AbstractIntermittent compressive (IC) forces (96 mm Hg, 0.3 Hz) inhibit by 35–60% the serum stimulated increase in ornithine decarboxylase activity (ODC) in chick embryo epiphyseal cartilage cells and rat chondrosarcoma cells. IC had no effect on mouse fibroblast L‐cells ODC. The dose‐response pattern of the IC effect indicated an all‐or‐none response with a threshold at 80 mm Hg, a pressure roughly equivalent to the in vivo weight bearing force. The kmof the cartilage cell ODC, measured at four hours, was about 0.1 mM and was not affected by IC. The Vmax, on the other hand, was significantly reduced by IC which is consistent with less enzyme or non‐competitive inhibition. IC also produced a significant increase in cAMP levels in both cartilage explants and isolated cells in the presence and absence of serum and a significant reduction in3H‐thymidine incorporation into DNA. The findings show that cellular cAMP, on one hand, and ODC and DNA synthesis, on the other hand, change in opposite directions following exposure to serum and/or IC. Investigation of the IC effect on DNA synthesis in serum‐deprived synchronized cartilage cells revealed that IC reduced the number of cells going into S but did not lengthen the G1phase. Exposure to IC early in G1(0–13 hours) produced the full effect, whereas IC application between 13 to 24 hours (pre S) had no effect. IC had no effect on3H‐thymidine incor
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