Ultrastructural study of the effects of cytochalasin D administration on the structure and acid trimetaphosphatase activity of osteoclast.

To determine the role of the cytoskeleton consisting of the microfilaments in osteoclasts, cytochalasin D (CD) conjugated with PBS was administered intravenously to rats at a dose of 1 mg/100 g b.w. Control rats were given only PBS. Then the osteoclasts in the metaphyses of the humeri were examined ultrastructurally, as well as acid trimetaphosphatase (TMPase) cytochemistry. The plasma calcium (Ca) concentrations before and after CD and calcitonin administration were also measured. CD administration first caused a prominent reduction of ruffled borders and spreading of clear zones associated with the occurrence of large pale vacuoles in adjacent cytoplasm. At 1 hr after CD administration, the osteoclasts mostly lacked both ruffled borders and clear zones, but still maintained a normal intracellular organization of cytoplasmic organelles. Morphometric analysis confirmed that CD administration resulted in about 70% reduction of both ruffled border and clear zone areas. TMPase secretion from osteoclasts towards the resorbing bone surfaces was strongly inhibited by CD administration. At 1 hr after administration, although CD caused a decrease of plasma Ca concentrations in 4 of 7 examined rats similar to that caused by calcitonin treatment, there was a slight increase of plasma Ca concentrations in the other 3 rats. These results suggest that the structure and bone-resorbing function of the ruffled border-clear zone complex of the osteoclast is highly regulated by cytoskeleton consisting of microfilaments containing F-actin.