The Anti-Bone-Resorptive Agent Calcitonin Also Actsin Vitroto Directly Increase Bone Formation and Bone Cell Proliferation*

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Abstract
The studies summarized in this report were intended to determine whether salmon calcitonin had direct effects one bone formation indices in vitro. The results of these investigations demonstrate acute effects of calcitonin on skeletal tissues derived from embryonic chickens to increase calvarial cell proliferation ([4H]thymidine incorporation into DNA) and bone matrix synthesis ([3H)proline incorporation into collagen, as [3H.sbd.hydroxyproline) in intact calvaria and tibiae. The effects of calcitonin on [3H]thymidine incorporation were significant at 1 mU/ml (0.08 nM; P<0.05), additive with respect to the action(s) of F (calcitonin increased the maximum effect of F, and F increased the effect of low dose calcitonin; P<0.01 for each, associated with an increase in total cell protein (r = 0.82; P<0.02), and inversely dependent on osteoblastic differentiation (r =-0.96; P<0.005). The effects of calcitonin to increase bone matrix synthesis ([3H]hydroxyproline incorporation, 139% and 155% of untreated control values for tibiae and calvaria, respectively; P<0.005 for each) were maximal at approximately 5 mU/ml (0.4 nM) and associated with a proportional increase in alkaline phosphate activity in the bones (r = 0.71; P<0.05 for tibiae). These effects of calcitonin were not dependent on continuous exposure. [3H]Thymidine incorporation was increased in calvarial cells 16 h after a 4-h limited (inductive) exposure to calcitonin (at 3 mU/ml; P < 0.01). [3H]Proline incorporation in embryonic chicken calvaria was also increased during 3 days of limited exposure (i.e. 4h/day) to 10 mU/ml calcitonin (P < 0.02). The proliferative action(s) of calcitonin was not unique to chicken osteoblastine cells. Salmon calcitonin also increased [3H]thymidine incorporation in the transformed murine calvarial cell lines MMB and MC-3T3-E1 and in primary cultures of cells prepared from newborn mouse calvaria (P < 0.05 for each). Furthermore, these effects were observed at calcitonin doses (3-30 mU/ml) that also decreased murine bone resorption (i.e. 45Ca release from prelabeled neonatal mouse calvaria; P < 0.01).