Effects of different magnitudes of tension-force on alkaline phosphatase activity in periodontal ligament cells.

Alkaline phosphatase (ALP) activity is involved in the process of calcification in various mineralizing tissues, and it is found at much higher levels in the periodontal ligament (PDL) than in other connective tissues. Since the PDL lies between hard tissues and functions as a cushion mitigating mechanical stress, such as occlusal and orthodontic forces, this stress may modulate ALP activity in PDL cells, which themselves may affect adjacent alveolar bone metabolism. The objective of this study was to determine the level of ALP activity and the gene expression of liver/bone/kidney (L/B/K) ALP in human PDL fibroblasts in response to cyclic tension-forces. Human PDL cells were cultured on flexible-bottomed plates and placed on a Flexercell Strain Unit. Cells were flexed at 6 cycle/min (5 sec strain, 5 sec relaxation) at 6 levels of tension-force (9%, 12%, 15%, 18%, 21%, and 24% increase in surface area) for 5 days. There was no significant difference in cell proliferation between the cells subjected to the tension-force and the controls. There was a 10% and 42% decrease, respectively, in the ALP activity in PDL cells exposed to low (9%) and high (24%) tension-forces, and these decreases were dependent on the magnitude of the tension-force. The finding of inhibited ALP activity in response to tension-force was consistent with the observation that L/B/K ALP mRNA levels were decreased in response to cyclic tension-force. These results suggest that tension-force may affect PDL metabolism, depending on the functional role of ALP.