Effects of phosphate on calcium release, lysosomal enzyme activity in the medium, and osteoclast morphometry in...

The relationship between changes in medium phosphate concentration and three indices of cell-mediated resorption in fetal rat bone cultures--calcium release, the activity of the lysosomal enzyme beta-glucuronidase in the medium, and the morphology of osteoclasts--has been investigated. Bones treated with either 1 mM or 4 mM phosphate, with or without parathyroid hormone, were examined. After 2 h of culture we found the predominant effect of changes in medium phosphate to be on non-cell-mediated resorption. However, after 24 h changes in medium phosphate affected both cell-mediated and non-cell-mediated resorptive mechanisms. The 24 h effects of phosphate were not associated with either a change in the activity of beta-glucuronidase in the medium or in the area of the ruffled border of osteoclasts, but 4 mM phosphate did prevent parathyroid hormone from increasing the area of the clear zone of osteoclasts. These results imply that changes in medium phosphate alter cell-mediated resorption by affecting mechanisms that are independent of increases in beta-glucuronidase activity or changes in the ruffled border of osteoclasts but that may involve effects on the clear zone of osteoclasts.