[Effects of periodic tension on osteoblast-like cells for cell differentiation and alkaline phosphatase activity]

Bone remodeling requires bone resorption and formation. Bone formation involves osteoblastic activity. Both intermittent and continuous mechanical forces have been proved to affect osteoblast differentiation in vivo. It is evident in the present investigation that periodically applied tension alter cell division, protein synthesis and alkaline phosphatase activity. An initial seeding density of 50,000 cells/well was used for both the mechanically stimulated cells and controls. For the experimental cultures, the cells were subjected to different levels of tension applied at different frequencies; 1. Low frequency (0.3 cycles/minute) at moderate tension (17% elongation); 2. Moderate frequency (3 cycles/minute) at moderate tension (17% elongation); 3. High frequency (30 cycles/minute) at moderate tension (17% elongation); 4. Low tension (9% elongation at moderate frequency (3 cycles/minute); 5. High tension (22% elongation) at moderate frequency (3 cycles/minute). Cell numbers, protein synthesis and alkaline phosphatase activity were measured for each treatment combination, as well as for the unstressed controls. The cells were harvested at 24, 72, and 120 hours after the start of force application. Analysis of variance revealed that periodically applied tension stimulated cell activity in cultured UMR 106-01 cells: 1. Seventeen percent elongation at 3 and 30 cycles/minute was the optimum condition for stimulating osteoblastic cell division; and 2. High frequency stimulation at moderate tension and high tension at moderate frequency stimulated alkaline phosphatase activity. Given these findings, the following conclusions seem warranted: 1. There may be an optimal magnitude of tensional force that stimulates osteoblast activity. Beyond this optimal level, osteoblasts may lose potential to have cell divisions; 2. The optimal condition of tension might be different for cell division and other cell functions. Although high tension (22% elongation) did not stimulate cell division, alkaline phosphatase activity showed significantly high measurement at this condition; and 3. in the light of both the present study and previous investigations, periodically applied tension may stimulate both collagenase (a parameter of bone resorption) and alkaline phosphatase (a parameter of bone formation) production.