Effect of parathyroid hormone and calcitonin on the cytoplasmic spreading of isolated osteoclasts.

Osteoclasts, the major agents of bone resorption, were isolated from neonatal rat bone, and the cytoplasmic spreading of these cells was measured after incubation in the presence or absence of hormones or other cell types. Salmon calcitonin, which inhibits osteoclastic bone resorption, reduced spreading in a dose-dependent manner and caused significant inhibition at concentrations as low as 6.7 pg/ml. Parathyroid hormone (PTH) had no effect on the spreading of isolated osteoclasts but if osteoblasts and osteoclasts were co-cultured the addition of PTH caused a marked increase in spreading at concentrations of 0.025 i.u./ml and above. The results suggest that while calcitonin is a direct inhibitor of osteoclastic activity, PTH may stimulate osteoclasts through a primary action on osteoblasts.