The Sensitivity of Isolated Osteoclasts to Morphological Transformation by Calcitonin*

We recently developed a technique for isolating osteoclasts from other bone cells which allows the direct effect of hormones on these cells to be assessed. We found that calcitonin (CT), which inhibits osteoclastic bone resorption, abolished cytoplasmic movement in these cells. In this paper we use the shape-change which accompanies immotility to define the sensitivity of osteoclasts to human and salmon CT. Concentrations of human CT above 62 pg/ml and salmon CT above 3 pg/ml regularly induced the shape-change in osteoclasts which corresponds to complete inhibition of cytoplasmic motility. Morphological transformation of osteoclasts by such low concentrations implies that CT may inhibit bone resorption by inhibiting the motility of the bone-resorptive cell. Even relatively high concentrations of other hormones had no effect on the sensitivity of osteoclasts to CT. This suggests that the response of isolated osteoclasts to CT is sufficiently specific and sensitive to be used as a bioassay, and we used the response to compare the biological potency of several samples of known immunoassayable CT content.