Ultrastructural localization of concanavalin-A binding sites on bone cells: effects of con A on osteoclastic bone resorption.

When osteoclasts on the endocranial surface of 10-day old rat calvaria are treated with concanavalin-A and hemocyanin, con A binding sites can be visualized by scanning electron microscopy (SEM). The hemocyanin label was uniformly distributed on the dorsal (unapposed) surface of all osteoclasts observed by SEM; it could be visualized on smooth surfaces, microvillous projections and filopodia. Osteoblasts were also labeled with the hemocyanin marker, demonstrating con A binding sites. 45Ca bone release assays of prelabeled calvaria incubated in varying concentrations of con A were conducted to determine whether or not con A has any functional effects on osteoclasts. At low concentrations of con A (0.1 micrograms/ml), there was an increase in bone resorption at 24, 48 and 72 hours of incubation. In contrast, calvaria incubated in high concentrations of con A (50 micrograms/ml and 100 micrograms/ml) showed a marked decrease in bone resorption significantly different from that of controls (incubated without the addition of con A) at each of the three 24 hour time periods. These differences in the release of 45Ca may indicate direct, dose-dependent effects of con A on osteoclastic bone resorption.