Effects of phosphate on calcium release,lysosomal enzyme activity in the medium,and osteoclast morphometry in cultured fetal rat bones

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 β-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 β-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 β-glucuronidase activity or changes in the ruffled border of osteoclasts but that may involve effects on the clear zone of osteoclasts.