Effects of ultrasound on the metabolism of periodontal ligament cells

Objectives: Periodontal ligament (PDL) plays important roles such as nutritive, homeostatic, and formative functions in enhancing cellular reactions for the remodeling of periodontal tissue. In addition, PDL cells have an ability to differentiate to osteoblast or cementoblast. PDL cells are thus essential for regenerative periodontal therapies. The purpose of this study was to investigate effects of ultrasound (US) stimulation on proliferation and metabolism of PDL cells. Methods: An immortalized human PDL cell line (HPL) with a low ALP and mineralization ability was used. HPL cells were subjected to US exposure (frequency = 1 MHz; pulsed 1:4; intensities = 30 or 150 mW/cm2) for 15 min per day. Expression levels of mineralization related genes including alkaline phosphatase (ALP), type I collagen (Col-I), bone-sialoprotein (BSP), osteocalcin (OCN) and osteopontin (OPN) mRNAs were analyzed by real-time PCR. Furthermore, ALP activity and collagen synthesis were examined after 6-day US exposure (one exposure per each day). Results: Both mRNA and protein levels of ALP and Col-I were significantly (p<0.01 or p<0.05) increased by 30 mW/cm2 US exposure. US with 150 mW/cm2 induced significant (p<0.01) increase in the syntheses of collagen and ALP compared with the controls. Whereas BSP, OCN and OPN, the mRNA levels showed no marked increase. Conclusion: The present results demonstrate that low-intensity US, in particular, enhances ALP activity and collagen synthesis of PDL cells, which would provide an insight into the regeneration of periodontal tissue.