Effects of physiological concentrations of parathyroid hormone on acid phosphatase activity in cultured rat bone cells.

The mechanism by which parathyroid hormone (PTH) induces osteoclastic bone resorption is still incompletely understood. Recent evidence suggests that the hormone exerts its effects indirectly, via the osteoblasts. Bone cells isolated from fetal rat calvaria by enzymatic digestion were used. Two heterogeneous cell populations were isolated by equilibrium density centrifugation on Percoll gradients and maintained by differential culture conditions. These two populations, which are morphologically distinguishable from one another by light and electron microscopy, have been characterized previously both biochemically and with regard to their hormonal (PTH and calcitonin) responses. We have called them type C cells (containing cells with some of the properties of osteoclasts) and type B cells (containing osteoblast-like cells, as well as fibroblasts, chondrocytes and other stromal cells). In the present study, we have further characterized the functional relationship between the two cell populations, with particular regard to the hormonal responses of type C cultures. Acid phosphatase, measured cytochemically in individual cells, was used as a marker for C cell responses. C cells had significantly higher levels of acid phosphatase activity than either B cells or spleen macrophages. Calcitonin (0-10 pg/ml) decreased C cell acid phosphatase activity but was without effect on B cells or spleen macrophages. Co-culture of C cells with B cells produced increased enzyme activity only in the former; this effect could be mimicked if fibroblasts replaced B cells and cell contact was essential for this response. PTH (0-10 pg/ml) raised enzyme activity further in C cells only when they were cultured with B cells. When C cells were cultured so that they shared medium, but were not in contact, with B cells, PTH (2 pg/ml) still increased enzyme activity in the former. Fibroblasts were ineffective in this system. Spleen macrophages were also unresponsive to PTH when substituted for C cells. Calcitonin (10 pg/ml) blocked the effects of PTH on C cells. These results indicate that macrophages are probably not a significant proportion of the C cell population, and that PTH may produce increased acid phosphatase activity in C cells via a humoral factor produced by cells present in B cell cultures.