removed by EDTA, suggesting that endocytosis of crystal had occurred. We conclude that cell association of BCP is not sufficient for its dissolution, and that endocytosis precedes solubilization of BCP crystals by macrophages.