Comparison of the effects of stimulators and inhibitors of resorption on the release of lysosomal enzymes and radioactive calcium from fetal bone in organ culture

The release of lysosomal enzymes, collagenase, and previously incorporated 45Ca from fetal rat long bones cultured in a chemically defined medium is compared. Parathyroid hormone (PTH) and prostaglandin E2 increased the release of β-glucuronidase, acetylglucosaminidase, and cathepsin D, but showed little effect on collagenase activity in the medium at 48 h. The dose-response relations for β-glucuronidase and 45Ca release were similar. However, the increase in lysosomal enzyme release was proportionally greater and occurred earlier than the increase in 45Ca release. PTH also caused a significant increase in total β-glucuronidase activity in bone plus medium. Several agents which stimulate 45Ca release at an optimal concentration, but not at a higher concentration, including dibutyryl cAMP, isobutylmethylxanthine, and the calcium ionophore, A23187, all increased lysosomal enzyme release at the concentration which increased 45Ca release. Three inhibitors of bone resorption (calcitonin, cortisol, and colchicine) blocked lysosomal enzyme release at the same time that 45Ca release decreased. When the bones escaped from calcitonin inhibition, both 45Ca and lysosomalenzyme release increased. While colchicine blocked both lysosomal enzymes and 45CA release, it actually increased the release of bone collagenase, and together with PTH or prostaglandin E2 caused a large increase in free collagenase activity in the medium. These data indicate that lysosomal enzyme release is closely linked to bone resorption and suggest that lysosomal enzymes may have a primary role in initiating resorption, perhaps by acting on noncollagenous matrix or tissue components before mineral removal and collagen degradation